Blown-in or batts, for an attic floor

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

In brief

Blown loose-fill and batts are two ways to do one job. On an open attic floor, blown wins most of the time. Loose fiber flows into odd shapes, so it covers the whole ceiling and buries the joists. Batts are cut rectangles. They sit between the joists and leave the joist tops bare.

At 16 inch centers that framing is about 9.4 percent of your ceiling. Gaps cost more than their size suggests. Leave 5 percent of a ceiling uncovered and a target of R-49 can perform like R-34.

Batts still win in a clean, walkable attic you want to work at your own pace, on knee walls, and on small areas. Blown needs a machine, which Home Depot and Lowe's lend free with 10 bags. Batts need a knife. At shelf prices, loose-fill costs less per square foot than batts at the same R-value.

This page is about the format, not the material. Loose fiber against cut pieces, on the floor of an attic. If your question is cellulose against fiberglass, that is a material question, and the cellulose against fiberglass page answers it. Fiberglass is sold in both formats, so you can settle the format first.

Coverage, and what a gap really costs

An attic floor is not a tidy grid. Joist spacing wanders in older houses. Truss webs cross the bays. Wires, junction boxes, pipes and bath fan ducts sit on the ceiling. Loose fiber lands in all of that and takes its shape. A batt has to be cut to fit around each one, and every cut is a chance to leave a gap.

Gaps matter far more than their size. Heat takes the easy path, so two paths side by side do not average out. The bad one carries much more than its share. Here is the arithmetic. Say part of your ceiling is left uncovered. Say heat moves through that part five, ten or twenty times faster per square foot than it moves through the insulated part. The table gives what the whole ceiling is then worth, as a percentage of the R-value you paid for.

Share of ceiling left uncovered	Gap leaks 5x faster	Gap leaks 10x faster	Gap leaks 20x faster
1%	96% of target	92% of target	84% of target
2%	93% of target	85% of target	72% of target
5%	83% of target	69% of target	51% of target
10%	71% of target	53% of target	34% of target

Read the middle column. Leave 5 percent of a ceiling bare, at 10 times the heat flow, and a target of R-49 performs like about R-34 across the whole attic. That is 31 percent of what you paid for, lost to one twentieth of the area. You bought the depth everywhere and got the result of a much thinner attic.

Two things about that table. It is arithmetic on the definition of R, not a measurement: heat flow through a layer runs as one over R, and two paths add up by area. And it does not condemn batts. A batt fitted tight in a regular bay leaves no gap at all. What it says is that coverage beats depth, and that a floor full of obstacles is where the format choice gets decided.

The joists, and which format covers them

Every ceiling has two surfaces. The bays hold the insulation. The wood between them does not. Joists run the full length of the ceiling, and heat that moves through framing instead of through insulation is called thermal bridging. It is the one loss you cannot fix by buying a thicker batt.

This is the clearest split between the two formats. Batts drop into the bays, so the wood stays exposed unless you add a second layer over the top. Blown fiber buries the joists in the same pass that fills the bays. How much of your ceiling is wood comes straight from the framing. Dressed 2x lumber is 1.5 inches thick under the American Softwood Lumber Standard PS 20.

Joist spacing	Clear bay	Ceiling area that is wood
16 in on center	14.5 in	9.4%
24 in on center	22.5 in	6.3%

So a batt attic at 16 inch centers has roughly 9.4 percent of its ceiling in bare wood, and one at 24 inch centers about 6.3 percent. Wood is not a hole. It slows heat much better than an open gap and much worse than a foot of insulation, so the loss sits well below the numbers in the gap table above.

The fix for a batt attic is a second layer, unfaced, laid across the joists rather than along them. That layer is what covers the wood. It is also where most of the depth has to go: reaching R-49 in fiberglass batts takes about 12.9 to 16.9 inches, against a deepest common bay of 9.25 inches in a 2x10. The fiberglass batts page works that through. Blown-in reaches the same place in one pass, which is the plainest practical difference between the formats.

