Mould does not arrive in November. It arrives in September, quietly, and you notice it in November. The gap between those two months is the whole game.
Once outdoor temperatures drop, wall surfaces cool, the heating goes on intermittently, windows stay shut and the moisture your household produces every day stops escaping. By the time black speckling appears on a window reveal, the colony has usually been establishing for six to eight weeks.
Which makes late August the right time to act. Here is what to do now, in rough order of how much difference it makes.
1. Test every extractor fan
Hold a single sheet of tissue paper against the grille with the fan running. It should hold there unassisted. If it drops, the fan is not moving meaningful air and it is not protecting you.
The usual causes, in order of likelihood: a grille clogged with dust and lint, a crushed or sagging flexible duct in the loft, a backdraught shutter stuck closed at the outside terminal, or a fan that has simply worn out. All four are cheap to fix in August and miserable to discover in January.
While you are there — is the fan actually ducted outside? A surprising number vent into a loft void, which just relocates the problem somewhere you cannot see it.
2. Clear the gutters and check the downpipes
An overflowing gutter soaks the wall beneath it every time it rains. Do this before the leaves fall and again after. Look for staining on the render, plant growth on the wall face, and joints that drip rather than run.
Then check where the downpipe discharges. If it is spilling onto a path that slopes back towards the house, you are irrigating your own wall base.
3. Walk the outside of the building
Fifteen minutes, ideally after rain, looking for:
- Cracked or hollow-sounding render
- Missing or eroded mortar pointing, especially on the weather-facing elevation
- Slipped or cracked roof tiles
- Failed sealant around window and door frames
- Ground level or decking that has crept above the damp proof course
- Soil or bark chippings piled against the wall
Penetrating damp fixed in September is a pointing repair. Left until February it is a pointing repair plus replastering plus redecoration.
4. Deal with what is already there
If you had mould last winter, treat those areas now while they are dry and while you can open windows. Trying to remediate in December means you are working in a cold room you cannot ventilate, with treated surfaces that will not dry.
Treat the full area plus a generous margin beyond the visible growth, respect the product's contact time, and wear a mask and gloves. Then, once fully dry, consider a fungicidal or anti-mould coating on the surfaces that were affected — it buys you a genuine margin on cold spots.
5. Move the furniture
Anything hard against an external wall — wardrobes, headboards, sofas, boxes in an unheated back bedroom — creates a pocket of still, cold air. That pocket reaches dew point long before the rest of the room does.
Fifty millimetres of clearance is enough. It looks like nothing and it changes the surface temperature meaningfully.
6. Sort out where the washing dries
A typical load of laundry puts around two litres of water into your air as it dries. Do that three times a week on radiators in a closed-up house and you have created a humidity problem that no amount of surface treatment will survive.
Best case, use a vented or condenser dryer, or dry outside while the weather still allows. If neither is possible, dry in one room, keep the door shut, and either run an extractor or a dehumidifier in that room.
7. Buy a hygrometer
They cost less than a takeaway and they turn guesswork into information. Put one in the room that had mould last year.
Target range is 40–60% relative humidity. Above 70% sustained, mould is close to inevitable on any cold surface. Watching that number move after you shut a door or open a vent teaches you more about your own house than any general advice can.
8. Plan the heating pattern, not just the temperature
Short bursts of high heat feel efficient and are, for mould purposes, the worst pattern. Surfaces never warm through, so they spend most of the day below the dew point of the air around them.
A lower, steadier background temperature keeps wall surfaces above dew point for more hours of the day. In poorly insulated rooms that difference is decisive.
9. Check the loft
Go up on a cold morning with a torch. Look for dampness on the underside of the roofing felt, damp insulation, or dark staining on rafters. Check that eaves ventilation has not been blocked by insulation stuffed into the corners, and that bathroom ducting runs to an outside terminal rather than stopping short.
10. Write down what you find
If you rent, this matters more than anything else on the list. Photograph every affected area with a date, keep a note of humidity readings, and report issues to your landlord or agent in writing rather than by phone. A dated written record is what turns "I mentioned it once" into an evidenced repair request.
The point of doing it now
Every item on this list is easier, cheaper and less disruptive in warm dry weather. Ventilation faults can be tested and fixed. External repairs can be done and can cure. Treated surfaces can actually dry. None of that is true once the clocks change.
Shop mould treatment kits — everything you need for the treatment step, PPE included.
