You notice it most in winter: the converted room never quite reaches the same temperature as the rest of the house, no matter how the heating is set, and there's often a faint musty smell that air fresheners don't shift.
This is a very common outcome from garage conversions done to a basic standard, where the priority was closing up the opening and putting down a floor covering rather than properly upgrading the original garage build-up.
Separating what you feel from what's wrong
The cold and the smell are two related symptoms, not two separate problems. Cold surfaces encourage condensation, and condensation on an unventilated surface is what produces the musty smell — so both usually point back to the same underlying build-up.
It's worth being clear that the room being cold is not simply 'needs more heating'. If the fabric of the floor and walls is uninsulated, heating input just leaks away faster than a normal room, and the smell won't disappear until the moisture source is dealt with.
Likely causes, and why they occur
Garages are built to a much lower specification than habitable rooms because they were never meant to be lived in.
- The original garage slab left in place with no insulation and no damp-proof membrane beneath it
- The garage door opening infilled with blockwork or a stud wall that lacks a correct damp-proof course and insulation detail
- Flank walls left without insulation because they were originally just garage walls
- Ventilation that existed in the garage — an air brick, gaps under the door — blocked up during conversion with nothing put in its place
How to check which one applies
Start with the floor. Lift a corner of the floor covering if possible, or check the conversion drawings if you have them, to see whether the original slab was overlaid with an insulated, membraned build-up or simply carpeted or tiled directly.
Look closely at the infilled door opening from outside and inside. A colder patch of wall exactly where the door used to be, or visible cracking along its edges, points to a poorly detailed infill panel.
Feel the flank walls compared to a wall you know is insulated elsewhere in the house. A noticeably colder surface, especially near external corners, suggests missing insulation rather than a heating shortfall.
Check whether any air bricks were bricked up flush during the conversion. If they were sealed rather than adapted, the room may simply be trapping moisture with nowhere for it to go.
Self-diagnosis before calling anyone out
Before booking an inspection, a few checks cost nothing and narrow down what a surveyor or builder will be looking at first. Note which surfaces feel coldest to the hand — floor, the infilled door wall, or the flank walls — and whether the smell is stronger near the floor or higher up, since damp air sinks and a floor-level smell often means the slab is the main contributor.
Keep a simple record over a week or two: room temperature at the same time each day, and whether condensation appears on windows or the coldest wall overnight. This kind of pattern, rather than a single observation, is genuinely useful information to hand to whoever assesses the room properly.
What a proper survey of the room should record
A useful survey doesn't stop at 'it's cold and damp' — it should record surface temperatures at several points, relative humidity in the room compared with the rest of the house, and a visual and physical check of the infilled opening for cracking or a cold bridge running through it.
It should also state plainly whether the original slab has any insulation or membrane beneath the floor covering, based on either opening a small inspection area or documented evidence from the conversion, rather than being left as an assumption in the write-up.
Common questions
Is a musty smell in a converted garage dangerous?+
Persistent damp and mould are a health concern, particularly for anyone with respiratory issues, and should be taken seriously rather than masked. It's not usually an emergency, but it shouldn't be ignored either.
Can I just add a dehumidifier and better heating?+
That will mask the symptoms while it's running, but it doesn't address the cold slab or uninsulated walls causing the condensation. Once the dehumidifier is off, the moisture problem returns.
How do I know if the slab has a damp-proof membrane?+
Older garage slabs very rarely had one, since garages weren't built to habitable standards. If the conversion didn't specifically add a new membrane and insulated floor build-up over the top, you should assume there isn't one.
Request a free assessment of your garage conversion
We'll check the floor build-up, the infill panel and the wall insulation, and set out in writing what's actually causing the cold and damp.