U-value of a room-in-roof flat ceiling (collar ceiling)

The flat ceiling at the top of a loft room, with a small loft above it, and what reaches 0.15.

The flat ceiling at the top of a loft room — 100mm of PIR between the ceiling joists or collars and 82.5mm PIR insulated plasterboard (70mm of PIR) under them — has a U-value of about 0.15 W/m²K, meeting the 0.15 Approved Document L sets for a new roof element in an existing home (0.146). A 62.5mm board gives 0.17 and misses. Where the small loft above can be reached, quilt is the cheaper route: 100mm between the joists and 200mm over them gives about 0.13. BR 443 calculates this ceiling as a loft, with the roof above it counted as one resistance (R 0.2).

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The build-up (outside → inside)

Ceiling joists at BR 443’s default 12.8% (§4.5.1) and the roof space above as R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR between the joists only≈ 0.31
+ 25mm PIR under (37.5mm board)≈ 0.21
+ 50mm PIR under (62.5mm board)≈ 0.17Misses 0.15
+ 70mm PIR under (82.5mm board)≈ 0.15As pre-filled: 0.146
150mm PIR between 150mm joists + 25mm under≈ 0.16Misses 0.15 (0.163)
100mm quilt between + 200mm quilt over the joists≈ 0.13Where the loft above can be reached

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

BR 443 (2019) §8 says an insulated flat ceiling near the top of the pitch is calculated as a roof insulated at ceiling level, so U-Monkey treats it as a loft: the void above, the roof timbers and the tiles count as one resistance from BS EN ISO 6946 Table 9 (0.2 for tiles on felt or an underlay), and the joists take BR 443’s 12.8%. The layer under the joists covers them and does more per millimetre than extra depth between them. The small loft above is a cold roof space, so keep it ventilated, seal the ceiling well, and carry the insulation unbroken round the corner where the flat ceiling meets the sloping one — that junction is the usual weak point.

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 flat ceiling (collar ceiling)?

The flat ceiling at the top of a loft room — 100mm of PIR between the ceiling joists or collars and 82.5mm PIR insulated plasterboard (70mm of PIR) under them — has a U-value of about 0.15 W/m²K, meeting the 0.15 Approved Document L sets for a new roof element in an existing home (0.146). A 62.5mm board gives 0.17 and misses. Where the small loft above can be reached, quilt is the cheaper route: 100mm between the joists and 200mm over them gives about 0.13. BR 443 calculates this ceiling as a loft, with the roof above it counted as one resistance (R 0.2).

What U-value does the flat ceiling of a loft conversion need?

In England, 0.15 W/m²K treated as a new roof element in an existing home (Approved Document L 2021, Table 4.2); some building control bodies ask for 0.15 across the whole roof of a loft conversion — CNC Building Control’s 2025 loft conversion guide does. Where the existing roof is treated as a retained element being upgraded instead, Table 4.3’s improved value is 0.16, with a lesser standard possible where meeting it would limit headroom (note 3). The pre-filled build-up meets both.

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.

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