A typical uninsulated cavity wall — brick, clear 50mm cavity, dense block, plaster — has a U-value of about 1.6 W/m²K, and older brick-brick versions are similar. That is roughly six times today’s new-build standard, which is why cavity fill is usually the cheapest big win in a retrofit.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- 102.5mm brick outer leaf
- 50mm clear cavity
- 100mm dense block inner leaf
- Plaster
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| Brick / 50 cavity / dense block | ≈ 1.6 | Typical 1930s–70s wall |
| Same wall, cavity filled (blown wool) | ≈ 0.5–0.6 | The classic retrofit step |
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 clear cavity contributes almost nothing (R≈0.18). Filling it transforms the wall; block density and any later lining decide the rest. Model your actual wall, then try the filled version — the difference is the business case.
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 an uninsulated cavity wall??
A typical uninsulated cavity wall — brick, clear 50mm cavity, dense block, plaster — has a U-value of about 1.6 W/m²K, and older brick-brick versions are similar. That is roughly six times today’s new-build standard, which is why cavity fill is usually the cheapest big win in a retrofit.
How do I know if my cavity is already filled?
A borescope inspection through a drilled joint is definitive; drill dust that comes out fibrous or beaded suggests fill. Homes built after the mid-1990s usually had insulation built in. If in doubt, model both cases in U-Monkey to see what is at stake.
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 timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a solid wall with external insulation
- 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.