A modern extension is built to be airtight, and that is exactly why it can feel stuffy and run damp. Seal a room well, glaze most of its external wall with fixed doors, and put the only opening low down, and warm moist air from cooking, drying and breathing has nowhere to go. The fix is rapid ventilation at the highest point of the room, which is where an opening rooflight earns its place. It clears humidity and stale air directly, it counts towards the Building Regulations ventilation requirement, and it works with the stack effect rather than against it. This page explains how the ventilation is calculated and where the rooflight fits.
Why sealed extensions trap air
Two things have changed in how extensions are built, and together they create the problem.
Airtightness has improved. Membranes, tapes and better detailing mean far less air leaks through the fabric than it did in an older room, which is good for heat loss and bad for getting rid of moisture, because the accidental ventilation that older houses relied on is gone. You now have to ventilate on purpose.
And the plan has changed. A side-return or wrap-around kitchen extension typically has one glazed external wall with bi-fold or sliding doors, and a deep plan running back under the existing house. The doors ventilate the front of the room. The back of the plan, often where the hob and the kitchen sit, is metres from any opening and gets no air movement at all. Warm air rises, hits a sealed ceiling, cools, and drops its moisture on the coldest surface it can find, which is usually the glazing or a north wall.
The tell-tale signs. Morning misting on the inside of the glass, a musty smell that lingers, black spotting of mould in ceiling corners away from the doors, and condensation that does not clear until mid-morning. None of these is a fault in the build. They are the symptom of a room that is sealed but not ventilated.
How extension ventilation is calculated
Building Regulations Part F splits ventilation into two duties, and a habitable room needs both. They do different jobs and one does not substitute for the other.
Purge ventilation
Purge ventilation is rapid ventilation for clearing pollutants quickly: cooking smells, moisture spikes, the stale air that builds up overnight. The working requirement is an openable area of at least 5 per cent of the floor area of the room, provided by windows, doors or rooflights. So a 20 square metre kitchen extension needs at least 1 square metre of openable area.
Bi-fold doors nominally provide a lot of openable area, but people rarely fling the whole run open in a Suffolk winter, and the doors do nothing for the deep part of the plan. This is where an opening rooflight over the far end of the room adds real, usable purge area exactly where the doors cannot reach.
Background ventilation
Background ventilation is a continuous trickle of fresh air, measured in equivalent area in square millimetres, provided by trickle vents in window and door frames or by a mechanical system. It runs all the time at a low rate to keep the air from going stale between purges. Trickle ventilation and purge ventilation are not interchangeable: a trickle vent cannot clear a moisture spike, and an opening rooflight left shut does nothing for background air. We set out why on the electric rooflight versus trickle ventilation page.
| Duty | What it does | How it is measured | Typical provision |
|---|---|---|---|
| Purge (Part F) | Clears pollutants and moisture quickly | Openable area, at least 5% of floor area | Windows, doors, opening rooflights |
| Background (Part F) | Continuous fresh-air trickle | Equivalent area in mm² per room | Trickle vents or mechanical system |
| Overheating (Part O) | Removes solar and casual heat gain | Openable area credited from high-level openings | High-level opening rooflights, cross-flow |
Worked example: a side-return kitchen
Take a common Ipswich job, a side-return kitchen extension of about 22 square metres with bi-fold doors across the garden end and the hob and sink at the deep end under the old house wall.
The purge requirement is 5 per cent of 22 square metres, so 1.1 square metres of openable area. The bi-folds satisfy the number on paper, but they only ventilate the garden third of the room, and in January they stay shut. The deep end, where the moisture is actually generated, gets nothing.
Add a 1200 by 1200mm opening rooflight over the deep end. Opening 300mm, it provides genuine purge area right where the cooking moisture rises, and because it sits at the highest point of a room with a warm surface below, it drives the stack effect: warm moist air out high, cooler air in through the doors or a low vent, with no fan. On a mild evening you can clear a steamy kitchen in minutes rather than leaving the doors open to the cold. In a Suffolk summer the same opening vents overnight heat that would otherwise sit under the ceiling.
Where the opening rooflight goes
Position matters more than size for ventilation, because warm air stratifies at ceiling level.
- At the highest point. On a sloping or mono-pitch ceiling, put the opening unit high. Hot, moist air collects there, and a vent at that point removes it directly instead of letting it mix back down.
