What is the best insulation for an attic?

For a typical open, accessible attic floor, blown-in fibreglass or blown-in cellulose. Both are loose-fill materials blown to an even depth over the whole floor, both fill the gaps and odd framing that defeat batts, and both sit in the lowest cost tier per unit of R-value. The choice between them is close: cellulose carries about R-3.2 to R-3.8 per inch against fibreglass at about R-2.2 to R-2.7, so cellulose reaches the same target at a shallower depth, while fibreglass is lighter and does not hold water the way a wet cellulose layer can. Spray foam, mineral wool and batts each win in specific situations, not in the general case.

Is cellulose better than fibreglass for an attic?

Neither is better across the board. Cellulose gives more R per inch, so it fits a higher target under low rooflines, and its density blocks air movement through the layer somewhat better. It settles more, so installed depth has to allow for it, and it absorbs water if the roof leaks. Blown fibreglass settles less, weighs less on the ceiling, and dries out better after an incidental leak, but needs more depth for the same R-value. On an open attic with generous clearance, either is a sound choice, and the installer you can actually get is a bigger factor than the material.

Is spray foam worth it in an attic?

Only when the job is the roofline rather than the floor. Foaming the underside of the roof deck creates a conditioned attic, which is the right move when ducts live up there or the attic is finished space. On an ordinary vented attic floor, closed-cell foam at roughly R-6 to R-7 per inch buys the same warmth as loose-fill at several times the installed cost, and open-cell buys it at a smaller premium with no depth problem to solve. Foam is also contractor-only work: it is a two-part chemical reaction with cure times, ventilation requirements and no honest do-it-yourself route at attic scale.

Can I mix insulation types in the same attic?

Yes, and topping up is the normal case. New loose-fill goes straight over old loose-fill or over old batts, and unfaced batts can be laid across the joists at right angles over existing material. The one rule is the vapour facing: never put a kraft or foil facing anywhere except directly against the ceiling below, because a facing buried between layers can trap moisture where it condenses. Use unfaced material for any layer that is not the first. A radiant barrier is the exception to mixing: it is not bulk insulation and does not add to the R-value of anything beneath it.

Does a radiant barrier replace attic insulation?

No. A radiant barrier reflects radiant heat rather than resisting conduction, so it has no useful R-value per inch and cannot be counted toward an R-value target. Bulk insulation and radiant barriers are measured differently and the two do not add together. In a hot, sunny climate a barrier under the roof deck can cut summer heat gain into the attic, which helps ducts and cooling load, but the house still needs its full recommended depth of bulk insulation on the floor. In a cold climate the barrier does little, and money spent on it is better spent on depth or air sealing.

How many inches of insulation do I need to reach R-49?

It depends on the material. From the ranges this site carries, R-49 takes about 12.9 to 15.3 inches of blown cellulose, about 18.1 to 22.3 inches of blown fibreglass, and about 7 to 8.2 inches of closed-cell spray foam. Treat the loose-fill figures as estimates rather than measurements: DOE notes that settled density rises as installed thickness rises, so loose-fill R-value does not scale strictly in proportion to depth. The coverage chart on the bag of the specific product you buy is the authority, and installers work to that chart, not to a generic R per inch.

Best attic insulation: every material compared

By Insulation Report Editorial. Last reviewed August 2026. How we research this.

For a typical open, accessible attic, the best insulation is blown-in fibreglass or blown-in cellulose, and the honest difference between the two is small. Both are loose-fill, both are blown to depth over the whole floor, both fill irregular framing far better than batts, and both cost the least per unit of R-value of anything on this page. Fibreglass tolerates damp slightly better; cellulose gives more R per inch, so it reaches the same target at a shallower depth. That default is wrong in four situations. A cathedral ceiling or a conditioned attic needs spray foam or dense batt work at the roofline. A rodent history needs cleanup before anything is added. A chronically damp attic needs the moisture fixed first, not insulation chosen around it. And a radiant barrier is not an alternative to any of these, because it has no useful R-value. If the existing material is dry and clean, topping it up beats replacing it, whatever it is.