Fitting batts around what is actually up there

A batt is held by nothing but its own width. It is made about half an inch wider than the bay it is meant for, so the edges press against the joists when you push it in. That is a friction fit. Nothing staples, nothing glues.

Framing spacing	Clear bay	Batt width	Grip
16 in on center	14.5 in	15 in	0.5 in
24 in on center	22.5 in	23 in	0.5 in

That grip only works if your bays measure what the batt expects. Modern trusses are consistent. Framing cut on site 60 years ago is not. Measure ten bays across the attic before you buy anything. If the readings scatter, every batt becomes a hand cut, and each cut edge is a place where a gap can open.

Then there is everything crossing the bay. Wiring runs, plumbing, a bath fan duct, cross bracing between joists. Cut the batt to length and around each one. Where a cable runs through the middle of a bay, peel the batt apart into two thinner layers, and slide one under the cable and one over it. Do not fold a batt over an obstacle. Folding leaves a doubled lump on one side and bare ceiling on the other, which is the gap arithmetic above in miniature.

Compression is the other trap, and it does not just make a batt thinner. Squeeze a batt into a space shallower than its loft and it stops delivering its labeled R-value. Gaps and compression cost real R-value. A poorly fitted batt underperforms its label. Label R-values are measured flat and uncompressed, under the FTC R-value Rule (16 CFR Part 460), by test methods such as ASTM C518.

Two numbers show how easily this happens. The R-30 fiberglass batt priced below is 9.5 inches thick, and the R-38 one is 12 inches. The deepest common ceiling joist is 9.25 inches. So neither batt fits inside a bay without standing proud of it, and neither can be pushed flush without losing part of what you bought. That is fine on an open attic floor, where the batt is allowed to sit tall. It is not fine under a plywood deck, where the bay depth is the hard limit and anything thicker gets crushed the day the boards go back down.

What each format costs

Material first, because it is the part with a receipt. Every price below was read off a live Home Depot product page on 2026-08-11 and converted to dollars per square foot of ceiling at a stated R-value. That is the unit that lets two formats be compared: a bag price on its own says nothing, because a bag of batts and a bag of loose fiber do not cover the same area.

Material and format	Per sq ft at R-30	Per sq ft at R-38
Blown cellulose, loose-fill	$0.58 to $0.62	$0.80 to $0.84
Blown fiberglass, loose-fill	$0.99	$1.28
Fiberglass batts	$1.67	$1.68
Stone wool batts	$3.24	no anchor read at this R-value

The order does not change between the two targets. Loose-fill is the cheaper material at the same R-value, and cellulose is the cheapest of all. On a 1,200 square foot attic at R-38 that is roughly $960 to $1,010 in cellulose, about $1,540 in blown fiberglass, and about $2,010 in fiberglass batts. Stone wool batts cost more again, which is what you pay for its fire and sound behavior rather than for R-value.

Five limits ride with those figures. They are material only, with no labor, delivery or disposal. They are shelf prices, and a contractor buys below shelf. They come from one retailer on one day, and prices move by store and by ZIP. One product is one point, not a market: a second R-30 batt on the same site that day worked out cheaper per square foot. And the cellulose coverage above R-19 comes from the Greenfiber attic data card rather than from the retail listing, because retailers publish only the R-19 figure.

Installed, by a contractor

One job here can be priced from parts rather than guessed at: blowing fiberglass into an attic. On a 1,200 square foot attic to R-49 the model returns $2,622 to $3,190, or $2.18 to $2.66 a square foot. It is built from the BLS mean wage for insulation workers, May 2025, times a burden multiplier of 2 to 3, plus 9 to 12 crew-hours per 1,000 square feet, plus material at shelf price. The multiplier and the crew hours are estimates, not measurements, and they are labeled that way everywhere they appear.

There is no matching installed figure for batts here, so this page publishes none. Nobody has measured batt crew hours for us, and a number invented to fill the column would be worth less than the blank. What can be said is structural. A batt attic to a modern target is two layers, the second laid crosswise, and it is hand work in a hot space with a knife. Loose-fill is one pass with a hose. When you compare two quotes, compare the scope and the target R-value first. The blown-in cost page decomposes the installed side.

