A thermal bridge — often called a cold bridge — is a place in the building envelope where heat escapes considerably faster than through the surrounding area. The insulation is interrupted, thinner, or the geometry works against you. The result is a cold spot on the internal surface.
Two kinds
- Material thermal bridge — a highly conductive material passes through the insulation layer. Classic examples: a concrete floor slab continuing out into a balcony, a steel beam or column penetrating the facade, a window frame profile with no thermal break, a floor slab bearing against the outer leaf of a cavity wall.
- Geometric thermal bridge — the shape itself is unfavourable. At an external corner, the outside surface giving off heat is larger than the inside surface supplying it. Every corner is therefore naturally colder than the middle of the wall, even with the insulation fully continuous.
Why it is more than heat loss
Heat loss costs money, but the real problem is moisture:
- The internal surface becomes colder than the rest of the wall.
- If its temperature drops below the dew point of the indoor air, water vapour condenses on it.
- On a persistently cold, damp surface, mould grows — often well before visible condensation appears.
That explains why mould keeps turning up in the same kinds of place: behind a wardrobe in the external corner of a bedroom, along the junction of floor and external wall, around a window reveal or at a roof edge.
How it is assessed
The thermal quality of a junction is expressed as the temperature factor f_Rsi:
f_Rsi = (T_surface − T_external) / (T_internal − T_external)
The higher the value, the warmer the surface stays. UK guidance on limiting the risk of surface condensation and mould growth commonly sets a minimum temperature factor of 0.75 for dwellings and 0.50 for most other building types. The additional heat loss of a linear thermal bridge is separately expressed as a psi value (ψ), in W/mK per metre run of junction.
Solutions
- Break the bridge thermally — an insulating element within the construction itself: an insulated balcony connector, a thermally broken aluminium profile, an insulating course under a wall bearing on a slab.
- Keep the insulation continuous — the gain lies in continuity. An insulation envelope with a gap of a few centimetres performs far worse than the arithmetic suggests.
- Load-bearing insulation at foundation junctions — cellular glass or a comparable load-bearing, moisture-resistant product where the insulation has to carry weight.
- Take extra care with internal insulation — it makes the existing construction colder and moves the condensation plane inwards. Floor and wall junctions then become the critical points; have them assessed.
- Find them with thermography — an infrared survey with enough temperature difference between inside and outside shows thermal bridges immediately.
Related terms
- Insulation
- Condensation
- Dew point
- Conduction
- Cavity wall
- Internal wall insulation
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