Glossary
Thermal bridging and cold spots in London extensions
A path of heat loss through poorly insulated junctions. Modern detailing aims to minimise.
A path of heat loss through poorly insulated junctions. Modern detailing aims to minimise.
A thermal bridge is a path through the building fabric with higher heat loss than the surrounding construction — a lintel, a jamb, a balcony, a wall-to-floor junction. Bridging is quantified as a psi value in W/mK, and unaddressed junctions can add 10 to 30% to a dwelling's total heat loss.
The head, jamb and cill of every opening; the eaves and the wall-to-roof junction; the wall-to-ground-floor junction; the party wall junction; and any steel or concrete element crossing the insulation line. In a rear extension with large sliding doors, the door threshold and jambs dominate.
Uninsulated cavity closers and steel lintels bridging the cavity are the two most common site failures, and both produce visible mould patterns on the internal reveal within a winter or two.
SAP either applies conservative default psi values or uses accredited construction details, or calculated psi values from a specific thermal model. Defaults are punitive: using accredited details or calculated values regularly recovers several SAP points and can be the difference between passing and failing Part L.
Surface condensation and mould at the cold spot, higher running costs, and — in a new dwelling — a failed compliance calculation. Approved Document L requires reasonable provision to limit heat loss at junctions, so the detail is a regulatory requirement rather than an optional refinement.
Thermal bridge in a Hammersmith and Fulham context: London Borough of Hammersmith and Fulham (Hammersmith & Fulham Council) runs 45 conservation areas including the Hammersmith Grove Conservation Area and the Brackenbury Village Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how thermal bridge is handled: £548 and 8 weeks for a Hammersmith and Fulham householder application, or £274 for the Lawful Development Certificate that records thermal bridge as lawful. The law behind thermal bridge sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Measured as | Psi value, W/mK |
|---|---|
| Typical share of heat loss | 10–30% if unaddressed |
| Common bridges | Lintels, jambs, cills, eaves, floor junctions |
| Calculation options | Default, accredited details, or calculated psi |
| Regulatory basis | Approved Document L |
| Symptom | Surface condensation and mould at reveals |
Almost always an uninsulated cavity closer or a bridging lintel creating a cold reveal.
Yes — they replace punitive default psi values and often recover several SAP points.
A regulation. Part L requires reasonable provision to limit heat loss at junctions.
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