U-value of a room-in-roof sloping ceiling

An existing room-in-roof slope with 100mm rafters, 50mm of quilt between them and a plasterboard ceiling has a U-value of about 0.79 W/m²K. Under an old bitumen felt, a high-resistance underlay, BS 5250 asks for a ventilated gap between the felt and the insulation — usually 50mm — which leaves 50mm of the rafter depth to insulate. With 50mm of PIR there and 100mm of PIR under the rafters the slope reaches about 0.15, inside the 0.16 that Approved Document L sets for a roof being renovated; 75mm under the rafters gives only 0.19. Re-covering the roof with a vapour-permeable underlay lets you fill the rafters instead: 100mm of PIR between and 60mm under reaches 0.16.

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The build-up (outside → inside)

Tiles on battens (ventilated, not counted)
Bitumen felt underlay, type 1F (not counted)
50mm ventilated gap under the felt (not counted)
50mm PIR between 100mm rafters (~12% timber)
75–100mm PIR under the rafters (continuous)
Polythene VCL (warm side)
Plasterboard

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
As built: 50mm quilt between the rafters	≈ 0.79	Renovating it brings in Table 4.3’s 0.16; worse than the 0.35 threshold too
50mm PIR between + 50mm under	≈ 0.24	Well short of the 0.16 renovation target
50mm PIR between + 75mm under	≈ 0.19	Still short of 0.16
50mm PIR between + 100mm under	≈ 0.15	Meets the 0.16 for a renovated roof
Vapour-permeable underlay, no gap: 100mm PIR between + 60mm under	≈ 0.16	Meets 0.16 (0.159) with 40mm less lost headroom

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

The ventilated gap is what makes this slope hard. BS EN ISO 6946 disregards it and everything above it, so only the insulation inboard of the gap counts, and in shallow rafters that is not much. Every millimetre under the rafters comes off the room — 100mm of PIR and the plasterboard bring the ceiling 112.5mm further in — which is why the underlay matters: replace the old felt with a vapour-permeable one when the roof is re-covered and the full rafter depth can be filled. Either way, BS 5250 expects an air and vapour control layer on the warm side, and U-Monkey’s free condensation check will not pass the slope without one; as pre-filled, with the VCL behind the plasterboard, it finds next to no condensation, a conditional pass that depends on the assumed vapour values, so enter your products’ declared figures. The other parts of a room in the roof — the stud walls, the flat ceiling and the insulation in the eaves voids — are separate elements: model each one.

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 room-in-roof sloping ceiling?

Do I have to reach 0.16 when I insulate a room in the roof?

If the work renovates the roof — stripping the sloping ceiling back to the rafters, for example — Approved Document L Table 4.3 asks for its improved value of 0.16 W/m²K where that is technically, functionally and economically feasible; where it is not, the slope is improved as far as is feasible within a 15-year simple payback. That holds whatever the slope starts from: the table’s 0.35 threshold is for a roof kept in a conversion (paragraph 4.12(b)), and at about 0.79 this one would be upgraded then too. Headroom is usually what limits it: model the options in U-Monkey and agree the spec with building control before you start.

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.