U-value of a wall to an unheated corridor or stairwell in flats

The wall between a flat and an unheated access corridor or stairwell, and the Ru SAP adds for the space beyond.

A 215mm dense blockwork wall between a flat and an unheated access corridor, lined on the flat’s side with 82.5mm insulated plasterboard (70mm of PIR), has a U-value of about 0.24 W/m²K with the corridor counted the way SAP 10.2 counts it, and about 0.28 taking the corridor as outside air. SAP adds the corridor as an extra resistance, Ru, outside the wall — 0.6 m²K/W for a corridor whose facing wall is exposed, with corridors above and below (Table 3.3) — using U = 1/(1/Uo + Ru). Next to an unheated stairwell the allowance is much bigger, Ru 2.1 to 2.5, which takes the same wall to 0.16–0.17. If the corridor is heated — with controlled fixed heaters, in SAP’s conventions (5.03) — the wall counts as a party wall instead, and SAP gives a solid one a U-value of 0.

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The build-up (outside → inside)

Ru from SAP 10.2 (and 10.3) Table 3.3, applied as SAP’s formula (3), U = 1/(1/Uo + Ru), where Uo is the wall calculated as if it faced outside air. The pre-filled link opens with “On the other side” set to this corridor. The insulated plasterboard is modelled as 70mm of PIR bonded to 12.5mm plasterboard, on dabs bridging 20% of the gap (BR 443 §4.6.1).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Facing outside air≈ 0.28Uo: 0.276, which misses the 0.26 limit
Access corridor, facing wall exposed, corridors above and below (Ru 0.6)≈ 0.24As pre-filled: 0.237
Access corridor, facing wall exposed, corridor above or below (Ru 0.5)≈ 0.240.243
Access corridor, facing wall not exposed, corridors above and below (Ru 0.9)≈ 0.22
Access corridor, facing wall not exposed, corridor above or below (Ru 0.7)≈ 0.23
Unheated stairwell, facing wall exposed (Ru 2.1)≈ 0.17Inside the 0.18 notional value: 0.175
Unheated stairwell, facing wall not exposed (Ru 2.5)≈ 0.16
60mm PIR (72.5mm board), corridor Ru 0.6≈ 0.26Misses 0.26 even with the corridor: 0.264
50mm PIR (62.5mm board), corridor Ru 0.6≈ 0.30
Unlined: 215mm dense block plastered both sides, corridor Ru 0.6≈ 0.992.43 facing outside air

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. Rows that give an Ru include the unheated space beyond the element as an extra resistance, U = 1/(1/Uo + Ru) (SAP 10.2 §3.3 formula (3); BR 443 §5), which the calculator applies under “On the other side”.

What changes the number

Ru depends on which surfaces of the corridor lose heat. SAP 10.2 Table 3.3 asks whether the corridor’s facing wall — the one opposite the flat — is an external wall, and whether there are corridors both above and below or only above or below (SAP’s Figure 3.1 shows the layouts); a stairwell gets far more, Ru 2.1–2.5. SAP 10 recalculated these values at one air change an hour, where SAP 2012 had used three, because buildings had become more airtight (BRE consultation paper CONSP:09), so its figures are much higher than SAP 2012’s 0.28–0.43 for corridors and 0.82–0.90 for stairwells: an older assessment of the same wall shows a higher U-value. The lining does the work either way — the 215mm of block is worth about 0.19 m²K/W, the 70mm of PIR about 3.2. The SAP conventions also give a window onto an unheated corridor a reduced U-value (5.17) and multiply the Ψ-values of junctions at the corridor or stairwell by 0.71–0.85 (5.12). U-Monkey’s condensation check takes the corridor as outside air; it finds no condensation in any month, but rates internal insulation on solid masonry as a screening result only.

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 wall to an unheated corridor or stairwell in flats?

A 215mm dense blockwork wall between a flat and an unheated access corridor, lined on the flat’s side with 82.5mm insulated plasterboard (70mm of PIR), has a U-value of about 0.24 W/m²K with the corridor counted the way SAP 10.2 counts it, and about 0.28 taking the corridor as outside air. SAP adds the corridor as an extra resistance, Ru, outside the wall — 0.6 m²K/W for a corridor whose facing wall is exposed, with corridors above and below (Table 3.3) — using U = 1/(1/Uo + Ru). Next to an unheated stairwell the allowance is much bigger, Ru 2.1 to 2.5, which takes the same wall to 0.16–0.17. If the corridor is heated — with controlled fixed heaters, in SAP’s conventions (5.03) — the wall counts as a party wall instead, and SAP gives a solid one a U-value of 0.

What U-value does a wall to an unheated corridor need?

It is a thermal element: it separates the flat from an unconditioned part of the building (Building Regulations 2010, regulation 2(3)). In England, in a new block that means the 0.26 W/m²K limiting value for a wall (Approved Document L 2021, Table 4.1), and the notional dwelling that sets the energy targets gives “external walls including semi-exposed walls” 0.18 (Table 1.1). Where the common areas are unheated, Approved Document L also asks for their own fabric elements to meet the Section 4 standards (paragraph 0.4). SAP counts the corridor’s Ru in the energy calculation, and BR 443 §5 says the effect of an unheated space can be incorporated into the U-value of the element; whether your building control body checks the limiting value with it is worth agreeing first. The pre-filled wall meets 0.26 only with the corridor counted (0.237, against 0.276), and 60mm of PIR misses it even then (0.264).

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

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Floors

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