U-value of a party wall (0.0, 0.2 or 0.5)

Why a cavity party wall loses heat, the U-values SAP assigns by construction, and how to get 0.0.

A party wall’s U-value is assigned by its construction, not calculated from its layers: 0.5 W/m²K for an unfilled cavity with no effective edge sealing, 0.2 for an unfilled cavity effectively sealed at all its exposed edges, and 0.0 for a solid wall or a cavity that is fully filled and effectively sealed (SAP 10.2 Table 3.10; Approved Document L Table 2.1). The heat is not conducted to the neighbour: it is carried by air moving up through the cavity and out at the top — the party wall bypass, measured at Stamford Brook as an effective 0.50–0.63. Part L 2021’s notional dwelling assumes 0.0; the limiting value is 0.20.

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The build-up (outside → inside)

U-Monkey assigns a party wall’s U-value from the construction you pick, as SAP 10.2 (Table 3.10) and Approved Document L (2021 edition, paragraph 2.14 and Table 2.1) do: the layers are shown, and used for thermal mass, but do not change it. The pre-filled link opens with the cavity fully filled and sealed.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Solid wall (masonry, or a SIP)0.0SAP 10.2 Table 3.10
Cavity fully filled and effectively sealed at all exposed edges0.0As pre-filled
Unfilled cavity, effectively sealed at all exposed edges0.2
Unfilled cavity, no effective edge sealing0.5Worse than the 0.20 limiting value
Measured, unfilled cavities (Stamford Brook, 2007)0.50–0.63The effective U-value of the bypass
Home Energy Model: unfilled, unsealed, dry-lined0.6Behind wet plaster: 0.2 (HEM-TP-05)

Party wall U-values are assigned by the construction (SAP 10.2 Table 3.10; Approved Document L 2021, Table 2.1), not calculated from the layers, and the pre-filled calculator applies the one you pick. The measured and Home Energy Model rows are figures from the sources named, for comparison.

What changes the number

Sealing and fill are the whole story. Sealing the cavity at the top, the bottom and the vertical edges — in line with the insulation of the roof, floor and external walls — stops the stack effect, and filling it stops air moving within it; only both together earn 0.0, and Approved Document L asks that the edge sealing be likely to be robust under normal site conditions (Table 2.1, note 1). Part E still applies: the fill and the seals must not spoil the wall’s sound insulation, and several Robust Details masonry separating walls (E-WM-17, E-WM-20 and E-WM-24, for example) now include a full-fill option with a named party wall roll. Under the Home Energy Model, one of the two routes in the 2026 edition of Approved Document L, the lining matters as well: an unfilled, unsealed cavity behind plasterboard on dabs counts as 0.6, behind wet plaster 0.2.

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 party wall (0.0, 0.2 or 0.5)?

A party wall’s U-value is assigned by its construction, not calculated from its layers: 0.5 W/m²K for an unfilled cavity with no effective edge sealing, 0.2 for an unfilled cavity effectively sealed at all its exposed edges, and 0.0 for a solid wall or a cavity that is fully filled and effectively sealed (SAP 10.2 Table 3.10; Approved Document L Table 2.1). The heat is not conducted to the neighbour: it is carried by air moving up through the cavity and out at the top — the party wall bypass, measured at Stamford Brook as an effective 0.50–0.63. Part L 2021’s notional dwelling assumes 0.0; the limiting value is 0.20.

Do I have to fill a party wall cavity?

To claim 0.0, yes: fully filled and effectively sealed at every exposed edge. Sealing alone gets 0.2, and an open, unfilled cavity 0.5, which is worse than Approved Document L’s 0.20 limiting value for a party wall (Table 4.1). The fill has to be compatible with the wall’s sound insulation under Part E, so use a Robust Details specification with a full-fill option, or a design that is sound-tested.

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 — a party wall takes the U-value SAP 10.2 (Table 3.10) and Approved Document L (Table 2.1) assign to its construction; everything else uses the combined method of BS EN ISO 6946:2017 with BR 443 conventions, BS EN ISO 13370 for ground 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.

Your build-up won't be exactly this one.

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