Airtight construction means creating one continuous airtight layer around the heated part of a building, so that no uncontrolled air flows through joints, laps and junctions. It is not about taping up ventilation grilles; it is about eliminating the draughts you have no control over.
The building physics rule of thumb: insulating without air sealing is half a job. A gap of a few millimetres running right through the construction loses more heat than a decent area of poor insulation — and worse, it carries moist indoor air into the structure.
Why it matters
- Heat loss — warm air that leaks out has to be reheated
- Moisture damage — the biggest risk. Warm indoor air flowing through a gap into the cold construction cools down there and deposits its moisture as condensation, which in time causes rot and mould in insulation or roof boarding
- Draughts and cold spots — more often caused by air leaks than by poor glazing
- Sound — an air leak is also a sound leak
- Ventilation that works — a mechanical ventilation system with heat recovery only reaches its efficiency if the air passes through the system and not through the cracks
The rule: tight inside, open outside
The airtight and vapour-control layer goes on the warm inner side; a water-resisting but vapour-open layer goes on the cold outer side. Moisture that does get in can then dry outwards. The classic mistake is a vapour-tight layer on the outside: moisture goes in but cannot get out.
Leakage points in order of importance
- Window frame to wall junction
- Roof plane — laps in the vapour-control membrane and the junctions at wall plate and ridge
- Penetrations — electrical boxes, pipes, ventilation ducts, roof penetrations
- Ground floor to external wall junction — the perimeter strip
- Loft hatch, crawl space hatch and meter cupboard
- Floor and ceiling junctions where timber joists are built into the wall
Getting it right on site
- Draw the airtight layer first — you must be able to trace it around the whole building in one continuous line without lifting the pen
- Use the right products — airtightness tape for membrane laps, grommets for penetrations, pre-compressed sealing tape at window frames. Ordinary duct tape lets go within a few years
- Plaster counts — a fully plastered internal wall is airtight, but only if the plaster continues behind the skirting down to the floor
- Test during construction — have the blower door test done as soon as the airtight layer is complete but still accessible. After finishing, repairs are expensive and sometimes impossible
- Plan the ventilation — airtight construction without controlled ventilation trades heat loss for poor indoor air quality
Related terms
- Airtightness
- Blower door test
- Vapour control layer
- Airtight tape
- Condensation
- Construction moisture
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