What is it?
A rigid foam insulation board fitted within the ground floor build-up, beneath the screed, to insulate the floor rather than the walls.
- 1Fireplace (brickwork)
- 2Blockwork (inner leaf)
- 3Rigid Insulation
- 4Rigid Insulation upstand
- 5Socket first-fix
- 6DPM
- 7Cavity Closer
- 8External Door
What does it do?
It forms the main thermal barrier under the floor, stopping heat escaping down into the ground or through a suspended floor void. Where it sits beneath the screed rather than beneath the slab, it also means the screed above it isn't bonded to the structure below. See Screed for how that changes the screed's own thickness and detailing.
Where is it used?
Within the ground floor build-up, typically between the concrete slab and the screed, and continued up at the perimeter to meet the wall's own insulation line so the two don't leave a cold gap between them.
How to specify
Thickness is set the same way as cavity insulation: by calculation, against the floor's target U-value under Building Regulations. It needs to be coordinated with the overall floor build-up depth, since it directly affects how much screed thickness is left and, in turn, the level the door threshold has to resolve.
Interacting elements
- Concrete slab: the insulation sits above the slab
- Screed: laid as a floating layer over the insulation, since it isn't bonded to the slab beneath