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Flat Roof Vapour Control Layer

When constructing a flat roof, a vapour control layer sits between the insulation and the roofing deck. Its main purpose is to shield the roof from the effects of condensation, which can damage it. The condensation occurs when warm air condenses and comes in contact with the cooler areas above the condensation. Hence, a vapour control layer is necessary to ensure your roof lasts long. The following are more details about the need for a flat roof vapour control layer.

Vapour Control Layer: What Is It?

A vapour control layer is one of the most important components of construction because it reduces the amount of moisture coming into contact with the roof. The VCL prevents mould build-up, which can cause respiratory issues. As your roof accumulates moisture, it damages the wooden frames or corrodes other materials, thus causing structural problems. The process might take a long period, but eventually, it would compromise your building's structure.

The VCL also prevents the insulation from accumulating moisture, a situation which could damage its thermal properties. The vapour control layers needed depend on the extent of humidity coming into contact with the roof.

How to Install Vapour Control Layers

When installing a vapour control layer, an individual could choose a vapour check or barrier. Here is a guide regarding the installation of either option.

Vapour Barriers

The vapour barrier is important in determining whether or not your roof will accumulate moisture. Therefore, every penetration needs sealing and overlap binding to prevent moisture accumulation, especially in highly humid areas. It's also advisable to isolate the barrier from any movement because this enhances its integrity. Using two layers is the best way to make your barrier last for a long period.

Vapour Checks

Builders use single layers to construct vapour checks and don't require partial bonding. The partial bonding is unnecessary because small movements don't cause many changes in the vapour check's function. Penetrations and unsealed overlaps would also cause minimal or unnoticeable damage.

How to Choose a Vapour Control Layer

The choice of a VCL depends on the material on which it will be used. Different types of substances are used in constructing a flat roof; therefore, choosing the right VCL ensures maximum protection against damage. The following are the common materials used on these roofs and what to consider when choosing the vapour control layer.

Concrete Decks

A double-layer system is ideal for installing a vapour control system on concrete decks. If you choose two layers, ensure that the first layer is polyester or 5U. Choose a glass-based substance for the second layer because it ensures maximum sealing of the vapour control system. It also allows for efficient lap security.

When using a single layer, ensure it has a special metal lining the vapour barrier substance. The layer should be fully bonded to offer the best results. If you choose mastic asphalt, use a 13mm layer on the glass tissue as the VCL.

Bitumen bonding facilitates the vapour check process and offers reliable results when placed on glass-based roofing. Polyester also offers similar results.

Timber Decks

When constructing in an area with high humidity, it's recommended to avoid timber roofs. If necessary, use high-grade timber because it has a high resistance to humidity, making your roof last longer.

The vapour control material chosen should also facilitate the installation of two layers to offer the best results. Ensure the first layer comprises nailed polyesters or other materials with the same strength. The second layer should also comprise high-performance materials such as BS 747 type 3B.

Metal Decks

Most of the buildings constructed in places with high humidity have metal decks. In this case, the builders bond the vapour control barrier onto the deck. The VCL has two layers because the troughs in metal decks make it challenging to ensure lap security. Polyester makes the first layer, while the second comprises a glass-based material to offer maximum results.

The troughs also make using a single layer with a metal-lined barrier challenging. However, if one must use a single layer, the longitudinal laps should measure 100mm and a 150mm sheet to support the vapour barrier. Also, high-quality work is essential to facilitate lap security. A second layer offers better results if the building is located in an extremely humid area.

Wood Wool and Plywood

These decks tend to experience a lot of movement around the joints. Therefore, the first layer of VCL should consist of a perforated glass base and an additional layer of polyester.

Part bonding offers maximum results when using a metal-lined barrier, especially on decks requiring a lot of movement. Type 3G provides an effective method to achieve part bonding on the first layer.

The Need for Flat Roof Vapour Control Layer

The use of a building plays an important role in determining the need for a vapour control layer. For instance, buildings likely to accumulate a lot of moisture need a lot of expertise when installing the VCL. The roof could accumulate moisture if poorly installed, thus compromising the structure's strength.

Here is an analysis of different buildings and their need for a vapour control layer on flat roofs.

Residential Homes

Residential houses tend to accumulate a lot of moisture due to daily activities. While the ventilation system allows much of this vapour to escape the building, some of it accumulates in the roof. Hence, a vapour check is enough to prevent moisture accumulation and damage to your roof. If the house requires frequent heating and has poor ventilation, it's best to have two layers for additional strength.

Commercial Buildings

The requirements for these buildings vary, and therefore, it's best to have it assessed before installing a vapour barrier. Some houses might need a vapour check, while others would require a barrier. For instance, sports halls tend to accumulate a lot of moisture, so their vapour control requirements would vary from those of offices.

Warehouses and other factories which don't involve wet processes don't need vapour control layers. However, those involving moisture would require a vapour check or barrier, depending on the building's use. The scenario explains why some building owners wouldn't allow certain businesses to operate on these premises to avoid compromising their integrity.

If you plan to construct a commercial building, it's best to have a vapour control layer on its flat roof to cater to diverse future uses. Kitchens and toilets would need sufficient ventilation and vapour control layers because they involve a lot of moisture which could damage the roof.

Vapour Control Layer vs Breather Membrane

When talking about vapour control layers, some people confuse them with breather membranes. The difference is that a vapour control layer is placed inside the insulation. It prevents warm air from the building from damaging the roof. The vapour control layer should always have sufficient warmth to facilitate its function. Hence, enough insulation is crucial to control the VCL's temperature. If its temperature drops below the dew point, the vapour control layer won't prevent your roof from moisture damage.

A breather membrane is placed on the outside of the insulation. It acts as a weather barrier and allows moisture to escape buildings. A breather membrane is ideal in areas that experience adverse weather conditions such as snow or frequent rain. If not used, the flat roof could get damaged from the outside.

Vapour Control Layers vs Vapour Barriers

A vapour control layer reduces air and water movement into the flat roof through a membrane, thus enhancing the roof's integrity. It's considered air and vapour permeable. A vapour barrier ensures no air or water movement onto the roof, thus offering the roof complete protection from moisture. It doesn't limit these elements to a controlled level.

A vapour barrier will offer an ideal choice if you seek an effective way to prevent vapour movement in a building. It prevents all air and moisture from entering the roof and, therefore, is an assurance that your roof will never get damaged by these elements. They are ideal for buildings that accumulate a lot of moisture, such as bathrooms and kitchens.

Vapour barriers also make a good fit for houses with minimal ventilation, which hold many people for long hours. Such buildings accumulate a lot of moisture, which could cause a lot of damage to the roof if not controlled well.

Is a Vapour Control Layer a Necessity?

A vapour control layer protects a flat roof from moisture inside the house. If the house has a lot of moisture which doesn't escape through ventilation, it’ll accumulate on the roof, thus weakening the structure. If the issue isn't solved, the roof could collapse in the long run.

While a vapour control layer isn't always necessary, it's needed in houses with a lot of moisture or in highly humid areas. It's best to have an expert assess a house's condition to determine whether it will need a vapour control layer.

Vapour control layers play an important role in enhancing a flat roof's integrity. Therefore, it's best to have the best quality installed depending on the materials used in constructing the flat roof. Roofing experts will be able to advise you on best VCL materials ideal for your building.