Insulation · Detail
Thermal bridges in a dormer
A high R-value is wasted if the insulation is interrupted somewhere. Thermal bridges are those interruptions — the small cold spots that turn a well-insulated dormer into a draughty one with stains in the corners.
6 min leestijd·Onafhankelijke informatie
Short answer
A thermal bridge is a point in the build where insulation is broken — at a frame, a corner, the lead apron or the loft floor. Those points stay colder than the rest of the wall, lose heat faster and pull condensation toward them. Good detailing prevents them; correcting them after the fact is much harder.
What a thermal bridge actually is
An insulated dormer is, in effect, a continuous warm envelope around a cold outside. A thermal bridge is any point where that envelope is interrupted — by a structural timber that bypasses the insulation, by a window frame fitted without a thermal break, or by a corner where two insulation panels meet imperfectly. Heat takes the path of least resistance, which is straight through the bridge.
Where they form in a dormer
- Lead apron line. Where the dormer meets the existing roof tiles, insulation often stops short of the lead — a classic cold spot.
- Side cheek corners. The 90° angle where the side wall meets the roof panel is hard to insulate continuously.
- Window frame edges. Frames without a thermal break carry heat from inside to outside straight through the aluminium or PVC.
- Dormer base. The transition from the dormer floor to the existing loft floor is often forgotten in basic installs.
- Lintels and headers. Solid timber lintels above the window run uninterrupted from inside to out.
Risk by location
| Location | Frequency | Visible sign |
|---|---|---|
| Lead apron line | Very high | Dark line above the apron inside |
| Side cheek corners | High | Stains or mould in upper corners |
| Frame edges | High | Cold to the touch, condensation on glass edge |
| Dormer base | Medium | Cold floor where dormer meets loft |
| Lintels | Low | Subtle line of staining above window |
How good detailing prevents them
- Continuous insulation that wraps every corner without breaks.
- Frames with a verified thermal break, fitted with insulating tape around the perimeter.
- Lead apron detailing that maintains the insulation line behind the lead, not just below it.
- Vapour control layer (VCL) on the warm side, sealed at every penetration.
- Properly designed ventilation so the inside air does not stay humid in the first place.
FAQ
FAQ
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01Where do thermal bridges usually appear in a dormer?
At the lead apron, at the corners where the dormer meets the existing roof, around the window frames and at the join between the dormer base and the loft floor. These are the points where insulation is hardest to keep continuous.
02Do thermal bridges always cause condensation?
Not always, but they raise the risk significantly. A thermal bridge is a cold spot on a warm wall — humid indoor air condenses there before it would anywhere else. Over time that shows up as staining or mould.
03Can a thermal bridge be fixed after the build?
Sometimes. Adding internal insulation to the corner, replacing the frame seal or improving ventilation can help. Major bridges usually need the original installer back on site.
Conclusie
Thermal bridges are the quiet reason a 'well-insulated' dormer can still feel cold and stain in the corners. The fix is detailing, not more insulation — and detailing is the part of the quote you cannot see from the brochure photo.