By creating a continuous hermetic airtight layer, enclosing the complete envelope of the house, enclosed by the roof, external walls, windows and ground floor. This should be addressed by treating the house like a ‘balloon’ with no leaks or punctures for air to escape out. Warm, vapour-laden air, that’s generated in the inside of the house, under a driving pressure to the outside, should be prevented from seeping out, by providing a continuous membrane or air tight surface, preventing it from convecting and diffusing through the interior, warm side of the insulation, or through multiple small gaps that may occur at joints and junctions.

This air-tightness also resists vapour from driving into the colder building element where it would cool and condense (below it’s ‘dew-point’, saturation condition) to form dampness in the insulation, thereby leading to further heat loss, mould growth and risk of decay from sustained and trapped dampness within the structure.

When dry lining the external wall on the interior surface, a difficult, concealed place to tackle air tightness, is where a timber upper floor structure or an internal stud-partition meets the external wall.  Plasterboard ceilings and similar wall finishes are porous and do not provide sufficient air tightness or vapour resistance at these critical junctions. This very difficult detail can be addressed, by inserting a continuous airtight membrane through these junctions, sealing with the insulated dry lining. Not alone will this reduce heat-loss, but protect the timbers that may extend into the external wall for support. Their end-bearings can be vulnerable to interstitial condensation, which may eventually cause decay and weaken their support.

Next blog topic: Controlling ventilation and maintaining good indoor air quality.

-Duncan Stewart