Vapor barrier under corrugated board - the choice and types of vapor barrier films. Waterproofing under corrugated roofing Film for cold roofing

Roof construction is a crucial stage of construction work, which needs special attention. As soon as it comes down to it, the following questions immediately arise: what kind of roof to make, what is its design, what to use as a flooring, insulate it or make it cold? These questions are up to you. We would like to draw your attention to the fact that there are plenty of materials for roof sheathing. All of them have their pros and cons. One of these popular materials is corrugated board. It has a lot of advantages, which is why it is so appreciated by builders. What about vapor barrier? Is it necessary if you use corrugated board? What materials are best suited for this purpose?

Let's take a closer look at the answers to these and other questions.

A bit about the cold roof

It is known that when constructing a roof, work is carried out to insulate it. This is done so that heat does not leave the building, and cold does not get inside. It is especially important to make insulation when the roof is an attic. But, there is another concept, the so-called "cold roof". At the same time, insulation work is not carried out, it is enough to make a crate and lay the material. The use of this type is advisable if the roof does not need to be insulated. The advantage of a cold roof is high efficiency, ease of operation and high speed of construction.

When is a cold roof suitable? For example, in a country or private house there is an attic that you will not use as an attic. Or, you need to close off a building that is not intended for permanent residence. In this case, it is rational to make the roof cold. At the same time, it is important to insulate the ceiling inside the building itself, and block everything using corrugated board. But the question arises: what to say about the vapor barrier, is it needed under the corrugated cold roof? Let's first understand what vapor barrier is generally needed for.

It performs the most important role - it protects the heat-insulating material from moisture penetration. After all, warm and humid air will accumulate in the upper part and on the roof. If the vapor barrier is not done correctly, it will lead to early repair of the roof and reduce the life of the building. Yes, and wooden structures under the influence of moisture will rot. But, there is one nuance here. All this is rational when the attic or attic is insulated, as for the cold roof, everything is not so simple here. Is a vapor barrier necessary in this case? After all, we learned that its main goal is to protect the insulation layer. It is necessary, and there are reasons for that.

Although the room will be well ventilated, it will still suffer from precipitation. Profiled 100% cannot exclude the fact that moisture will penetrate under the roof. Snowfall, heavy rain along with the wind can do their job, and the liquid will get inside. This is fraught with the fact that the metal elements inside will rust and fail. The same applies to wooden products (built, crate, etc.), which will rot and deform.

So, we learned that vapor barrier under corrugated board is still needed, even with a cold roof. But what to do? Let's look at the main vapor barrier materials that can be used under corrugated board.

Glassine as a vapor barrier

All vapor barrier materials are made in the form of films. The material consists of several layers that perform their role or task. One such material is glassine. This, one might say, is a classic for both warm and cold roofs. This material has been used for decades. It is quite reliable and does its job well. In addition, such a vapor barrier will cost you the cheapest. However, it is better not to use material for residential buildings.

If there is a dwelling immediately under the roof, then it will suffer from the properties of glassine. We are talking about the fact that when the material is heated, it emits a not very pleasant aroma. For life, it is not dangerous, just like a pair of roofing material, but for permanent residence it is not quite an ideal option. Moreover, corrugated board is used as a roofing material, which, like all metals, will get very hot under the influence of the sun, especially in summer.

Another disadvantage of glassine is its heavy weight and narrow rolls, so installation will be quite problematic and inconvenient. Summing up, we can note all the pros and cons of the material.

Pros:

• strength;
• reliability;
• flexibility;
• durability;
• low cost.

Minuses:

• when heated, an unpleasant odor appears;
• has a lot of weight;
• installation complexity.

We can conclude: for a living space, the material is not the best option, but it may fit for a summer residence with a non-residential attic or a bath.

Polyethylene film

Ordinary household polyethylene is also suitable for vapor barrier roofs made of corrugated board. It is quite famous and has been used for many years. Polyethylene is able to perfectly retain moisture, water vapor, protecting and isolating the structural elements of the roof from the inside. One of its main advantages is that it is cheap, so you will need less money to install a roof. Polyethylene is able to provide good vapor barrier, no worse than specialized films and various membranes.

But, these are all the advantages of this material. Cheapness is fraught with complexity and impracticality. As you know, polyethylene is not the strongest material, so during installation it can be easily damaged. To avoid this, experts recommend using a thicker whip. This leads to the fact that it will be difficult to fix it without outside help. And yet, such a weight becomes an obstacle in that it will be difficult to attach it to corrugated board in standard ways.

When you are limited in funds, then polyethylene is quite suitable for a vapor barrier layer. It is attached with a construction stapler to the crate inside. The step of fixing the film depends on its thickness. Usually it is about 1-1.5 m. To prevent condensate from getting inside, it is important to seal all places with punctures and joints with construction tape.

Note! In order for the vapor barrier layer to be reliable and fully perform its functions, it is better to fix it in two layers.

Now let's summarize and understand the advantages and disadvantages of polyethylene.

Pros:

• low cost;
• very low permeability.

Minuses:

• insufficient strength;
• short service life;
• big weight;
• complexity of installation;
• a lot of condensate forms, it drains and does not linger on the surface.

Polyethylene is not the best option, but in the absence of funds, you can resort to it.

Reinforced polyethylene vapor barrier film

Roof vapor barrier films, as already mentioned, consist of several layers. It is based on polyethylene, which is reinforced with auxiliary materials. The most budgetary of them is reinforced film. It consists of several layers of polyethylene film, and is also necessarily supplemented with an additional reinforcing mesh made of polypropylene or fabric. Thanks to this structure, the material is strong, it will be difficult to tear it with your own hands. In addition, an accidentally dropped tool will not damage it either, which is quite practical.

In addition to everything, such a film has a greater rigidity than conventional polyethylene. But, despite the large number of layers, its weight is not so great. Therefore, it will be easy for you to install the material on the roof of corrugated board. And you can fix and connect the film with special self-adhesive tapes.