The equipment question

This is the practical difference people feel first. Batts need a knife, a straightedge, a board to kneel on and protective gear. Blowing needs a machine, and the machine has to be rented or borrowed.

At attic quantities the machine is usually free. Both retailers publish the same kind of term, quoted here as they state it. Home Depot, for its cellulose blower: "The first day's rental is free with the purchase of 10 bags of Cellulose loose-fill insulation." Home Depot again, for blown fiberglass: "Free 24 hour blower machine rental with the purchase of 10 bags." Lowe's, for the same product line: "Free 24-Hour machine rental with purchase of 10 bags or more AttiCat."

Ten bags is not a real hurdle for a whole attic. The 1,200 square foot reference attic takes about 23 bags of the fiberglass product priced above to reach R-38. Neither retailer publishes a paid daily rate on the rental listing, so this page carries none, and a deposit or a card hold at the counter is normal and is not published either.

What the machine asks of you is not money. The rental unit Home Depot lists runs 450 pounds an hour through 100 feet of hose, weighs 165 pounds and is rated at 85 decibels. So you need a vehicle that can carry it, a second person, and hearing protection on top of the respirator. One person feeds the hopper at ground level while the other works the hose in the attic. It is not a solo job, and a starved hopper blows thin. Batts, by contrast, are a job you can start on a Saturday morning and stop at lunch.

Which format for your attic

Work down the table and stop at the first row that describes your attic. The rows that overrule everything else come first.

Your attic	Verdict
Loose gray-brown pebbly material that could be vermiculite	Neither. Disturb nothing. EPA guidance is to treat it as asbestos-bearing until tested. Read the vermiculite page first
Wet, moldy or rodent-fouled material, or an active roof leak	Neither, yet. Fix the cause and clear the mess first. Either format laid over it buries the problem and you pay twice. Remove or add over runs that test
Knob-and-tube wiring, or fixtures with no rating you can read	Neither, until an electrician has looked. Burying wiring you cannot identify is the one mistake here with a fire consequence
Dry, clean loose-fill already on the floor and you are adding depth	Blown. Loose fiber matches the surface it lands on. A stiff rectangle on a soft, uneven bed leaves voids under it
Target is R-49 or R-60, which is most of zones 4 to 8	Blown. One pass reaches it. Batts take two layers, the second laid crosswise, and most of the depth ends up above the framing anyway
Low pitch, no walkable framing, ducts filling the space	Blown, and hire it. The hose reaches where a person should not go, which is most of what the crew is being paid for
Bay widths scatter, or bracing and cable cross every bay	Blown. Every batt would be a hand cut, and the cut edges are where the layer leaks
Knee walls, a cathedral slope, or the hatch itself	Batts. Loose fiber will not stay on a vertical or sloped surface. Support the batt on both faces so it cannot sag out
One room's ceiling, or a small patch	Batts. Machine setup and the trip outweigh the job below a certain size, and there is less to get wrong in a small space
Floored or decked, and you want to keep using the floor	Batts in the bays under the deck, and accept that the bay depth caps the R-value. Do not crush a thicker batt to make it fit
Clean, walkable, bays regular, and you want to work weekends at your own pace	Batts. This is the case they are for. Cross-lay the second layer unfaced, and take your time on the cuts

The recommendation. For a whole open attic floor, blown. For anything vertical, small, or that you need to keep walking on, batts. When both rows look reasonable, pick blown, because of how each format fails. Blown fails as uneven depth, which you can see with a ruler and fix with a few more bags. Batts fail as gaps you then covered up, which nothing later will show you.

If you landed on the do-it-yourself side, the kit is short and the order matters more than the brand. The gear page lists it by job, with what to look for and when to skip an item, and the do-it-yourself comparison prices the gap you are working for. Baffles go in at the eaves before any loose fiber does: without them the material drifts into the soffit vents and blocks the intake. Whichever format you choose, wear a fitted respirator, sealed eye protection and gloves. OSHA's respirator standard binds an employer over the crew it sends, so a contractor's people work under it. In your own attic, nobody is covering that but you.

