What Thickness of PIR Do I Need for a Suspended Timber Floor?

Insulating a suspended timber floor is one of the most effective upgrades you can make in an older house, and PIR boards fitted between the joists are a popular way to do it. The question is how thick the boards need to be. This guide gives typical thicknesses for work in England, explains why the answer changes from house to house, and covers how to fit the boards so they perform as they should. For floors, walls and roofs in general, see our main guide to what thickness of PIR insulation you need.

Quick answer

• Renovating an existing suspended timber floor (target 0.25 W/m²K): typically 70–90mm of PIR between the joists.
• A new suspended timber floor in an extension (target 0.18 W/m²K): typically 120–140mm.

These figures come from the example U-values in Recticel's BBA certificate for Eurothane GP (see the table below) and assume a typical floor-perimeter-to-area ratio. They are a starting point, not a substitute for a U-value calculation for your floor.

Check your own floor: our free PIR U-value calculator has a suspended timber floor option. Enter your joist depth, joist centres and floor dimensions and it shows the U-value for every board thickness we stock.

The U-values you need to hit

Approved Document L, Volume 1 (2021 edition, incorporating 2023 amendments) applies in England. For floors in existing homes it sets:

Situation	Limiting U-value (W/m²K)
New floor, for example in an extension (Table 4.2)	0.18
Renovated or upgraded existing floor (Table 4.3, improved)	0.25

If meeting the figure would cause significant problems with adjoining floor levels, Approved Document L allows a lesser standard, which you should agree with Building Control. Wales has its own Approved Document L, so check the Welsh guidance for projects there.

Why the perimeter matters

Ground floors lose most of their heat around the edges, so the U-value depends on the ratio of the floor's exposed perimeter to its area (the P/A ratio). Only count walls that face outside or an unheated space, not a party wall with a neighbour.

For example, a semi-detached house with a ground floor 8m wide and 10m deep has an area of 80m². If the 10m side is the party wall, the exposed perimeter is 8 + 10 + 8 = 26m, giving a P/A ratio of 26 ÷ 80 = 0.33. Detached houses and small floors have higher ratios, which means more insulation is needed for the same U-value.

Typical thicknesses by P/A ratio

Recticel's BBA certificate 24/7254 gives these Eurothane GP thicknesses for a suspended timber floor, with the boards between 47mm joists at 400mm centres under a 22mm chipboard deck:

Target U-value	P/A 0.2	P/A 0.4	P/A 0.6	P/A 0.8	P/A 1.0
0.25 (renovation)	45mm	70mm	80mm	85mm	90mm
0.22	60mm	90mm	100mm	105mm	110mm
0.18 (new floor)	90mm	120mm	130mm	135mm	140mm

Joists at 600mm centres, or deeper joists, will change the result slightly, which is why a calculation for your own floor is worth doing.

Check your joist depth first

The boards sit between the joists, so the joist depth limits the thickness you can fit. Lift a couple of floorboards and measure the depth, width and spacing of the joists before you order. Many older houses have joists deep enough for 90mm or more, but if yours are shallow, options include fitting the thickest board that fits and agreeing a lesser standard with Building Control, or adding a thin layer over the joists if you can accept a small rise in floor level.

How to fit PIR between floor joists

1. Support the boards. Use saddle clips at no more than 1m intervals along the joists, or preservative-treated timber battens nailed to the sides of the joists at the right depth.
2. Measure every gap. Joist spacings vary, so measure each bay and cut the boards to fit tightly. A 2400 × 1200mm sheet cuts into three strips for a typical 400mm-centre floor.
3. Fill the gaps. Fill small gaps with strips of board or expanding PU foam. Gaps let cold air bypass the insulation.
4. Keep the void ventilated. The BBA certificate calls for a ventilated sub-floor void (at least 150mm between the underside of the floor and the ground) or a DPM. Keep air bricks clear.
5. Deal with services. Run services beneath the insulation where possible, and pre-lag any water pipes below it.

Frequently asked questions

Do I need a vapour control layer on a suspended timber floor?

According to Recticel's BBA certificate, a separate vapour control layer is not needed on a suspended timber ground floor as long as the sub-floor has adequate cross-ventilation.

Is 100mm PIR enough for a suspended timber floor?

For a renovation it usually is: the BBA figures show 90mm reaching 0.25 even at a P/A ratio of 1.0. For a new floor at 0.18 you will usually need 120mm or more.

Order your floor insulation

We stock Recticel Eurothane GP 90mm, Eurothane GP 120mm and the rest of the range, along with our full selection of floor insulation boards, including Celotex insulation boards and Kingspan insulation boards. Prices are shown ex VAT, and we deliver across mainland England and Wales (see delivery information). Call us on 03330 908479, email sales@insulationwarehousedirect.co.uk, or order online.

Sources: Approved Document L, Volume 1 (GOV.UK), Tables 4.2 and 4.3; Recticel Eurothane GP BBA certificate 24/7254, Product Sheet 2, Table 8, sections 9.1 and A.23–A.26. Figures are indicative and correct at the time of writing.