A thermal bridge occurs where geometry or materials change heat flow relative to the surrounding assembly. Assess both additional loss and surface temperature.
Linear and point bridges
Floor and window junctions are commonly assessed along a line; an individual fixing can have a point effect. Complex three-dimensional geometry may not be represented adequately by a flat section.
Where to look
Inspect slab edges, plinths, lintels, balconies, parapets and equipment supports. Pay attention where insulation stops or becomes thinner.
Heat loss and psi
The linear coefficient psi represents additional heat flow per unit length and temperature difference under a stated dimensional convention. Values using different geometric boundaries are not directly comparable.
Surface temperature
A cold internal surface can cause local discomfort. Assess its temperature under stated indoor and outdoor conditions rather than infer it from the wall U-value alone.
Condensation and mould
Humidity and occupancy conditions also matter. Acceptable overall heat loss does not rule out a problem in one cold corner.
Why wall U-value is insufficient
A one-dimensional layer calculation does not represent heat bypassing insulation through a concrete element. Significant junctions need a validated comparable detail or a separate thermal model.
What to check on site
Compare actual geometry, thicknesses and fixings with the assessed detail. Photograph concealed layers before covering them so later review has usable evidence.
| Item | Parameter | Check / result |
|---|---|---|
| Slab edge | Continuity | Section |
| Window | Installation psi | Head, jamb, sill |
| Bracket | Point effect | Geometry and material |
What this means for your project
Create a junction register with lengths. Prioritise repeated details and locations with local moisture risk.
Sources and scope of review
- PHI Passipedia — Thermal bridge quality assurance
- PHI Passipedia — Window installation thermal bridges
Related reading
- Blower Door Test: When, How and Why to Test a Building
- ZeroHouse Lab: Comparing Four External Wall Concepts
- Insulated Raft Foundation: When It Makes Sense
- Installing Windows in the Insulation Layer