Glass for your dormer: complete buyer's guide
Glass covers 50–70% of a dormer's façade and largely determines insulation, indoor climate and living quality. Since Bouwbesluit 2021 HR++ is minimum; BENG level needs triple (HR+++). Above that come functional variants: solar control (essential on south/west), acoustic (busy roads) and safety glass (mandatory near floor level and in certain combinations). Wrong choice costs €1,500–€3,000 in cooling, noise or unnecessary shading.
- HR++ (double, argon)
- Ug 1.1 W/m²K, €70–€95/m²
- HR+++ (triple)
- Ug 0.6 W/m²K, €130–€180/m²
- Solar control HR++ (g-value 0.30)
- €110–€140/m²
- Acoustic (Ra,tr 38–42 dB)
- €130–€180/m²
- Laminated safety glass (33.2)
- €150–€210/m²
- Bouwbesluit minimum
- HR++ (Uw ≤ 1.65)
HR++ vs triple: when worth it?
HR++ is double glazing with low-E coating and argon fill. Ug 1.1 W/m²K, typical build-up 4-16-4 (24 mm). Comfortably meets Bouwbesluit and is standard in 90% of quotes.
Triple (HR+++) is three layers, two low-E coatings, argon or krypton. Ug 0.5–0.7 W/m²K. Typical 4-12-4-12-4 (36 mm) — heavier frames and hardware needed. Glass surcharge €60–€100/m², plus €200–€400 for heavier frames.
Triple payback on a 3-metre dormer (8 m² glass): ~€60–€90/year on gas. Payback 20–25 years — only rational if aiming for BENG or Net Zero Energy.
Solar control glass — essential on south and west
A south-facing dormer absorbs up to 600 W/m² of solar radiation in July/August. Without shading the attic reaches 35–40°C. Two options:
Solar control glass: special coating lets light through (LT 60–70%) but blocks heat (g-value 0.25–0.35). €30–€50/m² above HR++. Pros: always active, no maintenance. Cons: slight tint (green/blue); less free winter warmth.
Or plain HR++ with external shading (screens, awnings). More effective (blocks 90% before the glass), pricier with motor (€800–€1,500). Best solution in 80% of south/west cases.
Acoustic glass — not always needed
Standard HR++ delivers Ra,tr 30–32 dB. Enough in quiet streets. On busy roads (50+ km/h, 5,000 vehicles/day) you need 35–38 dB to reach indoor ≤30 dB per Bouwbesluit.
Acoustic glass has asymmetric build-up (e.g. 6-16-44.2) with PVB film. Ra,tr 38–42 dB. €40–€80/m² extra. Always pair with acoustic 'suskast' vents; an ordinary vent is the weak link and cancels 5–10 dB of the gain.
Safety glass — when mandatory
NEN 3569 mandates safety glass when: (1) glass extends below 85 cm above floor, (2) glass in doors, (3) fall-hazard areas without 1.0 m balustrade. Relevant for French balconies in attics with wooden floors.
Types: laminated (33.2 = two 3 mm panes with 0.76 mm PVB) breaks safely, hangs in film. Tempered (ESG) breaks into small grains. Laminated is pricier but safer and more acoustic. €40–€80/m² laminated; €25–€50/m² tempered.
FAQ
- Is HR++ mandatory or is regular double glazing ok?
- HR++ is mandatory. Plain double glass (Ug 2.8) fails the Bouwbesluit Uw requirement for new build or major renovation, which includes a dormer.
- What does triple cost extra on a 3-metre dormer?
- At 8 m² glass: €480–€800 surcharge on glass plus €200–€400 for heavier frames. Total €700–€1,200 extra. Payback 20–25 years.
- Solar control glass or external screen — which is better?
- An external screen blocks ~90% of solar heat before the glass — more effective. Solar control glass is maintenance-free and cheaper to install. Combining both is overkill. Neither needed on north side.
- How long does HR++ glass last?
- 20–30 years. Argon slowly leaks (15–20% in 25 years) and internal condensation may appear — replacement needed. €200–€350 per pane.
- Which g-value is ideal?
- North: g 0.50–0.60 (free heat welcome). South/west: g 0.25–0.35 (prevent overheating). The g-value is stamped on the glass (CE marking).