U-value of a floor over an unheated cellar

Insulating the floor above an unheated cellar, and what the cellar itself is worth: BS EN ISO 13370’s unheated-basement method.

A suspended timber floor over an unheated cellar — 140mm of Rockwool Flexi between 175mm joists, a breathable membrane under it and a plasterboard ceiling to the cellar — has a U-value of about 0.26 W/m²K taken as if it faced outside air, and about 0.21 with the cellar counted the way BS EN ISO 13370 §7.4 counts an unheated basement: through its floor and walls to the ground, its walls above ground, and its ventilation. That is for a mid-terrace front cellar of 18m² with 4m of wall to the outside, 1.8m deep, at the standard’s default of 0.3 air changes an hour, the least cautious choice; at the 3 an hour SAP and BR 443 assume for an unheated space of unknown airtightness it is 0.23. The cellar matters most before the floor is insulated: bare boards over it read 2.64 taken as outside air, but 0.70–1.2 with the cellar counted, depending mostly on how draughty it is.

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The build-up (outside → inside)

The cellar is counted with U-Monkey’s “unheated cellar” option under “On the other side”: BS EN ISO 13370 §7.4 as the Passive House Institute and the Home Energy Model’s reference code set it out, with 0.3 air changes an hour and the cellar’s volume A(h + z) by default, and the floor above taken with the inside surface resistance 0.17 on both faces, as BR 443 §9.2 does for a suspended deck. The cellar here: 330mm brick walls (R 0.43), an uninsulated floor, sand or gravel (λ 2.0). The pre-filled link opens as an upgraded floor with the cellar counted; choose “Outside air” for the exposed figure.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Taken as outside air (RdSAP’s convention, and the cautious case)≈ 0.26RdSAP treats a semi-exposed floor as fully exposed
The cellar counted, 0.3 air changes an hour (the default)≈ 0.21As pre-filled
The cellar counted, 1 air change an hour (small vents)≈ 0.22
The cellar counted, 3 air changes an hour (open vents, a coal hole)≈ 0.23SAP’s default for an unheated space of unknown airtightness
Before: bare boards on joists, no ceiling, as exposed≈ 2.64
Before: bare boards, the cellar counted≈ 0.700.704; 0.89 at 1 air change an hour, 1.21 at 3
Uninsulated, plasterboard ceiling under the joists, the cellar counted≈ 0.581.43 taken as outside air
100mm mineral wool quilt between the joists, the cellar counted≈ 0.26Misses the 0.25 for a renovated floor (0.263)
160mm wood-fibre batts (STEICOflex 036), the cellar counted≈ 0.19
140mm slab + 25mm PIR under the joists, the cellar counted≈ 0.16
End terrace: 8.5m of exposed cellar wall, the cellar counted≈ 0.22
A whole-house cellar, 50m² with 30m of exposed wall (P/A 0.6), the cellar counted≈ 0.23

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 count the cellar add it as BS EN ISO 13370 §7.4 does for an unheated basement — its floor and walls to the ground, its walls above ground and its ventilation — which the calculator applies under “On the other side” → “An unheated cellar or basement”.

What changes the number

There are two ways to count the cellar that give an answer, and they differ. BS EN ISO 13370 §7.4 treats an unheated basement as a space that loses heat through its floor and walls to the ground, through its walls above ground to outside air, and by ventilation: 1/U = 1/Uf + A/(A·Ubf + z·P·Ubw + h·P·Uw + 0.33·n·V), where Uf is the floor above with an inside surface resistance on both faces — U-Monkey’s “unheated cellar” option, which needs the cellar’s area A, exposed perimeter P, depth z, the height of the floor above ground h and its air change rate n. SAP 10.2’s general formula for an unheated space (formula 4) leaves ground floors out, so it can’t represent a cellar, and SAP points to BS EN ISO 13370 for basements; RdSAP treats the floor as exposed — the cautious figure. The cellar’s numbers then matter: P counts only the cellar wall with outside ground or air behind it — U-Monkey’s reading leaves out party walls to a neighbour’s cellar and the wall to the ground under the rest of the house — so an end-terrace cellar (8.5m) does worse than a mid-terrace one (4m); and the air change rate runs from the standard’s default of 0.3, the least cautious figure, to the 3 an hour SAP and BR 443 take when an unheated space’s airtightness isn’t known (SAP Table 3.5 gives 0.5 for well-sealed spaces, 1 with small vents, 10 with many openings). A floor over a cellar is a thermal element — it separates the house from an unconditioned space (regulation 2(3)) — so insulating more than half of it brings in Approved Document L’s 0.25 for a renovated floor (Table 4.3), with a lesser standard where meeting it would cause problems with adjoining floor levels (note 8) and the general 0.7 backstop (paragraph 4.13). Heat the cellar or open it to the house and it becomes part of the dwelling: then the basement floor and cellar wall pages apply.

Condensation and damp over a cellar

U-Monkey’s free BS EN ISO 13788 check keeps outside air beneath the floor — colder than any cellar, so the cautious case — and gives the pre-filled floor a conditional pass: no interstitial condensation in any month, as long as the floorboards’ vapour resistance factor is at least μ 30 (μ 50 used, an assumed value for softwood). The membrane under the insulation should be windtight but vapour-open, so the floor can dry downwards.

Insulating the cellar’s ceiling also makes the cellar colder, so its air will be damper and the joists’ lower edges and their bearings sit on the cold side of the insulation: keep the cellar’s vents and gratings open, deal with any damp in it first, and lag pipes running under the insulated floor, as Ecological Building Systems’ guide advises.

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 (fRsi) 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 floor over an unheated cellar?

A suspended timber floor over an unheated cellar — 140mm of Rockwool Flexi between 175mm joists, a breathable membrane under it and a plasterboard ceiling to the cellar — has a U-value of about 0.26 W/m²K taken as if it faced outside air, and about 0.21 with the cellar counted the way BS EN ISO 13370 §7.4 counts an unheated basement: through its floor and walls to the ground, its walls above ground, and its ventilation. That is for a mid-terrace front cellar of 18m² with 4m of wall to the outside, 1.8m deep, at the standard’s default of 0.3 air changes an hour, the least cautious choice; at the 3 an hour SAP and BR 443 assume for an unheated space of unknown airtightness it is 0.23. The cellar matters most before the floor is insulated: bare boards over it read 2.64 taken as outside air, but 0.70–1.2 with the cellar counted, depending mostly on how draughty it is.

Why does a floor over a cellar have three different U-values?

Because the cellar can be counted two ways, and the second depends on how draughty the cellar is. Taken as outside air — RdSAP’s convention and the cautious one — the pre-filled floor is 0.26 W/m²K. Counted as BS EN ISO 13370 §7.4 counts an unheated basement, with its losses to the ground and its ventilation, it is 0.21 for a typical mid-terrace front cellar at the standard’s default ventilation, rising to 0.23 at the 3 air changes an hour SAP assumes when it isn’t known. SAP’s own formula for unheated spaces leaves the ground out and can’t be used for a cellar. The difference is biggest before insulating: bare boards read 2.64 as exposed and 0.70–1.2 with the cellar counted — which matters where a floor kept in a conversion is judged against the 0.70 threshold (Approved Document L paragraph 4.12(b)).

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.

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