Airtightness in Passive HousesAchieving the Passive Standard is to ensure air tightness. The traditional notion of house building never envisaged air tightness as an issue. Today the achievement of the Passive standard is crucially intertwined with air tightness. Air tightness effects
energy use
and
comfort in the home.
So what do we mean by air tightness. The official explanation is
The air tightness of a dwelling, or its air permeability, is expressed in terms of air leakage in cubic meters per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascal’s (m3/(h.m2)@50Pa).
The dwelling envelope area is defined in this context as the total area of all floors, walls and ceilings bordering the dwelling, including elements adjoining other heated or unheated spaces.
Air leakage is defined as the flow of air through gaps and cracks in the building fabric. 
Uncontrolled air leakage increases the amount of heat loss as warm air is displaced through the envelope by colder air from outside. Air leakage of warm damp air through the building structure can also lead to condensation within the fabric (interstitial condensation), which reduces insulation performance and causes fabric deterioration.In effect air tightness is expressed by the amount of air in cubic metres that leaks out of a house every hour.
The default value is 10m3/Hour.m2 whereas 3m3/hour.m2 is an excellent figure. The passive standard is 0.6 air changes per hour. The effects of poor air tightness is simply heat loss by convection. The aim of the passive house is to eliminate air loss and reduce heat loss as a result. This is very important to the build process particularly in the area of quality control. So how is it achieved? The simple answer by care design and control in the build environment. Air loss normally happens at junctions such as wall to floors and walls to ceilings. These can be summarised as follows • Plastering not finished to floors. • Poorly positioned sockets and lighting • Pipe work through walls and ceilings • Windows poorly fitted • Doors poorly fitted • Window sills poorly fitted. • Poor choice of materials.
2. Noisy house in windy weather 3. Unequal heating 4. Uncomfortable rooms The passive house is designed to eliminate these errors by sealing the wall and floor joints and by sealing the wall to ceiling joints. Passive houses eliminate air loss at windows and doors. Passive houses eliminate air loss at floor joists. This attention to detail will ensure that air leakage is minimised throughout the house
The proof of the pudding is in the eating and the air test will determine if the workmanship has been up to standard. To achieve the air tightness values some different construction methods are developed. For example the junction between the ceiling and walls is normally constructed with a 2” lath on to the roof truss and the ceiling plaster board screwed to the lath. The light socket is screwed to the plaster board. As you can guess this is not very efficient in terms of air leakage. Now by screwing 20mm plywood board to the roof truss which is sealed at each joint and at the walls and then screwing a 50mm lath to form a space for electrics and then screwing the ceiling plaster board to the lath you will get an acceptable sealed ceiling to wall joint. One thing we haven’t discussed is the loss of chimneys in the building. You can have chimneys as long as they do not interfere with the air tightness and thermal bridge requirements. You also can have a room sealed balanced wood stove. In passive homes chimneys are not needed but a stove may be put in place for aesthetic reasons.
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