Insulation · Basics

R-value (Rc) for a dormer, explained

R-value is the single most useful number on a dormer quote. It tells you how warm the dormer will feel in winter, how cool it will stay in summer, and how often you will think about your energy bill.

5 min leestijd·Onafhankelijke informatie

Short answer

R-value (Rc in Dutch quotes) measures how well a piece of construction resists heat loss. The higher the number, the better. For a new dormer, aim for R ≥ 4.5 m²K/W on the roof and side cheeks. The minimum that the Dutch building code allows is lower than the number a comfortable dormer needs.

What R-value actually measures

R-value is the thermal resistance of a build-up — roof panel, side wall, frame seal, whatever — at a given thickness and material composition. It is the inverse of how fast heat flows through it. Doubling the R-value roughly halves the heat loss through that surface, all else being equal.

Sensible targets for a new dormer

Roof panel: R ≥ 4.5 m²K/W (a comfortable target is 5.0–6.0).
Side cheeks: R ≥ 3.7 m²K/W as a floor, 4.5+ if you can.
Glazing: U ≤ 1.1 W/m²K for HR++, U ≤ 0.7 W/m²K for triple.
Frames: a low U-value and a good thermal break — ask for the system data sheet.
No thermal bridges. The R-value of the panel is meaningless if the lead, frame or corner detail is uninsulated.

What different R-values feel like

R-value (roof)	Real-world feel	Typical use
3.5	Just legal, noticeable cold spots in winter	Older dormers, bargain quotes
4.5	Code-comfortable, neutral feel	Most modern new builds
5.0–6.0	Markedly warmer, quieter, lower bills	Comfort-led upgrades
6.5+	Borderline overkill on a small dormer	Energy-label-driven projects

How to read it on a quote

A serious quote states the R-value of the roof panel and the side cheeks, the U-value of the glazing and the frame, and confirms that thermal bridges are detailed out at the lead apron and corners. A vague "well insulated" or "Rc value 3.5" is a warning sign, not a specification.

FAQ

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01What R-value should a new dormer aim for?

Aim for R ≥ 4.5 m²K/W on the roof and side cheeks. Going to R 5.0–6.0 is common, costs a little more and pays back in comfort, not just energy savings.

02Is the building code minimum good enough?

Technically yes, but the minimum is a floor, not a target. A dormer at the minimum will feel noticeably warmer in summer and colder in winter than one a step above.

03Does higher R-value mean a thicker wall?

Usually yes, but modern insulation materials (PIR, resol foam, mineral wool composites) deliver higher R-values per cm than older products. Thickness creeps up by a few centimetres, not dramatically.

Conclusie

R-value is one of the few numbers on a dormer quote that maps directly to how the room will feel for the next thirty years. Spend a little more here than you would in other places — it is one of the rare upgrades you will notice every winter and every summer.