The comparison, in one table

Every figure below is a range, because R per inch genuinely varies with density, product and installed condition, and a single number would be false precision. The depth column shows the inches needed to reach R-49, which is the ENERGY STAR recommendation for an attic in climate zones 4 to 8 that already has 3 to 4 inches of material (a bare attic in those zones is recommended R-60), and it is also the level the 2021 IECC model code accepts in place of its R-60 ceiling figure where the insulation carries at full height over the wall top plate, which normally means a raised-heel truss. What your state enforces is a separate question and varies widely; codes govern new and permitted work, not a voluntary top-up. For choosing your own target by climate zone, see the R-value guide.

Material	Form	R per inch	Inches to R-49	Notes
Blown-in fibreglass	Loose-fill (blown)	2.2 to 2.7	18.1 to 22.3	Settles. Coverage per bag is product-specific; read the bag label.
Blown-in cellulose	Loose-fill (blown)	3.2 to 3.8	12.9 to 15.3	Settles more than fibreglass. Installed depth should allow for it.
Loose-fill mineral wool	Loose-fill (blown)	2.8 to 3.7	13.2 to 17.5	Non-combustible. Less widely stocked than fibreglass or cellulose.
Fibreglass batts	Batt	2.9 to 3.8	12.9 to 16.9	Gaps and compression cost real R-value. A poorly fitted batt underperforms its label.
Mineral wool batts	Batt	3.3 to 4.2	11.7 to 14.8	Dense, holds its shape, non-combustible, sheds water rather than absorbing it.
Open-cell spray foam	Spray foam	3.5 to 3.7	13.2 to 14	Contractor-installed. Vapour-open, so the assembly design matters.
Closed-cell spray foam	Spray foam	6.0 to 7.0	7 to 8.2	Contractor-installed. Acts as a vapour retarder and adds rigidity.

R per inch ranges are aged, settled values from published material data, carried in this site's reference data with sources recorded; the depth column is computed from those ranges, best case to worst case. Two caveats. For loose-fill, DOE notes that settled density rises as installed thickness rises, so R-value does not scale strictly in proportion to depth and the bag label's coverage chart is the authority for any specific product. For spray foam, the R-49 column is arithmetic, not practice: foam is specified by the contractor to a designed thickness, usually at the roofline rather than piled on a floor. Labelled R-values for insulation sold in the United States are determined by standard tests such as ASTM C518 under the FTC R-value Rule, which is why the label, not a generic table, settles any disagreement.

Blown-in fibreglass

Molten glass spun into loose tufts and blown through a hose to an even depth over the attic floor. It is the material in the machine at most big-box rental counters, and products such as Owens Corning AttiCat are packaged specifically for that route, so it is the loose-fill a homeowner is most likely to install personally.

Where it wins. Open attics with room to build depth. It is light, so an older ceiling carries a full R-60 blanket of it without complaint. It is inorganic, so it does not feed mould, and after an incidental leak it dries out better than cellulose. It settles less than cellulose does.

Where it fails. Depth. At R-2.2 to R-2.7 per inch it needs 18.1 to 22.3 inches to reach R-49, which is more than some low rooflines can hold near the eaves. Very light fibreglass can also lose some effectiveness to air movement through the layer in cold, windy attics unless it is installed at proper density.

Install caveats. Air seal the ceiling plane first, baffle the soffit vents before blowing near the eaves, and work to the bag's coverage chart rather than to inches alone. Wear a fitted respirator, goggles and gloves; the fibres are an irritant.

Blown-in cellulose

Shredded recycled paper treated with borate fire retardants, made to specifications such as ASTM C739, and blown in exactly the way fibreglass is. It is the other half of the honest default, and the denser of the two.

Where it wins. R per inch. At R-3.2 to R-3.8 it reaches R-49 in about 12.9 to 15.3 inches, which matters wherever clearance is tight. Its density also slows air movement through the layer, and it is usually the cheapest path to a high R-value in the lowest cost tier on this page.

Where it fails. Water. Cellulose absorbs it, dries slowly, and a soaked layer mats and loses loft for good. It is the wrong choice over a roof with a known leak history until the roof is fixed. It also settles more than fibreglass, and it is heavier, which is worth checking on an old lath-and-plaster ceiling.

