Insulation R-value per inch: spray foam vs XPS vs rock wool vs fiberglass

Short answer: per inch of thickness, closed-cell spray foam gives about R-5.5 to 7, polyiso (PIR) board R-6.5, XPS foam board R-5, EPS R-3.8 to 4.4, rock (mineral) wool R-3.7 to 4.2, open-cell spray foam R-3.5, cellulose R-3.2 to 3.8 and fiberglass R-3.0 to 3.7. To reach R-49 in an attic you need about 9 in of closed-cell foam but 16 in of fiberglass batts.

R-value per inch, material by material

The R-value measures how well a layer resists heat flow: higher is better, and it adds up with thickness. In Europe the same property is printed as λ (lambda), the thermal conductivity, where lower is better. They convert as R per inch = 0.144 ÷ λ (US units) and R (m²K/W) = thickness in m ÷ λ.

Product figures vary, so the R-value or λ printed on your own pack always wins. The insulation calculator uses the less favourable end of each range, so it does not under-estimate the thickness.

How thick for R-13, R-19, R-30 and R-49?

Worked out with the calculator's values (thickness in inches):

US XPS boards are labelled R-5 per inch, so with those 2 in gives R-10. In metric, for R 6 m²K/W (about the UK's 270 mm loft recommendation) you need about 156 mm of closed-cell foam, 210 mm of XPS, 228 mm of rock wool or 264 mm of glass wool.

Which one should I use?

• Attic floor (loft): blown cellulose or fiberglass, or rolls of glass or rock wool. Cheap per R, and depth is rarely a problem. Do not squash it — compressed insulation loses much of its R-value.
• Stud walls (2×4 or 2×6): the cavity sets the depth. A 2×4 wall (3.5 in) holds R-13 to R-15 of batts or about R-19 to R-21 of closed-cell foam; a 2×6 wall (5.5 in) holds R-19 to R-21 of batts.
• Basement walls, slabs and below ground: XPS or closed-cell foam, because they cope with damp.
• Roofs with little depth (rafters, flat roofs): PIR or closed-cell foam give the most R per inch.
• Sound between rooms: rock wool.

Spray foam: board feet and kits

Spray foam kits are sold by board feet: one board foot is 1 ft² at 1 in thick. A 600 board-foot kit covers 600 ft² at 1 in, 300 ft² at 2 in or 200 ft² at 3 in. That is the maker's theoretical yield; cold tanks, wind and overspray lower it, so keep a generous extra. In the calculator, choose closed-cell or open-cell foam and a 600 or 200 board-foot kit: it counts the inches (passes) and the kits.

Spray foam gives off harmful vapours while it cures. Wear a proper respirator, goggles and gloves, ventilate, keep others out and follow the kit's re-entry time — and hire a trained installer for large areas. Where the rules require it, exposed foam must be covered with a fire barrier such as drywall.

R-value or U-value?

The R-value is one layer; the U-value is the whole wall, roof or floor with all its layers, and lower is better. Building rules in Europe usually set U-values. To check a full build-up — brick, insulation, plasterboard — use the U-value calculator.

Frequently asked questions

What insulation has the highest R-value per inch?

Among common products, polyiso (PIR) boards and closed-cell spray foam, at about R-5.5 to 7 per inch. XPS comes next at about R-5, then EPS and rock wool at around R-4.

Is spray foam better than fiberglass?

Per inch, closed-cell foam gives about twice the R-value and also seals air leaks, which fiberglass does not. It costs much more per R, though, so fiberglass or cellulose is usually the better buy where there is room for depth, such as an attic floor.

Is rock wool better than fiberglass?

Rock wool gives a little more R per inch (about R-4 against R-3 to 3.7), does not burn and blocks sound better. Fiberglass is lighter and cheaper. Both work well when fitted without gaps or squashing.

How thick should XPS foam board be?

For R-10 you need 2 in of US XPS (R-5 per inch); for R-15, 3 in. With European λ values (0.033–0.038), R 2 m²K/W needs about 65–75 mm.

Does R-value add up when I stack layers?

Yes. Layers add: R-19 of batts plus R-10 of foam board gives about R-29, as long as nothing is compressed and there are no gaps.