Pros:

• high strength;
• light weight;
• several layers provide reliability and durability;
• relatively low price;
• ease of installation;
• has low vapor permeability.

Minuses:

• as in the case of ordinary polyethylene, a lot of condensate forms, it drains and does not linger on the surface.

If you need to make a good vapor barrier, but you do not have huge funds, then reinforced film is the best solution. This is an ideal option in terms of price-quality ratio for your residential building.

Polypropylene film

It is made exactly according to the same scheme as multilayer reinforced polyethylene. Polypropylene is used to increase the strength of the film. Its installation is also simple. A feature of polypropylene films is the presence of an additional relief layer. What is it for? A lot of condensate forms on the foam, which negatively affects the structural elements.

So that moisture does not harm the insulation (if any) or structural elements, an additional anti-condensation layer is applied on the polypropylene film, which consists of a mixture of cellulose and viscose. The layer easily absorbs condensate, preventing the formation of drops of condensate. This layer is rough, and during installation it should be placed towards the room, with the smooth side outward.

Advice! Before starting work, read the instructions from the manufacturer.

Pros:

• light weight;
• high strength;
• reliability and practicality;
• ease of installation;
• low vapor permeability.

Minuses:

• material without an anti-condensation coating spoils the insulation and structural elements.

Vapor barrier film with foil

This is one of the types of previous films. But, the material not only has an additional reinforcing layer, but is also equipped with an aluminum layer that reflects heat. What does it give? This layer reflects infrared rays, preventing them from leaving the house. This creates a kind of "thermos effect". Foil film is intended for baths and greenhouses, but it began to be used as a vapor barrier for a roof made of corrugated board and other materials. It is highly efficient, reducing heat loss through the roof by 10-15%. This allows you to significantly save energy that is spent on heating. For example, the use of Izospan under corrugated board increases the temperature in the room by only 2˚, while the costs are the same.

Note! The reflective effect is possible only in the air, so when mounting a film with foil, it is important to make a gap of 4-5 cm.

Pros:

• light weight;
• high strength and reliability;
• durability;
• low vapor permeability;
• not harmful to humans;
• reflects infrared radiation, retaining heat.

Minuses:

• high price.

Foil film is one of the best, but as they say, you have to pay for quality. If you have enough funds, then it is better to make a vapor barrier under corrugated roofing from it. However, it is up to you which choice to make.

Today, more and more developers are asking this question. The fact is that a cold roofing system allows you to build a residential building for relatively little money, when an insulated roof requires enormous costs both financially and in terms of time and effort. Realizing that such a system is in a sense better, many developers have wondered if a vapor barrier is needed under a cold roof?

Cold roof construction in a residential building

Many developers who want to save some money on construction seek to purchase the cheapest materials, but at the same time, so that the quality of the entire building does not suffer much from this. This is possible if primitive two slopes are used as a rafter system, corrugated board as a covering, and the roof is made cold.

The design features of such a roof can be listed on the fingers, since there is no need to lay heat-insulating material, a vapor barrier layer and other components that are often found on insulated roofs. If you all doubt the savings, then it is safe to say that the cold version will cost 50-60% less than the insulated design. In addition, the work can be carried out independently, therefore, the savings will increase even more.

A cold roof pie, in most cases, includes the following products:

• rafter legs
• waterproofing material
• Control lathing
• crate
• Profiled metal sheet

Do I need waterproofing under the metal roof of a cold roof?

It is worth noting that all metal surfaces have one similar feature, they collect condensate. Waterproofing, which is part of the roofing pie with a cold roof, will correct this problem and protect the room from incoming moisture, both from the outside and from the inside. Many developers, when building an insulated roof, advise their friends not to lay a vapor barrier layer if they are building a building with a cold type of roof. They think that the cold will remove the condensation that is visible on metal products, but they are greatly mistaken.

Condensation occurs due to the temperature difference between the roof space and the external environment. Naturally, when the attic is not heated and not insulated with anything, the difference will be small, but still it is sufficient to provoke the release of moisture from the air. As you can see, condensate will form regardless of the type of roof, therefore, the question posed at the beginning of the paragraph can be answered unequivocally: yes, it is needed.

An example is outbuildings, the owners of which do not care much about their service life. In such buildings, the roofing cake is the most primitive and even violates building codes and regulations. Most often, a roofing cake consists of rafters, discharged lathing and roofing. All layers of vapor barrier and waterproofing were simply thrown out. Despite this fact, this building can stand for a significantly long period, or vice versa, fall apart after a couple of years. Here you are already lucky, but why take risks if you can do everything in a quality way?

Risks cannot be allowed on residential buildings, as someone's life may depend on it. I am not exaggerating, because if the condensate affects the rafter system and other roof elements for a long time, it will simply destroy them, which can lead to the fall of the entire roofing pie.

IMPORTANT: If at the beginning of construction you decided to build a cold roof, but later on after a certain period you will insulate it, then it is best to lay a microperforated film as a waterproofing product. Its properties are practically no different, but according to the price tag, it is much lower than membranes.

Waterproofing and ventilation of a cold roof

If you are laying a micro-perforated product, then such a layer will block moisture from the outside, but water vapor can pass through this area without problems. It can be said that the installation of vapor barrier products on roof slopes is not necessary in this case. After the steam penetrates inside, it is between the waterproofing material and the metal coating, from where it is removed by natural ventilation.

IMPORTANT: When a dense waterproofing film is used that retains water vapor, this leads to a significant increase in air humidity and, as a rule, dampness in the room.

With increased air humidity, condensation processes begin, so moisture accumulates on the reverse side of the film and penetrates into all roofing materials. In connection with this fact, it is not recommended to use materials such as polyethylene and polypropylene, roofing felt, and glassine for the construction of a cold roof.

