Insulation Calculator
Insulation thickness for a target R-value or RSI, and packs needed.
About this tool
Choose US or metric units, a material and whether you know the target or the thickness. The tool gives the thickness or R-value, R per inch, the layer's U-value and, from the area and the coverage on the pack, how many packs you need.
R-values, RSI and thickness
Insulation slows heat flow, and its performance is its thermal resistance. In the US that's the R-value (ft²·°F·h/BTU); elsewhere it's RSI in m²·K/W (1 US R = 0.176 RSI), or the U-value, which is 1 ÷ RSI. Resistance is thickness divided by the material's thermal conductivity λ, so better materials need less depth. Fibreglass batts give roughly R-3.3 per inch, mineral wool about R-3.9, rigid PIR or polyiso board about R-6.5, and closed-cell spray foam about R-6. Reaching the R-49 often recommended for attics in cold US climates takes about 15 inches of fibreglass but only 7.5 inches of polyiso. In the UK, the recommended 270 mm of mineral-wool loft insulation gives RSI 7.3 (about R-41). Add your area and the coverage printed on the product to estimate how many rolls or bags to buy. Honest limits: λ values here are typical, so use the figure on your product's label. Compressed batts, gaps and heat bridging through joists and studs reduce real-world performance, and required values depend on climate zone and local building codes. Not building advice. The BTU calculator and the attic ventilation calculator are related.
Frequently asked questions
How thick should loft insulation be?
The UK recommends 270 mm of mineral wool; many US cold-climate attics aim for R-49 to R-60, about 15 to 19 inches of fibreglass.
What is the difference between R-value and U-value?
R-value measures resistance to heat flow (higher is better); U-value is heat flow per area and degree (lower is better). U = 1 ÷ RSI.
How do I convert R-value to RSI?
Multiply the US R-value by 0.1761. R-30 is RSI 5.3.
Which insulation has the highest R per inch?
Rigid foam boards like PIR, polyiso and phenolic, and closed-cell spray foam, at around R-6 to R-8 per inch.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.