Condensation and ventilation advice

Condensation and ventilation advice for UK homes with mould: extractor fans, trickle vents, PIV, dehumidifiers and when to get a damp survey.

In short: Condensation and ventilation advice is a practical assessment of why moisture is building up in a home and what will stop the mould returning, from extractor fans and trickle vents to heating, drying habits and whole-house ventilation. It suits anyone whose mould keeps coming back after cleaning. In the UK a mould or damp inspection typically costs around £100 to £300.

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Most mould in UK homes is caused by condensation rather than a building defect, and the fix is about balancing the moisture produced indoors against how much is removed. A typical household can put several litres of water into the air every day through cooking, showering, drying clothes and breathing. If extractor fans are weak, trickle vents are shut and rooms are left to go cold, that moisture condenses on the coldest surfaces and mould follows within weeks.

Good advice looks at the whole home. It checks whether extractor fans actually move enough air, whether there is background ventilation in each room, how the home is heated, where washing is dried, and whether there are cold spots from missing insulation or thermal bridges. It also rules out leaks, penetrating damp and rising damp, and recommends a specialist damp survey where the signs point that way. The aim is a short list of practical changes that will keep the mould from returning.

What is included

Room-by-room check of where mould and condensation are appearing
Humidity and surface moisture readings in affected rooms
Check of extractor fans, run-on timers, ducting and cooker hoods
Check of trickle vents, air bricks and window ventilation
Review of heating patterns, laundry drying and other moisture sources
Look for cold spots such as missing loft insulation above ceilings
Advice on dehumidifiers and positive input ventilation where suitable
A recommendation for a damp survey where condensation is not the cause

Who it is for

Mould that returns within weeks or months of cleaning
Homes with streaming windows and damp-smelling rooms in winter
Flats and new-build homes that feel airtight and humid
Tenants and landlords wanting an independent view of the cause
Anyone unsure whether they have condensation or a damp problem

How much does it cost?

As a guide, a mould or damp inspection typically costs around £100 to £300. A replacement bathroom extractor fan supplied and fitted is usually around £200 to £450, and a loft-mounted positive input ventilation unit roughly £500 to £1,200 fitted. See our full cost guide.

How it works

Talk it through

We ask how the home is used, where washing is dried, how it is heated and when the mould is worst, because the pattern usually points to the cause.

Measure

Humidity and surface moisture are measured, and fans, vents and heating are checked in each affected room.

Rule out damp

Walls are checked for signs of leaks, penetrating damp or rising damp, so a building defect is not mistaken for condensation.

Clear recommendations

You get a short, prioritised list of changes, from habits that cost nothing to fan upgrades, PIV or insulation, with rough guide costs.

Condensation and ventilation advice: FAQs

What humidity should my home be?

Indoor relative humidity of roughly 40 to 60 percent is a sensible target for most homes. Mould tends to grow when surfaces stay at a high relative humidity, roughly 70 to 80 percent or more, for long periods, which can happen on cold walls even when the room reading looks lower. A cheap digital hygrometer in a bedroom and bathroom shows quickly whether moisture is building up.

Is a dehumidifier worth it for mould?

A dehumidifier can make a real difference where washing is dried indoors or a room has persistent condensation, and it is a useful short-term fix while ventilation is improved. Compressor models are cheaper to run in warm rooms, while desiccant models work better in cold rooms and unheated spaces. Empty or drain it regularly and run it with doors closed in the room that needs it most.

What is positive input ventilation?

Positive input ventilation, or PIV, is a fan unit usually fitted in the loft that gently pushes filtered air into the home through a ceiling diffuser on the landing. This slightly pressurises the house so moist air is pushed out through gaps and vents. It can work well for condensation in houses, but it does not fix leaks or rising damp and needs suitable loft space.

When do I need a damp survey instead?

Get a damp survey if mould sits in one defined patch, follows a tide mark, appears low on walls with flaking or salts, or appears after rain rather than in cold weather. Mould on a wall next to a leaking gutter, cracked render or a bridged damp-proof course also points to damp. Choose a surveyor who explains the evidence rather than recommending injection treatment on sight.