The ventilation system of cold roofs is quite simple. To create it, gaps are left, which, as a rule, are located along the cornice overhang. Air masses pass through them, collect all the humidified air and remove it through a cold triangle located in the ridge part of the roof.

The counter grille is a design to provide additional ventilation. Most often it can be found on complex roofing systems, where products with a high degree of waterproofing are used as a coating, for example, metal, shingles and others. The gap that is created by installing a counter-batten allows fresh air to dry the roofing pie from both sides, so structures with such a pie last much longer than the others.

Cold roof. Do buildings need waterproofing?

First of all, it should be noted that outbuildings differ significantly from residential buildings both in their structure and volume. The vast majority of developers are trying to save on the construction of secondary structures, so the issue of waterproofing is more relevant than ever. Before answering the question posed, let's deal with the coverage. To date, corrugated board is one of the cheapest materials, so I will talk about buildings covered with it.

Manufacturers from different countries are striving to produce such products so that the whole world begins to use it. If you are a fan of corrugated board, then today, such material is a sheet of metal with an anti-condensation coating.

In its appearance, it practically does not differ from its counterparts. A synthetic composition is applied from the inside, it looks like felt. Due to the large number of pores, this material is able to retain up to 1 liter of water per square meter. After the surface has been saturated with moisture, ventilation comes into play and if it is done in accordance with all the rules and regulations, then the drying process will not take much time.

Thanks to the laying of such a “cunning” coating, the need for laying a vapor barrier and waterproofing layer is removed, but it should be understood that the weight of such a coating will vary depending on weather conditions. When making the necessary calculations, it is very important to take into account these parameters and use them in the calculations. It is worth noting that the finished roof will be much cheaper, because it will not have almost half the roofing cake.

Installation of waterproofing for a cold roof

So, based on the information obtained above, you realized that the waterproofing material is laid regardless of the type of roof and the functional purpose of the building. However, if you want to save money, you can use roofing with an anti-condensation layer, but not all people like metal coatings, so I will now tell you the general principle of laying waterproofing.

• First of all, an inexperienced roofer must repeat the safety precautions and the principle of working at height. After that, he puts on a special uniform, which should include the following: personal protective equipment, good shoes with non-slip soles and a mounting belt.
• After the rafter legs are fixed in place, you can start laying the vapor barrier layer. It is attached to the rafters with a construction stapler and pressed against the crate. With a small slope, the strips of material are placed across the slope, and along the strongly sloped slopes. To improve the quality of laying this layer, the joints are smeared with bitumen or glued with double tape.

IMPORTANT: Before installing the crate, it is very important to treat its elements with special protective solutions that will increase the degree of ignition of the wood and protect it from decay.

• Next, the counter-lattice is laid. It creates the necessary air gap, thanks to which moisture will be removed from the roofing cake.
• A waterproofing material is laid on top of the konroblattice.
• After that, proceed to the installation of sheets of corrugated board.

Most developers strive to cover as much length as possible with one strip of profiled sheet. This is justified by the fact that in this way fewer joints are obtained, therefore, the waterproofing qualities of the entire roof increase significantly. You can do all the work yourself, but to increase efficiency it is better to invite 1-2 partners.

Waterproofing is a necessary element of the roofing pie, regardless of how the corrugated roof is equipped: according to the principle of a warm or cold roof.

No matter how well the roof is made of corrugated board, moisture can penetrate into the under-roof space through the joints of the roofing sheets and at the points of their attachment to the crate, causing damage to the supporting structures and ceiling finishes.

The reasons why waterproofing is installed under a cold roof from corrugated board

Precipitation

Decking cannot 100% exclude the possibility of moisture getting into the under-roof space of the house. Heavy rain or snowfall combined with intense wind can cause moisture to enter under the roof, which in turn will provoke corrosion.

Wear of fasteners and roofing modules

The peculiarity of the corrugated board is that its sheets are subject to thermal deformation, as a result of which the roof can pass moisture along the junction line of the roofing modules. Often, water penetrates under the roof and at the points of attachment of the corrugated board to the truss system, since over time the tightening of the screws weakens, and the rubber sealing washers are destroyed. The ingress of water into the under-roof space is fraught with wetting of the truss system with its subsequent decay, and as a result, the destruction of the roof frame.

Condensation

With a high-quality installation of a corrugated roof, when the roofing reliably protects the house from atmospheric moisture and leaks, theoretically, you can do without waterproofing.

But sources of moisture are not only rainfall from the outside. It is also condensation that forms on the inner surface of the roof due to the temperature difference inside and outside the house.

The likelihood of condensation becomes even higher when utilities pass through the attic: heating and hot water pipes, ventilation ducts or a fireplace chimney. Liquid and gaseous media circulating through such pipelines usually have an elevated temperature and heat up the air in the under-roof space, contributing to a more intense appearance of condensate.

What is suitable for waterproofing a cold roof under corrugated board

The main conditions that a hydrobarrier for a cold roof made of corrugated board must meet are moisture impermeability, mechanical strength, and resistance to temperature effects.

The most budgetary, but short-lived version of waterproofing is plastic film. Reinforced polyethylene membranes have slightly better strength characteristics. But against their background, polypropylene films and anti-condensate membranes significantly benefit, equipped with a non-woven viscose-cellulose layer that actively absorbs excess moisture.

As an under-roof waterproofing for non-insulated metal roofs made of corrugated board or metal tiles, a waterproofing membrane is optimally suited

The air of a heated dwelling always contains a certain amount of moisture in the form of water vapor. Most of this moisture is removed from the room through the ventilation system, but some of it remains inside the house. It is to protect the insulation from these excess moisture that a vapor barrier film is used under the corrugated board.

Installation of a vapor barrier film on a corrugated roof with a special adhesive tape

Do I need a vapor barrier under the corrugated board?

