Roof Insulation: How do you choose between glass wool, rock wool, or PIR?

Everyone knows heat rises. What many building owners underestimate is how much energy literally escapes through the roof. A poorly dak isoleren can account for up to 30 percent of total heat loss. Put roof insulation at the top of your renovation list and you usually get the highest savings per euro invested. Glass wool is made from fibred recycled glass and binders. It is light, springy, and relatively inexpensive. Rock wool comes from melted basalt. It offers excellent fire resistance and a higher density, which improves sound absorption. PIR boards (polyisocyanurate) are rigid foam panels with a very low thermal conductivity; you reach the same Rc value with a thinner build-up. So performance differs across insulation value, thickness, installation speed, and cost. If you plan to insulate a pitched roof from the inside, the new insulation needs to fit between purlins or rafters and still allow adequate vapor permeability. For above-deck insulation under the tiles (sarking), other demands apply, especially compressive strength and water shedding.

Glass wool: Affordable and flexible

For keen DIYers, glass wool is a friendly choice. It wedges easily between rafters or studs without lots of fixings. With a lambda around 0.035 W/(m·K), you reach Rc ≈ 6.0 m²K/W with roughly 210 mm. That sounds thick, but during renovations where interiors are being redone anyway, the space is not always an issue. If you are insulating a timber roof and want better acoustic comfort, glass wool’s higher absorption helps. Downsides: the fibres can irritate skin and lungs, so protective clothing is a must. It is not water-repellent either; one leaking tile can draw moisture into the layer.

Rock wool: Robust fire rating and acoustics

Rock wool looks similar to glass wool, but denser fibres make it tougher. Lambda is 0.036 0.038 W/(m·K), so slightly less insulating per centimetre. In return, it keeps its form up to about 1,000 °C, which is why hotels and apartment projects often choose rock wool when they insulate roofs and need to meet fire spread requirements. The extra mass also damps traffic noise and rain on metal sheeting more effectively. Expect roughly 20 percent higher price per cubic metre than glass wool, and keep an eye on weight in structural checks.

PIR: Thin build-up, high Rc

Low ridge height, or a dormer you do not want to thicken? PIR rigid foam becomes attractive. With lambda around 0.022 W/(m·K) you can reach Rc ≈ 6.0 with only 130 mm. Boards are load-bearing and usually come with aluminium or glass-fleece facings that act as a vapor retarder, so you can finish inside with gypsum or OSB straight away. PIR excels on the outside of pitched roofs, because the continuous layer removes thermal bridges. Trade-offs: higher material cost and less sound insulation. PIR also has a higher environmental footprint. For circular projects, some clients prefer mineral wool. Even so, many HOAs (VvE) choose PIR for over-roof work because the quick install fits tight renovation schedules.

Vapor control and moisture safety

Do you always need a vapor control layer for internal insulation? In short: yes, unless your board facing already provides a reliable retarder. Warm indoor air carries moisture that will condense once it hits the dew point inside the build-up. This is why Smits dakkundig onderhoud fits a vapor control layer with Sd ≥ 5 m on every project insulated from the inside, sealed airtight with tapes at all edges. For external insulation, a vapor-open, water-tight membrane replaces the foil so moisture can escape while rain stays out.

Fire safety in practice

For tiled pitched roofs the Dutch Building Decree requires resistance to fire spread. Glass wool and rock wool carry the top A1 non-combustible rating. PIR is class B under EN 13501: hard to ignite, but it will soften at very high temperatures. In dwellings this is generally acceptable. For storage or care facilities, mineral wool is often preferred. Always get project-specific advice if the building has a sensitive occupancy.

Installation time and cost factors

On real projects, speed saves money on scaffolding and disruption. Glass and rock wool roll out quickly, although you still cut each piece to size. PIR boards are heavier but larger. Two installers can place about 70 m² of sarking in a day, battens included.

As ballpark figures (materials plus labour):

• Glass wool: €30–35 per m²
• Rock wool: about €40 per m²
• PIR sarking: €55–60 per m²

A standard 100 m² roof insulated from the inside typically takes about three working days. Above-deck board systems often make a roof wind- and watertight within two days.

Subsidies and tax incentives

Since 2023, the ISDE scheme refunds €15 per m² for Rc ≥ 3.5, and €20 per m² for Rc ≥ 5.0. Combine roof insulation with wall or floor insulation and the contribution rises to €30 per m². Smits dakkundig onderhoud supplies the documentation to support your application. HOAs (VvE) can use SVVE funding to recover up to 30% of the invoice, provided there is a multi-year maintenance plan.

Environmental impact and circularity

Mineral wool is largely recyclable. PIR less so, although pilot projects are turning foam back into polyols. If you want a low CO₂ score while insulating the roof, pick glass or rock wool with a high recycled content. Smits dakkundig onderhoud takes demolition material back for processing, which keeps the site clean and anchors circularity in the handover documents.

Conclusion: Quality pays off over time

There is no single perfect insulation for every roof. Your situation decides. Glass wool is affordable and flexible. Rock wool delivers top-tier fire safety and good acoustics. PIR achieves high values with minimal thickness. Whatever you choose, professional installation, correct vapor control, and thorough up-front inspection are non-negotiable. Smits dakkundig onderhoud guides you from first assessment to subsidy application, installation, and aftercare. That way roof insulation does not just save energy. It improves comfort and increases property value in a way that is durable, safe, and ready for the future.

Let the team at Smits dakkundig onderhoud help you find the right solution.