PIR U-Value Calculator
Work out the U-value of a floor, wall or roof insulated with PIR boards, check it against Part L, and see the thinnest board we stock that meets the target. Pick the construction, adjust the details if you know them (the defaults are typical UK builds), then enter the area to price the boards.
Prices are live and ex VAT. Delivery to England & Wales in 2–3 working days, free over £500 ex VAT.
Estimated U-value
Boards for your job
U-value for every thickness we stock
How we worked it out
BS EN ISO 6946 combined method (average of upper and lower limits) for timber-bridged layers, BS EN ISO 13370 for ground floors (slab ignored, BR 443 defaults for ventilated voids), BR 443 conventions and typical material values. Foil facings next to an unventilated air gap: emissivity 0.05 (BBA 24/7254). Wall ties included for cavity walls (2.5/m² stainless steel); other fixings and air-gap corrections are ignored, as in the manufacturer tables we checked against.
What is a U-value?
A U-value measures how much heat passes through one square metre of a floor, wall or roof for every degree of temperature difference between inside and out. It is given in W/m²K, and lower is better. The U-value depends on every layer in the construction: the insulation does most of the work, but timber joists, studs and rafters let heat bypass it (thermal bridging), and air gaps next to a foil facing add a little extra resistance.
Part L U-value targets (England)
Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments) sets these limits for work to existing homes:
| Element | New element or extension (Table 4.2) | Renovating an existing element (Table 4.3) |
|---|---|---|
| Roof | 0.15 | 0.16 |
| Wall | 0.18 | 0.30 (internal or external insulation), 0.55 (cavity fill) |
| Floor | 0.18 | 0.25 |
Values are in W/m²K. New homes are assessed differently (on the whole-dwelling energy calculation), and Wales has its own Part L. The 2026 edition of Approved Document L, which takes effect in March 2027, keeps the same values for work to existing homes.
How thickness affects the U-value
The thermal resistance of a PIR board is its thickness in metres divided by its conductivity (λ). A 100mm board at λ 0.022 W/mK gives R = 0.10 ÷ 0.022 = 4.55 m²K/W. Doubling the thickness doubles the board's resistance, but the U-value improves less each time because the rest of the construction and the timber bridging stay the same. That's why going from 50mm to 100mm makes a big difference, while going from 150mm to 200mm makes much less.
Where the insulation sits between timbers, the timber carries heat past the PIR. A continuous layer across the timbers, such as insulated plasterboard under rafters or over studs, is often the thinnest way to hit a low target. Ground floors are different again: heat escapes mostly around the edges, so a small floor with a high perimeter-to-area ratio needs more insulation than a large one.
Getting it right for Building Control
Use this calculator to choose a thickness and price the job, then ask the board manufacturer for a free U-value calculation for your actual construction. Recticel, Kingspan and Mannok all offer this, and it usually includes a condensation risk check. If you'd like a hand choosing boards, call us on 03330 908479. Prices shown are ex VAT, and we deliver to England and Wales in 2–3 working days, free on orders over £500 ex VAT.
More help: What thickness of PIR insulation do I need? · Shop PIR insulation boards
PIR U-value questions
What thickness of PIR do I need for a U-value of 0.18?
It depends on the construction. As a guide with 0.022 PIR: a solid ground floor with a P/A of 0.5 needs about 90mm; a brick and dense block cavity wall about 100mm of partial-fill board; a suspended timber floor about 130mm between the joists. Use the calculator above for your build.
What U-value do I need for an extension?
In England, new walls and floors in an extension should reach 0.18 W/m²K and roofs 0.15 W/m²K (Approved Document L, Volume 1, Table 4.2). Your Building Control body may accept a trade-off across the whole extension, so check with them.
Is this calculation accepted by Building Control?
No. It is an estimate to help you choose a thickness. Building Control usually wants a U-value calculation for your actual construction, which the board manufacturers provide free of charge.
Why does a floor’s U-value depend on its size?
Heat escapes from a ground floor mostly around its edges, so a small or narrow floor (high P/A) loses more heat per square metre than a large one and needs thicker insulation to reach the same U-value.