Underfloor Insulation: Suspended & Solid

Floor insulation is consistently underrated in the hierarchy of UK home improvements. Loft and cavity wall insulation take priority in most programmes because their payback is fastest, but uninsulated floors, particularly suspended timber ground floors above a cold void, can account for around 15% of the heat lost from an uninsulated home.¹ In a house with good roof and wall insulation already in place, the floor moves up the priority list significantly.

The approach differs substantially depending on floor type. Suspended timber floors and solid concrete slabs require different materials, different access strategies, and have different implications for floor level and finishes.

Suspended Timber Floors

Ground floors in many UK homes built before the 1960s are suspended: floor joists span between the external walls and internal sleeper walls, with a ventilated void beneath.² This void is essential, it prevents moisture accumulation under the timber structure. Any insulation strategy must preserve cross-ventilation of the void.

Access Options

Lifting floorboards: the most disruptive but most thorough method. Boards are lifted systematically, insulation is installed between and flush with the top of the joists, and boards are relay back. If the boards are original tongue-and-groove or are in good condition, this approach also allows a full inspection of the joist structure below. Allow for some boards to be damaged in lifting; budget for 10–15% replacement.

From below via crawl space: if there is enough room in the void to crawl and work, it is possible to work from below without lifting any boards.² A plumber or insulation specialist can work under the floor, fitting rigid boards or suspending mineral wool batts in netting stapled to the underside of the joists. This is only practical where there is a usable access hatch or opening, and where the void is dry and free of debris.

Insulation Materials

Rigid PIR/PUR boards (e.g. Celotex, Kingspan, or generic equivalents) give the best thermal performance per millimetre. 100mm between joists (a common joist depth in older houses) achieves roughly U = 0.20–0.23 W/m²K; Hertfordshire Building Control’s tables show that about 110–140mm of PIR between joists is needed to reach 0.18 W/m²K on a typical floor.³ Boards should be cut to fit snugly between joist flanks with no gaps at the edges, as air bypass at the perimeter significantly degrades performance.² A DPC strip or rigid insulation retaining batten at the underside prevents boards from dropping.

Mineral wool batts are lower cost and easier to fit around irregular joist spacings and services, but require greater depth for the same performance: Hertfordshire Building Control’s tables show 200–250mm of mineral wool roll between joists to reach 0.18 W/m²K on most floors, so 200mm achieves roughly U = 0.20 W/m²K.³ Batts should be supported by netting or galvanised wire strung between joists, they must not rest on the sub-floor void as this blocks airflow.

Breathability and the Ventilated Void

The sub-floor void must remain ventilated. Building Regulations Approved Document C’s guidance for suspended timber floors is for ventilation openings in two opposing external walls, so that air has a free path across the void.⁴ A common and damaging mistake is fitting rigid insulation tight to the underside of the boards without netting, then blocking airbricks in a misguided attempt to stop draughts. The result is condensation in the void and eventual joist rot. Airbricks must be kept clear and cross-ventilation preserved.⁴

A vapour control layer (VCL) on the warm (room) side of mineral wool batts is required by some manufacturers to prevent interstitial condensation; with foil-faced PIR boards, follow the manufacturer’s guidance, which often relies on the foil facing and taped joints instead of a separate VCL.

Solid Concrete Floors

Many homes built from the 1950s onwards, and many ground-floor extensions, have a solid concrete ground-bearing slab.² Newer homes and extensions may instead have a beam-and-block floor, which is suspended rather than solid. Insulating these floors invariably raises the floor level.

Floating Floor System with PIR

The standard approach is to lay rigid XPS (extruded polystyrene) or PIR boards directly on the existing slab, then lay a new screed, chipboard floating floor, or direct tile finish on top. 100mm PIR (lambda ~0.022 W/mK) achieves approximately U = 0.15–0.17 W/m²K, depending on the floor’s size and shape.³ 75mm XPS (lambda ~0.033 W/mK) achieves approximately U = 0.25 W/m²K, which meets the Part L 2022 renovation target.⁵

Approved Document L Volume 1 (2021 edition, in force from 15 June 2022) floor U-values, and it is worth separating the two numbers that get quoted for new build. The limiting value, the worst you are allowed to build, is 0.18 W/m²K for a floor in a new dwelling (Table 4.1) and also for a new floor in an existing one (Table 4.2). 0.13 is the floor in the notional dwelling of Table 1.1, the reference building the target rates are calculated from, so it is what a design usually ends up near rather than a value you must not exceed. An existing floor being renovated is 0.25 W/m²K (Table 4.3), where that is technically and economically feasible.⁵

Floor level rise is the main constraint. 100mm PIR plus a 65mm sand:cement screed adds 165mm to the finished floor level, enough to require extending door linings, adjusting stairs, and lifting skirting boards. Where this is not acceptable, thinner boards (50–60mm PIR with a liquid screed or direct tile) can be used, accepting a slightly worse U-value.

