Start with the space you want to heat
Insulated roof slopes enclose a heated attic; an insulated attic floor separates living rooms from cold storage. A flat roof needs insulation and waterproofing designed together.
| Your situation | Option to investigate | Check first |
|---|---|---|
| Living space beneath a pitched roof | Insulate the roof slopes | Roof underlay, timber and available space |
| Unheated storage attic | Insulate the attic floor | Future use, floor construction and access hatch |
| Flat roof or extension | Warm or inverted roof system | Waterproofing, drainage and structural capacity |
Roof slopes or attic floor?
Insulate the slopes for a heated attic and consider the floor for an attic that stays unheated. Plan for future use before adding a finished storage floor.
Is a grant threshold the same as a building standard?
No. Project-specific EPB or PEB requirements, grant conditions and minimum housing-quality rules serve different purposes. R describes an insulating layer, while U describes the complete roof. Identify the region, work and applicable procedure before setting the required performance.
For example, the Flemish minimum housing-quality roof standard and the new-insulation requirement for Mijn VerbouwPremie are different tests. Neither should be presented as a universal Belgian roof design. See the regional grant overview for the funding context.
What needs to connect around a roof window?
The insulation must meet the window detail, the inner air-control layer must be sealed, and the outer weathering must drain water correctly. Each connection has a separate job. Increasing insulation thickness cannot repair a leaking flashing or an open air path.
Follow the appropriate construction guide for a pitched roof or a flat roof.
Materials, thickness and thermal performance
Mineral wool, cellulose, wood fibre and rigid boards such as PIR are used in different roof systems. Choose a suitable construction before selecting a product. A board beneath flat-roof waterproofing has different requirements from a flexible material fitted between rafters.
Ask for both the thickness and the declared thermal resistance, R or Rd. The U-value describes heat transfer through the complete construction. A thickness alone is not a reliable comparison. Air sealing, vapour control and room ventilation must also be considered together.
Coordinate insulation with other renovation work
Have leaks and damaged timber investigated before covering the construction. If the roof covering needs replacement, request a combined roofing and insulation proposal. Mention future external wall insulation so that roof edges, gutters and junctions remain compatible. Windows, penetrations and internal finishes should have named responsibilities in the quote.
Costs and price factors
Internal roof insulation with 14 to 18 cm of mineral wool has an indicative cost of €25 to €40/m² including installation and 6% VAT where the reduced rate applies. Warm flat-roof insulation is approximately €70 to €115/m² installed, excluding VAT. Sarking with new tiles has an indicative budget of €150 to €220/m² excluding VAT. These methods have different scopes; internal finishes, access equipment, edges and repairs may add to the cost.
Stroomversneller is a quote request platform for insulation companies. Describe your home and intended work; the company assesses what is suitable and prepares its proposal.
Request a quote for this projectGrants: check the region and timing
Belgian support depends on the property’s region, applicant, work and application date. UK grant schemes do not apply automatically. Check the procedure before committing to the work, including any required prior audit. Read our dated overview for Flanders, Wallonia and Brussels. A quote request is not a grant application or confirmation of eligibility.
Frequently asked questions
How thick should roof insulation be?
Calculate thickness in metres by multiplying the target thermal resistance by the product’s lambda value. The complete roof build-up and available space must also be suitable.
Can the old insulation stay?
Its condition, moisture content and existing membranes need assessment. Adding insulation must not create an unsuitable combination of layers.
Should I insulate both the roof and attic floor?
Not automatically. First define the heated rooms and future attic use, then compare the options.