Why is it important to use an AVCL on a warm flat roof?
- Read time: 2 minutes
- Date: 15 Jun 2022
- Flat Roofing
An Air and Vapour Control Layer (AVCL) is used within a warm roof system to prevent interstitial condensation from forming below the dew point. An AVCL is made from a material that prevents water vapour from warm air passing through it and is installed on the warm side of the insulation.
What is the dew point?
The dew point is the temperature at which water vapour in warm air turns back into water droplets. This happens because warm air containing moisture from people’s breath or cooking, etc, rises through the building structure, where it cools becoming less able to ‘hold’ this moisture. If there is no AVCL installed the water vapour will penetrate the substrate and insulation and condense on any component below the dew point.
Is an AVCL really necessary?
Absolutely, interstitial condensation can cause mould growth which can affect the health of building occupants. Over time, condensation will also damage the structure of the building by rotting timbers or causing corrosion. In some cases, condensation can be so severe that occupants may confuse it with a leak in the roof. This is because when the temperature drops below the dew point, condensation can form on metal fasteners that penetrate the surface of the cold deck where it drips simulating a rainwater leak. This condition can be identified because it is worse in very cold weather, or when relative humidity is high, but may not occur during heavy rainfall.
