Why is there damp or cold where my extension joins the house?

What this usually means

When an extension is built onto a house, two separately constructed envelopes have to be joined, and the join is where things commonly go wrong. The extension's wall, roof and floor insulation must connect continuously with the original house's, and the air barrier of the new must seal continuously to the old — but in practice the detailing at this interface is awkward and often imperfect, leaving a gap in the insulation (a thermal bridge) and a path for air leakage exactly along the line where the two structures meet. That line then behaves differently from the surfaces either side of it.

A thermal bridge at the junction conducts heat out and runs colder on the inside than the surrounding wall or ceiling, so when warm, moist indoor air meets that colder line it condenses, and over time mould grows along it. If there is also an air-leakage path at the join — very common where new meets old — cold draughts add to the chill and carry moisture into the junction. So the symptoms cluster along the join: a cold strip, condensation, mould, sometimes staining, and occasionally cracking where the two structures move differently. It looks like a mysterious damp patch but is a predictable consequence of a discontinuous junction.

Because the cause is discontinuity at the interface, the fix is to restore continuity. That means identifying exactly where the insulation is bridged and where air leaks at the junction, then making the insulation continuous across the join and sealing the air barrier between the new and old structures — typically by carrying insulation across the interface and sealing the gaps internally. Treating only the symptom (wiping the mould, repainting) fails because the cold, leaky line remains. Diagnosing the junction with thermal imaging and air-leakage testing locates the bridge and the leak precisely, so the detailing can be corrected and the join brought up to the temperature of the surrounding surfaces.

Common causes

Signs and symptoms

A junction between an extension and the existing house is a classic site for a thermal bridge, because continuity of the insulation layer across the interface is geometrically and practically difficult to achieve. Where the insulation is interrupted at the join, heat takes the more conductive path through the bridge, lowering the internal surface temperature along that line. Condensation forms wherever a surface falls below the dew point of the adjacent air, so the colder junction line reaches the dew point before the surrounding surfaces and accumulates moisture, with mould following once surfaces are repeatedly damp.

Air leakage frequently coincides at the same location. Joining the air barrier of a new structure to that of an existing one is one of the hardest details to execute, so the interface commonly leaks; the resulting draughts both chill the junction further and transport humid air into the cold zone, accelerating condensation. The combination of a thermal bridge and an air-leakage path along the join is why the symptoms concentrate so tightly on the line where the two structures meet, rather than spreading evenly across a wall.

Correcting it requires restoring both continuities. Thermally, the insulation must be carried across the junction so there is no interrupted path — which may mean overlapping or returning insulation at the interface to keep the internal surface above the dew point. For airtightness, the air barriers of the new and old construction must be sealed together along the join to close the leakage path. Diagnosis with thermal imaging under a temperature difference locates the cold bridge, and smoke or a blower door test locates the leakage, so the specific detailing failure can be targeted. Treating the surface alone leaves the underlying cold, leaky line, which is why symptom-only repairs recur; making the junction continuous is what permanently raises the join to the temperature of its surroundings and stops the condensation and mould.

How to fix damp where an extension joins the house

Restore continuity at the junction: locate the thermal bridge and air-leakage path, then make the insulation and air barrier continuous across the interface between new and old.

1. 01

   Confirm it follows the join

   Establish that the damp, cold or mould runs along the line where the extension meets the house.

2. 02

   Locate the bridge and leakage

   Use thermal imaging and air-leakage testing to find the cold bridge and the leak at the junction.

3. 03

   Make the insulation continuous

   Carry insulation across the junction so there is no interrupted path leaving a cold line.

4. 04

   Seal the air barrier

   Seal the extension's airtightness to the original house's along the join to close the leakage path.

5. 05

   Address any movement cracks

   Repair and detail cracks from differential movement so they do not re-open and admit moisture.

6. 06

   Verify the join warms up

   Confirm the junction line is no longer cold or damp and the mould does not return.

How to prevent it coming back

• Detail the junction for continuous insulation and airtightness when extending.
• Carry insulation across the interface between new and old.
• Seal the new air barrier to the existing one along the join.
• Allow for differential movement so cracks do not open at the join.