The amount of moisture that can be contained in the air depends very much on its temperature - the higher it is, the more humidity. Warm, and, therefore, more humid air is lighter than cold air and always accumulates in the upper part of the room under the ceiling or roof. There it gradually cools down, giving off the moisture released at the same time to the surrounding building structures.

If from the sides the interior of the house is protected by walls, then from above the only protection against the cold is a layer of insulation. And due to the fact that heat-insulating materials have a porous structure, they very intensively absorb the moisture contained in the air. At the same time, their thermal insulation characteristics are greatly reduced. The dependence of the thermal conductivity of the insulation on its humidity can be seen in the table below.

This is where the vapor barrier of corrugated roofing comes to the rescue. It protects the insulation layer from moisture contained in warm air. That is why a vapor barrier is laid below the insulation from the side of a warm room. At the same time, it also protects the building structures from the ingress of condensate, which can form on the lower surface of the thermal insulation layer.

On the types of vapor barriers, as well as on which vapor barrier is best for the roof and under what conditions, we will describe in detail in the next section.

What is the vapor barrier under the profiled sheet?

Vapor barrier materials are produced in the form of films. Most often they consist of several layers, each of which performs its own, special task. The most common types of roof vapor barrier are discussed below.

Glassine as a vapor barrier under corrugated board

Glassine is one of the classic options for vapor barrier, which has been used for decades. It is durable, reliable, strong and perfectly retains water vapor. In addition, using glassine for vapor barrier is one of the cheapest options.

However, it is better not to use this material for residential buildings, especially if there is a dwelling under the roof. The fact is that when heated, glassine begins to emit a not very pleasant smell. Of course, it is not dangerous, like a pair of roofing material, but for a house it is not the best option. Since a metal profile is used as a roofing, the vapor barrier will heat up quite often, especially in summer.

Glassine in a standard roll, which is mounted as a vapor barrier

In addition, glassine as a vapor barrier is not very convenient to install, since it weighs quite a lot and is produced in narrow rolls. Still, it is designed to be laid on top of a solid crate as a waterproofing.

Advantages:

• Durability;
• Reliability;
• Strength;
• Flexibility;
• Low cost.

Disadvantages:

• Great weight;
• Even with a slight heating emits an unpleasant odor;
• The complexity of installation as a vapor barrier.

If you don’t know which vapor barrier to choose for the roof of a bathhouse or a cottage with a non-residential attic, then glassine will be an excellent option in all respects. For a residential home, this would not be the best choice, unless of course cost is your priority.

Vapor barrier device made of polyethylene film

Quite often, the choice of a vapor barrier film stops at ordinary household polyethylene. This is the most famous and very long used vapor barrier material. It perfectly retains water vapor, completely isolating the insulation and other elements of the roofing pie from moisture from inside the room. In addition, ordinary polyethylene film for vapor barrier is very cheap - at least several times cheaper than specialized films of the same material, and an order of magnitude cheaper than various membranes.

A roll of ordinary polyethylene film that can be used as a vapor barrier

However, this is where the benefits of using this material end. Polyethylene film as a vapor barrier is very cheap, but difficult and impractical. It is not durable and is easily damaged during installation. In order to avoid this, it is necessary to use a very thick film, which makes it almost impossible to fix it without help. In addition, due to its weight, the polyethylene film under the metal profile is poorly attached using standard methods.

If for you, nevertheless, the question of price is fundamental, then you can make a vapor barrier layer from a polyethylene film. It must be fastened with a construction stapler to the inner crate. The fastening frequency depends on the thickness of the film and is usually 1-1.5 meters. Places of punctures and joints of the film must be glued with a special construction adhesive tape. In order for such a vapor barrier under corrugated board to be reliable, it is recommended to make two layers.

Advantages:

• Cheapness;
• Extremely low vapor permeability.

Disadvantages:

• The complexity of installation;
• Great weight;
• Low strength;
• fragility;

Thus, if cheapness is the main criterion based on which you decide which vapor barrier to choose, then using a simple polyethylene film is justified. In all other cases, this is far from the best option.

Reinforced polyethylene vapor barrier film

Modern vapor barrier films for roofing are often multi-layered and consist of the same polyethylene, which is reinforced with auxiliary materials.

The most budgetary and common option is a reinforced vapor barrier film. It consists of several layers of polyethylene with an obligatory reinforcing layer of polypropylene mesh or fabric. Such a structure provides the film with fairly good strength - it is difficult to tear it with your hands or damage it with an accidentally dropped tool.

Vapor barrier polyethylene film reinforced with polypropylene

In addition, reinforced vapor barrier has greater rigidity than ordinary polyethylene film, and, despite its multi-layered nature, is quite light. These properties make it easy to assemble and also make it possible to connect and fasten it with the help of special self-adhesive tapes.

Advantages:

• Good strength;
• Ease;
• Low vapor permeability;

Disadvantages:

• A large amount of condensate may form, which drains without holding on to the surface.

Reinforced polyethylene vapor barrier is the best vapor barrier film if you are looking for an inexpensive but high quality solution for your residential building. The best option in terms of value for money.

Polypropylene film for vapor barrier is made according to the same principle as multilayer polyethylene. The use of polypropylene is due to the need to increase the strength of the vapor barrier film in order to facilitate its installation.

In addition to the material used, polypyrene films have a slightly different structure - they have an additional relief layer. It is necessary because a large amount of condensate forms on these films, which can lead to damage to the insulation.

Multilayer polypropylene film for vapor barrier with anti-condensation fabric layer

In order to prevent such a development of events, an additional anti-condensation layer is applied to polypropylene films, usually consisting of a mixture of viscose and cellulose. It absorbs condensation, preventing droplets from breaking off, which then evaporates as soon as the humidity in the room decreases.

As a rule, films with an anti-condensation surface are mounted with the rough side towards the room, and the smooth side outward. However, it all depends on the manufacturer, so read the instructions carefully before installation.

