U-value of an uninsulated loft (a ceiling with no insulation)

A bare ceiling under a pitched roof, and how fast the first insulation pays.

A plasterboard ceiling with no insulation over it has a U-value of about 2.5 W/m²K. That includes the roof above it: BR 443 counts the ventilated loft, the roof timbers and the tiles as one resistance, 0.2 m²K/W for a roof with felt or an underlay (BS EN ISO 6946 Table 9); leave it out and the ceiling alone would be 5.0. RdSAP assumes 2.3 for a roof with no insulation. 100mm of mineral wool between the joists brings it to about 0.43, and 270mm to about 0.15 — a sixth of the heat loss for the first 100mm, a seventeenth for 270mm.

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The build-up (outside → inside)

(The ventilated loft, the roof timbers and the tiles: one roof-space resistance, R 0.2 — BR 443 §8)
12.5mm plasterboard ceiling

BR 443 (2019) §8 gives everything above the insulation of a ceiling-level loft one resistance from BS EN ISO 6946 Table 9: 0.2 for tiles or slates on felt, boards or an underlay, 0.06 with nothing beneath them, 0.3 for a boarded and felted roof or one with a foil underside; U-Monkey adds it, with Rse 0.04 beyond it. The pre-filled link opens as an existing roof, judged against Approved Document L’s 0.35 threshold (2021 edition, Table 4.3).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Plasterboard ceiling, tiles on felt or an underlay	≈ 2.50	As pre-filled (roof space R 0.2)
Plasterboard ceiling, tiles with nothing under them	≈ 3.85	Roof space R 0.06
Plasterboard ceiling, roof boarded and felted	≈ 2.00	Roof space R 0.3
20mm lime plaster ceiling (lath and plaster), felted roof	≈ 2.71	The laths themselves not counted
RdSAP 10 default, no insulation	2.3	An assumed value, allowing for the roof space
+ 100mm mineral wool between the joists	≈ 0.43	See the 100mm loft insulation page
+ 270mm (100 between + 170 over)	≈ 0.15	See the 270mm loft insulation page

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.

What changes the number

With nothing on the ceiling, the roof above is most of the resistance there is, which is why the roof type (chosen in the box above the build-up in U-Monkey) moves the figure from 2.0 to 3.9. Every millimetre of insulation then swamps it: at a 20°C difference an uninsulated ceiling loses about 50 W per square metre, 100mm of quilt cuts that to about 8.5 and 270mm to about 3. Air leaking through the hatch, downlights and pipe holes is not in a U-value at all — seal them as you insulate, and keep the eaves ventilation clear.

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 an uninsulated loft (a ceiling with no insulation)?

Does an uninsulated loft count as a thermal element to upgrade?

Yes. At about 2.5 W/m²K it is far worse than the 0.35 threshold for a roof in Approved Document L (Table 4.3), so if the roof is renovated — re-covered over more than half its area, for example — it should be brought to the improved value of 0.16 so far as that is technically and functionally feasible with a simple payback of 15 years or less (paragraph 4.13).

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

Roofs

Walls

Floors

Different build-up? See the full roof 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.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

Open this build-up pre-filled or call 01202 623236