How Thick Should Insulation Be? Thickness for a Target U-Value
The thickness of insulation for a target U-value comes from the resistance you have to add. To go from U 2.0 to 0.16 you must add 5.75 m²K/W, which is 127 mm of PIR board with a λ of 0.022. The insulation thickness calculator does it for your case.
Thickness for each target
In mm, before thermal bridging, from a U-value of 2.0 (a solid wall) or 0.5, with PIR (λ 0.022), and from 2.0 with mineral wool (λ 0.044):
| Target U-value | PIR from 2.0 | PIR from 0.5 | Mineral wool from 2.0 |
|---|---|---|---|
| 0.3 | 62 mm | 29 mm | 125 mm |
| 0.25 | 77 mm | 44 mm | 154 mm |
| 0.18 | 111 mm | 78 mm | 222 mm |
| 0.16 | 127 mm | 94 mm | 253 mm |
| 0.15 | 136 mm | 103 mm | 271 mm |
Thickness for each insulation, from U 2.0 to 0.16
| Insulation | λ (W/mK) | Thickness |
|---|---|---|
| Phenolic board | 0.018 | 104 mm |
| PIR board | 0.022 | 127 mm |
| Mineral wool loft roll | 0.044 | 253 mm |
The λ is the figure on the product, and brands differ, so check it. A better insulation (a lower λ) needs less thickness for the same result.
The targets in Approved Document L (England)
- New home: roof 0.16, wall 0.26, floor 0.18.
- New element on an existing home: roof 0.15, wall 0.18, floor 0.18.
- Existing element that is upgraded: roof 0.16, wall with internal or external insulation 0.30, cavity wall 0.55, floor 0.25.
Things to remember
- The calculation is for a continuous layer. Studs and joists add thermal bridging, so allow more thickness.
- Use the U-value of the wall you have from a survey or an EPC, since a guess makes the answer wrong.
- Wales and Scotland have different targets from England.
- Insulating inside a solid wall can cause damp if it is done badly: follow the system maker.
See also the loft insulation calculator and the heat pump calculator. These are estimates: confirm the details with a designer or building control.
Frequently asked questions
How thick should insulation be?
It depends on the starting U-value, the target and the insulation: 127 mm of PIR board (λ 0.022) takes a U-value of 2.0 to 0.16, before thermal bridging.
How do you work out the thickness from the U-value?
Take 1 divided by the target U-value, less 1 divided by the U-value now. That is the resistance to add. Multiply it by the λ of the insulation, in W/mK, to get the thickness in metres.
What are the U-value targets 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. For a retained roof that is upgraded it is 0.16, for a wall with internal or external insulation 0.30 and for a floor 0.25.
Does this work for Scotland and Wales?
No: the targets are for England. Scotland and Wales have their own rules, so check them.
Why is the real thickness more?
Studs, joists and fixings carry heat past the insulation, and gaps let air through. A real job allows for this, so add some thickness or have it calculated.
More guides
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.