U-value of a wall between a house and an integral garage

A cavity wall between a house and its integral garage — a 100mm dense block garage leaf, a 50mm clear cavity, 75mm of PIR partial fill and an aircrete inner leaf with plasterboard on dabs — has a U-value of about 0.20 W/m²K with the garage counted the way SAP counts it, and 0.21 taking the garage as outside air. SAP 10.2 treats an integral garage as an extra thermal resistance, Ru, outside the wall: 0.35 m²K/W for a single garage that shares a side wall, an end wall and the floor with the house and sits outside its insulation (Table 3.1), applied as U = 1/(1/Uo + Ru). On a well-insulated wall that takes off only about 0.015; on a weak one it can decide a pass, as 50mm of PIR misses the 0.26 limiting value facing outside air (0.273) and meets it with the garage counted (0.249). An attached garage, as opposed to an integral one, is disregarded altogether.

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The build-up (outside → inside)

(The integral garage beyond: Ru 0.35, SAP 10.2 Table 3.1)
100mm dense concrete block garage leaf
50mm clear cavity
75mm PIR partial-fill board
100mm aircrete block inner leaf (mortar joints counted)
Plasterboard on dabs

Ru from SAP 10.2 (and 10.3) Table 3.1, applied as SAP’s formula (3), U = 1/(1/Uo + Ru), where Uo is the wall calculated as if it faced outside air; BR 443 §5 says the effect of an unheated space can be incorporated into the U-value of the element. The pre-filled link opens with “On the other side” set to this garage. The 50mm cavity beside the foil-faced board counts as a low-emissivity cavity (BR 443), worth about 0.01 here (0.197 against 0.207). Building control bodies may differ on whether the limiting values are checked with or without the garage, so agree the basis first.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Facing outside air (or an attached garage, which SAP disregards)	≈ 0.21	Uo: 0.211
Single garage, fully integral: side wall, end wall and floor, outside the insulation (Ru 0.35)	≈ 0.20	As pre-filled: 0.197
Single garage, fully integral: one wall and floor, outside the insulation (Ru 0.25)	≈ 0.20	0.2005; a half-integral double garage takes the same Ru
Single garage, partially integral, displaced forward (Ru 0.30)	≈ 0.20	0.199
Single garage, fully integral, inside the insulation (Ru 0.70)	≈ 0.18	0.184: the insulated envelope goes round the garage
50mm PIR partial fill, Ru 0.35	≈ 0.25	Meets the 0.26 limit only with the garage counted (0.273 without)
Full fill: 100mm mineral wool (λ 0.032) between dense block and aircrete, Ru 0.35	≈ 0.23	Fails U-Monkey’s condensation check: see below
Single leaf: 100mm dense block + 82.5mm insulated plasterboard (70mm PIR), Ru 0.35	≈ 0.26	Meets 0.26 only with the garage counted: 0.258 (0.284 without)
Timber stud wall: 89mm mineral wool between the studs, VCL, plasterboard both sides, Ru 0.35	≈ 0.43	0.51 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

Pick the Ru that matches the plan. SAP 10.2 Table 3.1 covers single garages (about 3m × 6m) and Table 3.2 double garages (about 6m × 6m), both ventilated at 3 air changes an hour; the values depend on how much of the garage the house wraps round, and for a single garage roughly double when the house’s insulated envelope goes round the outside of it. The same Ru helps a weak wall more than a good one: 0.35 m²K/W is worth about 8mm of PIR (0.35 × λ 0.022), which takes 0.27 down to 0.25 but 0.21 only to 0.20. A garage much bigger or smaller than SAP’s typical one takes its own Ru from SAP’s formula (4), Ru = Ai / (Σ Ae·Ue + 0.33·n·V), which U-Monkey takes as a custom value under “On the other side”. The junctions at the garage are sheltered too: the SAP 10 conventions (5.12) multiply their Ψ-values by 0.83–0.94. And the wall, floor and door between the garage and the house have to provide the fire separation Part B asks for.

Condensation risk in a wall to a garage

U-Monkey’s free BS EN ISO 13788 check takes the garage as outside air, the cautious case, and the pre-filled wall passes it with no condensation in any month: the PIR’s foil facings stop vapour from the rooms on the warm side of the cavity, before it reaches the cold garage leaf. Fill the same cavity with 100mm of mineral wool instead and the check fails it. Condensation collects on the back of the dense block every winter, peaking at about 1.2 kg/m² in April, and about 830 g/m² of it is still there at the end of the year, because dense block resists vapour about four times as much as aircrete or brick (μ 40 against 10, from BS 5250).

An aircrete garage leaf lets the same full-filled wall dry out each summer (a peak of about 220 g/m²); brick gets a conditional pass. A timber stud wall needs its vapour control layer on the house side of the wool: leave it out and the check finds over 3 kg/m² of condensation on the back of the garage-side board by March, which dries out in summer but is not something to build. The partial-fill pass rests on the foil facings being continuous, with the boards’ joints taped and their edges sealed: taken as a vapour check of only sd 5m, the PIR lets about 140 g/m² of seasonal condensation form, and at sd 2m the check fails. A real garage is milder than outside air, so these are screening results, not predictions — but they are a reason to keep vapour resistance on the warm side of the insulation, as the pre-filled wall does.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (f_Rsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 between a house and an integral garage?

What Ru do I use for an integral garage in SAP?

SAP 10.2 Table 3.1, for a single garage outside the house’s insulation: 0.35 m²K/W when it is fully integral, sharing a side wall, an end wall and the floor with the house, 0.25 when it is fully integral but shares one wall and the floor, and 0.30 when it is partially integral, displaced forward. When the house’s insulated envelope goes round the outside of the garage instead, they become 0.70, 0.55 and 0.60. Table 3.2 gives a double garage 0.35, 0.25 and 0.25 outside the insulation, and 0.60, 0.35 and 0.30 inside it. The values apply to the elements between the house and an integral garage only — an attached garage is disregarded — and the adjustment can be left out where the garage covers less than 10% of the house’s external wall area (or of its heat-loss floor area, for a garage under a floor). SAP 10.3 has the same tables.

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.