Perimeter Insulation

A thermal bridge forms at the junction of the floor slab and the external wall if the floor insulation does not extend to the perimeter. The slab edge is uninsulated concrete in direct contact with the outside air. Perimeter or edge insulation, 50mm PIR in the cavity at the wall-floor junction, or insulated plinth boards, keeps the insulation continuous at that junction, which is what Part L asks for, and should be incorporated on all solid floor refurbishment projects.⁵

Combining with Underfloor Heating

Solid floor refurbishment is the ideal time to install underfloor heating (UFH). Electric mat systems can be laid directly on the insulation before screed or tile adhesive; water-based wet systems require a minimum screed thickness over the pipes (typically 65–75mm for sand:cement screed, and less, often from about 35–50mm, for a specialist fast-drying liquid screed). Adding UFH to a solid floor refurbishment adds roughly £50–100/m² for electric mat or £80–150/m² all-in for wet pipe systems installed under screed.

Comparison Table

Energy Savings and Payback

In an uninsulated house, around 15% of the heat is lost through the floor.¹ For a three-bedroom semi-detached burning around 9,000–12,000 kWh of gas a year for heating, this equates to roughly £110–145/year at the capped gas rate of 7.97p/kWh for 1 October to 31 December 2026.⁶ Insulation does not remove all of that loss: the Energy Saving Trust puts the saving from insulating under the floorboards at about £55 a year for an average house in Great Britain, and up to £85 a year for a detached house.²

The Energy Saving Trust puts a typical suspended floor installation at £1,400–£2,500, depending on house type, so against those savings payback runs roughly 16–45 years, longer than loft insulation, and warmer floors and fewer draughts are a large part of the case.²

Grants and Funding in 2026

ECO4 (Energy Company Obligation 4) requires larger energy suppliers to fund measures such as insulation, which can include floor insulation, for low-income, fuel-poor and vulnerable households.⁷ Coverage is subject to the energy supplier’s scheme rules; contact your energy supplier directly or use the government’s ECO referral route. ECO4 runs to 31 December 2026.⁷

Great British Insulation Scheme (GBIS) closed on 31 March 2026 and is no longer accepting applications.⁸ It targeted properties in EPC bands D–G and floor insulation was an eligible measure, so it is worth checking whether work already quoted to you was priced on the assumption of GBIS funding.⁹⁸

Warm Homes: Local Grant is the live scheme that replaced HUG2 in England, delivered by local authorities for owner-occupiers and private tenants in EPC band D to G homes with a gross household income below £36,000 or on a qualifying means-tested benefit.¹⁰ Check whether your local authority is delivering it.¹⁰

Local authority retrofit programmes in some areas offer further top-up grants or whole-house retrofit funding. Check with your local authority.

Work funded under ECO4 or, previously, GBIS goes through TrustMark, the government-endorsed quality scheme that oversees these retrofits, so use a TrustMark-registered installer.⁷ Always request a pre- and post-installation EPC if taking grant-funded work to evidence the improvement.

Common Mistakes

Blocking sub-floor ventilation is the most serious error on suspended floors and can lead to joist rot within a few years. Every airbrick must remain functional and unobstructed after insulation installation.

Not leaving an expansion gap at the perimeter on floating solid floors (chipboard or screed) allows the floor to buckle or crack when it expands in warm weather. A 10mm gap around the perimeter, covered by skirting, is standard.

Ignoring the moisture barrier on solid floors: a 1,200-gauge polythene DPM (damp-proof membrane) should be laid between the existing slab and the insulation boards where the existing slab has no integral DPM (common in older construction).⁴ Omitting this allows ground moisture to migrate into the insulation and reduce its performance over time.

Using the wrong insulation in wet areas: PIR boards must be protected from long-term moisture; standard PIR is not suitable for use under wet room tanking or areas subject to standing water without a sacrificial screed above.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.