Condensation and Mould

Condensation is a tenant’s responsibility to manage and prevent, and where damage is caused by condensation during a tenancy, the cost of putting it right falls to the tenant. In the UK it is overwhelmingly a winter problem. This page explains why it happens and what to do about it.

Condensation is not damp

This is the most important distinction on the page, and the one most often got wrong.

Damp has structural causes: a failed damp-proof course, penetrating water through the fabric of the building, a defective gutter. Its causes are numerous, they are generally structural, and they are usually the landlord’s responsibility.

Condensation is a product of how a property is ventilated, which is a matter of how it is lived in. Condensation in itself is not the problem; the black mould (Stachybotrys chartarum) that can grow as a result of persistent condensation is.

If you believe the problem in your property is genuine damp, report it, with photographs, to maintenance@emluk.com.

Why modern properties are worse

Cases of black mould in rental property, particularly older rental property, have increased over recent decades. The cause is counter-intuitive: it is largely the result of improving those buildings.

Better insulation, UPVC double glazing, effective draught exclusion and wall insulation have made properties markedly more airtight. Older buildings leaked air constantly, and with it, moisture. Sealing them up conserved heat and trapped the water vapour that used to escape.

How it forms

Air holds water vapour, and warm air holds more of it than cold air. When moist warm air meets a colder surface, or colder air, it can no longer retain the same amount of moisture and the excess is released as liquid water.

In a property, that moisture seeks out the coldest available surface, typically the inside face of an external wall, the back of a wardrobe standing against one, or a window reveal. Those grey patches that appear on a wall are frequently assumed to be rising damp. Occasionally they are. Far more often, they are condensation.

Where the water comes from

A five-person household puts roughly ten litres of water into the air every day, before any heating is taken into account:

Two things stand out. The largest single source by a wide margin is drying clothes indoors. Showers are heavy emitters of water vapour, and nothing beats opening a window, which is exactly what most people are least inclined to do in January, and least inclined to do at all in a ground-floor flat.

We understand the instinct. The competing priority is usually to keep the place warm and the energy bill down. But a window opened for a few minutes after a shower costs very little heat and prevents a great deal of trouble.

Twelve things that work

1. Pull wardrobes and furniture away from external walls so air can circulate behind them. Mould appears first in exactly these unventilated pockets.
2. Close the kitchen door and open a window when cooking, so moisture does not spread through the property.
3. Keep lids on saucepans. It reduces the moisture released, and the energy used.
4. Close the bathroom door when bathing or showering, and run the extractor fan. Leave it running; do not switch it off the moment you leave.
5. If you dry clothes on radiators, open a window. Drying outside is always preferable, as is a vented tumble dryer or a spin dry.
6. Use a dehumidifier where the problem is persistent. They cost very little to run.
7. Consider mould-inhibiting paint or spray. It suppresses growth, but it does not address the cause.
8. Deal with mould as soon as it appears, before it spreads. A fungicidal wash, or a diluted bleach solution, will remove it.
9. Dry windows, sills and any wet surfaces. Wring the cloth out thoroughly, and do not dry it on a radiator.
10. Keep the internal temperature reasonably constant. Sharp cycles between warm and cold create the cold surfaces that condensation needs.
11. Check your extractor fans are actually working. A fan that cannot hold a postcard against the vent when running is not moving enough air. Report it.
12. Use a dehumidifier daily if the property is genuinely prone to condensation. It draws the excess moisture out and keeps the problem under control.