Fixing a garage conversion that's always been cold and slightly damp is a fabric problem, and fabric problems need the fabric opened up and corrected — there's no shortcut that leaves the existing build-up in place.
The good news is that this is well-understood, straightforward work once you accept that the floor covering will need to come up and, usually, the infilled opening will need attention too.
What has to be opened up
The floor covering comes up first so the original slab can be seen and assessed. In most cases this reveals exactly what's suspected: a bare or barely-covered slab with no membrane and no insulation beneath the flooring.
Where the door opening was infilled to a poor standard, that panel often needs partial or full rebuilding, particularly if there's visible cracking or a cold bridge running through it.
The correct sequence of works
Work generally proceeds floor first, then walls, because the floor build-up needs to be checked and corrected before skirting and wall linings are refitted around it.
- Break out or overlay the existing slab area with a correctly detailed insulated floor build-up, including a damp-proof membrane lapped up the walls
- Rebuild the infilled door opening panel where necessary, with proper insulation and a continuous damp-proof course tied into the surrounding wall
- Insulate the flank walls, typically with a dry-lining system that includes a vapour control layer
- Reinstate ventilation appropriate to the room's new use — trickle vents or a controlled system rather than simply sealing everything
Why a cosmetic patch fails
Painting over the cold spots, adding a rug, or running a dehumidifier treats the symptom while the cold slab and uninsulated walls keep generating condensation underneath. As soon as the mitigation stops, the moisture returns, because nothing about the fabric has changed.
This is the same reason simply adding more radiators doesn't work: heat escapes through the uninsulated fabric faster than it's added, so the room never reaches a stable, comfortable temperature.
Interfaces with other trades
Electrical sockets and any underfloor wiring need to be planned around the new floor build-up before it goes down, since raising the floor level changes where cable runs and skirting sit.
If the room is being replastered after wall insulation, that follows once the insulation and vapour control layer are fixed and inspected, not before.
How you know the repair has worked
The room should reach and hold a comparable temperature to the rest of the house on the same heating input, rather than always lagging behind. The musty smell should not return once a full heating season has passed, including through cold, damp weather.
Common questions
Do I need to dig up the whole floor?+
Not always — in some cases an insulated overlay build-up can be installed on top of the existing slab, provided ceiling and door heights allow for the extra depth. Where headroom is tight, taking the slab out and rebuilding lower is sometimes the better route.
Is the infilled garage door always the problem?+
Not always, but it's worth checking specifically, because it's one of the most commonly poorly detailed parts of a garage conversion. If the wall there feels noticeably colder than the rest, it usually needs attention.
Will this work disrupt the rest of the house?+
It's generally contained to the converted room itself, though services running through it — heating pipes, electrics — may need brief isolation while the floor is opened up.
Get a written quote to fix your garage conversion floor
We'll open up the floor, assess the infilled opening, and quote the full correction in writing before any work starts.