What insulation has the highest R-value per inch?

Closed-cell spray foam at roughly R-6 to R-7 per inch, then rigid polyiso boards, then open-cell foam, mineral wool, and cellulose at around R-3.5, with fiberglass batts close behind. Per-dollar, the loose fills usually win.

Is spray foam worth the extra cost?

Where its air-sealing and per-inch density solve a real problem, rim joists, cathedral roofs, tight cavities, yes. As a whole-attic default, dense loose fill over a sealed ceiling delivers most of the result for far less.

Does cellulose insulation cause problems?

Modern borate-treated cellulose is fire-resistant, pest-resistant, and performs well; its honest caveats are settling allowance and, like all fibrous insulation, uselessness when soaked. Wet-history spaces want the moisture fixed first.

Why does insulation sometimes not work?

Installation: gaps, compression, and air leaking through or around it. Fibrous insulation filters moving air rather than stopping it, which is why air sealing comes first and why a gap of a few percent costs far more than a few percent.

Insulation Types Compared: Fiberglass, Cellulose, Foam, and Mineral Wool

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Insulation is sold by material and performs by installation: the four residential families, fiberglass, cellulose, foam, and mineral wool, differ in R per inch, air behavior, moisture behavior, and price, but every one of them is halved by gaps, compression, and the moving air it was never designed to stop. Insulation types compared honestly is therefore two comparisons: the materials against each other, and every material against its own installation. This reference covers both.

The scorecard, family by family

Fiberglass (batts and blown loose fill): the volume standard, R-3.2 to 3.8 per inch in batts, a bit less blown, cheap, ubiquitous, and fire-inert. Its weaknesses are behavioral: batts invite the cut-and-cram installation sins below, it filters rather than blocks moving air (wind-washed fiberglass at open eaves loses real R), and it does nothing for air sealing. Cellulose (blown, dense-packed): recycled paper with borate treatment, R-3.2 to 3.8 per inch, excellent cavity-filling conformity, better air-slowing density than fiberglass, strong per-dollar value, and the retrofit wall champion via dense-pack; caveats are settling allowance in open blows and, like all fibrous fills, total loss of function when soaked, with wet cellulose also slow to dry, which routes every moisture-history attic through its water fix first. Mineral wool (batts and boards): R-3.0 to 3.7 per inch, genuinely fire-proof, water-shedding rather than absorbing, dimensionally stiff (friction-fits without sag), sound-deadening, at a price step above fiberglass, the premium batt for rim joists’ fire-exposed cousins, party walls, and anywhere fire or sound votes. Foam, the family that also air-seals: closed-cell spray foam, R-6 to 7 per inch, its own air barrier and vapor retarder, structural stiffening, the premium answer for rim joists, cathedral ceilings, and cavities too thin for fiber’s math; open-cell spray, R-3.5 to 4, air-sealing but vapor-open, cheaper, with its own roof-assembly rules; and rigid boards (EPS, XPS, polyiso, R-4 to 6.5 per inch), the sheathing-and-basement-wall workhorses, exterior foam rewriting wall vapor rules helpfully by warming the sheathing. Foam’s honest ledger includes cost multiples over fiber, installation-quality dependence (bad spray jobs are expensive twice), and combustibility rules requiring thermal barriers in living spaces.

The comparison that outranks materials: installation

Fibrous insulation works by trapping still air, and everything that moves or removes that air removes the R. The sins, in field order: gaps, missed bays, un-fluffed corners, the batt short of the top plate, with the brutal math that small gap percentages cost disproportionate whole-wall performance; compression, the R-19 batt crushed to half thickness behind a shelf delivering a fraction of its label; cut-and-cram around wires and pipes instead of split-and-fit; wind washing at eaves without baffles; and, above all, air leakage through and past it, the reason air sealing precedes insulation in every competent scope, fiber over a leaky ceiling being a filter on a chimney. The corollary buys better results than upgrades do: a carefully installed mid-grade material beats a sloppy premium one, which is why installation grading, not material worship, is where inspection attention belongs.

Where each belongs

The working map: open attics, blown cellulose or fiberglass over a sealed ceiling to the regional depth, the value play of the whole field; walls, dense-pack cellulose in retrofits, batts or damp-spray in open framing, exterior rigid foam at residing time; rim joists, closed-cell foam or the cut-foam-board-and-seal method, per that reference; basement walls, rigid foam against the concrete (never bare fiberglass against masonry, the moldy-batt finding of a thousand basements) per the moisture rules; crawl spaces, wall foam inside the encapsulation assembly; cathedral roofs and tight cavities, foam’s per-inch density earning its price; and sound and fire walls, mineral wool’s home field. Old-house asbestos-suspect insulation, vermiculite above all, is the stop-and-test exception to every DIY paragraph here.

The bottom line

Fiber is cheap R that must be installed like it matters, foam is expensive R that also seals, mineral wool buys fire and water manners, and cellulose is the retrofit value champion. Seal first, fill completely, compress nothing, baffle the eaves, and grade the installer harder than the material, because in this category the workmanship is the product.