If the table pointed at hiring, the questions to put to a contractor are the same ones this page runs on. Which format, the target R-value in writing, how the joists get covered, and whether air sealing is in the price or out of it. The install guide lists what a correct job includes, so you can read a quote against it.

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You may not need either one

Measure before you buy anything at all. Push a rigid ruler down to the drywall at five spots spread across the attic, including one near the eaves, and take the typical reading rather than the deepest. The depth checker turns those inches into an R-value range in about a minute, and how much R-value you need sets the target for your zone.

If what is up there is dry, clean, and already within about R-10 of the level recommended for your climate zone, then neither format is your next job. Air sealing is. That threshold is this site's estimate rather than a measurement. The reasoning is in the arithmetic: heat flow drops as one over R, so the last step toward a target saves a small fraction of what the first inches did, while a bare hatch, open wall tops and unsealed plumbing chases keep moving air whatever the depth on top of them is.

For scale, ENERGY STAR recommends R-49 for zones 4 through 8 in an attic that already has 3 to 4 inches, and R-60 for a bare one. Warmer zones are lower. So an attic already at R-39 in zone 5 is inside that band, and more depth is a slow payback whichever format you use. We do not sell insulation and we do not install it, which is why this page can end on a caulk gun and a weekend instead of a machine or a pallet of batts. Air sealing is the cheaper, better job in that case.

Recommended levels above are ENERGY STAR retrofit guidance for a home that already exists, which ENERGY STAR states is based on the model 2021 IECC Table R402.1.3. Energy codes cover new building and work done under a permit. They do not usually cover adding insulation to an attic you already have. Which edition applies where you live depends on your state, and often on your city. Your local building department is the only authority on it.

Common questions

Is blown-in better than batts in an attic?

On an open attic floor, yes, most of the time. Loose fiber fills odd shapes and buries the joists in one pass, so it covers the whole ceiling. Batts sit between the joists and have to be cut around every wire, pipe and brace. Batts still win in three places: a clean, walkable attic where you want to work at your own pace, anything vertical like a knee wall, and a small area where hauling a machine out is not worth it.

Can I lay batts over blown-in insulation that is already there?

You can, and it is not the best pairing. Use unfaced batts only, laid across the joists rather than along them, because a second vapor facing traps water in the layer below. The real problem is the bed. Loose fiber is soft and uneven, and a stiff rectangle laid on it does not sit flat. The air pockets left underneath are the exact thing you are paying to avoid. Blowing more loose fiber over dry, clean loose fiber matches the surface it lands on.

Which is cheaper, batts or blown-in?

At store shelf prices, loose-fill is cheaper per square foot of ceiling at the same R-value. Read 2026-08-11 at one retailer: at R-38, cellulose runs $0.80 to $0.84 a square foot of ceiling and blown fiberglass $1.28, against $1.68 for fiberglass batts. Material only, no labor. On a 1,200 square foot attic that is roughly $960 to $2,010 depending on which you pick. Prices vary by store and ZIP.

Do I have to rent a machine to blow insulation?

You need one, and at attic bag counts it is usually free. Home Depot states, for its cellulose blower: "The first day's rental is free with the purchase of 10 bags of Cellulose loose-fill insulation." Lowe's states, for blown fiberglass: "Free 24-Hour machine rental with purchase of 10 bags or more AttiCat." A 1,200 square foot attic at R-38 takes about 23 bags of the fiberglass product priced here, so the threshold is not the binding part. A deposit or a card hold at the counter is normal and neither retailer publishes it. Batts need no machine at all: a knife, a straightedge and a board to kneel on.

How much does a small gap in the insulation actually cost me?

