Insulation R-Value Calculator

What is already there decides what the next 100mm is worth.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Insulation

Thickness

R-value already there

m²K/W

Target U-value

W/m²K

Total R-value

5.045 m²K/W

U = 0.1982 W/m²K · misses the 0.18 target

Total R-value5.0455 m²K/W

Resulting U-value0.1982 W/m²K

This insulation layerR = 4.5455

Existing constructionR = 0.5

Meets the targetNo

Thickness needed for the target111 mm

Heat loss reduction90.1%

MaterialPIR board

R-value is thickness divided by conductivity, and R-values add. That is why the existing construction matters so much: adding 100 mm to a wall that already has R 2.0 improves it far less than the same 100 mm on a bare one, because the marginal gain shrinks as the total grows. PIR and phenolic board give roughly twice the resistance of mineral wool per millimetre, which is what you are paying for. Where depth is constrained — a rafter, a floor build-up, an internal wall lining — that difference decides whether the target is reachable at all. Real construction also loses performance to thermal bridging through studs, joists and fixings, and to any gaps in the installation. A perfectly fitted 100 mm is worth considerably more than a hastily fitted 120 mm, which is not a figure any calculator can show you.

How the Insulation R-Value Calculator works

R-value and resulting U-value for an insulation layer over an existing construction, with the thickness needed to hit a target. What is already there matters more than most people expect.

Also known as: how thick should insulation be · r value of 100mm pir · insulation to reach 0.18 u value · pir versus mineral wool thickness

Where to go next

The Insulation R-Value question rarely arrives on its own. These are the ones that usually come with it:

Heat Transfer Calculator — R-value and U-value are the same physics, inverted.
Block Wall Calculator — With the three tonnes of blockwork nobody weighs.
Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Acceleration Calculator — Acceleration in m/s² and in g, which is the legible one.

Sources

ASTM International standards

Frequently asked questions

How do I calculate R-value?

Thickness in metres divided by thermal conductivity. 100 mm of PIR at 0.022 W/m·K gives R 4.55; the same thickness of mineral wool at 0.038 gives 2.63.

Why does the existing construction matter?

Because R-values add and the marginal gain shrinks as the total grows. Adding 100 mm to a wall that already has R 2.0 improves it far less than the same 100 mm on a bare one.

Is PIR worth the extra cost over mineral wool?

Where depth is constrained, yes. It gives roughly twice the resistance per millimetre. Where there is space, mineral wool is cheaper per unit of R, better acoustically and far more forgiving to fit.

What U-value should I aim for?

New-build standards typically require walls below 0.18 W/m²K and roofs below 0.13. Retrofit targets are usually looser, and the practical limit is whatever depth the construction allows.

Does a gap in the insulation matter?

Enormously. A 5% gap in a layer can cost 30% of its performance, because heat takes the easy path. A carefully fitted 100 mm is worth more than a hastily fitted 120 mm.

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The one-line version

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