What Thickness of PIR Insulation Do I Need? A UK Guide for Floors, Walls and Roofs

"What thickness do I need?" is the question we are asked most about PIR boards. The honest answer is that it depends on where the insulation is going, how the element is built and which U-value you have to hit. For the most common jobs, though, there are well-established starting points, and this guide sets them out for floors, walls and roofs.

Quick answer

For work on an existing house in England (extensions, loft conversions and renovations), typical PIR thicknesses taken from manufacturers' published U-value tables are:

• Solid concrete extension floor: around 80–100mm of PIR under the screed to reach 0.18 W/m²K.
• Suspended timber floor: around 120–140mm between the joists for 0.18 W/m²K.
• Cavity wall (partial fill): around 100mm of PIR cavity board for roughly 0.18 W/m²K, depending on the blockwork.
• Pitched roof on an extension: around 150mm in total, for example 75mm between and 75mm over the rafters, for 0.15 W/m²K.
• Loft conversion roof: for example 100mm between the rafters plus insulated plasterboard underneath, giving roughly 0.15–0.17 W/m²K.

These figures are indicative only. Always get a U-value calculation for your actual build-up and check with Building Control before you order.

Work it out for your build: our free PIR U-value calculator checks a floor, wall or roof against Part L, finds the thinnest board we stock that meets the target and prices it for you.

Why thickness depends on the U-value

Building Regulations do not set a minimum insulation thickness. They set a maximum U-value, which measures how much heat passes through a floor, wall or roof (in W/m²K). The lower the number, the better the insulation. The thickness you need is whatever gets your particular construction below that limit.

PIR is popular because it works hard for its thickness. The foil-faced PIR boards in this guide all have a declared thermal conductivity (lambda) of 0.022 W/mK. For example, Recticel's datasheet gives 100mm Eurothane GP a thermal resistance (R-value) of 4.50 m²K/W. But the board is only one layer: joists, rafters, blockwork, screed, air gaps and the ground all affect the final U-value, so the same board can give different results on different jobs.

The U-values you need to hit in England

The figures below come from Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments), which applies in England. They are area-weighted maximums in W/m²K.

Element	New element in an existing home, e.g. an extension (Table 4.2)	Renovated or upgraded element (Table 4.3, improved)
Roof	0.15	0.16
Wall	0.18	0.30 (internal or external insulation), 0.55 (cavity fill)
Floor	0.18	0.25

A few points worth knowing:

• The renovation standard applies when you renovate more than 50% of an element's surface, or when more than 25% of the whole building envelope is being renovated. It also applies to elements you keep during a loft or garage conversion if they are worse than the threshold values in Table 4.3.
• A lesser standard may be appropriate where meeting the figure would cut a room's floor area by more than 5% (walls), cause problems with adjoining floor levels (floors) or limit headroom (roofs). Agree this with Building Control.
• New-build homes have their own limits (Table 4.1), but some elements usually need to perform significantly better to pass the overall energy calculations.
• Wales has its own Approved Document L (2022 edition, published by the Welsh Government), so check the Welsh figures if your project is in Wales.

Floors

Ground floor U-values depend on the ratio of the floor's exposed perimeter to its area (the P/A ratio), as well as the floor type. Recticel's BBA certificate for Eurothane GP (24/7254, Product Sheet 2) gives these example thicknesses for a 0.18 W/m²K floor:

• Ground-bearing concrete floor (insulation on the slab, under a 65mm screed): 50mm at a P/A of 0.2, 80mm at 0.4, 90mm at 0.6, 95mm at 0.8 and 100mm at 1.0.
• Suspended timber floor (insulation between 47mm joists at 400mm centres): 90mm at 0.2, 120mm at 0.4, 130mm at 0.6, 135mm at 0.8 and 140mm at 1.0.

Small extensions often have a high P/A ratio, which pushes the thickness up. Approved Document L allows an extension's floor U-value to be calculated using the perimeter and area of the whole enlarged dwelling, which can reduce the thickness needed. For a renovated floor (0.25), the same certificate shows 45–65mm on a concrete floor and 70–90mm between timber joists at P/A ratios of 0.4 to 1.0.

