Installing insulation under a concrete slab is now standard in most new builds and extensions. It’s key to meeting building regulations, but also stops heat loss, reduces damp risk, and ensures your floor performs as it should. But slap it in wrong — with gaps, movement, or bridging — and it loses effectiveness fast. This guide shows you how to lay rigid floor insulation properly, the way it’s done on site: clean, tight, and built to last.
Before insulation goes down, make sure the hardcore is compacted and level. If a DPM is being used (which it almost always is), it should already be laid clean and tight on the sub-base. You don’t want sharp edges, loose rubble, or dips underneath. If needed, top with a sand blinding layer to create a smooth bedding. Any unevenness under the insulation will cause bridging and movement when the concrete is poured — especially with thinner boards.
PIR boards are the most common — typically 50mm, 75mm or 100mm thick depending on your U-value target. EPS and XPS polystyrene can be used for thicker slabs or where cost is a factor. Check your drawings or energy calcs before installing — the insulation must meet Part L requirements (usually around 0.22 W/m²·K or better). Boards must be compressive strength rated for floors (e.g. 100–150 kPa minimum). Don’t mix insulation types unless spec’d — it causes uneven settlement.
Start in one corner and lay boards in a brick-bond (staggered) pattern to prevent straight-line joints. Push them tight together — no gaps. If needed, trim edges with a handsaw or insulation knife. Gaps larger than 5mm should be filled with expanding foam or offcuts to prevent cold bridging. Butt boards up to walls and edges cleanly, and don’t leave floating pieces in corners — they’ll tip under load. The tighter the layout, the better the thermal and structural performance.
In most builds, rigid vertical insulation is placed around the edge of the slab to prevent thermal bridging at the wall-floor junction. Use thinner PIR or EPS (25–50mm), secured to the internal face of the foundation or formwork. If pouring against a block wall, tape insulation to the wall using foam adhesive or clips. This detail is often skipped on-site — but it’s required for Part L and makes a noticeable difference in heat loss and condensation resistance.
Not every spec requires taped joints — but if you’re installing a vapour control layer (VCL) or laying insulation under underfloor heating, taping joints helps reduce draughts and keeps moisture from tracking between boards. Use aluminium foil tape or insulation joint tape as specified. Press it down smooth to avoid raised edges or bubbles under the slab. If in doubt, check your build-up drawings or insulation supplier guidance.
Avoid walking directly on the insulation where possible. Use scaffold boards or plywood sheets to distribute weight if working across the surface. Tools, rebar, or boots can gouge or compress PIR boards, reducing performance. Don’t lay reinforcement mesh directly on the insulation — use chairs or spacers to hold it off the surface. If underfloor heating pipes are being fitted, mark zones and fix pipes in place before reinforcement goes in. Always double-check levels before pouring concrete.
Do a final walk of the slab zone: are all boards flat? Are there gaps or floating pieces? Have you installed perimeter insulation? This is your last chance to adjust before the pour locks everything in place. If working under Building Control, take photos or request an inspection before pouring. Once concrete hits the site, it’s too late to fix what’s underneath. From here, you’re ready for rebar, mesh or underfloor heating — and then the slab itself.
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