« 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
Insulation · Roof
Roof insulation: 30% of your heat loss escapes through the roof
The roof is the leading source of heat loss in a Belgian home. Insulating the roof means cutting your heating bill from the very first winter, improving comfort in summer and winter alike, and adding value to your property. From the inside, from the outside (sarking) or by blowing insulation into the attic: we choose the method that fits your roof.
- 30%
- of loss through the roof
- Umax 0.24
- grant standard
- Wool · PUR · cellulose
- all materials
−30%in heat loss
Umax 0.24W/m²K grant-eligible
Insulation methods
The three ways to insulate a roof
Each method has its strengths depending on whether the attic is converted or unused.
- Between rafters The most common
Panels or rolls of wool (glass, rock) or PUR placed between the rafters, beneath the roof covering. The standard method when renovating a converted attic.
- Sarking (from the outside) Maximum performance
Rigid panels laid above the rafters, beneath the new roof covering. Eliminates thermal bridges but requires redoing the roof covering.
- Blown-in for unused attics The fastest
Cellulose or wool blown onto the floor of an unconverted attic. The most economical and fastest method.
- Thermal resistance R ≥ 5 m²K/W
To reach Umax 0.24 W/m²K (the grant threshold), you need a resistance R of at least 5 m²K/W — that is 18–24 cm of wool or 12–16 cm of PUR.
- Thickness 12–24 cm
Thickness depends on the material: PUR (λ 0.022) is thinner than wool (λ 0.035). We optimise according to the space available.
- Fire reaction A1–A2 (wool)
Rock wool is non-combustible (class A1). PUR is combustible but treated with a flame retardant. The choice matters for fire safety.
* Indicative values. The exact thickness and material are determined during the on-site survey.
In full transparency
From the inside or from the outside?
Two main approaches, each with its strengths.
From the inside (between/under rafters)
- CheaperNo roof covering to replace — the budget focuses on the insulation itself.
- No exterior scaffoldingThe work is carried out from inside the attic.
- FastA few days for a typical house.
- Combines with conversionYou insulate and convert the attic at the same time.
From the outside (sarking)
- Eliminates every thermal bridgeContinuous insulation above the rafters removes the bridges that interior work cannot address.
- No loss of interior spaceThe thickness is added on the outside — the attic keeps all its volume.
- Ideal if the covering needs redoingYou take advantage of the roof renewal to insulate with no extra removal cost.
- Maximum thermal performanceSarking achieves the best U-values with no space constraint.
For which roof
Which method for your roof?
The choice depends on the condition of the covering and how the attic is used.
-
Converted attic
Insulation between and under the rafters is the standard. We fit the insulation, the vapour barrier and the finish in a single campaign.
-
Unused attic
Blowing cellulose or wool onto the attic floor: the fastest and cheapest solution. Done in a few hours.
-
Covering to be redone
If the roof covering is at the end of its life, sarking combines insulation and roof renewal in a single job. Maximum performance.
And wall insulation?
After the roof, the walls are the second source of heat loss (25%). And for the floor, see floor insulation.
Glissez la poignée pour comparer
Real case · 1960s house, Mons
A bare attic insulated in 3 days, the bill halved
A 1960s house with a converted but uninsulated attic — cold in winter, an oven in summer. We fitted 18 cm of rock wool between and under the rafters with a vapour barrier in 3 days. Comfort transformed from the very first winter.
- −35%on heating
- 3 daysof work
- R = 5.1grant-eligible
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 methods
Between rafters, sarking or blown-in?
Three methods, three situations. The choice depends on your attic and your roof covering.
| Critère | Between raftersVous êtes ici | SarkingVoir la gamme → | Blown-inVoir la gamme → |
|---|---|---|---|
| Performance | Very good | The best | Very good |
| Price | Mid-range | The highest | The gentlest |
| Duration of work | 2–4 days | 1–2 weeks | A few hours |
| Converted attic | Yes | Yes | No (unused attic) |
| Covering kept | Yes | No (redone) | Yes |
| Thermal bridges | Reduced | Eliminated | Reduced |
| Ideal for | Standard renovation | Covering to be redone | Unused attic, budget |
Indicative benchmarks. The optimal method depends on your roof — we assess it during the on-site survey.
- if attic is unused
Attic insulation
Blown-in is the fastest and most economical solution for an unconverted attic.
See blown-in → - next step
Wall insulation
After the roof, the walls are the second priority (25% of heat loss).
See wall insulation → - which material
Materials
Wool, PUR, cellulose, wood fibre: compare the insulation materials.
Compare materials → - from the outside
External wall insulation
Roof sarking is the counterpart of EWI for walls: insulation from the outside.
See EWI → - pair it with
Heat pump
Insulate first, then size the heat pump to an efficient home.
See heat pumps → - ↑ category
All insulation
An overview of our insulation solutions in Belgium.
Back to the insulation guide →
Indicative price 2026
How much does roof insulation cost?
The price depends on the method, the material and the surface area. Here are benchmarks per m² installed.
See detailed insulation prices →* Indicative ranges per m² installed. The exact price depends on the configuration and accessibility.
- Between rafters (wool) 30–55 €/m²material + installation
- Sarking (PUR) 80–150 €/m²excluding new covering
- Blown-in attic 15–30 €/m²cellulose or wool
- Renovation grant up to 35%depending on income and region
Roof insulation FAQ
Your questions about roof insulation
Where to start: roof, walls or floor? +
With the roof, almost always. It is the leading source of heat loss (~30%) and often the best cost-to-gain ratio. Then the walls (~25%), then the floor (~10%).
Do I need a vapour barrier? +
Yes, on the warm side (interior). The vapour barrier stops indoor humidity from penetrating the insulation and condensing inside it. Without one, you risk mould and degraded insulation.
How thick should the insulation be? +
To reach the grant threshold (Umax 0.24 W/m²K), you need roughly 18–24 cm of wool (R ≥ 5) or 12–16 cm of PUR. We calculate precisely during the survey, according to the chosen material.
Is sarking worth the extra cost? +
If the covering needs redoing, yes: you combine both jobs. If the covering is in good condition, interior insulation is the more sensible choice.
Does insulation also improve summer comfort? +
Yes, considerably. The insulation blocks the sun’s heat in summer just as it retains indoor heat in winter. Wood fibre offers the best summer comfort thanks to its thermal phase shift.
Which grants apply to roof insulation? +
In Wallonia and Brussels, significant grants exist for roof insulation reaching Umax 0.24 W/m²K. The amount depends on your income and the insulated surface. VAT at 6% for renovation.
Get your free roof insulation quote within 48h
Free technical visit, choice of material and method, grants calculated. Describe your project in under a minute.
- Free and no obligation
- Grants & 6% VAT handled for you
- Installed by our certified teams