A brick-clad timber frame wall with 140mm mineral wool between studs achieves a U-value of about 0.29 W/m²K once the ~15% timber bridging is properly counted — not the ~0.25 a naive calculation suggests. Adding a 50mm insulated lining board brings it to about 0.18. U-Monkey does the bridged (combined-method) calculation automatically.
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The build-up (outside → inside)
- 102.5mm brick outer leaf
- 50mm drained cavity
- 9mm OSB sheathing
- 140mm mineral wool between studs (~15% timber)
- Polythene VCL (warm side)
- Plasterboard
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 140mm mineral wool between studs only | ≈ 0.29 | Bridging is why it is not ~0.25 |
| 140mm + 50mm insulated plasterboard lining | ≈ 0.18 | Modern timber-frame spec |
Figures computed with the BS EN ISO 6946 combined method (bridging included) using BR 443 conventional lambda values — indicative for the generic build-up shown. Your products, thicknesses and (for floors) footprint will move the number: that's what the calculator is for.
What changes the number
The stud fraction is the story: every timber path (λ0.13) short-circuits the wool (λ0.038). A continuous insulated lining breaks the bridge, which is why it is worth more than its thickness suggests. The VCL goes on the warm side — U-Monkey’s free condensation check will show you why.
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):
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 timber frame wall?
A brick-clad timber frame wall with 140mm mineral wool between studs achieves a U-value of about 0.29 W/m²K once the ~15% timber bridging is properly counted — not the ~0.25 a naive calculation suggests. Adding a 50mm insulated lining board brings it to about 0.18. U-Monkey does the bridged (combined-method) calculation automatically.
Why is my timber frame U-value worse than the insulation thickness suggests?
Because roughly 15% of the wall is timber, not insulation, and heat takes the easy path through the studs. The combined method of BS EN ISO 6946 averages the two paths — U-Monkey applies it automatically, which is why its answer differs from a simple resistance-adder.
Is this calculator free?
Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard) and the condensation-risk screening check 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 and air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- U-value of a cavity wall with full-fill insulation
- U-value of a cavity wall with PIR partial-fill insulation
- U-value of a solid brick wall with internal insulation
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of 270mm loft insulation
- U-value of insulation between rafters (loft conversion)
- U-value of a warm flat roof
- How much insulation does a flat roof need?
- U-value of an insulated concrete ground floor
- U-value of an insulated suspended timber floor
- U-value of a beam and block floor with PIR
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.