Insulation Thickness Calculator (UK): Thickness for a Target U-Value

Enter the U-value of the wall, roof or floor now, pick a target from Approved Document L, and choose the insulation. You get the thickness needed, the next 5 mm up, and the U-value that thickness gives.

UK edition. Switch to the US edition for feet, inches and US gallons.

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Your project

U-value now

An uninsulated solid brick wall is about 2.1 and an uninsulated floor about 0.7. Use a figure from your survey or EPC if you have one.

Target U-value

The values are in Approved Document L, Volume 1 (England, 2021 edition, 2023 amendments). Wales and Scotland differ.

Insulation

λ (lambda) is the conductivity printed on the product. A lower figure insulates better.

Results

Example job shown. Change the numbers to match yours.

Insulation thickness needed

127 mm

λ 0.022 W/mK

Rounded up to the next 5 mmBoards come in set thicknesses: the next size up from this: 130 mm
U-value with that thicknessAdds the insulation to the construction you have: 0.156 W/m²K
Thermal resistance now1 divided by the U-value: 0.50 m²K/W
Resistance to addTarget resistance less the resistance now: 5.75 m²K/W

The thickness is the extra resistance you need multiplied by the conductivity of the insulation, which is how the U-value adds up for layers in a row. It ignores the thermal bridging of joists, studs and fixings, and any gaps, so a real job needs a bit more than this figure, and a framed wall is better calculated by a designer. The targets are the ones in Approved Document L, Volume 1 (existing dwellings and new homes, England). Other parts of the UK have their own limits: check yours before you buy.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

How this calculator works

Worked example: a roof at U 2.0 that must reach 0.16 needs a resistance of 1 ÷ 0.16 = 6.25 m²K/W, and it has 1 ÷ 2.0 = 0.5, so 5.75 m²K/W must be added. With PIR at λ 0.022 that is 5.75 × 0.022 = 127 mm, or 130 mm to the next 5 mm, which gives U 0.156. Phenolic board at 0.018 needs 104 mm, and mineral wool at 0.044 needs 253 mm. A wall at the 0.30 target needs 62 mm of PIR. A wall already at U 0.1 needs 0 mm.

PIR board (λ 0.022) needed to reach a target U-value

Thickness in mm, before bridging, from a starting U-value of 2.0 (a solid wall) or 0.5.

Target U-value	From 2.0	From 0.5
0.3	62 mm	29 mm
0.25	77 mm	44 mm
0.18	111 mm	78 mm
0.16	127 mm	94 mm

Tips before you buy

Take the U-value of the construction you have from a survey, an EPC or a recognised table, not from a guess.
Use the λ printed on the product: it is the figure the maker declares, and it differs between brands.
Add some thickness for the bridging of studs and fixings, or ask a designer to run the full calculation.
Check the limits for your part of the UK. These are for England.
Insulating a wall from the inside can cause condensation if it is done badly: use a vapour control layer or a breathable system as the product says.

Tools & materials for this project

Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.

PIR insulation board 50 mm PIR insulation board 100 mm Insulation tape Insulation fixings Thermal imaging camera

Frequently asked questions

How thick should insulation be?

It depends on the insulation and the target. To take a U-value of 2.0 to 0.16 you need 127 mm of PIR board (λ 0.022), before thermal bridging.

How do you calculate insulation thickness from a U-value?

The resistance to add is 1 divided by the target U-value, less 1 divided by the U-value now. The thickness in metres is that resistance multiplied by the λ of the insulation.

What U-value do the Building Regulations ask for?

In England, Approved Document L gives 0.16 for the roof, 0.26 for the wall and 0.18 for the floor of a new home, with 0.15, 0.18 and 0.18 for new elements of an existing home. A roof upgraded as a retained element must reach 0.16, a wall with internal or external insulation 0.30 and a floor 0.25.

What is the λ of insulation?

The conductivity, in W/mK, printed on the product. PIR board is about 0.022, phenolic about 0.018 and mineral wool loft roll about 0.044, but check the label.

Is this accurate for a stud wall?

No. The studs, joists and fixings let heat past the insulation, so a framed wall needs a full calculation. This is for a continuous layer.

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Related calculators & guides

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.