Best roof insulation types by roof material and form for UK homes

Part of our guide on how does insulation work in a house

Most advice ignores roof form—yet a flat roof and a pitched loft need different materials, different thicknesses and different fixing methods to hit the same U-value.

Why roof form decides insulation type before anything else

Insulation choice starts with how your roof is built, not which brand you prefer. A pitched roof with accessible joists lets you roll out mineral wool above the ceiling. A flat roof with no cavity demands rigid boards laid on top of the deck. A metal agricultural or industrial roof with exposed purlins needs something that fixes to the underside without thermal bridging through every fastener.

The U-value target (heat loss per square metre per degree) sets the minimum thickness, but the roof's geometry and load-bearing rules tell you which material can actually fit. A 270 mm mineral-wool quilt works beautifully between 400 mm joist centres but becomes a nightmare between 100 mm rafters in a Victorian terrace with 2.1 m headroom. That's where you switch to 100 mm PIR or phenolic foam, which delivers the same thermal resistance in half the depth.

Grant schemes (ECO4, the Boiler Upgrade Scheme, and the forthcoming Warm Homes Plan installers) all lock funding to 0.16 W/m²K for pitched roofs and 0.18 W/m²K for flat roofs. Miss those figures by 10 mm of insulation and the assessor will fail the job. Check what your EPC assessor or grant surveyor requires before you order materials, because the specification drives everything downstream.

Condensation risk changes with roof form too. A cold loft with insulation at ceiling level stays ventilated and dry. A warm roof with insulation between or above rafters traps moisture unless you fit a vapour control layer on the warm side and leave ventilation gaps on the cold side. Get the layer sequence wrong and you'll strip the roof again in five years to replace soaked timber. Before you commit to anything, it is worth looking at removing small foam patches.

Insulation for pitched roofs: between joists, between rafters or both

what type of insulation for roof. Pitched roofs are either cold lofts with insulation between ceiling joists or warm roofs with insulation between or above rafters.

Cold loft: lay 270 mm mineral wool in two layers. Fit 170 mm between joists and a 100 mm cross layer. That delivers about 0.16 W/m²K. Materials cost for a typical three-bed semi runs roughly £300–500.

Warm roof for a room in the loft loses headroom. Fit 100 mm PIR boards between 150 mm rafters plus 50 mm PIR over the rafters to reach 0.16 W/m²K. Mineral wool needs 170 mm total for the same U-value, so you may need to fur out rafters. People in this spot often ask about meeting UK insulation regulations as well.

Phenolic foam has lambda around 0.018 W/mK versus PIR at 0.022 W/mK. That saves 10–15 mm depth but costs 20–30% more. Foil-faced boards act as a vapour control if you tape every joint.

Spray foam seals gaps and sticks to uneven timber. It complicates future repairs and can stop the roof breathing if you don't add ventilation. Use spray foam only when sarking felt is missing or wind-driven rain must be sealed out.

• Cold loft: 270 mm mineral wool between and across joists, ventilated eaves, no VCL needed.
• Warm loft (room in roof): 100 mm PIR between rafters plus 50 mm continuous PIR over rafters, or 170 mm mineral wool.
• Cathedral ceiling: rigid PIR or phenolic above rafters, taped joints, counter-battens and tiles.
• Spray foam: seals but blocks repairs; use only where other options fail.

What you'll need

U Value Calculator

Calculates U-values for roof insulation thickness to help choose the right insulation by roof type.

See the one to get

Insulation options for flat roofs: above or below the deck

Flat roofs (garages, extensions, dormer roofs) are either warm-deck or cold-deck. A warm-deck roof has insulation above the structural deck, which keeps the timber or concrete warm and dry. A cold-deck has insulation below the deck, between joists, with a ventilated void above it. Warm-deck is the default now because cold-deck roofs condense and rot unless you get the ventilation perfect, which is hard when the void is only 50 mm deep. Before you commit to anything, it is worth looking at attic insulation you can walk on.

Rigid PIR or phenolic boards, laid directly on the deck and covered with a single-ply membrane or three-layer felt, are the standard warm-deck approach. You need 120–150 mm to hit 0.18 W/m²K, depending on whether you pick PIR (0.022 W/mK) or phenolic (0.018 W/mK). The boards carry foot traffic during installation, so use high-density grades (120 kg/m³ or above) or they'll dent. A 50 m² flat-roof extension needs roughly £600–900 of PIR and another £800–1200 for a roofer to fit it and re-membrane the surface.

Vapour control goes under the insulation, on the warm side, to stop moist air from the room below condensing inside the boards. Use a peel-and-stick bitumen VCL or a reinforced plastic sheet with every overlap taped. Miss this and the boards will wick moisture up from below, the thermal performance will drop by half, and the deck will rot. Membrane choice matters too: EPDM rubber, GRP fibreglass and torch-on felt all work, but GRP and EPDM handle standing water better if your fall is shallow.

