Thermal bridges and condensation in a dormer

Short answer

Thermal bridges are weak spots in the insulation where heat escapes and cold surfaces form. In a dormer, the junction with the existing roof is the biggest risk. Proper detailing and a continuous vapour barrier prevent condensation and mould.

Thermal bridges

Everywhere the new dormer touches the old roof, there is a risk of a thermal bridge. Wood, metal, and concrete conduct heat better than insulation, so these areas can become cold spots.

• Junction — where the dormer walls meet the roof slope.
• Frame — metal or hardwood frames conduct more heat than insulated walls.
• Fixings — metal bolts that penetrate the insulation can create local cold spots.

Condensation

Condensation happens when warm, moist air hits a cold surface. In a dormer, this often appears on the ceiling near the junction or on window frames in winter.

• Small, regular damp patches in cold weather.
• Mould near corners or behind furniture.
• Streaming windows on cold mornings.

Prevention

Good design and installation prevent most thermal bridge problems. The key is continuity: insulation should wrap around the structure without gaps.

• Continuous insulation at the dormer-roof junction.
• Thermal break strips in metal frames.
• Vapour barrier sealed to the existing structure.
• Adequate ventilation to remove moisture.

The full checklist

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