Fiberglass vs foam vs mineral wool: R-value per inch and what to order

Which insulation gives the most R-value per inch?

Rigid polyiso and high-density spray polyurethane foam, at about R-6 per inch in the DOE Building America table. XPS follows at R-5, EPS at R-3.8 to 4.4, mineral wool board at R-3 to 4, and fiberglass at R-2.5 to 4 (U.S. DOE Building America Solution Center (PNNL) and National Institute of Standards and Technology). Those are typical figures for each material type. The R-value printed on the product you buy is the one that counts for code.

R-value per inch matters most where depth is limited: a 2×4 wall, a cathedral ceiling, or continuous insulation over sheathing. In an open attic, where depth is free, it matters less, and the deciding questions are how the product is installed and how many bags or packs you need. This page compares what each type asks you to calculate and check. It does not compare price, and we hold no sourced fire or moisture ratings, so it leaves those out.

How do the three compare at a glance?

Fiberglass and foam are each several products. The rows below split them where the counting method differs.

How is each type counted?

Fiberglass batts and rolls

Match the batt to the framing, then divide the area by the package coverage. Batts for 16 in o.c. framing are 15 in (381 mm) wide and for 24 in o.c. framing 23 in (584 mm) wide; 16 and 24 in widths also exist. A 15 x 93 in batt covers 9.7 sq ft (0.90 m2). Package coverage varies by R-value because thicker batts pack fewer pieces, as the Owens Corning table shows.

Blown fiberglass

Count by the bag chart, and meet both columns. A stated blown-in R-value needs both the minimum bags per 1,000 sq ft and the minimum thickness. AttiCat needs at least 25 bags per 1,000 sq ft (93 m2) at 17 in (432 mm) for R-49. Code compliance uses the settled R-value (International Code Council). Cellulose, the main blown alternative, needs far more bags for the same R: GreenFiber INS515LD needs at least 56 bags per 1,000 sq ft for R-49 (14.82 in initial, 13.34 in settled).

Foam and mineral wool boards

Work out the thickness first: required R ÷ R per inch, rounded up to a stocked thickness. Then divide the area by the board's face area. We hold no board sizes, so read them from the product sheet. Spray foam is quoted and installed by a contractor, who works out the thickness for your target.

Illustration sources

• Owens Corning — EcoTouch PINK FIBERGLAS Insulation Manufacturers Fact Sheet (Pub. 10017881-A)
• Owens Corning — AttiCat Expanding Blown-in PINK FIBERGLAS Insulation Application Instructions (Pub. 10009393-E)
• GreenFiber — GreenFiber INS515LD Loose Fill Formula attic card (PM-6.3-324 Rev E)
• International Code Council — 2021 IECC Residential, Chapter 4 (Tables R402.1.2, R402.1.3; Sections R402.1.4, R402.2.1)

Worked example: R-13 in a 2×4 wall, or R-5 over the sheathing

A 2×4 stud bay is 3-1/2 in deep. R-13 fiberglass batts are 3.5 in (89 mm) thick, sized for 2x4 walls. So fiberglass fills the cavity to R-13 exactly. To reach the same R-13 in that depth with board, you would need at least 3.7 R per inch, which rules out the low end of fiberglass board and mineral wool board.

Continuous insulation is where foam's density pays. The IECC wall options in colder zones include R-20 cavity plus R-5 continuous (International Code Council). R-5 continuous is 1 in of XPS at R-5 per inch, about 0.8 in of polyiso (so a 1 in board), or 1.1 to 1.3 in of EPS. Mineral wool board at R-3 to 4 would need 1.25 to 1.7 in. The thicker board pushes out window and door trim, which is a detail to plan before ordering.

For the cavity, the insulation calculator counts batts or bags for your wall or attic area. Over insulation, the wall still needs its board: our plasterboard sheet count tool handles that step.

Illustration sources

• Owens Corning — EcoTouch PINK FIBERGLAS Insulation Manufacturers Fact Sheet (Pub. 10017881-A)
• International Code Council — 2021 IECC Residential, Chapter 4 (Tables R402.1.2, R402.1.3; Sections R402.1.4, R402.2.1)
• US Department of Energy — Continuous Rigid Insulation on Exterior Walls

What R-value does the code ask for?

In the US, the IECC sets minimum R-values by climate zone. IECC 2021 requires R-60 (RSI 10.6) ceiling insulation in climate zones 4-8. Code R-values count insulation only, not drywall, sheathing or air films; cavity and continuous insulation are checked separately. Any of the three materials can meet a ceiling target in an attic; walls are where depth limits the choice. The guide to insulation targets by zone covers each zone in detail.