Browse our floor insulation boards.

Walls

Cavity walls

For a new cavity wall on an extension, a partial-fill PIR board with a clear residual cavity is a common choice. Kingspan's published tables for Thermawall TW50 in a 102.5mm brick outer leaf, 100mm block inner leaf and plasterboard-on-dabs wall show:

• 80mm: 0.18–0.20 W/m²K, depending on the block type
• 100mm: 0.15–0.17 W/m²K

So 100mm is a typical starting point for the 0.18 target. Insulated plasterboard on the inner face is another way to reach it with a thinner cavity board.

Solid walls and drylining

If you are insulating an existing solid wall from the inside, the renovation target is 0.30 W/m²K. The result depends heavily on the existing wall, so get a calculation first. Insulated plasterboard is the usual choice where space is tight, and the 5% floor area allowance above can apply in small rooms.

See all our wall insulation boards.

Roofs

Pitched roofs on extensions

For a new pitched roof (0.15 W/m²K), Kingspan's tables for Thermapitch TP10 fitted between and over rafters at 600mm centres show 75mm + 75mm giving 0.14–0.15, and 70mm + 70mm giving 0.15–0.16, depending on rafter depth and whether there is a sarking board.

Loft conversions

In a loft conversion the existing roof is a retained element, so the usual target is 0.16. A common approach is PIR between the rafters with insulated plasterboard fixed underneath. For 150mm rafters, Kingspan's tables show 100mm of TP10 between the rafters plus 57.5mm insulated plasterboard giving around 0.17 in a ventilated roof (typical where you are not re-roofing) and around 0.15 in an unventilated roof with a breathable membrane. In the ventilated case, a 72.5mm insulated plasterboard brings it to around 0.15. A ventilated roof needs a clear air gap above the insulation, which limits how much board fits between the rafters.

Flat roofs use dedicated flat roof PIR boards and the thickness depends on the deck and waterproofing system, so ask for a calculation. Browse our loft, roof and ceiling insulation.

Before you order

• Get a U-value calculation. Many manufacturers offer U-value and condensation risk calculations free of charge, and Building Control will often ask for one.
• Check with Building Control before work starts, particularly for extensions, loft conversions and renovations covered by Part L.
• Use the right fixings. Over-rafter and cavity boards need fixings and wall ties suited to their thickness.
• Allow for cutting and waste when working out how many 2400 × 1200mm sheets you need.

Planning metal stud walls for a loft conversion or extension as well? Our drywall partition calculator will help you work out studs, track and boards.

Frequently asked questions

Is 100mm PIR enough for a floor?

Often, yes. On a ground-bearing concrete floor, Recticel's BBA data shows 100mm reaching 0.18 W/m²K even at a P/A ratio of 1.0. A suspended timber floor usually needs more, typically 120–140mm between the joists.

What is the minimum insulation thickness required by Building Regulations?

There isn't one. Approved Document L sets maximum U-values, not thicknesses. The thickness you need depends on the insulation material and the rest of the construction.

Can I use two thinner PIR boards instead of one thick one?

Yes, and it is common practice. Staggering the joints helps reduce gaps. Make sure your U-value calculation reflects the layers you will actually fit.

Do the same figures apply in Wales?

Not necessarily. Wales has its own Approved Document L, so check the Welsh guidance and your local Building Control. We deliver to both England and Wales.

Order your PIR insulation

Browse our full range of PIR insulation boards from Recticel, Kingspan, Mannok and more, with prices shown ex VAT. Not sure which thickness to order? Call us on 03330 908479 or email sales@insulationwarehousedirect.co.uk. Delivery is free on orders over £500 ex VAT to England and Wales.

Sources: Approved Document L, Volume 1 (GOV.UK), Tables 4.1–4.3 and paragraphs 4.11–4.13 and 11.3; Approved Document L, Volume 1 for Wales (GOV.WALES); Recticel Eurothane GP BBA certificate 24/7254, Table 8; Recticel Eurothane GP datasheet; Kingspan Thermawall TW50 brochure; Kingspan Thermapitch TP10 brochure. Figures are indicative and correct at the time of writing; manufacturers' data can change.