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What a wall U-value is
A wall U-value is the rate of heat loss through the external wall, per square metre per degree — the single biggest fabric element on most dwellings. It depends on the whole build-up: the masonry or framing, the insulation, any cavities, and repeating thermal bridges like timber studs or wall ties.
Build-ups it handles
U-Monkey builds up any wall — full-fill and partial-fill cavity, solid masonry with internal or external insulation, timber frame, steel frame and SIPs — with the bridged-layer fractions (studs, ties) handled to BR 443, and real product conductivities from the 480+ material database.
How it's calculated
It calculates the wall U-value by the combined method of BS EN ISO 6946, using BR 443 conventions for bridging, air cavities and the ΔU air-gap correction, and shows the full upper- and lower-limit working.
Every result is traffic-lit live against the Part L 2021 limiting and notional values and the Future Homes Standard targets, so you can see whether the wall passes — and by how much — as you design it.
Free — including the PDF
U-Monkey isn't a paywalled tool. Build up your wall, read the U-value and the full working on screen, and export a branded, Building-Control-ready PDF — all completely free, no sign-up. Prefer the whole SAP done for you? We're happy to help.
Condensation risk in walls
Where the insulation goes decides where the risk sits. In a timber or steel frame, the vapour control layer on the warm side and the sheathing and membrane on the cold side decide whether moisture that gets in can get out again. On the inside of a solid wall, insulation turns the masonry cold and changes how it wets and dries, so there BS EN ISO 13788 is a screening only and a full hygrothermal simulation to BS EN 15026 is the right next step. U-Monkey’s check says so rather than calling such a wall a pass.
U-Monkey runs a free BS EN ISO 13788 condensation check on every wall you build, alongside the U-value — where condensation would form, in which months, and whether it dries out again over the summer.
Worked examples: Wall to an integral garage · Aerogel internal wall insulation · SFS wall with rainscreen cladding · Thin-joint aircrete solid wall · Shipping container wall · Park home wall · Calcium silicate internal wall insulation · Cork insulation on a solid wall · Insulating plaster on a solid wall · Brick-slip external wall insulation · Tile-hung wall · Built-up metal wall cladding (twin skin) · Insulated composite panel wall · Rainscreen cladding on a concrete wall · Precast concrete sandwich panel
The condensation verdict appears just under the U-value as you build — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.
Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.
It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. It opens at the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air — so pick your nearest weather station: the result can change from one station to another.
Frequently asked questions
What is a wall U-value?
A wall U-value is the rate of heat loss through the external wall, per square metre per degree — the single biggest fabric element on most dwellings. It depends on the whole build-up: the masonry or framing, the insulation, any cavities, and repeating thermal bridges like timber studs or wall ties.
How do I calculate a wall U-value?
It calculates the wall U-value by the combined method of BS EN ISO 6946, using BR 443 conventions for bridging, air cavities and the ΔU air-gap correction, and shows the full upper- and lower-limit working. U-Monkey builds up any wall — full-fill and partial-fill cavity, solid masonry with internal or external insulation, timber frame, steel frame and SIPs — with the bridged-layer fractions (studs, ties) handled to BR 443, and real product conductivities from the 480+ material database.
Does it check Part L and the Future Homes Standard?
Every result is traffic-lit live against the Part L 2021 limiting and notional values and the Future Homes Standard targets, so you can see whether the wall passes — and by how much — as you design it.
Is the wall U-value calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. No sign-up, no catch.
