Under-Slab & Slab-Edge Insulation

The assembly

1. Compacted subgrade / engineered fill — or EPS geofoam block fill where the ground is soft or settlement-sensitive — Geofoam blocks are field-cut and placed dry without compaction; block fill weighs roughly 1% of soil, which is the point where bearing capacity or settlement governs. Sized by the project geotechnical/structural engineer against the @1% compressive resistance.
2. Sand blinding / regulating layer — A flat, stone-free bed. EPS board bridges local high spots poorly and will rock or crack under the pour if the base is not level.
3. Damp-proof membrane (DPM) — position per the project engineer — Commonly laid over the insulation so boards are protected from wet concrete, laps taped and turned up at the perimeter. Below-insulation DPM is also used; this is a moisture-design decision, not a Technopol specification.
4. Rigid EPS under-slab board (bulk insulation, cut-to-size flat sheet) — Grade selected for the sustained slab + imposed stress (@1% limit); thickness selected for the thermal target. R = thickness / lambda — no R/U table is published for the bulk board.
5. Slip / separation sheet — Polythene between insulation and concrete to prevent grout loss into board joints and to let the slab shrink independently.
6. Reinforced concrete slab — or a suspended rib-and-block slab (Technoblock void formers) where a ground-bearing slab is not viable — Designed by the structural engineer. EPS blocks in a rib-and-block slab are non-structural lost formwork and carry no slab load.
7. Vertical slab-edge / perimeter EPS board — The edge board is the piece that actually kills the bridge. Return it down the slab edge (and horizontally outboard as a wing where required) to the depth the psi-value calculation demands.
8. Protection to any slab-edge insulation above finished ground level — EPS above ground must be rendered/protected — never left exposed to UV or impact. Below ground it is protected by soil, backfill or paving.

Layers listed outside → inside.

Performance data & provenance

What these labels mean is defined on our data promise; every source report is on the register.

Installation

• Fix the design basis first: the sustained slab + imposed stress on the EPS (design to the @1% compressive resistance, never the @10% value) and the thermal target from the rational design or the heating/cold-store duty — XA sets no floor R minimum.
• Prepare a level, stone-free blinding over the compacted subgrade or the geofoam fill; board on an uneven base will rock and crack under the pour.
• Where geofoam replaces fill, place the field-cut blocks dry in staggered, close-butted courses without compaction, to the geotechnical engineer's layout.
• Lay the under-slab boards close-butted with staggered joints and no gaps; cut neatly around penetrations. Every open joint is both a heat leak and a grout path.
• Install the DPM/slip layer to the engineer's moisture design (commonly over the insulation, laps taped, turned up at the perimeter) so wet concrete cannot enter board joints.
• Fit the vertical slab-edge board around the full perimeter before the pour, to the depth the psi-value calculation requires, held tight to the shutter so no concrete bleeds behind it.
• Use only EPS-compatible, water-based adhesives, primers and membranes. Solvent-based primers, solvent bitumen and hot/torch-applied bitumen dissolve or melt EPS.
• Protect the boards: no site traffic or plant directly on bare EPS, no prolonged UV exposure, and render or backfill any edge board finishing above ground level.
• Pour and cure so the EPS ends fully encapsulated by slab, screed, paving or soil — it must not remain exposed in the finished works.

Project references