Foam concrete as a slab-on-grade sub-base
Foam concrete is increasingly used as the insulating layer under a ground-bearing slab rather than as the slab itself. It is not structural and it is not a substitute for rigid board insulation everywhere, but for irregular ground it solves a detailing problem boards cannot.
The build-up, ground to finish
A slab-on-grade incorporating a foam concrete insulating layer is built in the same order as a conventional ground-bearing slab, with one extra layer replacing part of the thermal build-up. Thicknesses below are realistic starting points, not a specification — every project needs its own thermal calculation and geotechnical input.
| Layer | Typical thickness | Function |
|---|---|---|
| Sub-grade | — | Existing or prepared ground, proof-rolled and blinded. Not a manufactured layer, but everything above depends on it being sound. |
| Sub-base | 150–300 mm | Compacted granular material (e.g. Type 1). Provides bearing and spreads load; unlike the layer above it, it is designed to be compacted. |
| Foam concrete insulating layer | 100–150 mm | Thermal break, poured to a level surface. Not load-bearing in the structural sense – see below. |
| Separation layer / DPM | ~0.25 mm (250 micron) | Polyethylene sheet. Breaks capillary and vapour paths between the foam layer and the slab above. |
| Structural slab | 100–200 mm | Reinforced, ground-bearing. Carries all imposed and dead load; sized by structural design, independent of the layers below. |
| Finish | 0–75 mm | Screed or direct floor finish, per the specified floor build-up. |
The foam concrete layer sits between two layers that are, deliberately, nothing like it: a compacted granular sub-base below, which relies on being densified to work, and a reinforced structural slab above, which relies on continuous tensile capacity that the insulating layer does not have. Getting the distinction right is most of what this page is about. This use sits alongside the other placement-driven applications of foam concrete – trench backfill, void filling and screeds – where the same self-levelling placement is the reason the material gets specified.
Quantities for the layer are worked out the same way as any other poured layer: area times thickness, plus a falls and placing allowance. The method, including the allowance figures, is set out on the screed volume page and applies here without change, since a sub-base insulating layer is placed exactly like an insulating screed.
Which density for the insulating layer
Density is chosen for the job the layer actually has to do, and there are two different jobs:
- 200–400 kg/m3, for thermal performance. This is the insulating and light non-structural classes: λ of roughly 0.06–0.18 W/(m·K), which is where the layer earns its place in the thermal design. Use this range where the layer only has to bear the wet weight of the slab poured on it and is protected from construction traffic during the pour.
- 600–800 kg/m3, where the layer must take construction traffic. Barrows, screed rigs and foot traffic before the slab is poured impose concentrated, wheel-scale bearing pressures that a 200–300 kg/m3 layer (0.3–1.0 MPa) has little margin against. The 400–800 kg/m3 band (0.8–3 MPa) gives real headroom for the same reasons it is specified for floor screeds generally. See density classes and properties for the full table.
Going denser than the job requires is a straightforward thermal-performance loss: λ roughly doubles between the 200–300 kg/m3 class and the 400–600 kg/m3 class. Specify the lower density unless construction traffic on the bare layer is unavoidable.
Why the foam layer is not structural
Do not add its thickness to the slab
The foam concrete layer contributes essentially nothing to the flexural or shear capacity of the structural slab above it. It has no continuous tensile capacity, no reinforcement, and a compressive strength one to two orders of magnitude below the structural slab's. A structural engineer sizing the slab for a given floor loading must treat the insulating layer as having zero structural depth – not as extra thickness, not as partial composite action, and not as a substitute for any part of the specified slab depth. Detail and design it purely as a bearing and thermal layer, separated from the slab by the DPM.
This is not a conservative simplification; it reflects what the layer physically is. Even at the top of its density range it is roughly a tenth the strength of the structural concrete above it, and it is not reinforced. Treating it as load-sharing would be a design error, not an approximation.
The DPM and protecting the layer from water
The separation layer does two jobs at once: it stops the wet structural slab from losing mixing water and cement fines into the porous layer below (grout loss, which weakens the slab's underside and can telegraph through as surface blemishes), and it breaks the capillary and vapour path between the ground and the insulating layer.
