- A vapour-tight layer on the outside that traps moisture in the insulation
- Gaps in the vapour control layer on the inside
- Thermal bridges along purlins and rafters when insulating from inside
- Too little ventilation under the tiles
Building detail · Roof
Insulating a pitched roof: from inside or outside
A pitched roof can be insulated from the outside, when the tiles are coming off anyway, or from the inside, when the roof stays in place. Both work, as long as the layers are in the right order.
- Insulation outside: one continuous layer without thermal bridges
- Insulation inside: the roof stays in place
- Vapour-open outside, vapour-retarding inside
Principle sketch
Build-up of the detail
Two build-ups of an insulated pitched roof, drawn flat. The numbers in the sketch refer to these parts:
- Roof tiles
- Tile battens and counter battens
- Vapour-open, waterproof membrane
- Insulation
- Vapour control layer
- Roof boarding
- Purlins or rafters
- Interior finish
Common mistakes
Where it often goes wrong
Leaks, condensation and cracks usually start at the junctions, not in the middle of a wall or roof.
In the drawing
What we look out for
We work out the detail for your home, based on a site survey and the chosen materials.
- The order: vapour-retarding inside, vapour-open outside
- Airtight connections at walls, ridge and penetrations
- The insulation thickness and the space left inside
- Ventilation under the tiles via the counter battens
- Connections to dormers, roof windows and the facade
Insulating the roof is often the first step in making a home more sustainable. Sustainable renovation
Detail FAQ
Frequently asked questions about this detail
Is insulating from the outside better?
In terms of building physics it is often simpler, because the insulation forms one continuous layer over the structure. It makes most sense when the tiles are being replaced anyway.
Why must the inside be vapour-retarding?
Warm indoor air contains moisture. A vapour control layer on the inside prevents that moisture from condensing in the insulation.
Can I insulate the roof of a listed building?
Often yes, with an adapted solution and usually with a permit. See also our page on renovating a listed building.
Can I use this principle sketch for my own home?
The principle sketch shows the build-up, but it is not a working drawing. Dimensions, materials and fixings depend on your home. We work out the detail for your situation.
Detail library