U-value of a solid stone wall with internal insulation

A 500mm stone wall lined inside — the traditional-building retrofit.

A bare 500mm sandstone or granite wall with lime plaster has a U-value of about 2.4 W/m²K, and the same thickness of limestone about 1.8. Lining it internally with 80mm of PIR and plasterboard brings it to about 0.25, inside the 0.30 that Approved Document L sets for a wall being renovated; 70mm gets there at 0.28, while 60mm falls short at 0.32. A vapour-open lining needs more thickness: about 120mm of wood fibre with a lime finish to reach 0.30. Internal insulation leaves the stone colder and slower to dry, so on a traditional wall the moisture question matters as much as the U-value.

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The build-up (outside → inside)

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Bare 500mm sandstone or granite, lime plaster≈ 2.4The starting point
Bare 500mm limestone, lime plaster≈ 1.8Limestone conducts less than sandstone or granite
+ 120mm wood fibre, lime plaster finish≈ 0.30Vapour-open; just inside the 0.30 target (0.2998)
+ 60mm PIR lining≈ 0.32Short of the 0.30 renovation target
+ 80mm PIR lining≈ 0.25Comfortable pass

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — 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 shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

Stone type and thickness set the bare figure — sandstone and granite (λ 2.3) conduct more than limestone (λ 1.4), and a 600mm wall of the same stone is about 2.2 rather than 2.4 — but once the wall is lined, the lining decides the U-value: the same 80mm of PIR gives 0.24 on limestone and 0.25 on sandstone. The figures take the wall as solid stone at BR 443’s conductivity; a rubble-filled wall with lime mortar may conduct a little less. The moisture question is the harder one. U-Monkey’s free condensation check models vapour diffusion only, so it flags internal insulation on solid masonry for a full hygrothermal assessment to BS EN 15026 rather than reporting a clean pass; on a vapour-open lining with no vapour control it predicts condensation building up behind the insulation, because it cannot see the capillary drying such linings rely on. On a traditional or listed wall, check the pointing, render and rainwater goods first and get that assessment before choosing a lining.

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 stone wall with internal insulation?

A bare 500mm sandstone or granite wall with lime plaster has a U-value of about 2.4 W/m²K, and the same thickness of limestone about 1.8. Lining it internally with 80mm of PIR and plasterboard brings it to about 0.25, inside the 0.30 that Approved Document L sets for a wall being renovated; 70mm gets there at 0.28, while 60mm falls short at 0.32. A vapour-open lining needs more thickness: about 120mm of wood fibre with a lime finish to reach 0.30. Internal insulation leaves the stone colder and slower to dry, so on a traditional wall the moisture question matters as much as the U-value.

What is the U-value of an uninsulated stone wall?

It depends on the stone and the thickness. Calculated as solid stone with lime plaster, a 500mm sandstone or granite wall is about 2.4 W/m²K and a 500mm limestone wall about 1.8; at 600mm, sandstone or granite comes to about 2.2. All are far worse than the 0.70 threshold in Approved Document L Table 4.3, so a wall like this has to be improved when it is renovated. Model your own wall’s stone and thickness in U-Monkey, free.

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

Walls

Roofs

Floors

Different build-up? See the full wall 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.

Open this build-up pre-filled or call 01202 623236