Substrates & Preparation
Preparing PIR Insulation Boards for Liquid Screed
PIR insulation gives strong thermal performance per millimetre under liquid screed, but its foil facings and joints need careful taping to control grout loss.
PIR (polyisocyanurate) insulation boards are popular under liquid screed where floor build-up depth is tight, since PIR achieves a lower U-value per millimetre than EPS or mineral wool alternatives. That thermal efficiency comes with a foil-faced board that behaves differently to open-cell foam boards when it comes to jointing and load spreading.
Preparation here is less about the substrate below the insulation and more about how the boards themselves are laid, taped and protected before the screed pour.
PIR's higher unit cost compared with EPS is usually justified where a build has a fixed finished floor level to hit — an existing threshold, a staircase bottom step already built, or a tight ceiling height upstairs — and every extra millimetre of insulation thickness has a knock-on cost elsewhere in the build. Where depth isn't under that kind of pressure, the choice between PIR and other rigid boards becomes more of a cost comparison than a technical necessity.
PIR Insulation Boards — the points that decide it
- PIR boards typically achieve a better U-value at a given thickness than EPS, useful where floor buildup depth is limited
- Foil facings mean joints need proper taping (usually with a compatible foil or reinforced tape) to stop grout loss between boards
- Boards are laid with staggered joints in a brick-bond pattern to avoid a continuous line of weakness across the floor
- PIR's compressive strength varies by product grade, so the specified grade needs to suit the anticipated floor loading and screed thickness
- Boards should sit tight to walls with no significant gaps, and any cut edges around penetrations need taping the same as full joints
- Manufacturer guidance on maximum point loads during installation (foot traffic, wheelbarrows) should be followed to avoid damaging boards before the pour
- Cut edges of PIR exposed at penetrations or perimeters should be sealed with tape in the same way as factory-cut board edges, since exposed foam is as vulnerable to grout loss as an untaped joint
- PIR boards are more rigid than EPS at a comparable thickness, which can make them easier to lay accurately across an uneven substrate without excessive point loading on high spots
- Storage on site should keep boards flat, dry and off bare ground until they're laid, since damp storage conditions can affect the facing before installation
Why joint taping matters so much with PIR
Liquid screed is a flowable material when poured, and any ungapped or untaped joint between insulation boards is a route for the wet mix to find its way down to the slab or deck below, taking cement paste with it and potentially causing an uneven fill and weak spot in the finished screed. PIR's foil facing doesn't itself provide a seal at the joint — it's the taping that does that job, and it's worth checking coverage carefully before the pour, especially around service penetrations and perimeters.
Load and compressive strength
PIR boards are manufactured in different compressive strength grades, and the correct one needs to be matched to the expected floor loading plus the load imposed by the liquid screed itself during the pour and cure. Under-specified boards can compress unevenly under load, leading to reflected dips or cracking in the finished screed over time.
The grade quoted on a datasheet reflects long-term performance under sustained load rather than just the brief point loads during installation, so it's worth checking the specified grade against the finished use of the room — a domestic living room and a garage conversion carrying vehicle weight have very different requirements.
Laying pattern and cutting around penetrations
Boards are laid in a staggered, brick-bond pattern across the floor rather than with joints running continuously in one direction, which avoids creating a single unbroken line of weakness where grout loss could travel unchecked across a large area. Around service penetrations, pipes and manifold boxes, boards are cut tightly and any resulting gaps packed and taped rather than left as an open void.
Perimeter boards often need trimming to an irregular wall line, and these cut edges — along with any factory edges — should be fully taped, since an untaped cut edge behaves no differently to an open joint once the screed starts to flow.
Site storage and handling before the pour
PIR boards should be stored flat, off the ground and protected from rain until they're laid, since a damp or degraded facing is harder to tape reliably. Once laid, boards should be protected from excessive point loading — stacking other materials directly on a small area, for instance — until the screed itself provides that load-spreading function.
A final walk-around immediately before the pour, checking every taped joint and any areas where earlier trades may have disturbed the insulation, is worth the short time it takes given how much harder any defect is to fix once covered.
Specifying insulation for a liquid screed floor?
We'll advise on board type, taping and compressive grade to suit your build-up before the screed goes down.
Or call our landline on 01392 237700
In short
PIR insulation gives strong thermal performance per millimetre under liquid screed, but its foil facings and joints need careful taping to control grout loss.
PIR Insulation Boards — your questions
Specifying insulation for a liquid screed floor?
We'll advise on board type, taping and compressive grade to suit your build-up before the screed goes down.