Insulating a loft — what, how, and what does it cost?

Rc-waarde explained

Rc expresses how well a construction holds back heat. Higher = better.

• Bbl requirement for a new roof: Rc ≥ 6,3 m²K/W (since 2021).
• Renovation minimum (ISDE subsidy): Rc ≥ 3,5.
• Practical advice: aim for Rc 6,3 or higher if you have everything open anyway. The difference in material costs is small, the energy savings are for decades.

To achieve Rc 6,3 you need:

• PIR: ~14 cm.
• Glass wool / rock wool: ~18 cm.
• Cellulose: ~18 cm.
• Wood fibre: ~20 cm.

Does it fit in your joists (usually 15–18 cm)? Otherwise 'build it up' with extra battens or a prefab Sarking system on the outside.

Which material to choose?

Four main types:

• PIR (polyisocyanurate): high insulation value per cm (λ 0,022), thin package. Disadvantage: synthetic, flammable upon exposure, poor summer performance.
• Glass wool / rock wool: price/performance champion (λ 0,032–0,035). Acoustically strong. Slightly thicker package needed. Complies with fire class A1.
• Cellulose (recycled paper): blown-in or in blankets. Sustainable, good summer performance, breathable. Slightly more expensive than glass wool.
• Wood fibre: best summer performance (long phase shift), 100 % natural. Most expensive of the four.

Rule of thumb: for pure performance and tight spaces → PIR. For budget + acoustics → glass wool. For sustainable + summer comfort → cellulose or wood fibre.

Airtight detailing is half the work

A poorly connected insulation of 20 cm performs worse than 15 cm that is airtight. Pay attention to:

• Vapour barrier foil on the inside — prevents moisture in the insulation.
• Tape over seams and penetrations (sockets, pipes, screws).
• Connections at the eaves, skylight, ridge points, and facades — draft-proof detailing.
• Penetrations (drainage, cables) sealed with collars or tape.
• No holes afterwards without repairing the vapour barrier.

A contractor who says 'insulation in and done' is fooling themselves. Ask specifically for airtight detailing — often a tape system from Pro Clima, Siga, or Isover Vario.

Do not forget: summer performance

Many people insulate against winter temperatures and are disappointed in July. Important:

• Mass in insulation keeps heat out longer. Wood fibre and cellulose have a high mass (phase shift 8–12 hours). PIR and glass wool much less (3–5 hours).
• Exterior sun shading on skylights works drastically better than interior sun shading (roller shutter > awning > interior roller blind).
• Night ventilation: an openable skylight with an automatic setting cools the loft quickly.
• Reflective underlays (alu-foil on the inside) help a little, especially on clear summer radiation days.

A well-insulated roof with exterior sun shading is often around 25 °C in the summer — without both, it easily goes up to 35–40 °C.

Subsidy and VAT

ISDE (Investeringssubsidie duurzame energie en energiebesparing - Investment subsidy for sustainable energy and energy savings) in 2026:

• Applies to a minimum of 20 m² of roof area with Rc ≥ 3,5.
• Compensation: € 8 – € 16 per m², depending on material performance.
• Condition: execution by a certified construction company (no DIY for subsidy).
• Application within 24 months after execution.

VAT: labour hours for insulation on homes older than 2 years fall under 9 % (renovation scheme). Materials remain at 21 %. Have the contractor itemize this — it saves € 300–800.

A complete roof insulation of 40 m² Rc 6,3: € 4.500 – € 7.500 including installation. Subsidy € 320 – € 640.

Frequently Asked Questions

Short, honest answers to frequently asked questions.

Can I insulate my loft myself?

Applying it between beams can be a DIY job with glass wool or cellulose blankets. For the subsidy and airtight detailing, a certified company is required. Blown-in cellulose is professional work by definition.

What does a Sarking roof (exterior insulation) cost?

During a full roof renovation. Count on € 150 – € 250 per m² including new roof battens and roof tiles. Advantage: no loss of space inside, no air leaks through the existing construction.

Is reflective foil enough?

No. Reflective foil has a low insulation value (Rc 1–1,5). Only useful as a supplement to standard insulation. Sellers of 'thermo-foil' often exaggerate the performance.

What if I do not have space for 18 cm?

Combine: 12 cm between beams (glass wool) + 6 cm underneath (PIR board). Or choose thinner PIR. Or consider a Sarking system on the outside.

How do I check the airtightness afterwards?

A building expert with a blower door test (€ 350 – € 600). Uncovers all leaks. Makes the difference between 'done well' and 'thrown away budget'.

Do I get a subsidy for loose packages from the DIY store?

Only via a certified company — so not for buying and installing it yourself. You do not get ISDE for DIY insulation.

What does insulation do to my WOZ value?

A better energy label = higher WOZ value (Dutch property valuation), but also higher taxes. When selling, an energy label A usually results in a 4–8 % higher selling price.

Does vapour-permeable vs vapour-retardant help?

Vapour-retardant is mandatory on the inside (prevents moisture in the insulation). Vapour-permeable on the outside allows any moisture to escape. Both are needed.

Summary

Good loft insulation stands or falls with the Rc-waarde (aim for 6,3), the right material (glass wool or cellulose for most situations), and above all, airtight detailing. Do not forget summer performance and exterior sun shading. With the ISDE subsidy and 9 % VAT on labour, a roof insulation of 40 m² quickly becomes affordable (€ 4.500 – € 7.500 incl. installation, minus € 320 – € 640 in subsidies).