« Isolation de la toiture et des combles. Différence ressentie dès l'hiver suivant. Petit retard de planning, mais résultat à la hauteur. »
Thomas R.Mons
Roof insulation · Between/under rafters
Under-rafter insulation: a warm pitched roof, insulated from the inside
The solution for lived-in lofts. We insulate the roof slope from the inside, between the rafters then beneath them: a double layer that breaks thermal bridges, reaches the grant-eligible R value and keeps the headroom. A vapour-check membrane on the warm side and careful airtightness avoid any condensation.
- R 4.5–6
- m²K/W targeted
- Vapour-check
- warm side, airtight
- Liveable lofts
- headroom preserved
Double layerthermal bridges broken
R ≥ 4.5 m²K/Wgrant-eligible level
Technical specifications
What under-rafter insulation is really worth
The benchmarks that matter for insulating a pitched roof from the inside, fitted by our teams. Concrete figures, not marketing.
- Thermal performance R 4.5–6 m²K/W
The R value measures the insulant’s thermal resistance. In renovation, grants often require R ≥ 4.5 m²K/W for the roof. We size the thickness to reach — or exceed — this threshold, the condition for a truly warm roof.
- Double layer Between + under rafters
A single layer between the rafters lets cold pass through the timber (a thermal bridge). We add a second continuous layer beneath the rafters: heat loss through the frame is broken and overall performance climbs noticeably.
- Thickness required 20–28 cm
To target R 4.5 to 6, you need 20 to 28 cm of insulant depending on the material (lambda). This is the main constraint when working from inside: we compose the two layers to limit the loss of headroom.
- Vapour-check membrane Warm side
A vapour-check (or vapour barrier) fitted on the inner side, with joined and taped strips, manages moisture migration and protects the insulant and the frame from condensation. The cornerstone of a durable install.
- Ventilated air gap Under the covering
We keep a ventilated air gap between the insulant and the under-roof, or fit a breathable membrane. Air circulates, evacuates residual moisture and preserves the tiles as well as the frame.
- Airtightness Careful work
Air leaks at the junctions (purlins, gables, openings) ruin good insulation. We treat every joint with tapes and grommets: fewer losses, more comfort, no cold draughts.
* Indicative values for a typical converted-loft renovation. The exact thickness and R value depend on the material, the frame and the available height — confirmed at the survey.
Full transparency
Upsides & limits of insulating from the inside
No solution is perfect for every job. Here, honestly, is where under-rafter insulation from the inside excels and where another method is better.
Where it shines
- Without touching the coveringWe work from inside: no removing the tiles, a simpler and cheaper job.
- Lofts made liveableThe pitched roof becomes a warm living space, summer and winter alike.
- Controlled costMarkedly cheaper than exterior sarking, at equivalent performance.
- Grant-eligibleBy reaching R ≥ 4.5, the job qualifies for renovation grants and 6% VAT.
What you should know
- Loss of headroomThe double layer reduces the liveable volume by a few centimetres — decisive under a shallow pitch.
- Demanding airtightnessA poorly fitted vapour-check creates condensation: the quality of the install is everything.
- Thermal bridges to treatWithout a second layer under the rafters, the roof timber lets heat escape.
- Frame not protectedUnlike sarking, the timber stays on the cold side: the covering isn’t renewed in the process.
For which project
Is under-rafter insulation right for you?
It’s the choice whenever you want to live under the roof without redoing the covering. A few cases where it shines:
-
Converted lofts
Bedroom, office or playroom under the roof: we insulate the slope from inside to turn a cold volume into a comfortable room.
-
Roof in good condition
If the tiles and frame are sound, there’s no need to strip everything: insulating from the inside is the most direct and economical route.
-
Defined renovation budget
When renovating step by step without exterior structural work, under-rafter insulation offers the best comfort gain per euro spent.
Covering due for renewal, or unused loft?
If you’re redoing the roof, exterior sarking insulates without losing height and protects the frame. And if the attic is unused, unused-loft insulation at floor level is simpler and cheaper.
Glissez la poignée pour comparer
Case study · loft, Namur
A freezing attic turned into a warm master suite all year round
The client wanted to gain a bedroom under the roof, but the loft was uninhabitable in winter and stifling in summer. We fitted two-layer insulation — wood fibre between the rafters then mineral wool beneath them — with a continuous vapour-check and careful airtightness. The room reaches R 5.5 and stays temperate in every season.
