With the trend towards greater energy efficiency in buildings, as driven by Building Regulations, air tightness plays a vital role in meeting these standards. While in the past little or no attention was paid to airtightness, in recent years it has come become clear how much air tightness can contribute to the energy balance of a building. Air tightness protects the insulation against air movement and humidity, both of which have a negative impact on thermal performance, and it has been demonstrated that poor airtightness can be responsible for up to 40% of heat loss from buildings.
- An envelope can be airtight only if it consists of ONE undisturbed airtight layer encapsulating the entire volume.
- For each construction element (walls, floors, ceilings, etc.), it must be specified, which plane will form the airtight layer.
- In a second step it has to be determined, how the airtight layers of the components will be connected to guarantee effective and continuous airtightness.
For many building designs, AVCLs have become the standard to meet building airtightness re- quirements throughout Europe. AVCLs help to enable compliance with Part L 2013, which reduces the limit for airtightness from 10m3/hm2 to 8m3/h.m2 at 50 Pa and which came into effect from April 2014.
Why is an AVCL is necessary?
An AVCL has two basic functions: Controlling the amount of vapour that migrates through the building envelope; and providing an airtight seal when properly installed and maintained. Various studies have demonstrated that fitting an AVCL is necessary in order to prevent moisture from the interior from ingressing to the walls or ceiling, a problem which could potentially result in mould formation, structural damage and the loss of thermal performance, thereby leading to increased energy costs.
Omitting an AVCL can run the risk of moisture accumulation in the construction, resulting from water vapour migration from wet trades during the construction process that have not completely dried out, or from vapour release arising from occupant activity. Simulations for specific building designs are possible by using standard calculation techniques, Glaser being a leading model.
Effective sealing of the building envelope requires both good workmanship and the right choice of AVCL. Water vapour, which can find many ways into the construction, is difficult to control and could result in risks to both the building and its occupants.
Effective drying out of a building can sometimes be difficult to achieve, especially as the moisture content of some construction materials is not always obvious. Once the building is in use, diffusion and convection are the typical methods of moisture migration into the structure. Diffusion takes place via pressure differences between the interior and the exterior. During winter, diffusion takes place from the building interior towards the exterior whereas, during the summer, the vapour drive is from outside to in. How much vapour can pass through the airtightness layer depends on its resistance to diffusion (MNs/g or sd-value expressed in m). Convection takes place by air flow, for example through poor joints. A high degree of convective vapour can result in high moisture accumulation in the insulation layer resulting potentially in a considerable risk of condensation.
Vapour diffusion can be calculated based on the likely environmental conditions and vapour resistance of the airtightnes layer. The extent of convective vapour drive, together with built-in moisture, however, is difficult to assess, but can be effectively limited through high quality installation.
AVCLs can differ significantly
Any AVCL should be airtight by definition. The degree of vapour diffusion, however, depends on the material composition. It can be distinguished by four different product technologies: Monolithic (solid) Polyethylene membranes, typically with high and fixed sd-values; Composite AVCL membranes with low to medium fixed sd-values; Reflective AVCLs; and AVCLs with variable sd-values by means of humidity concentration.
Under normal room humidity levels (40-70%), the water vapour resistance of a variable AVCL will not only block moisture transmission during the winter months, but also enables drying out of unwanted humidity during the summer months. In general, the higher the sd-value, the higher the moisture resistance during winter conditions, but conversely, the lower the drying out potential during summer weather. High vapour tightness reduces the capacity of a membrane to allow moisture trapped within the construction to dry out easily.
Many roof and wall assemblies are only durable if they can dry-out to the interior side as well4 In many cases moisture damage can be attributed to the fact that a vapour barrier is practically impermeable in both directions, i.e. it does not permit any drying out.5
Polyethylene membranes are typically offered with sd-values ranging from >=20m up to 100- 150m (>=100MNs/g up to 500-750 MNs/g and above). Due to their high vapour resistance, they can quickly become a moisture trap, as drying out of the construction toward the room side is rarely possible.
AVCLs with a variable sd-value offer excellent protection against condensation risk and potential damage due to the adaptation of their water vapour resistance by means of humidity concentration. During the winter the sd-value of the membrane increases as the relative humidity within the building interior increases, thus reducing moisture transfer towards the insulation when vapour control is needed most. The sd-value decreases as the relative humidity drops during the summer months, allowing the construction to dry out towards the interior.
The broader the sd-value range of a variable AVCL, the higher the humidity protection. The lower sd-value represents the drying out potential of a variable AVCL during summer months. A difference of 0.2-0.25m may sound small, but this can have a big impact if humidity is trapped within the construction. The upper sd-value represents the water vapour transmission during winter months, the higher the value the less moisture will penetrate from the room side towards the insulation.
DuPont has developed a special laminate technology enabling a lower sd-value of 0.05m (=25MNs/g) and an upper sd-value of 30m (=150MNs/g) for its adaptable AVCL DuPont™ Air- guard® Smart. DuPont™ Airguard® Smart provides outstanding humidity protection compared to other AVCLs, and helps to accelerate the drying out process of wet construction materials. Its capacity is approximately 1400-2000 times greater than Polyethylene membranes with an sd-value of 75-100m (375-500 MNs/g) and 5-6 times greater than comparable AVCLs with a variable sd-value.
For rooms with extreme humidity levels (>70%), AVCLs with high sd-values are recommended for installation after the drying out of construction materials. Special care should be given to the installation method to avoid moisture being trapped in the construction, since the high sd-value does not allow for drying out during summer months.
DuPont Building Innovations offers a range of AVCL products covering a broad range of applications: DuPont™ Airguard® Control, 25MNs/g, which is suitable for roofs, walls and floors with a diffusion open outer layer. This is a mechanically strong membrane with less risk of damage during installation, it is also translucent making it ideal for air-blown insulation applications and can be easily located for fixing; DuPont™ Airguard® Reflective, 10 000MNs/g: This is a 100% air and vapour tight AVCL. It has a metallised surface with a very low emissivity which significantly improves its thermal insulation performance, providing an additional 0.58-0.80 m2K/W to the R-value.6 It is ideal for applications with high humidity that require high vapour resistance; DuPont™ Airguard® Smart is an exceptional membrane with highly adaptable vapour resistance. It helps to protect against structural mould damage due to its high drying out potential, furthermore its flexibility combined with excellent robustness, makes it easy to install and handle.
The selection of an AVCL depends on a number of criteria, as for example room humidity level, the external sd-value or the diffusion openness of the insulation. DuPont has developed a product se- lector to help choose from its range of AVCL products. The technical hotline team is also available to assist you with further information on application specific questions.
