Airtightness 2
Posted by Christopher Hirst on
Insulation, Insulation, Insulation – everywhere we look we are bombarded by companies telling us that insulation is the way forward when we want to achieve a low energy, nearly zero or Passive House buildings.
However, insulation is nothing without airtightness, consider a really thermally efficient wall great, but if we have holes through the junctions of the wall or worse still a permeable wall structure then the thermal effect we are hoping to gain from will be lost to fresh air.
Part F does address this problem to a certain extent as all new houses require a defined figure to be included into the SAP calculation. The “blower door test” consists of sealing all the designed openings in a building and fixing a fan to a door frame. This fan then pressurises the inside of the dwelling to 50 Pascals which is equivalent to a 20mph wind. A dwelling can pass this test by losing 10M3 of air per M2 per hour.
A Passive House requires 0.6 air changes per hour nearly 20 times better than the limiting value for our building regulations. A number of houses are now being designed at 5 air changes but as we move to the proposed changes for 2013 and 2016 then we will see air changes coming down to below 3 as a standard. If the building is constructed from a permeable structure such as a lightweight frame or dense concrete then the structure will need to be sealed to prevent air leakage. This can be achieved by parging a block wall or by using specially designed membranes fixed to the frame.
This will of course only seal the structure of the building, it does not address the problem of air leakage through gaps in the building which have not been designed for example gaps around the window frames. It is important to identify every penetration through the building fabric and to ensure that each penetration has been sealed, this is when being a pedant is an asset.
A common area which creates a problem is the underside of a cold roof, electrical penetrations even through sealed plasterboard can be a problem. Although edge sealed plasterboard is an accepted form of air tightness barrier we have all seen cracks form in plasterboard ceilings which would break the air tightness barrier. There are two options for providing an air tight membrane in this situation, the first is to use a foil membrane stapled to the underside of the trusses / ceiling joists fix battens to form a service void onto which the plasterboard can be fixed. Alternatively any service wiring or ducting can be located above the foil membrane and brought through the membrane using the wire or ducting collars.
To help achieve this, a full range of products are available from Sirch Building Materials to prevent air leakage from a building such as tapes and membranes.
However, insulation is nothing without airtightness, consider a really thermally efficient wall great, but if we have holes through the junctions of the wall or worse still a permeable wall structure then the thermal effect we are hoping to gain from will be lost to fresh air.
Part F does address this problem to a certain extent as all new houses require a defined figure to be included into the SAP calculation. The “blower door test” consists of sealing all the designed openings in a building and fixing a fan to a door frame. This fan then pressurises the inside of the dwelling to 50 Pascals which is equivalent to a 20mph wind. A dwelling can pass this test by losing 10M3 of air per M2 per hour.
A Passive House requires 0.6 air changes per hour nearly 20 times better than the limiting value for our building regulations. A number of houses are now being designed at 5 air changes but as we move to the proposed changes for 2013 and 2016 then we will see air changes coming down to below 3 as a standard. If the building is constructed from a permeable structure such as a lightweight frame or dense concrete then the structure will need to be sealed to prevent air leakage. This can be achieved by parging a block wall or by using specially designed membranes fixed to the frame.
This will of course only seal the structure of the building, it does not address the problem of air leakage through gaps in the building which have not been designed for example gaps around the window frames. It is important to identify every penetration through the building fabric and to ensure that each penetration has been sealed, this is when being a pedant is an asset.
A common area which creates a problem is the underside of a cold roof, electrical penetrations even through sealed plasterboard can be a problem. Although edge sealed plasterboard is an accepted form of air tightness barrier we have all seen cracks form in plasterboard ceilings which would break the air tightness barrier. There are two options for providing an air tight membrane in this situation, the first is to use a foil membrane stapled to the underside of the trusses / ceiling joists fix battens to form a service void onto which the plasterboard can be fixed. Alternatively any service wiring or ducting can be located above the foil membrane and brought through the membrane using the wire or ducting collars.
To help achieve this, a full range of products are available from Sirch Building Materials to prevent air leakage from a building such as tapes and membranes.