U-value of a limecrete floor (insulated with foamed glass)

A breathable lime slab on a bed of foamed glass in an old house: what 100–250mm reaches, and why the floor’s size matters as much.

A limecrete floor on 150mm of foamed glass aggregate (Ty-Mawr’s Glapor, λ 0.078) has a U-value of about 0.33 W/m²K for a typical floor, with 30m of exposed perimeter to 50m² of area; 200mm gives about 0.27, still short of the 0.25 Approved Document L sets for an upgraded floor, which takes about 225mm (0.2496). The uninsulated solid floor it replaces is about 0.98. Foamed glass is a weaker insulant than PIR or EPS, so these floors are deep, but it carries the slab, doesn’t draw up ground water, and lets the floor be laid without a damp-proof membrane — which is why it is used in old buildings with solid walls.

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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 (sand or gravel), 300mm walls. The limecrete slab is entered as dense concrete, which BR 443 leaves out of a ground floor’s resistance because it conducts much like the ground, so its exact λ makes no difference. The pre-filled link opens as an upgraded existing floor, judged against Approved Document L’s 0.25 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Before: an uninsulated solid floor≈ 0.98
100mm foamed glass≈ 0.42
150mm foamed glass≈ 0.33As pre-filled; Ty-Mawr’s minimum
200mm foamed glass≈ 0.27
250mm foamed glass≈ 0.23Meets the 0.25 for an upgraded floor (225mm just does)
150mm MISAPOR foam glass gravel (λ 0.09)≈ 0.36U-Monkey’s library value
150mm at a design λ of 0.105 (MISAPOR, German approval)≈ 0.39
150mm, a larger floor (P/A 0.35)≈ 0.28
150mm on clay (ground λ 1.5)≈ 0.30

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

Three things decide the figure: the depth of foamed glass, its λ, and the floor’s shape. Ty-Mawr quotes 0.078 for Glapor without saying on what basis; MISAPOR’s European Technical Assessment (ETA-13/0549) gives 0.080 for its 10/75 grade and 0.093 for 10/50 with a moisture factor of 1.2 for use under a slab, and its German approval gives the 10/75 a design value of 0.105. If Ty-Mawr’s figure is a dry one, the same moisture factor (λ 0.094) takes 150mm to about 0.37; at MISAPOR’s 0.105 design value it would be about 0.39. Like any ground floor, a small or narrow one loses more through its edges: the same 150mm gives about 0.28 on a larger floor (P/A 0.35), and clay, which conducts less than the sand or gravel U-Monkey assumes, helps a little (0.30). Ty-Mawr’s minimum system is 250mm deep — 150mm of Glapor and 100mm of limecrete — before the finish, so an old floor usually has to be dug out, and that needs care: the government’s solid-floor guide warns that digging down next to shallow foundations could undermine the wall, and Historic England calls the excavation the principal disadvantage of these systems. Digging out and relaying a whole floor may also count as replacing it rather than renovating it, which in England brings in 0.18 (Approved Document L, paragraph 4.8 and Table 4.2, with a lesser standard where adjoining floor levels make it impractical, note 4) — about 350mm of Glapor — so agree with building control which applies.

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 limecrete floor (insulated with foamed glass)?

A limecrete floor on 150mm of foamed glass aggregate (Ty-Mawr’s Glapor, λ 0.078) has a U-value of about 0.33 W/m²K for a typical floor, with 30m of exposed perimeter to 50m² of area; 200mm gives about 0.27, still short of the 0.25 Approved Document L sets for an upgraded floor, which takes about 225mm (0.2496). The uninsulated solid floor it replaces is about 0.98. Foamed glass is a weaker insulant than PIR or EPS, so these floors are deep, but it carries the slab, doesn’t draw up ground water, and lets the floor be laid without a damp-proof membrane — which is why it is used in old buildings with solid walls.

Does a limecrete floor need a damp-proof membrane?

Not in the systems built for it. Ty-Mawr’s Sublime floor leaves the membrane out and relies on the foamed glass beneath the slab, which doesn’t draw water up, so the floor can let ground moisture evaporate. That is the point in an old building: Historic England’s guidance on insulating solid floors notes that a membrane laid across a floor can displace ground moisture to the base of the walls, where it can cause rising damp instead. U-Monkey’s condensation check doesn’t cover ground floors — moisture from the ground is a matter of the floor’s damp design, not interstitial condensation — so take the system maker’s and a conservation specialist’s advice on the details.

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.

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