Insulation Calculator

Going from R-4 to R-6 costs half again in thickness and saves a third as much

Heat loss falls with 1/R, so the savings are front-loaded. Doubling from R-2 to R-4 cuts loss by half; going on from R-4 to R-6 cuts it by only a further third — while the material gets 50% thicker. That is the whole economics of insulation in one line, and it is why there is always a sensible stopping point.

How it works

• Turns a target R-value into the thickness needed for the material you pick.
• Uses each material's thermal conductivity, its lambda value.
• Adds a waste margin to the area so the order allows for offcuts.

thickness (m) = target R-value × lambda
heat loss is proportional to 1 / R

typical lambda: PIR 0.022 · EPS 0.038 · mineral wool 0.037-0.045
(lower lambda means less thickness for the same R)

Worked example

Reaching R-4 in two different materials.

1. mineral wool at lambda 0.040 needs 4 × 0.040 = 0.160 m, so 160 mm
2. PIR board at lambda 0.022 needs 4 × 0.022 = 0.088 m, so 88 mm
3. PIR reaches the same R-value 45% thinner
4. at R-2 the relative loss is 0.50; at R-4 it is 0.25; at R-6 it is 0.17
5. so the second doubling buys half as much benefit as the first

160 mm of wool or 88 mm of PIR for the same thermal performance. Where depth is constrained — a floor build-up, a reveal, a loft hatch — the lambda value decides what is possible.

Reading the result

• The calculation gives the thickness of insulation alone, not the finished build-up. Battens, boards, membranes and a service void all add depth, and the fixings need to be long enough for the whole sandwich.
• Lambda values vary between products, and the figure printed on the pack is the one to trust over any general table. Performance also degrades if the material is compressed, so wool squeezed into a tight joist bay does not deliver its rated R-value.
• This ignores thermal bridging, which is often what actually limits the result. A well-insulated wall interrupted by uninsulated joists or a steel lintel can perform far below its calculated value, and that is a design question rather than an arithmetic one.
• Building regulations set minimum standards that differ by country and by element, and they change. Treat a target R-value as something to confirm against current local rules rather than something this page can tell you.

Common questions

How thick does insulation need to be for R-6?

It depends on the material's lambda value, because thickness = R × lambda. For R-6: mineral wool at 0.037 needs 222 mm, EPS at 0.038 needs 228 mm, glass wool at 0.040 needs 240 mm, wood fibre at 0.045 needs 270 mm, and PIR board at 0.022 needs only 132 mm. Use the lambda printed on the product you are buying, since it varies between brands.

How many rolls of insulation do I need for an 8 × 6 m loft?

7 packs if each covers 8 m². The floor area is 48 m², which becomes 52.8 m² with a 10% waste allowance, and 52.8 ÷ 8 = 6.6, rounded up. The coverage printed on a pack applies to that product's thickness, so if you build the depth up in two layers, count the packs for each layer separately.

Is a metric R-value the same as a US R-value?

No. This calculator uses metric R-values in m²·K/W, while US products quote R in ft²·°F·h/Btu, which is about 5.68 times larger for the same material. US R-30 is therefore roughly R-5.3 metric, and metric R-6 is about US R-34. Entering a US figure here would ask for a layer more than five times too thick.