Did making my house airtight cause condensation?

What this usually means

Older homes leak air through hundreds of small gaps, and that uncontrolled leakage does two jobs at once: it wastes heat, and it accidentally ventilates the home, carrying away the water vapour from cooking, washing, drying and breathing. When a retrofit seals those gaps to save the heat, it also removes the accidental ventilation — and unless deliberate ventilation is put in its place, the moisture that used to leave now stays indoors. Humidity climbs, the dew point rises, and condensation begins to form on cold surfaces that were previously fine.

This is why a home can feel stuffy and start growing mould shortly after draught-proofing, new windows or general sealing, even though every individual measure was sensible. The occupant has done the right thing for energy but only half of the measure: tightened the envelope without controlling the air change. The symptoms — misted windows, damp corners, a close stale feeling, mould behind furniture — are all signs of moisture that the home can no longer shed.

The correct approach treats airtightness and ventilation as a single design decision: seal the uncontrolled leaks so you choose where air comes in and out, then provide controlled ventilation — trickle vents and extract, continuous mechanical extract, or mechanical ventilation with heat recovery — sized to remove the moisture and supply fresh air. Done this way, the home is both efficient and healthy: it keeps its heat and loses its moisture, which is exactly what uncontrolled draughts could never do well.

Common causes

Signs and symptoms

Indoor humidity is set by a balance between the moisture occupants generate and the rate ventilation removes it. Air leakage contributes an uncontrolled, weather-driven share of that ventilation; under cold, windy conditions it is high, which is why leaky homes rarely suffer chronic condensation even though they are uncomfortable. Sealing the envelope cuts that uncontrolled share dramatically, so unless controlled ventilation replaces it, the moisture-removal rate falls, indoor humidity rises and the dew point climbs toward the temperature of ordinary cold surfaces.

Once the dew point rises to meet a surface temperature, condensation forms there and persists long enough for mould. The surfaces most at risk are the coldest and least ventilated — window reveals, corners, the wall behind furniture, the back of a wardrobe — which is exactly where post-sealing mould appears. The physics is identical to ordinary condensation; the retrofit simply pushed the humidity up by removing the leakage that used to keep it down.

The remedy is controlled ventilation matched to the tightened fabric: enough fresh-air supply and moist-air extract to hold humidity at a healthy level, ideally with heat recovery so the energy benefit of airtightness is kept. This is the meaning of 'build tight, ventilate right' — airtightness lets you choose and control the air paths, and ventilation then does the moisture removal that random leakage did badly. Measuring the airtightness achieved and designing ventilation to suit is what makes the pair work together.

How to fix condensation after sealing a home

Keep the airtightness — it saves energy — and add the controlled ventilation it needs. Do not reopen the draughts; replace them with ventilation you can control.

1. 01

   Measure airtightness and humidity

   Establish how tight the home now is and how high humidity runs, to size the ventilation needed.

2. 02

   Add background ventilation

   Fit trickle vents or a continuous supply so fresh air enters in a controlled way.

3. 03

   Ensure effective extract

   Provide working extract in kitchens and bathrooms to remove moisture at source.

4. 04

   Consider MVHR for tight homes

   Where the home is genuinely airtight, mechanical ventilation with heat recovery removes moisture while keeping the heat.

5. 05

   Manage moisture sources

   Reduce indoor drying and use extract while cooking and bathing to lower the load.

6. 06

   Verify healthy humidity

   Re-measure to confirm humidity and condensation have returned to safe levels.

How to prevent it coming back

• Plan ventilation whenever you seal a home or fit airtight windows.
• Always include trickle vents with new sealed windows.
• Keep kitchen and bathroom extract working and used.
• Aim for 'build tight, ventilate right' as one combined measure.