Install caveats. Installed depth must allow for settling: the bag chart gives an installed depth and a settled depth, and the settled one is the number that has to meet your target. The same air sealing, baffles and respirator rules apply as for fibreglass, and the dust is heavier.

Mineral wool, loose-fill and batt

Fibre spun from molten rock or steel slag, sold both as loose-fill and as dense, board-like batts. It occupies a middle position: more R per inch than blown fibreglass, denser than either default, and non-combustible without added retardants.

Where it wins. Fire performance, sound, and moisture behaviour. It sheds water rather than absorbing it and does not feed mould, which makes the batts a reasonable pick for knee walls and for damp-prone edges of a plan. The batts are stiff enough to friction-fit crisply and stay where they are put.

Where it fails. Availability and price. It sits a tier above fibreglass and cellulose on cost, fewer yards stock the loose-fill, and fewer crews blow it, so quotes are harder to gather. For a plain open attic floor it buys little that the cheaper loose-fills do not.

Install caveats. The batts cut cleanly with a serrated blade but must still be fitted around wiring and boxes rather than compressed over them; compression costs R-value in any fibrous batt. Gloves and a respirator, as with fibreglass.

Fibreglass batts

The pink or yellow rolls everyone pictures first. In an attic they are the right tool for specific geometry and the wrong tool for a big open floor.

Where it wins. Framing bays that are regular, accessible and empty: knee-wall backs, the attic hatch, a small addition. A second layer of unfaced batts laid across the joists at right angles is also a legitimate way to top up over existing material without renting a machine.

Where it fails. Real attics. Wiring, plumbing, bracing and irregular joist spacing force cuts, and every gap, void and compressed corner costs real R-value, so a poorly fitted batt underperforms its label. A blown material flows around all of it. Batt jobs look finished long before they perform, which is exactly the failure a homeowner cannot see from the hatch.

Install caveats. Fit is everything: batts must fill the bay fully, split around cables rather than squash over them, and butt tightly with no fold-back at the ends. Kraft or foil facing goes against the ceiling only, and never buried between layers, where it can trap moisture.

Open-cell spray foam

A two-part polyurethane foam that expands into a soft, vapour-open layer, at roughly R-3.5 to R-3.7 per inch. In attics its real role is the underside of the roof deck, where it insulates and air seals in one pass to create a conditioned attic.

Where it wins. Rooflines. When the ducts and air handler live in the attic, or the attic is finished space, moving the thermal boundary to the roof with open-cell foam is often the right call, and it is cheaper per inch than closed-cell.

Where it fails. The open attic floor, where it buys nothing that loose-fill does not at several times the price. Because it is vapour-open, moisture from the house can reach a cold roof deck through it in cold climates, so the assembly needs design, not just depth.

Install caveats. Contractor-only. This is a chemical reaction with mixing ratios, cure times and occupancy restrictions during and after spraying, and a bad batch cannot be un-sprayed. There is no honest do-it-yourself route at attic scale.

Closed-cell spray foam

The dense sibling, at roughly R-6.0 to R-7.0 per inch, the highest R per inch of any material on this page. It cures rigid, adds structural stiffness, and acts as its own vapour retarder.

Where it wins. Anywhere depth is the binding constraint: cathedral ceilings, flat roofs, the tight triangle at the eaves, and rooflines in cold climates where its vapour-retarding cure solves the problem open-cell creates. R-49 in about 7 to 8.2 inches is a capability nothing else here has.

Where it fails. Cost, again on the open floor. It is the most expensive material per unit of R-value on this page, and buried in it go your wiring, your roof deck access and any future leak diagnosis, because water shows itself slowly through a rigid foam layer.

Install caveats. Contractor-only, same as open-cell, with thickness per pass limited by the exotherm of the cure. Get the designed thickness and the target R-value in writing, because foam is quoted by the board foot and thin spots are invisible once the job is walked away from.

Radiant barrier is a different product, not a competitor

A radiant barrier reflects radiant heat rather than resisting conduction, so it has no useful R-value per inch and cannot be counted toward an R-value target. It is measured differently from bulk insulation and the two do not add together.

