DakkapelBedrijfKiezenDakkapelBedrijf · Kiezen
Insulation · Materials

Insulation materials for a dormer

In a dormer the cavity is shallow and the airtight detailing is fiddly, so the choice of insulation matters more than in a regular wall. Here's how the four common materials compare on the things that actually decide outcomes.
6 min leestijd·Onafhankelijke informatie

Short answer

PIR for shallow cavities where Rc must be high. Mineral wool when acoustic performance and fire behaviour matter. EPS only on a tight budget and only above grade. Natural fibres when moisture buffering and breathability are priorities. In all cases, the vapour barrier on the warm side is non-negotiable.

PIR (polyisocyanurate) boards

  • λ-value: ~0.022 W/mK — highest performance per centimetre.
  • Strengths: reaches Rc 6.0 in 140 mm; rigid; foil-faced for vapour control.
  • Trade-offs: poor acoustic performance; combustible without proper detailing; joints must be taped to be airtight.
  • Best for: shallow cavities, high-spec roofs.

Mineral wool (glass or stone)

  • λ-value: ~0.035 W/mK.
  • Strengths: excellent acoustic absorption, non-combustible, vapour-open, easy to cut around timbers.
  • Trade-offs: needs ~40% more cavity depth than PIR for the same Rc; settles if poorly installed.
  • Best for: dormers near airports, busy roads or used as bedrooms.

EPS (expanded polystyrene)

  • λ-value: ~0.034 W/mK.
  • Strengths: cheap; rigid.
  • Trade-offs: combustible; no acoustic benefit; lower compressive strength than PIR.
  • Best for: budget projects where higher-performing boards are unaffordable.

Natural fibres (wood fibre, hemp, cellulose)

  • λ-value: ~0.038–0.042 W/mK.
  • Strengths: excellent moisture buffering, low embodied carbon, good summer comfort thanks to high heat capacity.
  • Trade-offs: needs more cavity depth, higher material cost, requires experienced installers.
  • Best for: renovation projects on older houses with moisture-sensitive construction.

Vapour barrier and airtightness

FAQ

FAQ

Veelgestelde vragen

01Which insulation material is best for a dormer?
There is no single winner. PIR boards give the highest Rc per centimetre, mineral wool is best for acoustics and fire, and natural fibres (wood fibre, hemp) handle humidity best. The right choice depends on the available cavity depth and how the dormer is used.
02Is more insulation always better?
Up to a point. Going from Rc 4.5 to Rc 6.0 makes a clear difference; from Rc 6.0 to Rc 8.0 the heat-loss saving is small while the cost and risk of condensation rise sharply.
03Do I need a vapour barrier?
Yes. A continuous airtight vapour barrier on the warm side is essential — without it, warm humid indoor air condenses inside the insulation and the structure rots from the inside out.
Conclusie
Insulation choice is downstream of cavity depth, acoustic needs and how moisture-tolerant the existing structure is. Get those three answers first; the right material follows.