U-value of a cavity wall after cavity wall insulation (blown wool or bead)

Filling a typical 50mm cavity between brick and dense concrete block takes the wall from about 1.67 W/m²K to about 0.60 with blown mineral wool or white EPS bead at λ 0.040, or about 0.52 with graphite bead (λ 0.033). It is the biggest single improvement most cavity-walled homes can make, but it does not always reach the 0.55 Approved Document L gives for a cavity-insulated wall: λ 0.040 fill against dense block misses it, while graphite bead, a λ 0.034 wool or an aircrete inner leaf clear it. A 65mm cavity with the same wool reaches about 0.49.

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The build-up (outside → inside)

102.5mm brick outer leaf
50mm cavity, filled with blown mineral wool (λ 0.040)
100mm dense concrete block inner leaf
13mm dense plaster

BR 443 (2019) §7 lists no air-gap correction for a cavity filled by injection after construction, so the air-gap correction is level 0 here. The pre-filled link opens as a retained wall being upgraded, judged against Approved Document L’s 0.55 for cavity insulation (2021 edition, Table 4.3).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Before: 50mm clear cavity	≈ 1.67	See the uninsulated cavity wall page
50mm blown mineral wool, λ 0.040	≈ 0.60	As pre-filled: misses 0.55
50mm white EPS bead, λ 0.040	≈ 0.60	The same as λ 0.040 wool
50mm blown mineral wool, λ 0.034	≈ 0.53	Clears 0.55
50mm graphite EPS bead, λ 0.033	≈ 0.52	Clears 0.55
65mm cavity, blown wool λ 0.040	≈ 0.49	A wider cavity does more than a better fill
50mm blown wool, aircrete inner leaf	≈ 0.47	Mortar joints counted (BR 443)
50mm blown wool, brick inner leaf	≈ 0.57	Older brick-and-brick cavity walls

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

Three things set the result: the cavity width, the fill’s λ (0.034 to 0.040 for blown wool, about 0.033 for graphite bead, nearer 0.040 for white bead) and the inner leaf, where aircrete is worth more than any upgrade of the fill. BR 443 applies no air-gap correction to a cavity filled by injection, so none is added. The figures leave out the wall ties: add them in U-Monkey (it offers to), because BR 443 counts metal ties through insulation — about +0.005 W/m²K for 4mm stainless wire ties at 2.5 per m², and about three times that for galvanised steel ties of the same size, which leaves a λ 0.034 fill inside 0.55 with little to spare (0.545). Approved Document L’s 0.55 applies only to walls suitable for cavity insulation (Table 4.3, note 5), and whether yours is depends on its exposure to wind-driven rain, the cavity’s width and condition, and the outer leaf — the installer’s survey decides that.

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 cavity wall after cavity wall insulation (blown wool or bead)?

Is cavity wall insulation covered by the Building Regulations?

Yes: inserting insulation into a cavity wall is building work in its own right (Building Regulations 2010, regulation 3(1)(e)), so it needs Building Control approval or an installer registered with a competent person scheme for cavity wall insulation, who certifies it. The thermal target in Approved Document L is the “improved” U-value for a cavity-insulated wall, 0.55 W/m²K (Table 4.3); where a fill cannot reach it, paragraph 4.13 accepts the best that is technically and functionally feasible with a simple payback of 15 years or less. Part C matters as much: the fill must suit the wall’s exposure to driving rain.

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.