Making a home more airtight keeps heat in, but it also keeps moisture and other indoor pollutants in unless the home has enough ventilation. In a sealed-up home, condensation and mould become more likely, and in some parts of the UK the level of radon, a natural radioactive gas from the ground, can rise too.

None of this is a reason to avoid insulation, new windows or draught-proofing. The UK government's damp and mould guidance says an energy-efficient home is less prone to condensation, as long as it is adequately ventilated. The point is to plan the ventilation at the same time as the energy work, not afterwards.

Why does an airtight home get more condensation?

Everyday life puts a lot of water into the air: cooking, showering, washing, drying clothes and breathing. In an older, draughty house much of that damp air leaks out through gaps. Seal those gaps and the moisture stays inside until something removes it.

The government's guidance on the health risks of damp and mould says it plainly: if a home is inadequately ventilated, moisture in the air cannot escape and is more likely to cause condensation. It also warns that some energy efficiency work may reduce overall ventilation rates, and that if ventilation is not addressed this can increase the risk of damp and mould, poor indoor air quality and overheating in warm weather.

Condensation then settles on the coldest surfaces, such as window frames, corners and walls behind furniture, and mould grows where the surface stays damp. Our guide to what causes mould in a home explains the different kinds of damp in more detail.

Why does airtightness matter for radon?

Radon is a colourless, odourless radioactive gas that comes from uranium in rocks and soil. The UK Health Security Agency (UKHSA) explains on its UKradon website that buildings naturally draw air in from the ground through cracks and gaps in their floors, and that air contains radon. UKHSA's page on the health risks of radon says it causes over 1,100 lung cancer deaths a year in the UK.

When a home is made more airtight, less outdoor air comes in to dilute whatever radon has entered from the ground. A large observational study of 470,689 UK homes, published in the journal Indoor Air in 2019, found that homes with double glazing, loft insulation or wall insulation recorded on average higher radon measurements than homes with no recorded insulation or glazing work. The study shows an association, not proof that a particular home will change, and the effect matters most where radon levels are already raised.

The only way to know your home's radon level is to measure it. UKHSA's standard home test runs for three months with two detectors, one in a living area and one in a bedroom. If you're in a higher-radon area, read how radon testing works and when it's worth doing before or after major energy work.

Which ventilation does an airtight home need?

The government's damp and mould guidance and the Energy Saving Trust point to the same basics:

  • Extractor fans in kitchens and bathrooms. They should work, be powerful enough and actually be used. The government guidance suggests humidity-controlled fans, or fans with an "overrun" that keeps running after the light goes off.
  • Trickle vents. The government guidance notes that building regulations require trickle vents in all replacement windows. The Energy Saving Trust's draught-proofing advice says you should normally leave them open all the time.
  • Air bricks and wall vents. Keep them clear. The government guidance says roof and under-floor spaces should be ventilated so the timber stays dry.
  • Whole-house systems. If you have mechanical ventilation with heat recovery (MVHR), the government guidance says it should be regularly inspected and maintained by a qualified professional. Leave it running and change filters as the manual says.

The Energy Saving Trust adds a useful warning: if your home has no trickle vents, wall vents or mechanical ventilation, it may already be too airtight, and draught-proofing may not be suitable.

How do I draught-proof without causing damp?

Draught-proofing is worth doing, but some gaps are there on purpose. The Energy Saving Trust says not to block vents, chimneys or airflow needed for safety, and not to draught-proof gaps into rooms with open fires or open flues, or rooms that produce a lot of moisture, such as kitchens, bathrooms and utility rooms. Flueless gas appliances need ventilation to work safely, so never block their fixed vents.

For a practical walk-through, see this guide to draught-proofing windows, doors and floors without blocking the ventilation you need.

For bigger retrofit projects, the government guidance mentions PAS 2035, the industry specification for domestic retrofit. It takes a whole-house approach designed to reduce unintended consequences such as damp, mould and condensation. Ask your installer whether the work will follow it.

Does ventilation that helps with mould also help with radon?

Sometimes, but not reliably. Extra ventilation is good for condensation, but UKHSA's guidance on reducing radon levels says sealing floors and adding extra ventilation do not reduce radon on their own. UKHSA describes an active radon sump, a small pipe and fan that draw gas from under the floor, as the most effective way to reduce indoor radon.

Positive ventilation, where a small fan blows fresh air into the home, usually from the roof space, is one of the radon methods UKHSA lists. Whether it suits your home depends on your radon result and how the house is built. Our sister site explains what to ask before choosing positive ventilation for radon.

What should I check after energy work?

  • Look for new condensation on windows and cold walls during the first winter.
  • Check extractor fans still work and vent outside.
  • Make sure trickle vents are open and air bricks weren't covered by the work.
  • Clean any small patches of mould early. See preventing mould at home.
  • If you live in a radon Affected Area, consider a three-month radon test once the home is being lived in normally.

If mould keeps coming back despite good habits, there may be a ventilation fault, a cold spot or a leak. That's when a survey is worth it: see mould testing and damp surveys.

Common questions

Will new double glazing cause mould?

Not on its own. But new windows make a room more airtight, so moisture has to be removed in other ways. Use the trickle vents and extractor fans, especially in kitchens and bathrooms.

Should I avoid insulating my home because of radon?

No. Insulation makes a home warmer and less prone to condensation. If you live in an area where radon is more likely, test the home so you know the level, and fix it if it is above UKHSA's Action Level of 200 Bq/m³ as an annual average.

Can opening windows fix condensation?

Opening windows after cooking or bathing helps clear moist air. It won't fix a broken fan, a cold spot or a leak, so look for a cause if mould keeps returning.

Is a dehumidifier enough?

It can help in a damp room, but it treats the symptom. The government guidance says the causes, such as inadequate ventilation or building defects, need tackling too.

Get help with damp and mould

If you've made your home warmer and mould has started appearing, a specialist can find the cause. Get free quotes from local mould and damp specialists.

Sources

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ventilationairtightnesscondensationradondraught-proofing