Ventilation & damp — Eltham & Mottingham SE9
The cheapest change that actually works, and why it suits these houses
A lot of Eltham and Mottingham is still ventilated the way it was built to be: a passive vent in the wall, and somebody opening the kitchen window. That worked when the houses were draughty. It is also why this is the clearest case we see anywhere for continuous mechanical extract — modest cost, no redecoration, and a change you can measure in the rooms that actually make the moisture.
Inter-war and post-war housing was built with ventilation in mind, but the ventilation it was given was passive: an airbrick or a wall vent, a chimney, windows that did not seal, and the assumption that somebody would open one. None of that is mechanical, none of it is controllable, and all of it depends on weather and on habit.
It genuinely worked, for a while, because the rest of the house leaked too. What has changed since is not the vents — it is everything around them. Double glazing, draught-proofing, capped flues, insulation, and a household producing more moisture than the one the house was built for: showers rather than baths, washing dried indoors, a tumble dryer that may or may not be vented.
So the passive vent is still there, doing what it always did, in a building that no longer supports it. And on a cold day people close it, because it is a hole in the wall making the room cold — which is entirely reasonable, and is why passive ventilation fails in practice long before it fails on paper.
Why continuous extract suits this stock so well
Of everything we fit, this is the option with the best ratio of result to disruption — and on these houses it is usually the right answer rather than a compromise.
It runs all the time, quietly, at a few watts
A continuous extract fan sits on a low trickle rate permanently and lifts to a boost when the room needs it. Moisture is produced continuously, so removing it continuously is simply the right shape of answer — rather than four minutes of noise after somebody remembers to press a switch, by which time most of the water from a shower is already on the ceiling.
Nothing needs redecorating
In most of these houses the wet rooms have an outside wall behind them, so it is a like-for-like replacement in an opening that already exists. A couple of hours a room, no ceilings down, no making good, and the room is usable the same day. That is the practical difference between this and anything ducted.
You can prove it worked
We measure the rate at the grille with a meter when it is in and give you the figure in writing. That matters more than it sounds — a fan rated 15 l/s on the box can be moving half that on a poor duct, and it sounds identical while doing it. Noise is not airflow, and without a meter nobody knows.
But it needs air to get in
This is the half that gets skipped and it is why so many replacement fans disappoint. Extract can only pull if the house can let replacement air in. On this stock that usually means making the background ventilation work again — trickle vents where the windows have none, an airbrick reinstated where a flue was sealed, the existing wall vent unblocked rather than removed.
What it will not do, and when to spend more
Being straight about the limits is the only way the recommendation is worth anything.
- Extract fans do nothing at all for a bedroom. They serve the room they are in. If the mould is in bedroom corners rather than in the bathroom, a better bathroom fan will not fix it — that is a whole-dwelling problem and it needs a whole-dwelling answer.
- They throw away the heat with the moisture. That is the trade-off for the low cost, and at a continuous trickle rate it is small. If you want the warmth kept out of the air you are extracting, that is heat recovery, and it is a bigger job at a much bigger price.
- They do not fix a cold surface. If the wall is cold because of a bridge at a joist or a lintel, ventilation reduces the moisture reaching it but the pattern will still be the coldest thing in the room. We will tell you if that is what we are looking at.
- And they cannot fix a leak. If it persists through a warm dry summer, or it is a defined patch that tracks a defect outside, it is not condensation and no fan will touch it.
Where a house has no external wall behind the wet room — which happens where a bathroom was added off a landing, or an extension enclosed what used to be an outside wall — a wall fan is not an option and it needs an inline fan in the ceiling void with a duct run out. That is most of a day rather than a couple of hours, and it is the single biggest price difference in this work. We check it first and tell you which one you are before quoting.
Questions we get asked in Eltham and Mottingham
No, and it is usually the cheapest part of the whole conversation. A continuous extract fan on its trickle rate draws a few watts — less than a lightbulb — and the exact figure is on the product data for whichever unit is specified, which we will show you rather than estimating. We deliberately do not quote a pounds-per-year number here, because it depends on the unit and on your tariff and a plausible-looking figure invented on a website is worth nothing. The larger cost is the small amount of heat leaving with the extracted air, and at a trickle rate that is modest. Set against a bathroom that has to be redecorated every couple of years, it is not a close call.
Because it is passive, and passive ventilation depends on weather and on habit. It worked when the rest of the house leaked too — open flues, timber windows that never quite shut, a ventilated floor void. Since then the windows have been replaced, the chimney capped and the draughts sealed, all of which were sensible, and the vent is now doing a job that used to be shared by the whole building. It is also the thing people close on a cold day, entirely reasonably, because it is a hole in the wall making the room cold. That is why passive ventilation fails in practice long before it fails on paper. The vent still has a job — letting replacement air in for an extract fan to pull — but on its own it is not enough any more.
A like-for-like extract fan replacement where the existing duct is sound is £239 to £395 including VAT, and it is a couple of hours a room with nothing to redecorate afterwards. A continuously running dMEV fan is £340 to £450. Trickle vents are £175 for the first window and £70 for each one after that. Where it gets more involved is if the duct needs replacing or the wet room has no outside wall behind it — then it is an inline fan in the ceiling void with a duct run out, £450 to £1,100, and most of a day with a ceiling open. We check which of those you are before quoting rather than after starting.
No, and anybody who tells you otherwise is selling you something. An extract fan serves the room it is in. If the mould is in a bedroom — typically the corner where two outside walls meet, or the wall behind a wardrobe — that is a whole-dwelling problem, and the moisture is largely being made by the people sleeping in there. Two adults asleep for eight hours put out around two thirds of a litre of water, into a room with the door shut and the window closed. The answer for that is whole-dwelling ventilation rather than a better fan in a different room. It is a bigger job and we would rather say so than take money for something that will not work.
There is a rough test you can do now: hold a single sheet of toilet paper flat against the grille with the fan running. If it will not hold up on its own, something is clearly wrong. Be clear about what that cannot tell you though — it proves the fan moves some air, not that it moves the right amount, and the gap between those two is the whole problem. The other check costs nothing and finds the worst fault there is: put your head through the loft hatch with a torch and follow the duct. If it does not end at a terminal in the roof, a wall or the soffit, it is discharging into your loft, and every shower has been going in there for years.
For a straightforward like-for-like replacement in an existing opening, generally not. Where it does come into it is when you are fitting out or altering a wet room, adding one, or converting a loft — that work triggers a requirement for mechanical extract meeting the Approved Document F rates in its own right. The gap worth knowing about is between specifying and proving: jobs are very often signed off because a fan of the correct rating was installed, rather than because anybody put a meter on the grille and read what it actually delivers. Those two figures routinely differ by a factor of two. Whoever does the work, ask for the commissioning figures.
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.