Advantages:

• High strength;
• Reliability due to several layers;
• Ease;
• Low vapor permeability;

Disadvantages:

• Films without an anti-condensate layer spoil the insulation.

Polypropylene film is the best vapor barrier for roofing in the budget segment. Due to its increased strength and anti-condensation coating, it is practically devoid of flaws and is the best option in terms of reliability and durability.

Vapor barrier with foil

Foil vapor barrier film is a special case of polyethylene or polypropylene film. In this case, it is not only reinforced, but also provided with a reflective layer of aluminum. Such an aluminum vapor barrier reflects infrared radiation, preventing it from leaving the house, thereby creating a kind of "thermos effect".

Foil vapor barrier material was originally intended for use in baths and greenhouses, but is now often used for ordinary residential premises. Its efficiency is quite high - on average, heat losses are reduced by 10-15%, which can significantly save on home heating. For example, bedding Izospan under corrugated board increases the temperature of a heated room by about 2 ° at the same cost.

It should be borne in mind that the effect of heat reflection is possible only in the air, so the vapor barrier with aluminum foil must be installed with a gap. Regardless of the finishing material, it should be 4-5 cm between the reflective part and the wall material. In addition, the aluminum vapor barrier should not be laid close to the insulation - you need to leave a small gap.

Advantages:

• High strength;
• Reliability due to several layers;
• Ease;
• Low vapor permeability;
• Infrared reflection and heat saving.

Disadvantages:

• Price.

Undoubtedly, foil vapor barrier film is one of the best options among non-breathable materials. If you are not very limited in finances and have firmly decided to use a vapor barrier film, and not a membrane, do not hesitate about which vapor barrier is better - take aluminum. It will not only perfectly fulfill its direct functions, but also keep the heat in your home.

How to choose a vapor barrier for corrugated board?

All common types of roof vapor barrier have already been discussed in detail above. Each material has its own scope and application features, so before choosing a vapor barrier film, you must decide exactly what result you want to get.

First of all, you must answer the question, what is more important to you: price or effect? If the price is of primary importance, then it is worth determining the budget and choosing the best option within it. If efficiency is more important, then which vapor barrier film to choose will depend on the operating conditions.

So, for a small country house with a cold roof, ordinary glassine is enough. For a rarely used cottage with an attic, you can use polypropylene or polyethylene film - if the house is not heated all winter, then it does not make sense to use expensive foil materials. If we are talking about a residential building, then, of course, the best option would be a vapor barrier with a reflective layer of aluminum.

Today, more and more developers are asking this question. The fact is that a cold roofing system allows you to build a residential building for relatively little money, when an insulated roof requires enormous costs both financially and in terms of time and effort. Realizing that such a system is in a sense better, many developers have wondered if a vapor barrier is needed under a cold roof?

Many developers who want to save some money on construction seek to purchase the cheapest materials, but at the same time, so that the quality of the entire building does not suffer much from this. This is possible if primitive two slopes are used as a rafter system, corrugated board as a covering, and the roof is made cold.

The design features of such a roof can be listed on the fingers, since there is no need to lay heat-insulating material, a vapor barrier layer and other components that are often found on insulated roofs. If you all doubt the savings, then it is safe to say that the cold version will cost 50-60% less than the insulated design. In addition, the work can be carried out independently, therefore, the savings will increase even more.

A cold roof pie, in most cases, includes the following products:

• rafter legs
• waterproofing material
• Control lathing
• crate
• Profiled metal sheet

Do I need waterproofing under the metal roof of a cold roof?

It is worth noting that all metal surfaces have one similar feature, they collect condensate. Waterproofing, which is part of the roofing pie with a cold roof, will correct this problem and protect the room from incoming moisture, both from the outside and from the inside. Many developers, when building an insulated roof, advise their friends not to lay a vapor barrier layer if they are building a building with a cold type of roof. They think that the cold will remove the condensation that is visible on metal products, but they are greatly mistaken.

Condensation occurs due to the temperature difference between the roof space and the external environment. Naturally, when the attic is not heated and not insulated with anything, the difference will be small, but still it is sufficient to provoke the release of moisture from the air. As you can see, condensate will form regardless of the type of roof, therefore, the question posed at the beginning of the paragraph can be answered unequivocally: yes, it is needed.

An example is outbuildings, the owners of which do not care much about their service life. In such buildings, the roofing cake is the most primitive and even violates building codes and regulations. Most often, a roofing cake consists of rafters, discharged lathing and roofing. All layers of vapor barrier and waterproofing were simply thrown out. Despite this fact, this building can stand for a significantly long period, or vice versa, fall apart after a couple of years. Here you are already lucky, but why take risks if you can do everything in a quality way?

Risks cannot be allowed on residential buildings, as someone's life may depend on it. I am not exaggerating, because if the condensate affects the rafter system and other roof elements for a long time, it will simply destroy them, which can lead to the fall of the entire roofing pie.

IMPORTANT: If at the beginning of construction you decided to build a cold roof, but later on after a certain period you will insulate it, then it is best to lay a microperforated film as a waterproofing product. Its properties are practically no different, but according to the price tag, it is much lower than membranes.

Waterproofing and ventilation of a cold roof

If you are laying a micro-perforated product, then such a layer will block moisture from the outside, but water vapor can pass through this area without problems. It can be said that the installation of vapor barrier products on roof slopes is not necessary in this case. After the steam penetrates inside, it is between the waterproofing material and the metal coating, from where it is removed by natural ventilation.

IMPORTANT: When a dense waterproofing film is used that retains water vapor, this leads to a significant increase in air humidity and, as a rule, dampness in the room.

With increased air humidity, condensation processes begin, so moisture accumulates on the reverse side of the film and penetrates into all roofing materials. In connection with this fact, it is not recommended to use materials such as polyethylene and polypropylene, roofing felt, and glassine for the construction of a cold roof.