That is why it is absent from the comparison table: there is no honest number to put in the R per inch column, and any seller quoting an "equivalent R" for foil is quoting a figure the FTC R-value Rule exists to prevent. What a radiant barrier can do is reduce summer radiant heat gain from a hot roof deck into the attic below it, which mainly helps houses in hot, sunny climates, especially where ducts run through the attic. It is a supplement stapled under the rafters, not a substitute for depth on the floor, and in a heating-dominated climate it is close to pointless. If a pitch for foil is competing against a pitch for inches, buy the inches.

What they cost, relative to each other

This table ranks; it deliberately does not price. This site publishes dollar figures only where they can be built from measured inputs, and the one measured material price it carries is a single anchor, Owens Corning AttiCat blown-in fibreglass at retail shelf price, read in August 2026. One anchor prices one material, not seven, so per-material dollar claims here would be the unsourced averaging this site exists to avoid. The built-up installed numbers, with the method and its estimates stated, are at the cost guide.

Material	Installed cost per unit of R	Who installs it
Blown-in fibreglass	Lowest tier	Homeowner with a rented blower, or a contractor
Blown-in cellulose	Lowest tier	Homeowner with a rented blower, or a contractor
Fibreglass batts	Low on material, expensive in fitting time done properly	Homeowner
Mineral wool, either form	A tier above the fibreglass and cellulose defaults	Homeowner for batts, contractor for loose-fill
Open-cell spray foam	High	Contractor only
Closed-cell spray foam	Highest	Contractor only
Radiant barrier	Not comparable: it buys no R-value	Homeowner staple-up is common

The decision, by attic

Find your row, take the recommendation. If two rows apply, the one describing a problem (moisture, rodents) wins, because material choice comes after the attic is sound.

Your attic	Do this	Realistic DIY?
Open, accessible, existing material dry and clean	Air seal the ceiling, then top up with blown fibreglass or cellulose to your zone's target	Yes, with a rented blower and a helper
Bare or nearly bare floor, good clearance	Air seal, baffle the soffits, then blow fibreglass or cellulose to the uninsulated-attic target for your zone	Yes, same route
Low clearance at the eaves or a shallow roofline	Blow cellulose for its higher R per inch; where full depth cannot fit at the perimeter, have a contractor spray closed-cell foam in that band	Partly; the foam band is contractor work
Moisture problem, past or present	Fix the roof or the ventilation first, then insulate; prefer blown fibreglass or mineral wool over cellulose near any spot with a leak history. See mould in attic insulation	The moisture fix usually is not
Rodent or pest history	Have the contaminated material removed and the deck cleaned before anything new goes in. See rodent-contaminated insulation	No; this is cleanup work, not insulation work
Cathedral ceiling, knee walls, or ducts in the attic	Move the boundary: closed-cell foam at the roofline in cold climates, open-cell in mild ones, mineral wool or high-density batts in accessible knee walls	No for foam; batt knee walls, yes
Loose vermiculite anywhere on the floor	Stop. Treat it as potentially asbestos-containing per EPA guidance and read the vermiculite page before touching anything	No, under any circumstances

If no row forced a change, the default from the top of the page stands: air seal, then blown fibreglass or cellulose over whatever dry, clean material is already there. Whether the old layer should ever come out first is its own decision, and the remove-or-add-over guide runs that test in five steps.

You may not need new material at all

A material comparison invites the assumption that something must be bought. Often nothing should be. If the existing insulation is dry, clean and free of pest damage, it is still doing its job, whatever unfashionable material it is, and the question is only whether there is enough of it. Measure before you shop: the depth checker turns inches into an R-value range and compares it with your zone's recommendation. Within about R-10 of the target, more depth is a slow payback, and air sealing the ceiling plane is almost always the better spend. Topping up an adequate base costs a fraction of a tear-out and replacement, and switching materials buys nothing on its own: R-49 of cellulose and R-49 of fibreglass hold heat the same. We do not sell insulation and we do not install it, which is why we can tell you that the cheapest material on this page is the one already in your attic.

