Where to Insulate: Guide to Every Part of a Building

Not sure what insulation goes where? This guide takes each part of a building in turn: lofts, roofs, walls, floors, partitions and suspended ceilings. It covers which products suit, the typical thicknesses that meet current Building Regulations U-values, and links to the right products, calculators and in-depth guides.

Quick answer

  • Loft: 270–300mm mineral wool roll
  • Pitched roof: around 150mm PIR between and over the rafters
  • Flat roof: around 120–150mm flat-roof PIR
  • Cavity wall: around 100mm PIR partial-fill board
  • Solid wall (inside): around 60–80mm insulated plasterboard
  • Timber frame: 140mm mineral wool plus 40–60mm PIR lining
  • Solid floor: 80–100mm PIR
  • Suspended floor: 120–140mm PIR
  • Partitions: 50–100mm acoustic mineral wool
  • Suspended ceilings: 50 or 100mm ceiling pads

Thicknesses are typical starting points for England (Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments) based on manufacturers' published U-value tables. Your result depends on the whole build-up, so always get a U-value calculation and check with Building Control. Wales has its own Approved Document L.

U-value targets at a glance (England)

Element New element in an existing home (e.g. extension) Renovated element
Roof 0.15 W/m²K 0.16 W/m²K
Wall 0.18 W/m²K 0.30 (internal/external insulation), 0.55 (cavity fill)
Floor 0.18 W/m²K 0.25 W/m²K

See our Building Regulations insulation guide for the details.

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Lofts (cold roof at ceiling level)

What to use: Mineral wool loft roll laid between and across the ceiling joists.

Typical thickness: At least 270mm of mineral wool. Most people fit 100mm between the joists and 200mm across (300mm in total). Approved Document L gives 0.16 W/m²K for a renovated roof insulated at ceiling level.

Shop: Loft insulation rolls (glass mineral wool) · Glass wool insulation

Tips: Keep eaves ventilation clear. Don't squash the insulation under boards. Insulate and draughtproof the loft hatch.

Tools and guides: Insulation calculator · What thickness of loft insulation do I need? · How to install loft insulation

Pitched roofs and loft conversions (warm roof at rafter level)

What to use: PIR boards between and/or over the rafters, often with insulated plasterboard underneath.

Typical thickness: New roof on an extension (0.15 W/m²K): around 150mm of PIR in total, for example 75mm between plus 75mm over the rafters. Loft conversion (0.16, or around 0.15–0.17 depending on the build-up): typically 100mm PIR between the rafters plus 57.5–72.5mm insulated plasterboard underneath.

Shop: Roof insulation · PIR insulation boards · Insulated plasterboard · Breather membranes and vapour control layers

Tips: A ventilated roof needs a clear air gap above the insulation. Fit a vapour control layer on the warm side unless you're using foil-faced insulated plasterboard with taped joints.

Tools and guides: PIR U-value calculator · What thickness of PIR insulation do I need? · How to install insulated plasterboard

Flat roofs

What to use: Flat-roof PIR boards laid over the deck as a warm roof, under the waterproofing.

Typical thickness: Typically around 120–150mm of flat-roof PIR, often in two staggered layers, for 0.15–0.18 W/m²K on a timber deck. The exact figure depends on the deck and the waterproofing system, so get a U-value calculation.

Shop: Roof insulation, including Kingspan Thermaroof TR27 and TR24 flat roof boards · Vapour control layers

Tips: Choose a board that suits your waterproofing (torch-on, single ply, cold-applied liquid or EPDM). Fit a vapour control layer on the deck under the insulation.

Tools and guides: PIR U-value calculator · What thickness of PIR insulation do I need?

External and cavity walls

What to use: Partial-fill PIR cavity boards for new masonry walls. Solid walls are usually insulated from the inside (see internal walls below).

Typical thickness: New cavity wall on an extension (0.18 W/m²K): typically around 100mm of PIR cavity board with a clear residual cavity, depending on the blockwork. Insulated plasterboard on the inner face lets you use a thinner cavity board.

Shop: Wall insulation · Wall insulation boards · Kingspan Thermawall TW50 and Recticel Eurowall Cavity partial-fill boards

Tips: Leave the residual cavity clear and use wall ties and retaining clips that suit the board thickness. Filling an existing cavity (blown fibre or beads) is a job for a registered installer.

Tools and guides: PIR U-value calculator · Cavity wall insulation guide · Solid wall insulation guide

Internal wall insulation and dry lining

What to use: Insulated plasterboard (PIR bonded to plasterboard) on dabs or battens, or PIR between battens with plasterboard over.

Typical thickness: Upgrading a solid wall (0.30 W/m²K renovation target): typically around 60–80mm of PIR insulated plasterboard, for example 62.5mm or 72.5mm boards, depending on the existing wall. A lesser standard can be agreed with Building Control if the work would lose more than 5% of a room's floor area.

Shop: Insulated plasterboard · Gypliner and wall lining systems · Plasterboard · Beads, tapes and fillers

Tips: Treat damp first. Tape the board joints or fit a vapour control layer, and allow for window reveals and services.