The ventilation system of cold roofs is quite simple. To create it, gaps are left, which, as a rule, are located along the cornice overhang. Air masses pass through them, collect all the humidified air and remove it through a cold triangle located in the ridge part of the roof.

The counter grille is a design to provide additional ventilation. Most often it can be found on complex roofing systems, where products with a high degree of waterproofing are used as a coating, for example, metal, shingles and others. The gap that is created by installing a counter-batten allows fresh air to dry the roofing pie from both sides, so structures with such a pie last much longer than the others.

Cold roof. Do buildings need waterproofing?

First of all, it should be noted that outbuildings differ significantly from residential buildings both in their structure and volume. The vast majority of developers are trying to save on the construction of secondary structures, so the issue of waterproofing is more relevant than ever. Before answering the question posed, let's deal with the coverage. To date, corrugated board is one of the cheapest materials, so I will talk about buildings covered with it.

Manufacturers from different countries are striving to produce such products so that the whole world begins to use it. If you are a fan of corrugated board, then today, such material is a sheet of metal with an anti-condensation coating.

In its appearance, it practically does not differ from its counterparts. A synthetic composition is applied from the inside, it looks like felt. Due to the large number of pores, this material is able to retain up to 1 liter of water per square meter. After the surface has been saturated with moisture, ventilation comes into play and if it is done in accordance with all the rules and regulations, then the drying process will not take much time.

Thanks to the laying of such a “cunning” coating, the need for laying a vapor barrier and waterproofing layer is removed, but it should be understood that the weight of such a coating will vary depending on weather conditions. When making the necessary calculations, it is very important to take into account these parameters and use them in the calculations. It is worth noting that the finished roof will be much cheaper, because it will not have almost half the roofing cake.

Installation of waterproofing for a cold roof

So, based on the information obtained above, you realized that the waterproofing material is laid regardless of the type of roof and the functional purpose of the building. However, if you want to save money, you can use roofing with an anti-condensation layer, but not all people like metal coatings, so I will now tell you the general principle of laying waterproofing.

• First of all, an inexperienced roofer must repeat the safety precautions and the principle of working at height. After that, he puts on a special uniform, which should include the following: personal protective equipment, good shoes with non-slip soles and a mounting belt.
• After the rafter legs are fixed in place, you can start laying the vapor barrier layer. It is attached to the rafters with a construction stapler and pressed against the crate. With a small slope, the strips of material are placed across the slope, and along the strongly sloped slopes. To improve the quality of laying this layer, the joints are smeared with bitumen or glued with double tape.

IMPORTANT: Before installing the crate, it is very important to treat its elements with special protective solutions that will increase the degree of ignition of the wood and protect it from decay.

• Next, the counter-lattice is laid. It creates the necessary air gap, thanks to which moisture will be removed from the roofing cake.
• A waterproofing material is laid on top of the konroblattice.
• After that, proceed to the installation of sheets of corrugated board.

Most developers strive to cover as much length as possible with one strip of profiled sheet. This is justified by the fact that in this way fewer joints are obtained, therefore, the waterproofing qualities of the entire roof increase significantly. You can do all the work yourself, but to increase efficiency it is better to invite 1-2 partners.

Roof construction is a crucial stage of construction work, which needs special attention. As soon as it comes down to it, the following questions immediately arise: what kind of roof to make, what is its design, what to use as a flooring, insulate it or make it cold? These questions are up to you. We would like to draw your attention to the fact that there are plenty of materials for roof sheathing. All of them have their pros and cons. One of these popular materials is corrugated board. It has a lot of advantages, which is why it is so appreciated by builders. What about vapor barrier? Is it necessary if you use corrugated board? What materials are best suited for this purpose?

Let's take a closer look at the answers to these and other questions.

A bit about the cold roof

It is known that when constructing a roof, work is carried out to insulate it. This is done so that heat does not leave the building, and cold does not get inside. It is especially important to make insulation when the roof is an attic. But, there is another concept, the so-called "cold roof". At the same time, insulation work is not carried out, it is enough to make a crate and lay the material. The use of this type is advisable if the roof does not need to be insulated. The advantage of a cold roof is high efficiency, ease of operation and high speed of construction.

When is a cold roof suitable? For example, in a country or private house there is an attic that you will not use as an attic. Or, you need to close off a building that is not intended for permanent residence. In this case, it is rational to make the roof cold. At the same time, it is important to insulate the ceiling inside the building itself, and block everything using corrugated board. But the question arises: what to say about the vapor barrier, is it needed under the corrugated cold roof? Let's first understand what vapor barrier is generally needed for.

It performs the most important role - it protects the heat-insulating material from moisture penetration. After all, warm and humid air will accumulate in the upper part and on the roof. If the vapor barrier is not done correctly, it will lead to early repair of the roof and reduce the life of the building. Yes, and wooden structures under the influence of moisture will rot. But, there is one nuance here. All this is rational when the attic or attic is insulated, as for the cold roof, everything is not so simple here. Is a vapor barrier necessary in this case? After all, we learned that its main goal is to protect the insulation layer. It is necessary, and there are reasons for that.

Although the room will be well ventilated, it will still suffer from precipitation. Profiled 100% cannot exclude the fact that moisture will penetrate under the roof. Snowfall, heavy rain along with the wind can do their job, and the liquid will get inside. This is fraught with the fact that the metal elements inside will rust and fail. The same applies to wooden products (built, crate, etc.), which will rot and deform.

So, we learned that vapor barrier under corrugated board is still needed, even with a cold roof. But what to do? Let's look at the main vapor barrier materials that can be used under corrugated board.

Glassine as a vapor barrier

All vapor barrier materials are made in the form of films. The material consists of several layers that perform their role or task. One such material is glassine. This, one might say, is a classic for both warm and cold roofs. This material has been used for decades. It is quite reliable and does its job well. In addition, such a vapor barrier will cost you the cheapest. However, it is better not to use material for residential buildings.

