This is not usually a fault in the insulation product. It is nearly always a junction: where the wall meets the floor, where a steel passes through, around a window reveal, or where the new build ties into the original house.
The usual suspects
Window and door reveals top the list. If the insulation in the cavity does not return into the reveal and meet the frame, the block around the opening becomes a direct path to the outside. You feel it as a cold strip beside the glass and often see condensation there first.
Steel beams come second. A steel that is boxed out in plasterboard without insulation behind it acts as a radiator to the outside where it bears on the external wall. Third is the wall to floor junction, where insulation under a screed does not meet the wall insulation and leaves a gap at ankle height.
- Uninsulated window and door reveals
- Steel beams and bearings boxed without insulation
- A gap between floor insulation and wall insulation at the perimeter
- Partial fill cavity insulation slipped or gapped during the build
- The junction where the new extension meets the original wall
Confirming it rather than guessing
The reliable check is thermal imaging on a cold morning with the heating on and the outside temperature at least ten degrees lower than inside. Cold bridges show as clear cold lines and blocks rather than general coolness.
Without a camera you can still learn a lot. Feel the wall with the back of your hand at a slow walking pace and note where it changes. Look for where condensation forms first on a cold night and where any mould appears, because both follow the coldest surface in the room.
What a real fix looks like
For reveals, the repair is to open up the plaster at the reveal, fit an insulated board or cavity closer that meets the frame, and make good. It is a day or two of work per opening and it removes both the cold strip and the condensation with it.
For a steel, we take the boxing off, insulate around the section, reinstate an airtight line and box back. For a perimeter gap at floor level, the pragmatic fix is usually to insulate at the skirting line internally, since lifting a finished screed is rarely worth it.
Where the cavity was partially filled and the boards have slipped, an inspection with a borescope tells you how widespread it is before anyone decides whether to inject or to line internally.
What we would not recommend
Thermal paint, foil behind radiators and thin insulated wallpapers do very little against a genuine cold bridge. They add fractions of a degree at the surface and leave the heat path untouched.
We would also be cautious about full cavity injection into a partial fill wall without an inspection first, because filling the residual gap in a wall designed to have one can create a damp path across the cavity. Look before you fill.
Common questions
Is a cold spot a building regulations failure?
Not automatically. Regulations set a limit on junction heat loss, and a poorly detailed reveal or an uninsulated steel would generally not meet current expectations, but proving it after the fact needs evidence such as thermal images.
Can I fix a cold reveal from the inside?
Yes, and that is normally how it is done. The plaster comes off the reveal, an insulated closer goes in against the frame, and it is replastered.
Why is the wall between the extension and the old house cold?
Because the original external wall was never insulated to the standard of the new one and the two build ups rarely line up. That junction is the most common cold bridge in any extension.
Will more heating solve it?
It will mask it and cost you every month. The surface stays colder than the room, so condensation and mould risk remain even when the air is warm.
Do you offer thermal imaging?
We arrange it where a job warrants it, and we will always look at the obvious junctions during a free site visit before anyone spends money on surveys.