Cold-deck flat roofs still exist in older extensions, but Building Control won't approve a new one unless you can prove 50 mm of clear ventilation between insulation and deck. That means cross-flow ventilation from eaves to eaves, which is impossible in a narrow porch or a three-sided dormer. If you have an existing cold-deck roof that's dry, you can top up the insulation between joists to 150–170 mm of mineral wool, but check for mould on the underside of the deck first: if it's there, the roof already has a condensation problem and needs rebuilding as warm-deck. Before you commit to anything, it is worth looking at materials for floor insulation.

Insulation types suited to metal roofs: agricultural, industrial and modern homes

Metal roofs (steel or aluminium sheeting on purlins) are common in barns, workshops, industrial units and some modern self-builds. They lose heat fast because the metal itself has no insulation value and every purlin is a thermal bridge. You need insulation that fixes to the underside of the sheeting or between the purlins without crushing, and it must handle condensation because single-skin metal roofs drip with moisture in winter.

Multifoil insulation (thin reflective quilts with bubble or foam cores) works when you fix it under the sheeting with a 25 mm air gap on each side. A typical 40 mm multifoil with trapped air spaces on both sides delivers an effective U-value around 0.18–0.20 W/m²K, enough for Building Regulations but not always enough for grants. It's light, quick to fit and costs £5–8 per m². The catch: it only works if the air gaps are clear - dust, cobwebs or sagging contact with the sheeting kills the reflective performance.

Spray polyurethane foam, applied to the underside of the sheeting by a specialist, seals every purlin and eliminates thermal bridging. You need 100–120 mm to hit 0.18 W/m²K, and it costs £25–35 per m² installed. The foam also stops condensation drips because it insulates the metal above dewpoint. It's the best solution for long-span industrial roofs, cold stores or workshops in Scotland and the north of England where winter dewpoint sits around 4–6 °C and single-skin sheeting condenses from October to March. People in this spot often ask about polystyrene insulation benefits as well.

Rigid PIR boards between purlins, with foil tape sealing every joint, are another option if the purlins are 100–150 mm deep. You lose the space between purlins for cable runs, and you must fix a vapour control layer under the boards or condensation will drip onto whatever you're storing below. This approach suits self-builds and modern agricultural buildings where the purlins are already sized for insulation, but it's fiddly and expensive in existing barns with shallow or irregular purlins.

• Multifoil: quick, light, cheap; needs 25 mm air gaps both sides or performance collapses; best for short-span farm buildings under 6 m.
• Spray foam: seals purlins, stops condensation, no thermal bridges; expensive and permanent; best for industrial roofs and cold stores.
• PIR between purlins: good thermal performance, easy to source; needs vapour control and regular purlin spacing; suits new-builds.

How to choose insulation by roof type: U-value, span and fixing method

Start with the U-value target. Building Regulations expect 0.16 W/m²K for pitched roofs and 0.18 W/m²K for flat roofs in new work and extensions. ECO4 and the Boiler Upgrade Scheme ask for 0.16 W/m²K for loft insulation.

Check rafter or joist depth and the span. Mineral wool works between 400 mm centres. It sags between 600 mm unless you support it. Rigid boards need timber or metal fixings. Curved, purlin-free roofs usually need spray foam.

Match thickness to lambda. Use 270 mm mineral wool at 0.040 W/mK or 100 mm PIR at 0.022 W/mK for 0.16 W/m²K. If rafters are 100 mm deep, fur them out or pick phenolic foam.

Add vapour control on the warm side when insulating between or above rafters. Leave ventilation gaps: 25 mm above rafter insulation and 50 mm in cold flat-deck voids.

1. Look up the U-value your grant or Building Control requires.
2. Measure rafter/joist depth, spacing and headroom.
3. Choose material by lambda and calculate thickness.
4. Specify VCL and ventilation gaps before you buy.

Vapour control, ventilation and condensation risk by roof form

Every insulated roof needs a moisture plan. Warm air carries vapour and it condenses on cold surfaces. Condensation soaks insulation and rots timber quickly.

Cold lofts: ventilate at eaves and ridge. Provide a continuous 10 mm eaves gap or equivalent airbrick area and a ridge vent or tile vent every 10 m of ridge. No vapour control layer is needed in the loft itself.

Warm roofs between or above rafters: fit a vapour control layer on the warm side. Tape every joint. Leave 25 mm clear air above the insulation to the sarking felt or membrane. Vent at eaves and ridge to carry moisture away.

Flat warm-deck roofs: place the VCL on the structural deck under the insulation. Lap joints 100 mm and use the manufacturer's tape. Metal roofs with multifoil need 25 mm air gaps both sides and vents at eaves and ridge. Spray foam acts as both insulation and VCL but hides existing rust.

Questions people still ask

I've stripped and re-insulated three loft conversions after DIY installs skipped the vapour layer: two had rotted rafters, one had soaked rockwool that weighed twice what it should and compressed to nothing.