More than its size. Heat takes the easy path, so two paths side by side do not average out. Leave 5 percent of a ceiling uncovered, and say heat moves through that strip 10 times faster per square foot than through the insulated part. A target of R-49 then performs like about R-34 across the whole ceiling. That is roughly 31 percent of the upgrade gone. It is arithmetic on the definition of R, not a measurement, and it is the main argument for the format that covers everything without a cut.

Do batts leave the joists uncovered, and does that matter?

Yes, and yes. A batt sits in the bay, so the wood beside it stays bare. Dressed 2x framing is 1.5 inches thick, so at 16 inch centers about 9.4 percent of your ceiling area is joist rather than insulation. Wood slows heat much better than a hole and much worse than a foot of insulation, so the loss is real and smaller than an open gap. The fix is a second unfaced layer laid across the joists. Blown fiber buries them in the first pass.

Sources, limits and corrections

Framing dimensions are dressed sizes under the American Softwood Lumber Standard PS 20: 1.5 inch nominal 2x thickness and joist depths of 5.5 in for a 2x6, 7.25 in for a 2x8, 9.25 in for a 2x10. Batt widths of 15 in and 23 in are manufacturer product dimensions for 16 in and 24 in on-center framing. R per inch ranges are this site's hand-curated reference data, carried as ranges because R per inch varies with density and installed condition: 2.9 to 3.8 for fiberglass batts, 2.2 to 2.7 for blown fiberglass, 3.2 to 3.8 for blown cellulose. The gap table is arithmetic on the definition of R and contains no measured input: heat flow runs as one over R, so a ceiling that is part uncovered is worth its target divided by one plus the uncovered share times the leak ratio less one. The leak ratios of 5, 10 and 20 are illustrative, chosen to bracket the range, not measured on any attic. Retail prices were read off live Home Depot product pages on 2026-08-11: Owens Corning R-30 unfaced fiberglass insulation batt, 16 in. x 48 in. (C84A) at $88.82, Owens Corning R-38 unfaced fiberglass insulation batt, 24 in. x 48 in. (W65A) at $107.21, Greenfiber SANCTUARY cellulose blown-in insulation, 25 lb bag (INSSANC) at $15.97, and ROCKWOOL Comfortbatt R-30 stone wool batt, 7.25 in. x 15 in. x 47 in. (RXCB301525) at $96.97. Cellulose coverage above R-19 is the Greenfiber SANCTUARY attic technical data card, not the retail listing. Blown fiberglass material is Owens Corning AttiCat blown-in fiberglass, 27.5 lb bag at $67.58 a bag, read 2026-08-07, with the manufacturer's published coverage chart. Retail shelf price. A contractor buys below it, so material in an installed quote is lower than this. The discount is not measured. Blower rental terms and the machine specification are quoted from the retailer listings that state them, read 2026-08-11. Neither retailer publishes a daily rate on the rental listing, so the cost of the machine on a job that clears ten bags is nothing. A deposit and a card hold are normal at the counter and are not published either. The installed figure is the Insulation Report Cost Model, August 2026: BLS Occupational Employment and Wage Statistics May 2025, SOC 47-2131, a burden multiplier of 2 to 3 and 9 to 12 crew-hours per 1,000 square feet. The last two are estimates rather than measurements. Recommended attic levels are ENERGY STAR "Recommended Home Insulation R-Values", which ENERGY STAR states is based on 2021 IECC Residential Provisions Table R402.1.3. Labeled R-values are governed by the FTC R-value Rule (16 CFR Part 460) using test methods such as ASTM C518; ASTM C739 covers cellulose loose-fill. Settling behavior per DOE guidance that settled density rises with installed thickness. Vermiculite handling per EPA guidance. Respiratory protection: OSHA's standard binds employers with respect to their employees, so it is cited as what covers a contractor's crew rather than as a rule on a homeowner. Limits: no installed price for batts is published here because none has been measured, one product is one price point rather than a market, prices vary by store and ZIP, the "within about R-10" threshold is an editorial judgement labeled as an estimate, and nothing here is a code requirement for your house. Page reviewed August 2026. Corrections: [email protected].