Choosing an insulation material comes down to three questions, in this order: where is it going, how much room is there, and what does the maker’s own sheet say the product is rated at in that thickness. Where depth is free, as on most attic floors, you add material until you reach the target; where space is fixed, as in a wall or rim joist, the R-value delivered in that thickness decides. Every figure in the guides below is tied to a maker’s sheet, a federal rule or an agency page and the date we read it; product names appear only as the source of a number, never as a pick, and we don’t price jobs.
General information, not professional advice. Building codes are adopted locally, with changes, and your building department decides what applies to your house. Stop and call a qualified professional for anything this page marks as a stop.
Start with the form, then the place
Home insulation comes in a handful of forms: batts and rolls, loose fill that is blown in, dense-pack for closed walls, spray foam, rigid foam board, and radiant barriers, which work differently from the rest. Our plain guide to insulation types goes through each: what it is made of, where it goes in an existing house, who usually installs it, and what one maker’s sheet rates it at. Its last table starts from the place and gives the types the sources name for it. Read that page first.
What an R-value is, and why “per inch” misleads
R-value measures how well insulation resists heat flow: the higher the number, the more it resists. On a package it is a regulated claim: the Federal Trade Commission’s R-value Rule (16 CFR Part 460) sets how it is tested, at a mean temperature of 75°F, and what the label must show.
Our R-value chart by material lists what each maker’s sheet prints: an R-value at a stated thickness. On the sheets we read on 10 November 2026, closed-cell spray foam is R-7.0 at 1 inch, foil-faced polyiso board R-6.0, XPS board R-5.0 and EPS board R-3.60 to R-4.50 depending on its type.
A single “R per inch” number is a shortcut the Rule distrusts. Makers, sellers and installers may not advertise one unless tests show it doesn’t drop as the product gets thicker (16 CFR 460.20). The sheets show why: one fiberglass batt sheet lists R-11, R-13 and R-15 all at 3.5 inches, and one closed-cell foam sheet prints R-7.0 at 1 inch but R-24 at 3.5 inches.
How thick is R-30, R-38 or R-49?
There is no single thickness for an R-value. On the makers’ sheets we read, R-30 runs from 7¼ inches as a stone wool batt to 10.8 inches in one blown fiberglass product. The R-value to thickness calculator takes a target and a material and gives an estimate from a named sheet, as installed and after settling. It reads the sheet’s rows and never multiplies a per-inch figure.
No target yet? The how much insulation do I need tool gives ENERGY STAR’s recommended level for your climate zone and the part of the house. For an uninsulated attic in an existing home, its table lists R30 in zone 1, R49 in zones 2 and 3 and R60 in zones 4 to 8 (ENERGY STAR, live page based on the 2021 IECC, read 10 November 2026). That is a recommendation, not a code.
Fibers: cellulose, fiberglass and mineral wool
Cellulose vs fiberglass compares the two common loose fills from the makers’ own bag charts. On the charts we read, loose-fill cellulose reaches R-49 at 14.3 inches blown and 13.3 inches settled, with 72.6 bags per 1,000 square feet. Loose-fill fiberglass needs 16.5 to 17 inches and 21.5 to 25 bags. The page also covers weight on the ceiling, settling, the federal fire rules for cellulose, very cold attics, and dust. Every loose-fill bag must carry a coverage chart (16 CFR 460.12(b)(2)), and both its bag count and its thickness must be met; the blown-in bag calculator works from those charts.
For batts, see mineral wool vs fiberglass. In a wall the sheets show a tie: both top out at R-15 in a 3.5-inch cavity and R-23 in a 5.5-inch one. Mineral wool is the denser batt, so R-30 takes 7.25 inches against 9.5 inches of standard fiberglass. Unfaced, both are listed as noncombustible under the same test (ASTM E136), and neither gets an exemption at recessed lights or flues.
