U-value of a boarded loft (loft boarding over insulation)

Boards for storage over loft insulation: raised clear of the quilt they cost nothing; laid on the joists they can nearly triple the heat loss.

A loft with 270mm of mineral wool quilt — 100mm between the joists and 170mm over them — has a U-value of about 0.15 W/m²K, and boarding it for storage costs nothing as long as the boards are raised clear of the quilt: BR 443 counts everything above the insulation as roof space, so the figure stays at 0.15. Lay the boards on the joists instead and only the 100mm between them is left, about 0.43 — nearly three times the heat loss. Squashing the full 270mm into the joist depth is little better: the National Physical Laboratory measured the quilt’s resistance falling from 6.11 to 3.09 m²K/W, which gives about 0.38.

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The build-up (outside → inside)

Joists at BR 443’s 12.8% (§4.5.1), and the boards, the air under them, the loft and the tiles as one roof-space resistance, R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). The squashed row takes the NPL’s measured R of 3.09 for 270mm compressed to 100mm (λ 0.0324). The pre-filled link opens as an upgraded existing roof, judged against Approved Document L’s 0.16 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
270mm of quilt, no boards≈ 0.150.149
270mm under boards raised clear, 50mm air gap≈ 0.15As pre-filled: the same
Boards on the joists, 100mm of quilt between them only≈ 0.43The quilt over the joists taken out
Boards on the joists, 270mm squashed into the joist depth≈ 0.38NPL: R 6.11 → 3.09
330mm under raised boards (100 between + 230 over)≈ 0.12The most LoftZone’s StoreFloor allows over the joists
350mm under boards on 300mm legs (100 + 250, 50mm gap)≈ 0.11Legs this tall on 100mm joists, with the gap kept

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

What matters is whether the quilt keeps its thickness. BR 443 (2019) §8 gives a loft insulated at ceiling level one resistance for everything above the insulation — the boards, the air under them, the ventilated loft and the roof — so boards held clear of the quilt change nothing, and U-Monkey (from engine v3.9) counts them that way. Squashing is the problem: compressed quilt loses resistance even though its λ improves, and boards on the joists usually mean the quilt over them has been flattened or taken out. Raised floors come in two kinds: LoftZone’s StoreFloor, which holds the boards 279mm above the joists and has a BBA certificate (15/5269), and loft legs such as Loft Leg’s 175mm leg, sold for boards “above 270mm of quilt”, and its 300mm XL. On 100mm joists a 175mm leg puts the boards 275mm above the ceiling, so 270mm of quilt leaves almost no gap; the BBA certificate asks for a 50mm air gap between quilt and boards, kept open at opposite sides, and with that gap a 175mm leg leaves room for about 125mm of quilt over the joists (about 0.18 in all), a 300mm leg for about 250mm (0.11). Quilt pressed up against the boards is the case U-Monkey’s condensation check fails. The joists have to carry the load too: the certificate takes ceiling joists as typically good for 0.25 kN/m², and more only where an engineer confirms it.

Condensation risk under loft boards

Boards over loft insulation sit on its cold side, so they are a condensation question as much as a heat one. Lay them on the joists, straight onto the quilt, and U-Monkey’s free BS EN ISO 13788 check fails the ceiling: moisture from the rooms condenses on the underside of the boards and builds up year on year, to around 9 kg/m² by the end of the first year in the calculation. Raise them on a raised floor or legs, with an air gap open to the loft, and that goes: the boards become part of the ventilated roof space, which is what the BBA certificate’s 50mm gap, open at opposite sides, is for.

A loft insulated at ceiling level is a cold pitched roof, which BS 5250:2021 treats by prescriptive rules rather than a calculation, because most of the moisture that reaches a loft gets there on air leaking through the ceiling. So U-Monkey reports the pre-filled loft as outside the scope of a calculation rather than as a pass, with no condensation in the ceiling itself in any month. Keep the eaves ventilation clear when you add quilt, seal round the loft hatch, pipes and downlights, and don’t lay polythene or foil-backed sheet over the insulation.

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 boarded loft (loft boarding over insulation)?

A loft with 270mm of mineral wool quilt — 100mm between the joists and 170mm over them — has a U-value of about 0.15 W/m²K, and boarding it for storage costs nothing as long as the boards are raised clear of the quilt: BR 443 counts everything above the insulation as roof space, so the figure stays at 0.15. Lay the boards on the joists instead and only the 100mm between them is left, about 0.43 — nearly three times the heat loss. Squashing the full 270mm into the joist depth is little better: the National Physical Laboratory measured the quilt’s resistance falling from 6.11 to 3.09 m²K/W, which gives about 0.38.

Does loft boarding reduce the effectiveness of insulation?

Only if it squashes the insulation or takes its place. Boards raised clear of the quilt, with an air gap under them, leave the U-value unchanged — BR 443 counts everything above the insulation as roof space — but boards laid on the joists leave room for only the quilt between them: about 0.43 W/m²K, against 0.15 with 270mm. Compressing 270mm of quilt into the joist depth roughly halved its thermal resistance in tests by the National Physical Laboratory for LoftZone (6.11 to 3.09 m²K/W). The Energy Saving Trust’s advice is the same: if you use mineral wool, don’t squash it.

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.

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