Ventilation: Why Airtight Homes Still Need Air

An airtight home still needs air because people, cooking, bathing, pets, cleaning products and building materials all add moisture and pollutants to indoor air, and that air must be replaced with fresh air at a controlled rate. In a leaky old home, that replacement happened accidentally through gaps, chimneys and ill-fitting windows, which wasted heat but rarely left air stale. When insulation, new windows and draught-proofing close those accidental routes, the home needs a deliberate route instead, or moisture, carbon dioxide, cooking odours and volatile compounds build up.

The point of ventilation is therefore not to make a home draughty again, but to give fresh air a planned path, at a planned rate, to the rooms that need it, while stale air leaves from kitchens and bathrooms. That planned path can be as simple as trickle vents plus extract fans, or as complete as a mechanical system with heat recovery.

Moisture is the most visible consequence of under-ventilation. A household produces litres of water vapour daily through breathing, cooking, showering and drying clothes indoors, and that vapour condenses on the coldest surfaces, which may be window glass, external wall corners or inside a wall build-up where it cannot be seen. Persistent condensation leads to mould growth, which affects health, damages finishes and can rot timber, so ventilation is a building protection measure as well as a comfort one.

Pollutants are less visible but equally relevant. Carbon dioxide builds up in bedrooms overnight, cooking produces particles and nitrogen dioxide from gas hobs, and cleaning products, furnishings and new materials release volatile compounds. Fresh air dilutes all of them, and a ventilation system that only runs when someone remembers to open a window will not dilute them reliably.

Check for signs of under-ventilation: condensation that persists beyond morning, mould in corners or behind furniture, stuffiness in bedrooms, cooking smells that linger for hours, and trickle vents or fans that have been taped shut or switched off because of noise or draughts. Note which rooms have vents, which have fans and whether fans actually run, including whether they are humidistat-controlled or manually switched.

Also note occupancy and moisture sources: how many people live there, whether clothes are dried indoors, whether showers are frequent, and whether any room is used as a drying space. Those patterns determine how much ventilation the home actually needs, beyond any generic rate.

In the EU, ventilation requirements are set nationally under the EPBD framework, with member states differing on required air change rates, on whether mechanical ventilation is required in new homes, and on how ventilation is assessed in energy certificates. Some countries set rates per person, others per room or per floor area, so a rate quoted from one country should not be applied directly to another.

In the UK, Approved Document F sets ventilation provisions for new and renovated homes in England, with devolved equivalents in Scotland, Wales and Northern Ireland. Those documents distinguish background ventilation, extract ventilation and purge ventilation, and they set different expectations for new homes, where airtightness is tested, and for existing homes, where work must not make ventilation worse.

A common mistake is treating ventilation as the enemy of warmth and blocking every vent to save heat, which saves a little energy while creating moisture and air quality problems that cost far more to fix. Another is relying on opening windows as the whole ventilation strategy, which works on a mild day when someone is home and remembers, but fails on a cold night, during a holiday or in a noisy street where windows stay shut.

A third mistake is fitting a powerful extract fan in a kitchen or bathroom without a fresh air inlet, so the fan struggles, pulls air from a chimney or a neighbouring room, or creates a negative pressure that affects a stove or boiler. Ventilation needs both an inlet and an outlet, planned together.

Keep the ventilation design or survey notes, fan and vent specifications, any airtightness test result and photographs of vent positions. Those papers let a later installer or assessor see how air was intended to move, rather than guessing from blocked vents.

Ask a qualified professional: how much background ventilation does this home need, where should fresh air enter and stale air leave, what happens when windows are shut in winter, and what maintenance will vents and fans need. A whole-home answer, not a single-room one, is the test.