Why a Converted Garage Floor Feels Cold

The slab was never insulated

Most garage slabs are unheated, uninsulated concrete laid directly on hardcore. In a habitable room that slab is a continuous heat sink into the ground and the surface stays cold to the touch all winter.

The correction is a new insulated build-up over the slab: damp-proof membrane, rigid insulation, then either a screed or a floating deck. That raises floor level, so it has to be planned around door thresholds and the level change into the house.

There is no damp-proof membrane

Older garage slabs often have no membrane at all, because a bit of ground moisture never mattered under a car. Under a carpet or laminate it produces cold, musty flooring, condensation under the covering and eventually mould.

A liquid-applied or sheet membrane lapped up to meet the wall damp-proof course is essential, and it must be continuous with the wall detail rather than stopping at the skirting.

The old garage door opening was closed badly

The infill panel where the door was is the coldest part of most conversions. Done properly it is a full-depth insulated wall built off a proper foundation, tied into the existing masonry and finished to match.

• Check the infill sits on a footing, not on the edge of the slab
• Look for a cold stripe or condensation line along that wall in winter
• Verify a cavity tray and DPC were installed at the junction
• Confirm the new window has been sealed and insulated around the reveal

Heating was never sized for the room

A garage conversion has more external wall area than an internal room of the same size, so it needs more output, not the same. Extending a single radiator circuit from a nearby room often leaves the space several degrees behind the rest of the house.

Where the floor is being rebuilt anyway, underfloor heating over the new insulation is worth considering — it deals with the cold-surface complaint directly and frees the wall space.