U-value of a solid stone wall (uninsulated)

A 500mm solid sandstone or granite wall with 20mm of lime plaster calculates at about 2.4 W/m²K with the stone at λ 2.3; limestone (λ 1.4) gives about 1.8, and a 600mm sandstone wall about 2.2. Measured walls usually do better than the calculation: the 21 stone walls in the SPAB’s in-situ study averaged 1.42 W/m²K, and of 57 traditional walls Historic Scotland measured, 44% came out below the range calculated for them and 42% within it. RdSAP 10 assumes about 1.7 for a 500mm sandstone or limestone wall and 1.9 for granite. Every one of those figures is far above the 0.70 at which Approved Document L counts a wall as one to upgrade.

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The build-up (outside → inside)

500mm solid stone (sandstone or granite, rubble or ashlar)
20mm lime plaster

Stone at BR 443’s conductivities: sandstone and granite λ 2.3, limestone λ 1.4. The pre-filled link opens as an existing element, judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
400mm sandstone/granite + lime plaster	≈ 2.68
500mm sandstone/granite + lime plaster	≈ 2.40	As pre-filled
600mm sandstone/granite + lime plaster	≈ 2.18
700mm sandstone/granite + lime plaster	≈ 1.99
500mm limestone (λ 1.4) + lime plaster	≈ 1.80	Softer stone conducts less
RdSAP 10, 500mm: sandstone or limestone / granite	≈ 1.7 / 1.9	Assumed values, from the stone-wall equations in RdSAP 10 (Table 12 of the June 2026 specification)
Measured: 21 stone walls (SPAB, 2012)	1.42 (mean)	The software over-estimated U in 77% of the SPAB’s cases

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

Thickness and the stone are what the calculation sees; it treats the wall as solid stone, though real walls are often two faces of stone with a rubble and lime core, which is one reason they measure better. The calculation is still what Building Control and a SAP calculation use (RdSAP, for EPCs, has its own defaults). For insulating it, the risk is moisture rather than heat: stone walls take in rain and dry to both sides, so internal insulation needs a vapour-open, capillary-active design or a full BS EN 15026 assessment. U-Monkey’s condensation check reports internal insulation on solid masonry as a screening result only for that reason. See the stone wall with internal insulation page for a lined version.

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 (uninsulated)?

Should I use the calculated or the measured U-value for a stone wall?

For a Building Regulations or SAP calculation, the calculated one: BR 443 and BS EN ISO 6946 are the agreed method, and RdSAP has its own defaults for EPCs. But know that it is often pessimistic for traditional walls: the SPAB found the calculation over-estimating the U-value in 77% of the walls it measured, and Historic Scotland found 44% of 57 walls below their calculated range and 42% within it, some far below. That matters when you weigh up insulating a stone wall, because the saving is the difference between the real U-value and the insulated one. An in-situ measurement (to BS ISO 9869) is the way to know your wall.

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.