U-value of a ceiling insulated from below with insulated plasterboard

When the loft can’t be reached: insulated plasterboard fixed under the existing ceiling, and what it achieves.

Where a loft can’t be reached or cleared, a ceiling can be insulated from below: 62.5mm of PIR insulated plasterboard (50mm of PIR) screwed through the existing plasterboard ceiling into the joists, under the 100mm of quilt already between them, takes the roof from about 0.43 W/m²K to 0.22 — the old quilt still counts, and the loft and roof above count as one resistance (BR 443 §8). Reaching the 0.16 Approved Document L asks of a renovated roof takes about 90mm of PIR, or a 92.5mm phenolic board (0.16); 82.5mm of PIR gives 0.18. Headroom is the usual limit, and the new board becomes the room’s vapour and air barrier, so its joints have to be taped and sealed.

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The build-up (outside → inside)

Insulated plasterboard entered as its two layers, PIR (λ 0.022) and 12.5mm plasterboard, as U-Monkey’s other insulated-plasterboard pages enter it; phenolic rows at λ 0.019 (Kingspan K118, Unilin SR019/TB-MF). The roof space above the loft is BS EN ISO 6946 Table 9’s 0.2 m²K/W for a felted or boarded roof (U-Monkey’s default; 0.06 with nothing under the tiles). The pre-filled link opens as an upgraded element.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
As it is: 100mm quilt between the joists, plasterboard ceiling≈ 0.43Worse than 0.35, the threshold for upgrading a roof kept in a conversion
+ 37.5mm insulated plasterboard (25mm PIR)≈ 0.28
+ 52.5mm (40mm PIR)≈ 0.24
+ 62.5mm (50mm PIR)≈ 0.22As pre-filled
+ 82.5mm (70mm PIR)≈ 0.18
+ 72.5mm phenolic board (60mm, λ 0.019)≈ 0.18Kingspan K118 or similar
+ 92.5mm phenolic board (80mm, λ 0.019)≈ 0.16Meets the 0.16 for a renovated roof (0.156)
Only 50mm of old quilt + 62.5mm board≈ 0.27
No quilt above + 82.5mm board≈ 0.29The old ceiling counts as part of the roof space here, so this is a little cautious

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

Three things carry the result: the board, what is already in the loft, and the loft itself. The board does most of it — each 10mm of PIR is worth about 0.02 on this roof — and a phenolic core (λ 0.019) goes further per millimetre, which is why the board that reaches 0.16 under 100mm of old quilt is a 92.5mm phenolic one. The old quilt still counts, bridged by the joists (12.8%, BR 443 §4.5.1); with only 50mm left, the same 62.5mm board gives 0.27. Everything above the quilt — the ventilated loft, the roof timbers and the tiles — is taken as one resistance, 0.2 m²K/W for a felted or boarded roof, as BR 443 §8 does for any loft. BR 443 also lets the air-gap correction drop to level 0 where a second insulation layer covers the joists; U-Monkey keeps level 1 here, because the old plasterboard separates the two layers, and reads 0.21 at level 0. Adding a layer counts as renovating the roof (regulation 2(1)), and renovating more than half of it brings in regulation 23: the whole roof to 0.16 (Approved Document L, Table 4.3) “in so far as that is technically, functionally and economically feasible” — on a ceiling, headroom and door heads usually set the thickness, and paragraph 4.13 asks for the lowest U-value that is practical, generally no worse than 0.7.

Condensation risk when insulating a ceiling from below

A loft with insulation at ceiling level is a cold, ventilated roof, which BS 5250:2021 handles with prescriptive guidance rather than a calculation — so U-Monkey’s free BS EN ISO 13788 check reports it as outside a calculation’s scope, with the Glaser result (no interstitial condensation in any month) shown for information only.

The guidance is what to follow: keep the eaves ventilation open, make the new board the air and vapour control layer by taping its joints and sealing every light, pipe and the loft hatch, and don’t fix it over an existing vapour check — Celotex allows its insulated plasterboard directly under an existing ceiling only where there is no vapour check layer, such as gloss paint or foil-backed plasterboard, already there.

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, a vapour-open underlay, a ventilated void, a warm deck — 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 ceiling insulated from below with insulated plasterboard?

Where a loft can’t be reached or cleared, a ceiling can be insulated from below: 62.5mm of PIR insulated plasterboard (50mm of PIR) screwed through the existing plasterboard ceiling into the joists, under the 100mm of quilt already between them, takes the roof from about 0.43 W/m²K to 0.22 — the old quilt still counts, and the loft and roof above count as one resistance (BR 443 §8). Reaching the 0.16 Approved Document L asks of a renovated roof takes about 90mm of PIR, or a 92.5mm phenolic board (0.16); 82.5mm of PIR gives 0.18. Headroom is the usual limit, and the new board becomes the room’s vapour and air barrier, so its joints have to be taped and sealed.

Can I insulate my loft from below?

Yes — by fixing insulated plasterboard to the underside of the existing ceiling, screwed through into the joists. Under 100mm of existing quilt, 62.5mm of PIR board takes the roof from about 0.43 to 0.22 W/m²K, and a 92.5mm phenolic board reaches the 0.16 Approved Document L asks of a renovated roof. Keep the loft ventilated at the eaves, tape the board’s joints and seal round lights and pipes, because the new board becomes the ceiling’s vapour and air barrier — and don’t fix it over an existing vapour check such as foil-backed plasterboard or gloss paint, which Celotex warns against for its boards.

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

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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