If there is a dwelling immediately under the roof, then it will suffer from the properties of glassine. We are talking about the fact that when the material is heated, it emits a not very pleasant aroma. For life, it is not dangerous, just like a pair of roofing material, but for permanent residence it is not quite an ideal option. Moreover, corrugated board is used as a roofing material, which, like all metals, will get very hot under the influence of the sun, especially in summer.

Another disadvantage of glassine is its heavy weight and narrow rolls, so installation will be quite problematic and inconvenient. Summing up, we can note all the pros and cons of the material.

• strength;
• reliability;
• flexibility;
• durability;
• low cost.

• when heated, an unpleasant odor appears;
• has a lot of weight;
• installation complexity.

We can conclude: for a living space, the material is not the best option, but it may fit for a summer residence with a non-residential attic or a bath.

Polyethylene film

Ordinary household polyethylene is also suitable for vapor barrier roofs made of corrugated board. It is quite famous and has been used for many years. Polyethylene is able to perfectly retain moisture, water vapor, protecting and isolating the structural elements of the roof from the inside. One of its main advantages is that it is cheap, so you will need less money to install a roof. Polyethylene is able to provide good vapor barrier, no worse than specialized films and various membranes.

But, these are all the advantages of this material. Cheapness is fraught with complexity and impracticality. As you know, polyethylene is not the strongest material, so during installation it can be easily damaged. To avoid this, experts recommend using a thicker whip. This leads to the fact that it will be difficult to fix it without outside help. And yet, such a weight becomes an obstacle in that it will be difficult to attach it to corrugated board in standard ways.

When you are limited in funds, then polyethylene is quite suitable for a vapor barrier layer. It is attached with a construction stapler to the crate inside. The step of fixing the film depends on its thickness. Usually it is about 1–1.5 m. To prevent condensate from getting inside, it is important to seal all places with punctures and joints with construction tape.

Note! In order for the vapor barrier layer to be reliable and fully perform its functions, it is better to fix it in two layers.

Now let's summarize and understand the advantages and disadvantages of polyethylene.

• low cost;
• very low permeability.

• insufficient strength;
• short service life;
• big weight;
• complexity of installation;
• a lot of condensate forms, it drains and does not linger on the surface.

Polyethylene is not the best option, but in the absence of funds, you can resort to it.

Reinforced polyethylene vapor barrier film

Roof vapor barrier films, as already mentioned, consist of several layers. It is based on polyethylene, which is reinforced with auxiliary materials. The most budgetary of them is reinforced film. It consists of several layers of polyethylene film, and is also necessarily supplemented with an additional reinforcing mesh made of polypropylene or fabric. Thanks to this structure, the material is strong, it will be difficult to tear it with your own hands. In addition, an accidentally dropped tool will not damage it either, which is quite practical.

In addition to everything, such a film has a greater rigidity than conventional polyethylene. But, despite the large number of layers, its weight is not so great. Therefore, it will be easy for you to install the material on the roof of corrugated board. And you can fix and connect the film with special self-adhesive tapes.

• high strength;
• light weight;
• several layers provide reliability and durability;
• relatively low price;
• ease of installation;
• has low vapor permeability.

• as in the case of ordinary polyethylene, a lot of condensate forms, it drains and does not linger on the surface.

If you need to make a good vapor barrier, but you do not have huge funds, then reinforced film is the best solution. This is an ideal option in terms of price-quality ratio for your residential building.

Polypropylene film

It is made exactly according to the same scheme as multilayer reinforced polyethylene. Polypropylene is used to increase the strength of the film. Its installation is also simple. A feature of polypropylene films is the presence of an additional relief layer. What is it for? A lot of condensate forms on the foam, which negatively affects the structural elements.

So that moisture does not harm the insulation (if any) or structural elements, an additional anti-condensation layer is applied on the polypropylene film, which consists of a mixture of cellulose and viscose. The layer easily absorbs condensate, preventing the formation of drops of condensate. This layer is rough, and during installation it should be placed towards the room, with the smooth side outward.

Advice! Before starting work, read the instructions from the manufacturer.

• light weight;
• high strength;
• reliability and practicality;
• ease of installation;
• low vapor permeability.

• material without an anti-condensation coating spoils the insulation and structural elements.

Vapor barrier film with foil

This is one of the types of previous films. But, the material not only has an additional reinforcing layer, but is also equipped with an aluminum layer that reflects heat. What does it give? This layer reflects infrared rays, preventing them from leaving the house. This creates a kind of "thermos effect". Foil film is intended for baths and greenhouses, but it began to be used as a vapor barrier for a roof made of corrugated board and other materials. It is highly efficient, reducing heat loss through the roof by 10–15%. This allows you to significantly save energy that is spent on heating. For example, the use of Izospan under corrugated board increases the temperature in the room by only 2˚, while the costs are the same.

Note! The reflective effect is possible only in the air, so when mounting a film with foil, it is important to make a gap of 4–5 cm.

• light weight;
• high strength and reliability;
• durability;
• low vapor permeability;
• not harmful to humans;
• reflects infrared radiation, retaining heat.

• high price.

Foil film is one of the best, but as they say, you have to pay for quality. If you have enough funds, then it is better to make a vapor barrier under corrugated roofing from it. However, it is up to you which choice to make.

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The roof plays a key role in protecting the home from rain and other adverse effects. To preserve heat and maintain a comfortable microclimate, a number of materials are used. Together they make up a roofing pie. So that the heated air does not leave the room during the cold season, heat-insulating materials are used under the corrugated board. Maintain a low degree of thermal conductivity will allow vapor barrier.

Why do you need a vapor barrier under corrugated board

Vapor barrier (as well as waterproofing) is arranged under corrugated board to achieve two goals:

1. Protect the thermal insulation layer from moisture. When wet, its thermal conductivity increases. Also, the penetration of liquid leads to the gradual destruction of the insulation.
2. Prevent the accumulation of moisture in the thermal insulation layer and direct the water vapor to the outside.