The second job matters more for this layer than for an ordinary sub-base, because of how sharply moisture degrades foam concrete's thermal performance. Water conducts heat roughly 25 times better than air, and published λ values for foam concrete are oven-dry figures; a layer that has picked up moisture from the ground, or from standing water before the DPM went down, can be conducting significantly worse than its datasheet value for a long time afterwards – the same effect covered for foam concrete generally on the density classes page. A layer specified for its thermal performance that is then allowed to sit wet has had that performance compromised before the slab is even poured.
Lap and seal the membrane properly at joints and upstands; a punctured or poorly lapped DPM defeats the purpose of specifying a low-density, high-porosity layer in the first place.
Strength to carry the wet slab
Work the self-weight load through as an actual number. A 150 mm reinforced structural slab at normal-weight concrete's wet density of roughly 2400 kg/m3 imposes:
load = 2400 kg/m3 × 0.15 m × 9.81 m/s2 = 3532 N/m2 ≈ 0.0035 MPa
Compare that with even the weakest insulating class, 200–300 kg/m3 at 0.3 MPa minimum: a margin of roughly 85 times. The wet slab's own weight, spread evenly over the full area of the layer, is not a meaningful load for any density class covered on this page.
What does matter is concentrated construction traffic before the slab is placed. A loaded wheelbarrow with an operator, say 150 kg total, bearing through a wheel contact patch on the order of 100 × 100 mm (0.01 m2), gives:
bearing pressure = 150 kg × 9.81 m/s2 ÷ 0.01 m2 = 147,150 Pa ≈ 0.15 MPa
That is comfortably inside the 0.3–1.0 MPa range of the lowest insulating class on paper, but the margin shrinks fast once realistic effects are added: dynamic and impact loading from a wheel crossing an uneven surface can add a factor of 1.5 or more, an unbedded stone under the wheel concentrates the same load over a smaller area, and a serviceable design wants headroom above the strength floor of the class, not a value sitting right on it. That is the practical reasoning behind specifying the 600–800 kg/m3 band wherever the bare layer will see barrows, rigs or foot traffic before it is protected: it is not the slab's self-weight that drives the higher density, it is what happens on top of the layer during construction.
Foam concrete versus rigid insulation boards
Rigid insulation boards conduct at roughly 0.022–0.038 W/(m·K), against foam concrete's best case of about 0.06 W/(m·K) in the 200–300 kg/m3 class. Per millimetre of thickness, boards win outright: to achieve the same thermal resistance R, required thickness scales with λ, so at λ = 0.03 for a board against λ = 0.075 for a mid-range foam concrete layer, the board needs only about 0.03 ÷ 0.075 ≈ 40 % of the foam concrete thickness for the same R-value – for example 45 mm of board against roughly 113 mm of foam concrete to reach R = 1.5 m2K/W. Where the build-up depth is tightly constrained, boards generally win.
Foam concrete's advantage is not thermal efficiency per millimetre; it is what happens where boards do not fit cleanly:
- Irregular sub-grades. A pumped or poured layer follows the actual surface; boards need a levelled bed or they bridge and crack.
- Non-combustibility. Foam concrete is mineral and Euroclass A1 without organic additions, where board insulation is frequently an organic foam with its own fire classification and end-of-life questions.
- No board joints. Every board joint is a thermal bridge and a potential air or moisture path; a poured layer has none.
- Awkward geometry. Pumped into a shape with changes of level, penetrations or service runs, foam concrete fills continuously where boards need cutting and infill pieces around every obstruction.
Where the build-up is thin and unobstructed, specify boards. Where the ground is irregular, the depth budget is generous, or non-combustibility and joint-free continuity matter more than the last few millimetres of build-up, specify foam concrete.
Curing before the slab goes on
The insulating layer needs to be dry enough, not just strong enough, before it is sealed under the DPM and loaded with the structural slab. Sealing a layer that still holds a large fraction of its mixing water traps that moisture against a membrane that is meant to be keeping water out, not in, and undermines the thermal performance the layer was specified for.
As a working minimum, allow the layer to cure sealed on its own surface for at least seven days before the DPM and slab go on – the same minimum given generally for foam concrete on the density classes page, and for the same reason: it controls both strength gain and drying shrinkage before the layer is loaded and covered. Foot traffic for setting out and light works can generally proceed sooner, once the surface has set, but full curing before covering is not the step to compress on a tight programme.
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