- Over the moisture source. The hob, a utility corner, a shower room. Vent where the moisture is made, not where it is convenient to wire.
- Away from the doors. The doors handle the front of the plan. The rooflight handles the part they cannot.
- Paired with a low inlet. A rooflight and an open low window or door together give a genuine air current. A rooflight alone still helps, but the stack effect needs an inlet.
Why the damp matters beyond comfort
A stuffy extension is not only unpleasant. Persistent high humidity has real consequences that are cheaper to design out than to fix later.
- Mould. Black spotting in ceiling corners and around the glazing is mould feeding on moisture that never clears. It stains decoration, it can affect health, and it comes straight back after cleaning if the underlying humidity is not dealt with.
- Condensation on the glazing. Water running down the inside of the glass most mornings sits on sills and reveals, and over time it damages finishes and can reach the structure below.
- A cold, clammy feel. Damp air feels colder than dry air at the same temperature, so a humid extension gets heated harder, which costs more and still does not feel right.
- Musty smell. The lingering smell of a room that never fully airs is stale, humid air with nowhere to go.
None of these is a fault in the building. They are the signature of a room sealed well but ventilated poorly, and an opening rooflight at the high point is usually the missing piece.
Where a rooflight stops and mechanical ventilation starts
An opening rooflight handles purge ventilation, the rapid clearing of moisture spikes, and it does it with no power and no running cost by driving the stack effect. What it does not do is provide continuous background ventilation, and it does nothing when it is shut overnight or while you are out. For a room with a heavy, constant moisture load, or where you want ventilation without opening anything, a mechanical system such as continuous extract or heat-recovery ventilation is the tool for the background duty. The two are complementary, not alternatives: the rooflight purges, the background system trickles. We fit rooflights, not mechanical ventilation systems, and we are clear about that boundary, but we will tell you honestly if your room needs the background side addressed as well as the purge. The regulation split between the two is on the ventilation Building Regulations answer, and why one does not replace the other on the electric rooflight versus trickle ventilation page.
Electric, solar or manual for an extension vent
If the opening rooflight is high on a sloping ceiling, which is exactly where you want it for ventilation, it is out of reach, so it needs to be motorised. That is the usual case in an extension. A manual pole-operated unit works if it is genuinely within reach, but a vent you cannot reach is a vent you will not use.
On a new extension with the ceiling still open, a mains electric unit is straightforward because the electrician is already on site. On a finished room, a solar unit avoids the cable route problem entirely. The trade-offs are covered on the main electric skylight installation page, and if you are weighing motorised against manual, the electric versus manual rooflight comparison lays out the thresholds. For the regulations side, the ventilation Building Regulations answer summarises exactly what an extension needs.
Common questions
Why does my new extension get so much condensation?
Because it is airtight but under-ventilated. Modern construction leaks very little air through the fabric, which is good for heat but means moisture from cooking, drying and breathing has to be removed deliberately. If the only opening is glazed doors at one end, the deep part of the plan gets no air movement, so warm moist air collects at the ceiling and condenses on the coldest surface. An opening rooflight at the high point clears it.
Do bi-fold doors count as enough ventilation?
On paper they often provide the openable area Part F asks for, but in practice they only ventilate the end of the room they are on, and few people open them in winter. Purge ventilation needs at least 5 per cent of the floor area as openable area, and it needs to reach the whole room. An opening rooflight over the deep end adds usable purge area exactly where the doors cannot help.
Will an opening rooflight fix an overheating extension?
It helps a great deal, because it removes heat at the point where it collects. Warm air stratifies at ceiling level, and a rooflight opening there vents it directly, especially paired with a low inlet to drive the stack effect. It works best alongside good glazing: solar control glass with a low g-value stops much of the heat arriving, which is a better problem to solve than removing it afterwards.
Get a fixed price for extension ventilation
Tell us the size and shape of the room, where the doors and the moisture sources are, and whether the ceiling is still open. We survey it, work out where an opening rooflight does the most for purge and overheating, and give you one written price covering the unit, the controls and the electrical connection, fixed after survey with a 10-year workmanship guarantee.
Request a fixed quote for an opening rooflight to ventilate an extension across Ipswich, Suffolk and north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.