Dormer insulation
The R-value, the glazing, the frames and — most importantly — the joints. The four things that decide whether your dormer is comfortable or cold.
Short answer
What makes insulation good?
Good dormer insulation is not just a thick layer of mineral wool. It is the combination of a high R-value, a continuous vapour barrier, properly sealed joints and matching glazing and frames. Skip one of those and the whole package underperforms.
Standards
What the numbers mean
| Item | Legal minimum (NL) | Comfortable target | Why |
|---|---|---|---|
| Roof R-value | 4.7 | 5.5+ | Lower heat loss, better comfort |
| Wall R-value | 3.7 | 4.5+ | Side cheeks lose heat fast if thin |
| Glazing U-value | 1.65 | 0.8 (triple) | Less heat loss, less condensation |
| Frame U-value | — | 1.3 or lower | Frame is half the window's loss |
| Air tightness | Required | Tested with smoke pen | Drafts ruin a high R-value |
Risks
Where insulation fails
Checklist
Before signing the quote
Checklist
Five must-ask questions
- 01What is the total R-value of the roof element?
- 02How are thermal bridges at the existing roof avoided?
- 03Which vapour barrier do you use and how is it sealed?
- 04What is the glazing U-value, and the frame U-value?
- 05How is ventilation handled to avoid condensation?
Read next
Related guides
Quick answers
Frequently asked questions
FAQ
Veelgestelde vragen
4 vragen · klik om te openen
01What R-value should a new dormer have?
The Dutch Building Decree requires at least R 4.7 m²K/W for roof elements on new builds. For a dormer, aim for R 5.0 or higher in walls and roof for real comfort.
02What are thermal bridges?
Spots where insulation is interrupted — typically at junctions between dormer and existing roof. Cold passes through there, causing condensation and heat loss.
03Is triple glazing worth it?
Often yes for comfort and sound, but only if the frames are good enough to match. Triple glass in a poor frame is wasted money.
04Do I need a vapour barrier?
Yes. Without a vapour barrier on the warm side, moisture from the house migrates into the insulation and slowly destroys it.
Conclusie
Insulation is the part of the dormer you never see and never think about — until it goes wrong. Spend the time on R-values, vapour barriers and joints up front; you will not get a second chance once the cladding is on.