Won't making my house airtight cause damp?

Done correctly — airtight AND properly ventilated — it does the opposite: it keeps moisture out of both the structure and the air, making the house drier and healthier. Damp problems arise when airtightness is increased WITHOUT adding ventilation, so the moisture occupants generate gets trapped. The solution to that is ventilation, never deliberately leaving the building leaky.

What is interstitial condensation?

It's condensation that forms inside the construction — within a wall or roof build-up — rather than on a visible surface. It's typically driven by warm, moist indoor air leaking into the cold parts of the structure (or, more slowly, diffusing through it) and cooling below its dew point. Because it's hidden, it can cause timber decay and mould before you notice. Air leakage is the main culprit, which is why airtightness controls it.

How do I know if air leakage is causing hidden condensation?

Signs include musty smells, staining at ceiling/wall junctions, soft or decaying timber (e.g. joist ends, roof timbers), and mould in concealed voids. Diagnosis combines a blower door survey (to find the exfiltration paths) with thermal imaging and, where needed, condensation-risk analysis of the build-up. We investigate the air paths and the moisture together, because they're the same problem.

Airtightness & Interstitial Condensation Risk Explained

There are two ways water vapour gets into a building's construction: it can diffuse slowly through materials (vapour diffusion), or it can be carried bodily by moving air (air-transported moisture, or convection). The second mechanism is far more powerful — and it is exactly what air leakage enables. Understanding this changes how you think about airtightness: it's not just about heat, it's about keeping the structure dry.

Air-transported moisture vs vapour diffusion

The numbers are stark. Studies of moisture transport repeatedly show that the quantity of water carried into a construction by air leakage through a small gap can be tens to hundreds of times greater than the amount that diffuses through the same area of intact material over the same period. Warm indoor air holds a lot of water vapour; when that air is driven through a leak in the envelope, it carries all that moisture with it.

Two routes for moisture into the structure

Mechanism	Driver	Relative quantity of water moved
Vapour diffusion	Vapour pressure gradient through intact material	Small — slow, distributed
Air-transported moisture	Warm moist air driven through a leak (convection)	Large — tens to hundreds× more

Exfiltration condensation — the hidden failure

Here's the mechanism that rots buildings from the inside out. In winter, the warm, moist indoor air is at higher pressure at high level (the stack effect), so it is driven outward through any leaks in the upper envelope — ceiling penetrations, the wall head, roof junctions. As this warm, humid air travels out through the cold construction, it cools. When it cools below its dew point, the water vapour it's carrying condenses — deep inside the wall or roof, on the cold side, where you cannot see it.

Warm, moist indoor air (say 20 °C, 50% RH) leaks into the construction through a gap in the air barrier.
As it moves toward the cold outer side, its temperature falls.
When it reaches its dew point, moisture condenses on the cold surfaces inside the build-up (interstitial condensation).
Repeated over a winter, this wets timber, sheathing and insulation — feeding rot, corrosion and mould, all hidden from view.

Why airtightness protects the structure

A continuous, well-positioned air barrier stops the warm, moist indoor air from entering the construction in the first place — removing the dominant moisture-transport mechanism. That's why, counter-intuitively, making a building more airtight generally makes it drier and safer, not damper: you've cut off the route by which most of the water was getting into the walls and roof. The key qualifier is 'well-positioned' — the air barrier and the vapour control strategy have to be designed together, especially in retrofit.

The dangerous mistake: sealing without ventilating

Here is where well-meant retrofits go wrong. If you make a building airtight but do NOT add controlled ventilation, you trap the moisture that occupants generate — cooking, washing, drying clothes, breathing — inside the living space. Indoor humidity climbs, and now you get the other kind of condensation: surface condensation and mould on cold internal surfaces, plus poor air quality. The building was relying on its draughts for ventilation, and you just removed them.

Airtight + ventilated (right): moisture is barred from the structure AND removed from the living space. Dry, healthy, efficient.
Airtight + NOT ventilated (wrong): structure protected, but indoor humidity soars → surface condensation, mould, stuffy air.
Leaky (old default): draughts remove some moisture but waste energy and drive air-transported moisture into the fabric. Inefficient and risky.

Designing it right — the joined-up approach

Moisture-safe airtightness means designing the air barrier, the vapour control, the insulation and the ventilation as one system — and verifying it. On our projects that means: a continuous air barrier positioned to keep warm moist air out of the cold construction; a vapour control strategy appropriate to the build-up (assessed by condensation-risk analysis where insulation is added); a designed ventilation system sized for the now-tight envelope; and verification by blower door testing and thermal imaging. Get those four working together and the building is drier, warmer, healthier and more durable than the leaky original — not less.

Airtightness and condensation risk — why leaky buildings rot from the inside.