Ventilation & damp — Catford, Bellingham & Downham SE6
Catford condensation is usually something that was taken away, not something that went wrong
A great deal of the housing here is inter-war cottage-estate stock, built solid brick throughout and designed around draughts nobody thought of as ventilation. Most of it has since been double-glazed and draught-proofed — correctly — and almost none of it had anything put back. That combination is behind most of the condensation calls we take from SE6, and it is usually fixable without touching the windows again.
Catford sits between two of the London County Council’s inter-war cottage estates. Bellingham was largely complete by 1923, laid out on cottage-suburb principles around Bellingham Green and built substantially for railway workers. Downham followed between 1924 and 1930 — around seven thousand homes, and the third largest of the eight cottage estates the LCC built on the edge of London in that decade.
They were good houses and they still are. But they were built to a construction that has one consequence worth knowing about: the LCC inter-war houses on these estates are solid brick throughout — the internal walls as well as the external ones.
That matters more than it sounds. A cavity wall has an insulating gap in it; a solid wall does not, so its inside face sits below room temperature through the winter. And where the internal walls are solid brick too, the cold surfaces are not confined to the outside of the building — there is a great deal of dense, slow-to-warm masonry in the middle of the house as well. Warm damp air finds all of it.
And then the draughts were sealed up
Every one of these was the right thing to do. Nobody was wrong. The problem is that they were done one at a time, by different trades, and none of them was responsible for the air.
The windows were replaced
Original timber casements leaked continuously around the frames, and that leak was background ventilation nobody had to think about. A modern sealed unit does not. Where the replacements went in without trickle vents — or with them, and they have been painted over or simply never opened — the room lost its air supply on the day of the swap. This is the single most common item we find here.
The chimneys were capped
An open flue is a permanently running extract in that room, free, all day. Capping it is right for heat loss and for keeping rain out. It also removes a substantial continuous air change from what is very often the room that later goes mouldy. Capped properly it keeps a vented cowl and an airbrick into the room, so the flue can still breathe; a great many round here were simply sealed.
The floors were sealed and the airbricks blocked
Suspended timber floors were built over a ventilated void, with airbricks feeding it. Laminate over the boards, a raised path outside, or a rendered plinth across the airbricks all stop that. It shows up as a cold floor and a musty smell before it shows up as mould.
The house got warmer and wetter at the same time
Insulation, better heating and modern living all arrived together: showers instead of baths, washing dried indoors because there is no time to hang it out, tumble dryers that are not vented. A household of four puts ten to fifteen litres of water into a home every day just by living in it. That number has not changed. What changed is where it can go.
What we actually do about it here
The good news is that this is the most fixable version of the problem, and it very rarely means touching the windows again.
- Put the background ventilation back, in a controlled way. Trickle vents where the windows have none, an airbrick where a flue was sealed, the underfloor void unblocked. Small, cheap, and it is what lets any extract fan actually pull — a fan cannot draw air out of a room that has no way of letting air in.
- Make the extract real. Most of the fans we find in these houses are moving a fraction of what is printed on them: four metres of sagging flexible duct, a stuck backdraught shutter, or a duct that stops in the loft rather than going outside. We measure at the grille and give you the figure.
- Continuous rather than intermittent, where it suits. A dMEV fan running quietly on trickle deals with moisture as it is produced, rather than for four minutes after somebody remembers to press a switch. On this stock it is usually the best value change there is.
- Deal with the cold surfaces we can. We are not a solid-wall insulation contractor and will not pretend to be. But pulling furniture off external walls, unblocking a chimney breast and getting the moisture out of the air does most of what is achievable without touching the fabric — and it is what stops the mould coming back in the same outline.
Where a house has already had internal wall insulation, the mould often reappears in a clean line around window reveals and at the wall-to-ceiling joint. That is a cold junction the insulation could not bridge, and it is a different conversation — we will tell you if that is what we are looking at rather than selling you a fan for it.
Questions we get asked around Catford
Not exactly, and the distinction matters because it decides what you do next. The windows are doing what they were fitted to do: they stop draughts and they keep heat in. What went with the draughts was a continuous, accidental air change that the house was designed around and that nobody thought of as ventilation. The moisture your household produces has not changed — ten to fifteen litres a day for a family of four — but the route out has. So the answer is not to regret the windows or replace them again, it is to put controlled background ventilation back: trickle vents where there are none, and extract that genuinely works in the rooms where the moisture is made. That is a far cheaper job than the windows were.
Because on much of the inter-war cottage-estate stock around Catford the internal walls are solid brick too, not stud. Dense masonry warms up slowly and holds cold, so a solid internal wall — particularly one on the shaded side of the house, or behind a wardrobe where room air never reaches it — can easily be a colder surface than the plaster on an outside wall that has a radiator under it. Mould grows where the surface is coldest and stays damp longest, not where the outside of the building happens to be. That is also why it returns in exactly the same outline after you clean it: the outline is a map of the cold part of the wall, and cleaning does not move it.
Usually yes, and on this stock it is often the single cheapest thing that changes the outcome. A fan can only push air out if the same volume of air can get in behind it. Seal a house completely and the fan simply pulls harder against nothing and moves less — it sounds exactly the same while doing it, which is why people assume it is working. Trickle vents are the controlled way of letting that replacement air in, and the amount of heat they cost is a fraction of what an open window costs. If yours exist but are painted shut or taped over, undoing that is free and worth doing before you spend anything.
Report it to your landlord first, in writing, and keep the reply. For social housing tenants the first phase of Awaab's Law has been in force for registered providers since 27 October 2025, covering damp and mould and emergency hazards, and it sets out timescales your landlord has to work to. If you are a private tenant instead, that phase does not apply to you yet — what does apply, today, is the Homes (Fitness for Human Habitation) Act 2018 and section 11 of the Landlord and Tenant Act 1985. We work for owners, landlords and managing agents rather than acting for tenants against them, so if you rent, the useful thing we can offer is a straight explanation of what is causing it that you can pass on.
On this housing type, one properly installed continuous fan in the bathroom and a working extract in the kitchen fixes a great many houses, and we would rather tell you that than sell you a system. Where it does not is where the moisture is being made across the whole house rather than in the wet rooms — washing dried on a landing every day, an unvented tumble dryer, a lot of people in a small house — or where bedrooms are already going mouldy in the corners. Extract fans do nothing at all for a bedroom. That is the point at which whole-dwelling ventilation earns its money, and the survey is what tells the two apart rather than a guess over the phone.
Usually not. On a straightforward job — replacing a fan, running it out through an existing external wall, fitting trickle vents — nothing structural is touched and the room is usable the same day. Where it does get more involved is when the wet room has no external wall behind it, which happens where a bathroom has been added off a landing or a house has been converted. Then it needs an inline fan in the ceiling void and a duct run out to the nearest outside face, which is most of a day with a ceiling open. That is the biggest single difference in price in this work and we tell you which one you are before quoting, not after.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, both sat in August 2026. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.