Insulation and screed depth
Specifying screed depth over insulation layers
The depth of screed needed above an insulation layer depends heavily on the insulation's compressive strength and how loads are expected to spread through it.
Insulation type has a direct bearing on the screed depth that can safely be specified above it, since a screed that is too thin can flex under load where the insulation beneath is compressible.
This is a common area of confusion because two projects with the same screed product can require different depths simply because different insulation has been selected.
Because two insulation products of the same thickness can carry very different loads, the specification should quote the insulation's compressive rating directly rather than leaving the screed installer to assume a generic figure applies.
Why compressive strength matters as much as thickness
It's tempting to focus purely on insulation thickness for thermal performance, but the compressive strength rating (often expressed in kPa) determines how well the insulation supports the screed under point and distributed loads. A thicker but softer insulation can sometimes need a deeper screed above it than a thinner, denser product.
Board fit and site practice
Even a well-specified depth can underperform if insulation boards are poorly butted, leaving gaps that the screed flows into unevenly, or if boards are laid over an uneven base. Specifying a tolerance for insulation flatness, and inspecting it before the pour, helps protect the intended screed depth.
Evidence for an insulation-backed pour
An insurance-backed warranty over insulation is typically conditional on the compressive strength rating specified having actually been installed, not just assumed.
Photographs of insulation board fit before the pour, alongside batch numbers, help demonstrate the specified build-up was actually installed.
Drying allowance for insulation-backed floors
Because the floating layer sits over insulation, moisture has only one way out, so drying times quoted for bonded work should not be assumed to apply here.
Someone on the project team should be named as responsible for running dehumidifiers and maintaining background heat until moisture readings confirm the floor is ready.
Depth over insulation — the points that decide it
- Rigid insulation with a higher compressive strength rating generally allows a thinner screed to perform adequately than a softer, more compressible product.
- The insulation manufacturer's own guidance on minimum screed thickness above their product should be checked, not just the screed manufacturer's generic figure.
- Two layers of insulation, sometimes specified for thermal upgrade works, need their combined behaviour under load considered, not just each layer individually.
- Loose-fitting or poorly jointed insulation boards can create local soft spots that effectively reduce the screed's real-world support, regardless of the specified depth.
- Where insulation is being upgraded on a renovation project, existing floor build-up and depth constraints should be checked before assuming the same screed depth will fit.
Choosing insulation and screed depth together?
Get in touch before the insulation order goes in and we can flag anything that might affect the screed depth.
Or call our landline on 01392 237700
Depth over insulation — your questions
In short
The depth of screed needed above an insulation layer depends heavily on the insulation's compressive strength and how loads are expected to spread through it.
Choosing insulation and screed depth together?
Get in touch before the insulation order goes in and we can flag anything that might affect the screed depth.