U-value of a solid floor with edge insulation

Insulating round the edge of an uninsulated slab — vertical or horizontal, 0.5m or 1m — and how far it gets you.

An uninsulated concrete ground floor of a typical size, with 30m of exposed perimeter to 50m² of area, has a U-value of about 0.94 W/m²K. Insulating just its edge — 100mm of XPS (λ 0.034) taken 0.5m down the outside of the foundation — takes it to about 0.68, and 1m down to about 0.56: a cut of about 27% to 40% without lifting the floor. It can’t reach the 0.25 Approved Document L sets for an upgraded floor, because the middle of the floor still loses heat into the ground. Laid flat instead, a horizontal strip needs twice the width to match vertical insulation of a given depth.

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The build-up (outside → inside)

BS EN ISO 13370 at U-Monkey’s default floor (30m of exposed perimeter, 50m², P/A 0.6), ground λ 2.0, 300mm walls, with the edge insulation applied as Annex D’s correction (BR 443 §9). The concrete slab is excluded from the floor’s resistance as BR 443 directs; the screed counts, which is why the bare floor here is 0.94 where the limecrete page’s, with no screed, is 0.98. The pre-filled link opens as an upgraded existing floor with the edge insulation set in the ground-floor panel (Edge insulation: vertical, down the foundation).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
No edge insulation≈ 0.94
Vertical, 0.5m deep≈ 0.68As pre-filled
Vertical, 1.0m deep≈ 0.56
Horizontal, 1.0m wide≈ 0.68The same as vertical 0.5m
Horizontal, 0.5m wide≈ 0.78
Vertical 0.5m, 50mm thick≈ 0.72
Vertical 1.0m, 150mm thick≈ 0.53
A smaller, more exposed floor (P/A 0.8), no edge insulation≈ 1.10
The same floor, vertical 1.0m deep≈ 0.60A cut of nearly half
An insulated slab (75mm PIR over it) + vertical 0.5m≈ 0.200.22 without the edge insulation

Figures computed to BS EN ISO 13370 with BR 443 conventions, exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up and the footprint and depth shown: your products, thicknesses, perimeter, area and ground will move the number, and that's what the calculator is for.

What changes the number

Much of a solid floor’s heat leaves near its edges, so insulating the perimeter does more than its area suggests. BS EN ISO 13370 Annex D works the effect out from the edge insulation’s depth (or width) and its thermal resistance, and BR 443 §9 says to calculate the floor without it first and apply it as a correction, counting only the more effective of the two where there is both vertical and horizontal insulation. Depth matters more than thickness: going from 0.5m to 1m deep takes 0.68 to 0.56, while 150mm instead of 100mm at 1m only reaches 0.53. Small, exposed floors gain most, because more of their heat goes out at the edge: on a floor with a perimeter-to-area ratio of 0.8, 1m of vertical insulation cuts the U-value by nearly half. On a floor already insulated over the slab it adds little (0.22 to 0.20). Digging alongside old walls needs care — the government’s solid-floor guide warns that excavating near shallow foundations could undermine the wall — and it notes that external wall insulation can be brought down below floor level, which treats the same junction.

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of a solid floor with edge insulation?

An uninsulated concrete ground floor of a typical size, with 30m of exposed perimeter to 50m² of area, has a U-value of about 0.94 W/m²K. Insulating just its edge — 100mm of XPS (λ 0.034) taken 0.5m down the outside of the foundation — takes it to about 0.68, and 1m down to about 0.56: a cut of about 27% to 40% without lifting the floor. It can’t reach the 0.25 Approved Document L sets for an upgraded floor, because the middle of the floor still loses heat into the ground. Laid flat instead, a horizontal strip needs twice the width to match vertical insulation of a given depth.

Is edge insulation enough to meet the Building Regulations for a floor?

Not if the floor itself is being renovated. In England, Approved Document L asks for 0.25 W/m²K when an existing floor is renovated — a new layer added to it or one replaced (Building Regulations 2010, regulation 2) — and edge insulation on an uninsulated slab takes a typical floor only to about 0.53 even 1m deep with 150mm of board, because the middle of the floor still loses heat into the ground. Insulation dug in round the outside of the walls leaves the floor itself unchanged, so check with building control whether it counts as renovating it at all. It is worth doing where the floor can’t be lifted — a cut of a quarter to two-fifths on a typical house, more on a small floor — and where the 0.25 does apply, the guidance allows a lesser standard if meeting it would cause significant problems with adjoining floor levels (Table 4.3, note 8), subject to the 15-year payback test (paragraph 4.13). The government’s solid-floor guide also recommends considering an insulated perimeter upstand, to warm the edge of the floor where its surface is coldest.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Floors

Walls

Roofs

Different build-up? See the full floor U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

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