Water vapor is always present in the air of a residential building. Partially they leave through the ventilation system, a certain proportion of them remains in the room. Warm air, saturated with vapor, rises up to the ceiling. There it cools, while moisture is given to roofing materials.

Due to its porous structure, the insulation absorbs moisture. As it gets wet, its thermal conductivity increases, and it retains heat in the room worse. To prevent the penetration of moisture into the pores of the insulation, use a vapor barrier film under the corrugated board. It is laid from the side of the room and is needed to protect the insulation and other structural elements from water vapor.

What to consider when choosing a vapor barrier

Many types of vapor barrier films for the roof are available in hardware stores - both cold and attic. To make the right choice, you need to be aware of the following factors:

• price;
• efficiency;
• expediency.

Before purchasing materials for corrugated board, you need to decide on the amount allocated for construction. If the price of the material is decisive, you need to choose from economical materials.

The second point is efficiency. Vapor barrier films have different performance characteristics, which are important to familiarize yourself with before buying.

It is worth choosing the material appropriate to the characteristics of the room. For a residential heated house, a foil film would be the best option, while for a cold roof of a country house, inexpensive glassine is quite sufficient. If the roof of a rarely visited mansion is being built, which is not heated for a long time, you need a simple vapor barrier made of polyethylene or polypropylene.

What materials are used for vapor barrier

The vapor barrier layer for corrugated board or any other roofing material is made in the form of a film. The following are the most common types of it, which are presented in hardware stores.

Polyethylene film. Household polyethylene has long been used as a vapor barrier material. He does not let the steam rising from the living quarters. Polyethylene is a cheap material compared to reinforced films or membranes. Its significant disadvantage is the lack of strength. It is easy to damage the film during installation. For reliable vapor barrier, it is laid in two layers and fixed on the inner crate with a construction stapler. Mounting step - from one to one and a half meters. If the film is still damaged, a patch is needed. The puncture site should be covered with construction tape. Tape must be fixed and joints. Given the above, plastic film is quite acceptable and cheap, but at the same time not the best option for vapor barrier.

Reinforced polyethylene film. Also inexpensive, but high-quality material for vapor barrier under corrugated board. It is made of several layers of polyethylene and reinforced with a polypropylene mesh. Such a film, with a low weight, has sufficient strength, it is more difficult to damage it during installation. Due to the reinforcing mesh, the material has greater rigidity. The film can be fixed with a special tape. This is the best combination of low price and good quality material, suitable for vapor barrier of residential buildings.

Glassine. This material has long been used for vapor barrier. It reliably retains moisture, is durable and has a long service life, but has a rather large weight. Although glassine is a cheap material, it is not recommended for living spaces. When heated, an unpleasant odor emanates from it, and under the corrugated board its temperature will often rise, especially in the warm season. Therefore, glassine should be used only for a bath or a cold roof of a non-residential attic. It is more convenient to use it as a waterproofing - then it will need to be laid on top of the crate.

Polypropylene film. Durable material for vapor barrier under corrugated board. It is made similarly to a multilayer polyethylene film. The two main differences are the use of polypropylene and the presence of a relief layer. The fact is that condensate accumulates on smooth plastic films, which collects in drops and periodically drips onto the floor. Rough anti-condensation coating is made on the basis of viscose and cellulose. It absorbs moisture and prevents it from collecting in drops. Subsequently, with a decrease in air humidity, the collected condensate evaporates. As a rule, a polypropylene film has an anti-condensate surface down towards the room. To clarify the installation procedure, you need to refer to the instructions from the manufacturer. This type of vapor barrier film is one of the best options for arranging a roof for corrugated board.

foil film. In fact, it is a kind of polyethylene or polypropylene film. In addition to the reinforcing mesh, it has a thin aluminum layer. It reflects infrared rays and helps keep the heat in the house. Although at first this material was used in the arrangement of the bath, it was also used for vapor barrier in residential buildings. After laying the foil film, heat losses are reduced by 10 percent or more. Its price is higher than other types of vapor barrier films, but it justifies itself by saving on heating costs. The foil film is laid with an aluminum layer towards the room. Be sure to make a gap between the material and the wall finish: you need a gap of about 5 cm. A small distance should also be left to the insulation. The film is fixed with self-adhesive tape, all joints are closed with aluminum tape. Then the vapor barrier layer will be airtight.

Features of a cold roof

Although the space directly under the cold roof is not heated, the temperature inside will still be different from the temperature outside. Therefore, condensation will begin to appear on the underside of the corrugated board. In view of this, a waterproofing layer is needed to prevent moisture from entering the wooden rafters and other structural elements of a cold roof.

Important! The waterproofing layer is laid on top of the rafters. Then the material is fixed with a counter-lattice, followed by the crate and the profiled sheet itself.

If the waterproofing layer does not prevent steam from escaping to the outside, a vapor barrier for cold roof slopes is not required. Water vapor, having risen, will penetrate through the film under the corrugated board, and there it will be dried under the action of air currents.

It is important to choose a material for waterproofing a cold roof that will not trap steam. In this case, polyethylene, polypropylene, roofing material or glassine will not work. For a cold roof, a non-woven vapor permeable membrane is recommended.

As you can see, vapor barrier is arranged only for a warm roof in order to protect the insulation from water vapor. There are a few tips to help you get the job done right.

1. It is better to place panels of vapor barrier material with an overlap of about 15 cm.
2. It is necessary to select a suitable connecting tape for the selected film or membrane.
3. Planks for vapor barrier can be made of both wood and metal. They must be small.

When arranging a warm roof, vapor barrier is an important component of the roofing pie. The material is chosen on the basis of financial and practical considerations, as well as guided by its feasibility.