Thermal Bridging in London Home Extensions: What It Is and How to Avoid It

The most significant thermal bridge locations in a typical London rear extension are: Eaves junction (flat roof to wall): The flat roof's insulation must connect without interruption to the wall's insulation at the junction where the roof meets the wall. If the roof insulation stops at the top of the wall and the wall insulation stops at the base of the roof, there is a thermal bridge at the junction. Correctly detailing this junction so that the insulation layers are continuous (for example, by using a proprietary insulated parapet closure or by continuing the roof insulation down the inner face of the parapet wall) reduces this bridging significantly. Floor/wall junction (threshold): At the level where the extension floor meets the external wall, the floor insulation must connect to the wall insulation. This junction is particularly difficult in extensions where the floor is a concrete ground-bearing slab — the edge of the slab typically contacts the external wall (and thus the outside environment) unless an edge insulation board is installed at the perimeter of the slab. Perimeter edge insulation (typically 50–75mm XPS board at the slab edge) is required to reduce this bridging. Lintel over window and door openings: A steel or concrete lintel over a window or door opening acts as a thermal bridge. The inside face of the lintel is at a lower temperature than the surrounding wall, creating a cold spot above the window or door frame. Using insulated boot lintels (which incorporate insulation on the warm side of the structural element) significantly reduces this bridging. Window frame jamb and cill: The window frame material (typically aluminium or uPVC) has a higher thermal conductivity than the surrounding insulated wall. Where the frame bridges between the inside and the outside, there is a thermal bridge at the frame perimeter. Thermally broken aluminium frames (which use a polyamide thermal break within the aluminium frame) significantly reduce this effect compared with solid aluminium frames.

On a London extension project, the following practical steps significantly reduce thermal bridging: Specify junction details at design stage: Ask your architect or designer to include junction details in the technical drawings for the extension that specifically address thermal bridging at eaves, threshold, and openings. These details should use the published Psi values from the NHBC/LABC Appendix K or should be custom-calculated. Include insulation continuity in the specification: The specification for insulation installation should explicitly require that the insulation layers are continuous at all junctions — no gaps at the eaves, no missing edge insulation at the floor perimeter, no uninsulated reveals at windows and doors. Use insulated lintels: Specify proprietary insulated boot lintels (such as Thermabate or Keystone) over all window and door openings rather than standard steel lintels. The cost premium is small and the thermal benefit is significant. Specify thermally broken frames: For aluminium window and door frames in a London extension, thermally broken frames should be specified rather than solid aluminium. Thermally broken frames are now the standard in any quality window specification and should not incur a significant premium over equivalent solid aluminium frames. Check the SAP calculation: The SAP calculation submitted with the Building Regulations application includes a thermal bridging assessment. Review this with your architect to confirm that the junction details specified produce a satisfactory result and that the extension will meet Part L requirements.