Tools and guides: Plasterboard and fixings calculator · How to install insulated plasterboard · Why insulated plasterboard works

Timber frame walls

What to use: Glass mineral wool frame rolls or PIR between the studs, plus an insulated lining or PIR layer to cut thermal bridging through the timber.

Typical thickness: For 0.18 W/m²K, a common build-up is 140mm studs fully filled with mineral wool, plus around 40–60mm of PIR or insulated plasterboard on the inside. The exact figure depends on stud size, spacing and sheathing.

Shop: URSA FRAMETEC frame rolls (90mm and 140mm) · PIR insulation boards · Insulated plasterboard · Breather membranes and VCLs

Tips: Fit a breather membrane on the outside of the sheathing and a vapour control layer on the warm side. Fill the studs fully with no gaps.

Tools and guides: PIR U-value calculator · Understanding thermal bridging

Solid (concrete) floors

What to use: PIR boards on the slab under a screed, or XPS/EPS where you need high compressive strength or moisture resistance.

Typical thickness: New floor on an extension (0.18 W/m²K): typically 80–100mm of PIR, depending on the floor's perimeter-to-area ratio. Renovated floor (0.25): around 45–65mm.

Shop: Floor insulation boards · PIR insulation boards · XPS and EPS boards · DPM and polythene · Screed and flooring

Tips: Fit a DPM, use perimeter edge strips to stop thermal bridging at the walls, and lay a separating layer under the screed.

Tools and guides: PIR U-value calculator · PIR vs EPS vs XPS floor insulation · Kooltherm K103 vs PIR

Suspended timber floors

What to use: PIR boards cut between the joists on battens or clips, or mineral wool supported on netting.

Typical thickness: New or replacement floor (0.18 W/m²K): typically 120–140mm of PIR between the joists. Renovated floor (0.25): around 70–90mm.

Shop: Floor insulation boards · PIR insulation boards · Glass wool insulation · Membranes

Tips: Keep the airbricks and underfloor ventilation clear. Seal gaps at the skirting and around pipes to stop draughts.

Tools and guides: PIR U-value calculator · What thickness of PIR for a suspended timber floor?

Partitions and acoustic insulation

What to use: Acoustic mineral wool slabs or rolls in the stud void, with acoustic or standard plasterboard on metal or timber studs.

Typical thickness: Partitions aren't U-value driven. Typically 50–100mm of acoustic slab or roll in a 70–146mm stud, with a double layer of acoustic plasterboard each side for the best results. Approved Document E covers sound between dwellings.

Shop: Acoustic mineral wool · Metal studs and track · Plasterboard · Resilient bar and fixing plates

Tips: Seal all gaps with acoustic sealant, stagger board joints, and use resilient bars or independent studs for higher performance.

Tools and guides: Drywall partition calculator · Plasterboard calculator · How to soundproof a stud wall · Understanding acoustic insulation

Suspended ceilings

What to use: Ceiling insulation pads laid over the tiles, plus acoustic ceiling tiles for echo control.

Typical thickness: Pads come in 50mm or 100mm. Use one 600 × 600 pad per 600 × 600 tile and two per 1200 × 600 tile. Where the ceiling forms the insulated layer under a cold roof void, 100mm pads (or a double layer) give the most benefit.

Shop: Encapsulated ceiling insulation pads (50mm and 100mm) · Ceiling tiles · Ceiling grid · Suspended ceilings range

Tips: Check the extra weight of the pads against the grid and tile specification. Leave space around light fittings, and keep fire-rated ceilings to the tested system.

Tools and guides: Suspended ceiling calculator · MF ceiling calculator · TVS CleanCare tiles

Frequently asked questions

Where should I insulate first?

In most homes the loft gives the quickest win, followed by cavity or solid walls and then floors. Draughtproofing at the same time makes every layer work better.

What thickness of insulation do Building Regulations require?

Approved Document L sets maximum U-values, not thicknesses. In England, new elements in existing homes need 0.15 W/m²K for roofs and 0.18 for walls and floors. Renovated elements need 0.16 for roofs, 0.30 for walls insulated internally or externally, and 0.25 for floors. Wales has its own Approved Document L.

Is PIR or mineral wool better?

PIR gives the most insulation for its thickness, so it suits floors, roofs and walls where space is tight. Mineral wool is non-combustible, cheaper per m² and better for sound, so it's the usual choice for lofts, timber frames and partitions.

Do I need a vapour control layer?

Usually yes, on the warm side of the insulation in roofs, timber frames and internally insulated walls. Foil-faced PIR and insulated plasterboard with taped joints can act as the vapour control layer, but follow the manufacturer's guidance.

Can you help me choose?

Yes. Call 03330 908479 with your build-up and we'll suggest suitable products. Use our calculators to work out quantities and price the job.

Ordering and delivery

Prices are shown ex VAT. Insulation and building materials have a £400 ex VAT minimum order with free delivery to mainland England and Wales. Suspended ceiling orders (tiles, grid and wire) have a £150 ex VAT minimum, with £35 ex VAT delivery under £350 ex VAT and free delivery from £350 ex VAT. Questions? Call 03330 908479 or browse all our guides.