What is an airtight house and what does airtightness mean?
Where air pressurisation tests are carried out on a sample of dwellings representing a specific dwelling type, the requirements outlined in TGD L for use of non-default permeability test results should be applied.
The air leakage from a building is specified as the flow of air coming through cracks and gaps out of the building fabric, which consist of floors, walls and roofs. The 2011 TGD part L have specified that a new building must have a maximum air pressure test of 7m3/(hr.m2). This means that no more than 7m3 can escape from the building per hour for every m2 of the building surface area when the buildings internal air pressure is at 50 Pascals.
Compared to PassivHaus (which is the gold standard for energy efficient buildings), which requires a figure less than 1m3/hr.
The main factors to consider while air tightening a house
When building an airtight house the three broad areas should be addressed:
The building envelope such as the walls, roof and floor. When air tightening a building’s fabric the type of building construction or material will determine the type of airtightness system used. Timber frame buildings will use an airtight membrane to seal the envelope. Block walls will use a combination of plaster and membranes. The junction of floor and walls with the different materials can cause problems and care will need to be taken while applying the membranes.
When installing windows and doors on a new building many people focus on the U-value of the window system used but more attention needs to put on the airtightness of the window and its installation. The windows will need to be air tightened when it is been installed in the new build.
Pipes and Cables passing through the building fabric will need to be air tight.
Open fires and Chimneys will need to be sealed in an air tight house, as large quantities of air can escape through the chimney hole. The main purpose of a chimney in a house is to draw large quantities of air from the building but this is against everything we are trying to achieve with an airtight house.
What is Airtight Construction?
When building an airtight house this means that the house is a draught free construction.When constructing a new building a clear plan at the design stage is essential to achieve an airtight house. All the air barrier membranes need to be continuous with overlaps and sealed on joints. Airtight construction does not need to be confusing to understand.
The building envelope will need to be sealed from unwanted draughts. This can be done by installing airtight breathable materials to form the air-tight barrier. Breathable membranes can and should be draught free, this will ensure the building is healthy. Energy efficiency is generally the main reason for air tightness of a new building but it is also vital to protect the building from moisture damage. An airtight house will also provide a greater indoor comfort and efficiency from the heat recovery ventilation (MHRV)
How to assess Airtightness
A blower door test is used to test the airtightness of a building. The blower door test will help to highlight where air leakage is occurring through the fabric of the building.