How to Insulate a Floor: Underfloor Insulation Options Explained
How you insulate a floor depends on what kind of floor you have. A solid concrete floor, a suspended timber floor and a floor you are laying over an existing one each call for a different method and different materials. This guide covers all three, with the products we stock, the points to check and a costed example, so you can plan the job before you order.
Quick answer
- Solid concrete floor: lay a DPM, then rigid insulation board, then a screed or floating deck. For a new extension floor, our PIR guide suggests around 80–100mm of PIR for 0.18 W/m²K.
- Suspended timber floor: fit rigid board or mineral wool between the joists, supported underneath. The PIR guide suggests around 120–140mm of PIR for 0.18.
- Over an existing floor: a thin foil underlay or rigid board with a new deck, where height allows.
These thicknesses are starting points; the right figure depends on your floor and your target, so check with our PIR U-value calculator and with Building Control. See what thickness of PIR you need.
Option 1: solid concrete floors
The usual build-up for a concrete floor is, from the bottom:
- Slab and DPM. A damp proof membrane stops moisture rising. We stock membranes from 500 to 2000 gauge, such as the BBA 1200 gauge DPM (£42.99 per 4m × 25m roll) and double-sided butyl tape (£8.16) to seal the laps. See our DPM and polythene.
- Insulation board. Choose PIR, phenolic, EPS or XPS. For example 100mm Recticel Eurothane GP (£36.83 per sheet, £12.79 per m²), 100mm Kingspan Kooltherm K103 (a phenolic floorboard, £37.91 per m²), or 100mm EPS 70 (£6.97 per m²). XPS such as GG300 is used where loads or moisture are a concern.
- Perimeter strip. An edge strip around the screed gives it room to move. See screed edging strip (£12.49).
- Screed or floating deck. A separating layer over the insulation, then screed or a floating board deck. Follow the manufacturer's guidance for the board you choose.
The materials are compared in PIR vs EPS vs XPS for floors, and the phenolic and PIR options in K103 vs PIR floor insulation. Browse our floor insulation boards.
Option 2: suspended timber floors
If you can reach the underside of the floor, the insulation goes between the joists and is held in place with netting, battens or support clips. Rigid board is cut to fit tightly between the joists; mineral wool is the alternative. For PIR, our guide suggests 120–140mm to reach 0.18 W/m²K, and the depth of your joists will decide what is possible. Ventilation under the floor must be kept clear, to protect the timbers. See what thickness of PIR for a suspended timber floor.
If you are lifting boards to do it from above, take care with services and make sure the insulation is supported and sealed at the edges. You can also fit thin multifoil under the floor covering, but follow the manufacturer's installation guidance; it relies on air gaps to work.
Option 3: over an existing floor
If you cannot raise the floor level much, a thin product like SuperFOIL SFUF underfloor foil (£84.00 per 1.5m × 8m roll) can go under a floating floor covering. Where you have height, rigid board with a floating deck, for example 22mm moisture resistant chipboard flooring (£10.83 per 1.44m² board), gives a higher level of insulation. Check door heights, thresholds and stairs before you commit.
Worked example: a 20m² concrete extension floor
20m² at 100mm Eurothane GP needs 7 sheets (20 ÷ 2.88 = 6.9), so 7 × £36.83 = £257.81 ex VAT, plus the DPM, tape and edge strip. Use the insulation calculator to adjust for your floor. Tape the foil joints with a product like Alpha Foil Tape 75mm (£6.08) where the system needs it.
Building Regulations and floors
Insulating a floor as part of an extension or a renovation is usually notifiable work, and the target depends on whether it is a new element or an upgrade to an existing floor. The usual figures in Approved Document L (Volume 1) are 0.18 W/m²K for a new extension floor and 0.25 for a renovated floor, but check the current edition and speak to Building Control before you start. See our building regulations guide.
Common mistakes
- Skipping the DPM on a concrete floor.
- Leaving gaps between boards or at the edges.
- Blocking the air vents under a timber floor.
- Forgetting the extra height at doors, thresholds and stairs.
- Not checking the whole build-up against the U-value target.
Frequently asked questions
What is the best insulation for a floor?
Rigid boards (PIR, phenolic, EPS or XPS) are the usual choice for solid floors. Between joists, rigid board or mineral wool. The best one depends on depth and budget.
How thick should floor insulation be?
That depends on the floor type and the target. For a new concrete extension floor, around 80–100mm of PIR, for a suspended timber floor around 120–140mm, as a starting point.
Do I need a DPM under floor insulation?
On a concrete floor, yes, you normally need damp protection. Your Building Control officer will confirm the details for your project.
Can I insulate a floor myself?
Many DIYers do. Working under a timber floor is awkward, so plan access and ventilation first.
Prices are shown ex VAT and vary, so check the product page before you order. Most orders over £400 + VAT are delivered free across England, with delivery to mainland Wales and Scotland too. Mainland Wales and mainland Scotland: Celotex insulation and Knauf loft roll have £40 ex VAT delivery on every order; ceilings have a £400 ex VAT minimum plus £40 ex VAT delivery; British Gypsum plaster and plasterboard have a £400 ex VAT minimum plus £40 ex VAT delivery. We do not deliver to Northern Ireland or the Scottish islands. Insulation and plasterboard orders have a £400 ex VAT minimum and free delivery (see delivery information). Outside England? Call us or contact us for a quote. Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.