U-value of a thatched roof

Water reed and long straw thatch: what a coat achieves on its own, over a room in the roof, and over an insulated loft.

A 300mm coat of water reed thatch with a plasterboard ceiling under the rafters has a U-value of about 0.28 W/m²K, taking the λ of 0.09 that Dorset Council’s thatch guidance gives for reed, from CIBSE Guide A; the thatch on its own is about 0.30. Thatch is a good insulator for a roof covering — 450mm of reed, or 350mm of long straw (λ 0.07), gives about 0.19, close to the 0.2 that guidance quotes — but a 300mm coat falls well short of the 0.16 Approved Document L asks of a renovated roof. Over an unused loft, 270mm of quilt at the ceiling gives about 0.15, as in any loft.

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The build-up (outside → inside)

Thatch λ from Dorset Council’s “Dorset Model” guidance (reed 0.09, straw 0.07, citing CIBSE Guide A); the East Anglian Master Thatchers Association gives the same for water reed and long straw. Rafters at 12%, the rafter void unventilated. The loft row counts everything above the ceiling insulation as one roof-space resistance (BR 443 §8), with the joists at 12.8%. The pre-filled link opens as an existing element.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
300mm water reed on its own≈ 0.30The thatch and its surfaces only
300mm water reed, plasterboard under the rafters≈ 0.28As pre-filled
350mm water reed≈ 0.24
450mm water reed≈ 0.19Dorset Council: 0.2
350mm long straw (λ 0.07)≈ 0.19Dorset Council: 0.2
450mm long straw≈ 0.16Meets 0.16: 0.155
300mm reed + 100mm wood-fibre batts between the rafters≈ 0.17
300mm reed over a loft: 270mm of quilt at the ceiling≈ 0.15The thatch becomes part of the roof space (R 0.2)

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 material and the thickness decide it. Water reed is the more conductive thatch (λ 0.09 in the CIBSE figures Dorset Council quotes), long straw and combed wheat reed about 0.07, and a new coat is typically 280–330mm of water reed or 400mm or more of long straw (East Anglian Master Thatchers Association guidance), thinning as it weathers. Under the coat, a room in the roof gains a little from the rafter void and the ceiling, and wood-fibre batts between the rafters take 300mm of reed to about 0.17 — but Historic England’s advice is that insulation at rafter level is “generally neither necessary nor appropriate” under thatch, and that where the roof space isn’t lived in, insulation at ceiling level may be considered. Over a loft, BR 443 gives everything above the ceiling insulation one roof-space resistance from BS EN ISO 6946 Table 9, whose rows are for tiled and sheeted roofs; there is none for thatch, and U-Monkey takes 0.2, the row for a covering with boards or felt under it, which gives 300mm of reed almost no credit — a cautious figure. RdSAP 2012, which does count the thatch, gave 0.35 for an uninsulated thatched roof and 0.12 with 270mm in the loft (Table S9; RdSAP 10 has since replaced it).

Condensation risk under thatch

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month in any of the rafter-level rows here. It still rates them “not a pass”, because BS 5250:2021 expects an air and vapour control layer on the warm side of a roof insulated at the slope, and a calculation can’t see the warm, moist room air that leaks up through a ceiling, which is how most moisture gets into a roof. Over a loft it treats the roof as a cold pitched roof, which BS 5250 handles by prescriptive rules instead of a calculation.

What matters most in practice is keeping room air out of the thatch: seal the ceiling’s joints, the loft hatch, pipes and light fittings. A fire barrier over the rafters — Dorset Council’s guidance asks for a 30-minute one on new thatched buildings and extensions within 12m of a boundary — changes both results, so add it as a layer and run the check again. Historic England warns that such a board may leave a ventilated void under the thatch; if that void is ventilated, the thatch sits outside it and counts for nothing in a U-value calculation (about 2.1 for this roof, against about 0.26 with the void sealed), so any insulation meant to count would have to go under the board — and whether to add it at all needs the care Historic England asks for.

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 thatched roof?

A 300mm coat of water reed thatch with a plasterboard ceiling under the rafters has a U-value of about 0.28 W/m²K, taking the λ of 0.09 that Dorset Council’s thatch guidance gives for reed, from CIBSE Guide A; the thatch on its own is about 0.30. Thatch is a good insulator for a roof covering — 450mm of reed, or 350mm of long straw (λ 0.07), gives about 0.19, close to the 0.2 that guidance quotes — but a 300mm coat falls well short of the 0.16 Approved Document L asks of a renovated roof. Over an unused loft, 270mm of quilt at the ceiling gives about 0.15, as in any loft.

Do I have to insulate a thatched roof when it is re-thatched?

Possibly, but not at any cost. In England, re-thatching adds or replaces a layer of the roof, so doing more than half of it counts as renovating a thermal element (Building Regulations 2010, regulations 2 and 23), which brings in Approved Document L’s 0.16 W/m²K (Table 4.3) where that is technically, functionally and economically feasible with a simple payback of 15 years or less (paragraph 4.13). For a historic or traditional building, Approved Document L adds that energy efficiency should be improved only where it won’t cause long-term deterioration of the fabric or fittings (paragraph 0.10), and listed buildings and those in conservation areas need only meet the standards so far as that won’t unacceptably alter their character or appearance (0.8–0.9). Historic England advises against insulating at rafter level under thatch. Agree the approach with building control, and with the conservation officer where the building is listed.

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.

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