- R 5.5m²K/W achieved
- −40%roof heat loss
- +1 roomof liveable space gained
Customer reviews
They chose wendows
4.5/5 · 2verified reviews
« Insufflation des murs creux, rapide et sans dégât. La prime régionale a couvert une bonne part. Maison nettement moins froide. »
David K.Huy
Compare the options
Insulate the rafters, or another way?
Under-rafter insulation wins on cost and simplicity when the covering is sound. Compare it point by point with sarking and loft insulation, then open the solution that suits you.
| Critère | Under-rafter insulationVous êtes ici | Sarking (from the outside)Voir la gamme → | Loft insulationVoir la gamme → |
|---|---|---|---|
| Indicative price | €€ — controlled | €€€€ — high | The gentlest |
| Liveable lofts | Yes | Yes | No (unused attic) |
| Headroom | Reduced | Preserved | Not applicable |
| Protects the frame | No (timber on cold side) | Yes | No |
| Covering removal | None | Roof redone | None |
| Achievable performance | Very good (R 4.5–6) | Excellent, continuous | Excellent at floor level |
| Best for | Converted lofts, sound roof | Roof due for renewal | Unused attic |
Indicative benchmarks for typical jobs. The right choice depends on the state of the roof and how the loft is used — we refine it together at the survey.
- vs rafters
Exterior sarking
Continuous insulation laid over the frame, with no loss of height and with timber protection. The reference when redoing the covering.
Discover sarking → - vs rafters
Loft insulation
For an unused attic, we insulate the floor: simpler, faster and the cheapest way to stop losses through the top.
See loft insulation → - overview
Roof insulation
Every method for insulating the roof: rafters, sarking, lofts. Which solution suits your project and budget.
Compare the methods → - to combine
Insulation materials
Mineral wool, wood fibre, PUR: lambda, performance and impact. Understand what lies behind the R value.
Choose your material → - to combine
Sprayed foam
An alternative that hugs the complex shapes of a frame and delivers airtightness in a single pass.
See sprayed foam → - ↑ category
All insulation
An overview of our insulation solutions in Belgium: roof, walls, floors, grants and audit on one page.
Back to the insulation guide →
Indicative pricing 2026
How much does under-rafter insulation cost?
The price depends on the targeted thickness, the material, loft access and the finish. Insulating from the inside remains one of the most affordable options. Here are some benchmarks for fitted insulation.
See the detailed insulation price grid →* Indicative ranges per m² of fitted under-rafter insulation, excluding specific finishes. A costed estimate is provided with your free quote.
- Mineral wool from €45/m²double layer, vapour-check fitted
- Wood fibre from €70/m²bio-based, summer comfort
- Renovation grant up to €35/m²depending on region and income
- Reduced VAT 6%on renovation (home > 10 years)
Under-rafter insulation FAQ
Your questions about insulating from the inside
What R value should you target for the grant? +
In Belgium, renovation grants for roofs generally require a thermal resistance R of at least 4.5 m²K/W. We size the insulant thickness to reach — or exceed — this threshold, which is also the level that makes lofts genuinely comfortable. The exact threshold depends on your region (Wallonia, Brussels, Flanders).
Why fit two layers of insulant? +
A single layer placed between the rafters lets the roof timber conduct cold: that’s a thermal bridge. By adding a second continuous layer beneath the rafters, you cover the frame and eliminate that leak. Overall performance and comfort improve noticeably.
Will I lose a lot of headroom? +
The double layer typically takes 20 to 28 cm depending on the material, part of which sits between the existing rafters. The net loss of headroom is often limited to a few centimetres. Under a shallow pitch, we choose a high-performance lambda insulant to reduce the thickness.
Is the vapour-check membrane really essential? +
Yes. Fitted on the warm (inner) side, with joined and taped strips, it regulates moisture migration and protects both the insulant and the frame from condensation. Under-rafter insulation without moisture management eventually rots the timber: it’s the cornerstone of a durable install.
Which material should you choose for the rafters? +
Mineral wool offers the best performance-to-price ratio. Wood fibre, bio-based, brings excellent summer comfort by delaying the heat. PUR reaches the same R value in less thickness, useful under a shallow pitch. We guide you based on the available height and the budget.
What grants are there for under-rafter insulation? +
As soon as the R value reaches the regional threshold (often R ≥ 4.5), the job qualifies for a renovation grant and 6% VAT for homes over ten years old. A prior energy audit often maximises the amount. We handle the grant application for you.
Get your free under-rafter insulation quote within 48h
Free home survey, R value and grants calculated, a single point of contact. Describe your project in under a minute.
- Free and no obligation
- Grants & 6% VAT handled for you
- Fitted by our certified installers