Common wall build-ups
Worked examples with typical U-values — each one opens in U-Monkey pre-filled, ready to edit:
- U-value of a cavity wall with full-fill insulation — typically 0.21 W/m²K
- U-value of a cavity wall with PIR partial-fill insulation — typically 0.17 W/m²K
- U-value of a timber frame wall — typically 0.30 W/m²K
- U-value of a solid brick wall with internal insulation — typically 0.25 W/m²K
- U-value of a one-and-a-half-brick (327.5mm) solid wall — typically 1.61 W/m²K
- U-value of a solid wall with external insulation — typically 0.25 W/m²K
- What is the U-value of an uninsulated cavity wall? — typically 1.67 W/m²K
- U-value of a timber frame wall with PIR insulation outside the studs — typically 0.17 W/m²K
- U-value of a solid stone wall with internal insulation — typically 0.25 W/m²K
- U-value of a cavity wall with external wall insulation — typically 0.22 W/m²K
- U-value of a cavity wall with a dense concrete block inner leaf — typically 0.23 W/m²K
- U-value of a solid brick wall (9-inch, uninsulated) — typically 2.10 W/m²K
- U-value of a solid stone wall (uninsulated) — typically 2.40 W/m²K
- U-value of a cavity wall after cavity wall insulation (blown wool or bead) — typically 0.60 W/m²K
- Insulated plasterboard on a solid wall: what thickness reaches a U-value of 0.30? — typically 0.28 W/m²K
- U-value of a solid wall battened out with insulation — typically 0.28 W/m²K
- U-value of a dormer cheek wall (loft conversion) — typically 0.17 W/m²K
- U-value of a garage conversion wall (insulating single-skin brick) — typically 0.28 W/m²K
- U-value of a SIP wall (structural insulated panels) — typically 0.20 W/m²K
- U-value of an ICF wall (insulating concrete formwork) — typically 0.16 W/m²K
- U-value of a light steel frame wall (SFS) — typically 0.17 W/m²K
- U-value of a timber frame wall with wood-fibre insulation — typically 0.17 W/m²K
- U-value of an I-joist timber frame wall (Passivhaus-level) — typically 0.11 W/m²K
- U-value of an extension cavity wall (what reaches 0.18) — typically 0.17 W/m²K
- U-value of a wide cavity wall (200mm, 250mm, 300mm) — typically 0.13 W/m²K
- U-value of wood-fibre internal wall insulation on a solid brick wall — typically 0.49 W/m²K
- U-value of external wall insulation with mineral wool — typically 0.27 W/m²K
- U-value of a knee wall in a loft conversion — typically 0.18 W/m²K
- U-value of a party wall (0.0, 0.2 or 0.5) — typically 0.00 W/m²K
- U-value of a basement wall (heated basement) — typically 0.16 W/m²K
- U-value of a cellar wall when converting a cellar (basement conversion) — typically 0.25 W/m²K
- U-value of a hempcrete wall (hemp-lime) — typically 0.23 W/m²K
- U-value of a wall between a house and an integral garage — typically 0.20 W/m²K
- U-value of a wall to an unheated corridor or stairwell in flats — typically 0.24 W/m²K
- U-value of a CLT (cross-laminated timber) wall — typically 0.16 W/m²K
- U-value of a straw bale wall — typically 0.18 W/m²K
- U-value of a cob wall — typically 0.82 W/m²K
- U-value of aerogel internal wall insulation — typically 0.58 W/m²K
- U-value of wood-fibre external wall insulation (EWI) on a solid wall — typically 0.26 W/m²K
- U-value of a log cabin wall — typically 1.97 W/m²K
- U-value of a garden room (garden office) wall — typically 0.34 W/m²K
- U-value of an SFS infill wall with rainscreen cladding — typically 0.16 W/m²K
- U-value of a solid aircrete wall (thin-joint blockwork) — typically 0.20 W/m²K
- U-value of a shipping container wall — typically 0.23 W/m²K
- U-value of a park home wall with external insulation — typically 0.25 W/m²K
- U-value of the wall between a house and a conservatory — typically 0.60 W/m²K
- U-value of calcium silicate internal wall insulation — typically 0.54 W/m²K
- U-value of cork insulation on a solid wall — typically 0.50 W/m²K
- U-value of insulating (thermal) plaster on a solid wall — typically 0.64 W/m²K
- U-value of brick-slip external wall insulation — typically 0.28 W/m²K
- U-value of a tile-hung wall — typically 1.28 W/m²K
- U-value of built-up (twin-skin) metal wall cladding — typically 0.25 W/m²K
- U-value of an insulated composite panel wall — typically 0.19 W/m²K
- U-value of rainscreen cladding on a concrete or blockwork wall — typically 0.23 W/m²K
- U-value of a precast concrete sandwich panel — typically 0.15 W/m²K
Other elements
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