When constructing a timber frame passive house there are two methods of achieving airtightness which are commonly used:
1. Using a service cavity:
This is the most common of method. The first step in this method is to make the timber frame itself airtight. The load bearing stud is insulated and an airtightness membrane is installed throughout the entire structure. In some structures there is an OSB board applied to the studs and then an airtightness membrane is fitted; this provides further airtightness as the OSB boards are airtight in their own right. The airtightness membrane which is applied prevents air penetration; ensuring airtightness (if fitted correctly). Installing this airtightness membrane relies on high quality workmanship and attention to detail; every joint must be carefully taped or sealed (with mastic) to ensure it is airtight.
Where there are any breaks in the airtightness membrane (for example from services which pass from the internal through the external leaf); grommets are applied to prevent air leakage. Grommets are rubber seals within an airtightness membrane or tape which make the penetration point airtight.
Once the airtightness membrane is installed; work usually begins on installing the service cavity. The service cavity is simply battens which create a small cavity outside the airtightness layer. It is important to note that services are not installed until the structure is checked to ensure its airtightness reaches the passivhaus standard of 0.6 ach-1 @50kPa.
Once the structure has succeeded in obtaining an airtightness level below 0.6 air changes work can begin in the service cavity. The services are installed and the service cavity is insulated. Plasterboard is then applied and skimmed; ensuring that all the surface area is covered, guaranteeing a high airtightness level.
2. Making the inner leaf airtight:
The most common method of making the inner leaf airtight is by using airtightness membranes.
This system does not use a service cavity, instead the services (conduits, pipes, etc...) are run within the loadbearing studs and then the studs are insulated. An osb board (airtight in its own right) and an airtight membrane are then applied or alternatively an airtightness membrane can simply be applied to the stud without the osb board (image on the right); as the membrane once installed correctly, can prevent airleakage.
Where the services penetrate the airtight membrane grommets are applied; these prevent air leakage through the penetration of the airtightness membrane. The plasterboard can then be applied and skimmed ensuring to cover the entire surface area; this further ensures airtightness in the structure.
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