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Common questions

What is the best insulation for an attic?: For a typical open, accessible attic floor, blown-in fibreglass or blown-in cellulose. Both are loose-fill materials blown to an even depth over the whole floor, both fill the gaps and odd framing that defeat batts, and both sit in the lowest cost tier per unit of R-value. The choice between them is close: cellulose carries about R-3.2 to R-3.8 per inch against fibreglass at about R-2.2 to R-2.7, so cellulose reaches the same target at a shallower depth, while fibreglass is lighter and does not hold water the way a wet cellulose layer can. Spray foam, mineral wool and batts each win in specific situations, not in the general case.
Is cellulose better than fibreglass for an attic?: Neither is better across the board. Cellulose gives more R per inch, so it fits a higher target under low rooflines, and its density blocks air movement through the layer somewhat better. It settles more, so installed depth has to allow for it, and it absorbs water if the roof leaks. Blown fibreglass settles less, weighs less on the ceiling, and dries out better after an incidental leak, but needs more depth for the same R-value. On an open attic with generous clearance, either is a sound choice, and the installer you can actually get is a bigger factor than the material.
Is spray foam worth it in an attic?: Only when the job is the roofline rather than the floor. Foaming the underside of the roof deck creates a conditioned attic, which is the right move when ducts live up there or the attic is finished space. On an ordinary vented attic floor, closed-cell foam at roughly R-6 to R-7 per inch buys the same warmth as loose-fill at several times the installed cost, and open-cell buys it at a smaller premium with no depth problem to solve. Foam is also contractor-only work: it is a two-part chemical reaction with cure times, ventilation requirements and no honest do-it-yourself route at attic scale.
Can I mix insulation types in the same attic?: Yes, and topping up is the normal case. New loose-fill goes straight over old loose-fill or over old batts, and unfaced batts can be laid across the joists at right angles over existing material. The one rule is the vapour facing: never put a kraft or foil facing anywhere except directly against the ceiling below, because a facing buried between layers can trap moisture where it condenses. Use unfaced material for any layer that is not the first. A radiant barrier is the exception to mixing: it is not bulk insulation and does not add to the R-value of anything beneath it.
Does a radiant barrier replace attic insulation?: No. A radiant barrier reflects radiant heat rather than resisting conduction, so it has no useful R-value per inch and cannot be counted toward an R-value target. Bulk insulation and radiant barriers are measured differently and the two do not add together. In a hot, sunny climate a barrier under the roof deck can cut summer heat gain into the attic, which helps ducts and cooling load, but the house still needs its full recommended depth of bulk insulation on the floor. In a cold climate the barrier does little, and money spent on it is better spent on depth or air sealing.
How many inches of insulation do I need to reach R-49?: It depends on the material. From the ranges this site carries, R-49 takes about 12.9 to 15.3 inches of blown cellulose, about 18.1 to 22.3 inches of blown fibreglass, and about 7 to 8.2 inches of closed-cell spray foam. Treat the loose-fill figures as estimates rather than measurements: DOE notes that settled density rises as installed thickness rises, so loose-fill R-value does not scale strictly in proportion to depth. The coverage chart on the bag of the specific product you buy is the authority, and installers work to that chart, not to a generic R per inch.

Sources: R per inch ranges, ENERGY STAR recommended attic levels and 2021 IECC model-code ceiling values from this site's reference data, hand-curated with citations: ENERGY STAR "Recommended Home Insulation R-Values", which states its guidance is based on 2021 IECC Residential Provisions Table R402.1.3, and the model code as published, including the R402.2.1 allowance for R-49 over the full ceiling with a raised-heel truss. Depth-to-R-49 figures computed from those ranges at build time. Loose-fill settling behaviour per DOE guidance; A radiant barrier reflects radiant heat rather than resisting conduction, so it has no useful R-value per inch and cannot be counted toward an R-value target. It is measured differently from bulk insulation and the two do not add together. Labelled R-values are determined under the FTC R-value Rule using standard test methods such as ASTM C518. Limits: cost is compared in tiers only, because this site carries one measured material price (a retail shelf anchor, August 2026) and will not publish per-material dollar figures it cannot source; state code enforcement varies and no statement here is a code requirement for your house. Page reviewed August 2026. Corrections: [email protected].