In England, Approved Document L sets U-values instead. New-dwelling walls in England must not exceed U = 0.26 W/m2K. A U-value covers the whole wall, every layer and the surface films included, so it cannot be turned into an insulation R-value by taking 1 ÷ U. Multiply an imperial R-value by 0.1761 to get RSI (m2·K/W); multiply RSI by 5.678 for imperial R. The metric and imperial conversion reference has the other factors.

How common is each type, and how well insulated are homes?

Mineral wool and glass fibre dominate lofts in England. 94% of English dwellings with loft insulation (just under 18 million) used mineral wool or glass fibre in 2024, against 4% rigid foam board and 1% spray foam (EHS home insulation fact sheet). Coverage still has gaps. In 2024, 42% of dwellings in England had 200mm or more of loft insulation, up from 39% in both 2014 and 2022 (English Housing Survey 2024-25, MHCLG). Over 2.7 million households in England lived in homes with less than 100mm of loft insulation in 2024-25; 1.2 million of them (43%) also had uninsulated solid walls (EHS home insulation fact sheet, May 2026). 54% of dwellings in England had cavity or solid wall insulation in 2024, up from 48% in 2014; among cavity-wall homes, 73% of owner-occupied but only 60% of private rented had it insulated (English Housing Survey 2024-25).

In the US, About 20% of US primary homes (26.8 million of 132.5 million) were reported as poorly insulated (23.3 million) or not insulated (3.4 million) in EIA's 2024 Residential Energy Consumption Survey (preliminary). Draughts are common too. About 14% of US homes (18.2 million) were reported too drafty in winter most or all of the time, and another 54.8 million some of the time, in EIA's 2024 RECS (preliminary). About 1.53 million US homeowners reported insulation projects in 2023, totalling about $3.5 billion (Harvard JCHS Table A-1, 2023 AHS).

Heating is why it matters. Space heating used 4,026 of 9,481 trillion Btu of US household site energy in 2020 (about 42%), and air conditioning 866 trillion Btu (about 9%), per EIA's 2020 RECS, the latest end-use edition (2024 RECS consumption data due 2027). Space heating took 61.5% of EU household final energy consumption in 2024 (space plus water heating 77.1%), and households used 26.0% of all EU final energy (Eurostat). ENERGY STAR estimates the return: ENERGY STAR estimates homeowners save an average of 15% on heating and cooling costs (11% of total energy costs) by air sealing and adding insulation to attics, crawlspace floors and basement rim joists. That is a modelled national average, not a promise for one house. More figures are on the home insulation statistics page.

How have insulation prices moved?

Faster than most building materials. US producer prices for insulation materials were 50.3% above the 2020 average in September 2026 and 2.7% higher than a year earlier (BLS PPI). Mineral wool on its own tracks the same path: the BLS series for mineral wool envelope insulation was 52.0% above January 2020 in September 2026 (US Bureau of Labor Statistics). In the UK, UK producer prices for thermal and acoustic insulating materials stood at 206.7 in August 2026, roughly double 2015 levels, and 4.3% higher than a year earlier (DBT Table 2, ONS data). These are producer prices, not shelf prices, so use them to judge how quickly a quote dates rather than to estimate cost.

When does this comparison not apply?

• Your product states a different R. The DOE ranges are typical; the label value is the one that counts.
• Blown insulation filled to depth but short on bags. The R-value will be lower than the chart claims.
• UK projects. Compliance is by U-value for the whole element, so a manufacturer's U-value calculation, not an R-per-inch sum, decides the thickness. We hold no UK roll sizes.
• Fire, moisture and vapour control. We hold no sourced ratings for these, and they can rule a material in or out regardless of R-value.
• Open-cell spray foam and cellulose R per inch. We found no government figure, so they are not in the table.

Which insulation suits your project?

• Open attic floor: blown fiberglass or batts. Depth is available, so R per inch matters little; meet both the bag count and the depth.
• 2×4 or 2×6 wall cavity: fiberglass or mineral wool batts sized to the stud depth, unless the target needs more R than the cavity allows.
• Continuous insulation over sheathing, or a thin roof build-up: foam board, because it reaches a given R in the least thickness.

All three can meet the IECC ceiling targets. The space, the code route and the installer decide the rest. The insulation and R-value tools turn your target into a quantity.

Questions about choosing insulation