U-value of the wall between a house and a conservatory

The house wall behind a conservatory: how SAP, BR 443 and Approved Document L treat it.

The wall between a house and a thermally separated conservatory keeps the U-value it had as an outside wall: SAP 10.2 calculates the dwelling “as if it were not present” (§3.3.3), so a filled 50mm cavity wall stays at about 0.60 W/m²K and a solid 9-inch brick wall at about 2.1. BR 443 does allow an unheated space such as a conservatory to be counted as an extra resistance, Ru — about 0.05–0.07 m²K/W for an older, leaky conservatory and 0.15 for a well-glazed, airtighter one, by SAP’s own formula (4) — which would take the cavity wall to 0.58 or 0.55: a small effect, and not one SAP uses. Take the doors out or extend the heating into the conservatory, and that wall stops being the boundary at all.

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The build-up (outside → inside)

Calculated as an external wall, as SAP 10.2 §3.3.3 does for a thermally separated conservatory; the Ru rows add BR 443 §5’s unheated-space resistance, U = 1/(1/Uo + Ru), for the example conservatory in “What changes the number” (SAP 10.2 formula (4), the floor left out as it says). The pre-filled link opens the wall facing outside air; U-Monkey takes an Ru under “On the other side” → “Another unheated space”.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Filled 50mm cavity, the conservatory disregarded (SAP 10.2)≈ 0.60As pre-filled: how SAP and RdSAP count it
Same, counting an older double-glazed conservatory (Ru 0.07)≈ 0.58Ru from SAP formula (4) for the example below
Same, with a well-glazed, airtighter conservatory (Ru 0.15)≈ 0.55
Unfilled 50mm cavity≈ 1.67
Unfilled 50mm cavity, Ru 0.15≈ 1.34
Solid 215mm brick, plastered≈ 2.10
Solid 215mm brick, Ru 0.15≈ 1.60Where an Ru makes most difference
A new separating wall: 100mm PIR partial fill, aircrete, plasterboard on dabs≈ 0.17Meets the 0.18 for a new wall in an existing dwelling (Table 4.2)

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

Two rules meet here. SAP 10.2 §3.3.3 says a conservatory is thermally separated when it is divided from the house by walls, floors, windows and doors with U-values similar to (or, for a new conservatory, no worse than) the dwelling’s own exposed elements, and new doors and windows draught-stripped to match; then “the calculation should be undertaken as if it were not present” — the wall behind it counts as an outside wall, and the conservatory’s floor area is left out. BR 443 §5, the U-value conventions, lists conservatories among the unheated spaces whose effect can be added to a wall as a resistance Ru, from Ru = Ai / (ΣAe·Ue + 0.33·n·V). For an example conservatory 3.5 × 3.0m with 2.4m walls under a roof averaging 2.5m (an 8.4m² wall to the house, 22.8m² of glazed walls, a 10.5m² roof, 26m³ of air), older double glazing at 2.8 and a polycarbonate roof at 2.4 with SAP’s default three air changes an hour give Ru ≈ 0.07 (single glazing, at about 4.8, gives 0.05); modern double glazing at 1.4, a roof at 1.5 and one air change an hour give about 0.15. Either way the effect on a filled cavity wall is a few hundredths, and on a solid brick wall larger (2.10 → 1.60); SAP leaves it out regardless. Approved Document L exempts a new conservatory of up to 30m² at ground level from the energy requirements if the wall, door or window between it and the house is kept (or replaced with one), and the heating is not extended into it (paragraph 0.14; regulation 21); remove that separation or heat the conservatory, and regulation 21(4) brings it within them — see GlazeMonkey’s conservatory guide.

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 the wall between a house and a conservatory?

The wall between a house and a thermally separated conservatory keeps the U-value it had as an outside wall: SAP 10.2 calculates the dwelling “as if it were not present” (§3.3.3), so a filled 50mm cavity wall stays at about 0.60 W/m²K and a solid 9-inch brick wall at about 2.1. BR 443 does allow an unheated space such as a conservatory to be counted as an extra resistance, Ru — about 0.05–0.07 m²K/W for an older, leaky conservatory and 0.15 for a well-glazed, airtighter one, by SAP’s own formula (4) — which would take the cavity wall to 0.58 or 0.55: a small effect, and not one SAP uses. Take the doors out or extend the heating into the conservatory, and that wall stops being the boundary at all.

Do I need to insulate the wall between my house and conservatory?

Not for SAP or an EPC: a thermally separated conservatory is disregarded, so the wall behind it keeps the U-value it had as an outside wall — about 0.60 W/m²K for a filled cavity wall, 2.1 for solid brick — and improving it helps exactly as much as improving any outside wall. The conservatory itself only buffers it a little (an Ru of about 0.05–0.15 m²K/W). The wall matters most when it is taken away: knock through, or extend the heating into the conservatory, and the conservatory’s own glazing, roof and dwarf walls become the outside of the house, with the Building Regulations’ energy requirements applying to them (regulation 21(4)).

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.

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

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