Then there is the paper. Kraft-faced vs unfaced insulation explains that the brown facing adds no R-value: its job is to slow water vapor, and the 2024 International Residential Code, a model code, names kraft-faced fiberglass batts as an example of a Class II vapor retarder. The paper faces the side that is warm in winter, it burns and is never left in view, and a layer added over existing insulation should be unfaced, as ENERGY STAR’s guide says. Whether a wall wants a retarder at all depends on your climate zone, which our page on vapor barriers and vapor retarders covers.
Foam: boards and spray
Foam board insulation compares the three rigid boards: EPS, the white beaded one; XPS, the smooth extruded one; and foil-faced polyiso. On the sheets we read, foil-faced polyiso lets almost no water vapor through (0.05 perms), XPS is 1.5 perms at most at 1 inch, and EPS is the most open. The page covers basement walls from the inside, rim joists and the attic hatch.
Spray foam insulation insulates and stops air in one step. That suits a few awkward places: gaps and holes (foam from a can, which ENERGY STAR’s guide uses for gaps from a quarter inch to about 3 inches), rim joists, and roofs built without vents, which need a design. It causes trouble over old wiring, against a flue, left bare where the code wants a covering, or when people move back in too soon. Two-part foam over any large area is a job for a trained installer, and Consumer Product Safety Commission staff advise occupants to stay out during installation and for at least 24 hours after. That is staff guidance, not a Commission rule; ask the installer for the maker’s re-entry time.
If foam is the plan, closed-cell vs open-cell spray foam sets the two kinds side by side from four data sheets. Closed-cell is dense and rigid, about R-7 at 1 inch, and a Class II vapor retarder at 1 to 1.5 inches on those sheets. Open-cell is light and spongy, about R-3.8 at 1 inch, open to water vapor, and by DOE’s wording not for use below ground.
One rule covers every foam here. Foam plastic burns, and the 2024 IRC requires it to be separated from the inside of the building by a thermal barrier such as half-inch gypsum wallboard (section R303.4; R316 in earlier editions), with separate rules for some attics, crawl spaces and rim joists. Ask your building department which edition it enforces before buying foam that will face a room.
Before any material goes in
Whatever you choose, these come first:
- Look before you touch. Loose, pebble-like granules on the attic floor may be vermiculite. EPA says to assume vermiculite insulation contains asbestos and not to disturb it, not to store things in that attic or let children play there, not to try to remove it yourself, and to hire a professional asbestos contractor before any work that would disturb it (EPA, read 10 November 2026). Pushing a ruler into it or laying insulation over it would disturb it. Read what EPA says to do about vermiculite first.
- Old wiring is a stop. No insulation of any kind goes over or around knob-and-tube wiring. A licensed electrician comes first.
- Keep clear of heat. ENERGY STAR says to keep all insulation at least 3 inches from recessed “can” lights unless the fixture is rated IC, and never to lay insulation over them; see recessed lights and insulation. At flues and chimneys, the vent’s own listing governs.
- Fix what is wrong first. ENERGY STAR’s stop list includes wet or damp insulation, moldy or rotted rafters or joists, kitchen, bathroom or dryer vents that blow into the attic, a history of ice dams, little or no attic ventilation, and many unsealed recessed lights.
- Seal, then insulate. Loose fill and batts are not an air barrier, and EPA recommends sealing the attic before adding insulation: start with air sealing the attic. After a house is tightened, ENERGY STAR says to have a heating and cooling technician check that gas and oil appliances (furnace, water heater, dryer) are venting properly. Keep working carbon monoxide alarms.
Your next step
Pick the place, measure the room you have and find your target for your climate zone. Then read the label or fact sheet for the exact product: the R-value at the thickness you will install and, for loose fill, the bags per 1,000 square feet and the minimum thickness. If you hire the work out, the R-value Rule requires a dated, signed contract or receipt showing the coverage area, thickness and R-value installed, and for loose fill the installed thickness, the settled thickness and the number of bags used (16 CFR 460.17).
Sources checked on 10 November 2026. Makers revise their sheets; the label on the product you buy governs.
