How much insulation do I need?
For batts, divide the net area by the coverage printed on the package. An R-13 kraft batt bag for 16 in stud bays covers about 106 sq ft, and the 24 in version roughly 163. Nothing more to it than that.
For blown-in, go by the bag chart. Owens Corning AttiCat takes about 15 bags per 1,000 sq ft to R-30, 19 to R-38, 25 to R-49 and 32 to R-60. Those are the bare label rates. The calculator adds your waste allowance on top, so it’ll usually read a bag or two higher.
What R-value do you actually need?
Ceilings under the 2021 IECC run R-49 in the southern zones 2–3 and R-60 in zones 4–8. The 2024 edition rolled these back to R-38/R-49, so treat the span as your range and check your local code year before you order. Walls run from R-13 in the deep south to R-20 plus continuous insulation in the north.
New insulation goes on top of dry, uncompressed old insulation, so you only pull it out if it’s wet, moldy or rodent-fouled.
Canada doesn’t use the IECC. Under NBC Section 9.36 the prescriptive ceiling target is about R-50 in climate zones 5–6 and R-60 in zone 7A. Canadian zones are set by heating degree days and numbered 4 to 8, and provincial amendments (Ontario SB-12, BC Energy Step Code, Quebec) override the national figures.
How many bags of blown-in insulation per 1,000 sq ft
Every bag of loose-fill sold in the US carries a coverage chart, and it isn’t marketing copy. The FTC’s R-value Rule, 16 CFR Part 460, requires it to state, for each R-value, the minimum initial installed thickness, the minimum settled thickness, the maximum net coverage one bag may be spread over, the minimum number of bags per 1,000 sq ft and the minimum weight per square foot.
Two of those bind you at the same time. You have to put down the bag count and reach the depth. Spread the material thin and fluffy and you can hit depth with too few bags, and every chart’s small print says the same thing about that: miss either one and the real R-value drops below the label. Count empty bags as you work, since that tally is about the only part of the job anyone can check afterwards.
The bag count for a given R-value varies far more than most people expect. Take a 40 x 25 ft attic, which is exactly 1,000 sq ft, and insulate it to R-49. Owens Corning’s professional ProCat chart calls for 21.5 bags per 1,000 sq ft at 16.5 in. The retail AttiCat chart wants 25 bags, but at 17 in. CertainTeed’s InsulSafe SP asks for 25.2 bags at 18.5 in, and a 30 lb bag of Greenfiber cellulose needs 67.3 bags at 14.7 in as blown. Round each of those up and the same attic to the same R-value takes 22, 25, 26 or 68 bags, depending purely on what’s on the pallet.
That spread exists because coverage is measured with a named machine, hose and setting. Owens Corning determined the ProCat figures with 100 ft of 2.5 in hose on the manufacturer’s preset attic settings, and states plainly that no gearing or gate adjustment is permitted on the AttiCat machine. A rental blower run on a coarser gate fluffs the same material further, so you reach depth early and finish light.
Use the number this attic insulation calculator gives you to decide how many bags to order and where to park the truck. Then, before anyone opens a bag, re-divide your square footage by the coverage per bag on the chart of the product you ended up buying.
R-value to inches: how deep R-30, R-38, R-49 and R-60 sit
There’s no single inches-per-R answer, because every material has its own R per inch. The Department of Energy’s Building America program puts settled loose-fill cellulose at R-3.2 to R-3.8 per inch, loose-fill fiberglass at R-2.2 to R-2.7, and loose-fill rock wool at R-3.0 to R-3.3.
Batts are simpler because thickness is fixed at the factory. R-13 and high-density R-15 are both 3.5 in, R-19 is 6.25 in, R-21 is 5.5 in, R-30 is about 10 in and R-38 about 12 in. So R-38 is roughly 12 in as a batt, 13.25 in in Owens Corning ProCat (13.5 in in AttiCat), 14.5 in in CertainTeed InsulSafe SP, and 11.5 in of cellulose as blown, which settles back to 10.3 in.
Going up the scale, R-49 is 16.5 in of ProCat, 17 in of AttiCat, 18.5 in of InsulSafe SP, or 14.7 in of 30 lb cellulose as blown. R-60 is 19.75 in, 20.5 in, 22 in and 17.9 in of the same four.
Those last figures are worth checking against your roof before you order. At 22 in, loose-fill buries the bottom chord of a truss under more than a foot of fill, swallows any walk boards and leaves the hatch, the wiring and the ceiling boxes unreachable without wading. Need storage or access up there? Frame and deck an island at the right height rather than compressing the insulation under a sheet of plywood. Squashing loose-fill into a fraction of its rated depth throws away most of what you just paid to blow in.
Topping up existing insulation is the same arithmetic run backwards. Push a ruler down to the drywall in six or eight places and take the average, then multiply by the R per inch of whatever’s up there. Six inches of old loose-fill fiberglass is usually R-13 to R-16, so getting to R-49 means adding about R-34.
Read that shortfall straight off the chart rather than the finished target. The ProCat table lists R-35 at 14.9 bags per 1,000 sq ft and 12.25 in, so a 1,000 sq ft attic takes 15 bags and 12 and a quarter inches of new material, finishing around 18 in deep. Never compress what’s already there to make room.
Cellulose vs fiberglass: bags, weight and settled depth
The two materials aren’t close on weight, and weight is what gets people into trouble. Building America gives settled cellulose a density of 1.5 to 2.0 lb per cubic foot against 0.5 to 1.0 for loose-fill fiberglass, and the label charts say the same thing in units you can act on.
To reach R-49, Greenfiber’s 30 lb cellulose chart demands a minimum of 2.02 lb per square foot, while ProCat needs 0.722 and InsulSafe SP 0.780. Over a 1,000 sq ft attic that’s 2,020 lb of cellulose (a shade over a US ton) against roughly 750 lb of fiberglass. And it’s sitting on your ceiling board rather than on the joists.
The limit is a real one. The gypsum industry caps the weight of overlaid unsupported insulation a ceiling membrane may carry, and USG’s Gypsum Construction Handbook prints the numbers everyone works to. It allows 1.3 lb per square foot for single-layer half-inch gypsum wallboard on framing at 24 in centers, 2.4 lb at 16 in centers, and 2.2 lb for five-eighths board at 24 in centers.
Blow cellulose to R-49 at 2.02 lb per square foot over half-inch board on 24 in trusses and you’re well over the published limit. Take it to R-60 at 2.59 lb and you’re over every single-layer figure in the table. Find out what your ceiling board is before you commit to deep cellulose. Blown fiberglass never gets near these numbers.
Settling is where the published sources genuinely disagree, so treat the range honestly. Building America says loose-fill cellulose settles 10 to 20 percent over time, and that fiberglass and rock wool settle only 2 to 4 percent. Greenfiber’s own chart implies about 10 percent, listing 11.49 in as blown against 10.34 in settled at R-38. Owens Corning goes further the other way and prints identical initial and settled thicknesses for its blown fiberglass, with a footnote calling settling negligible and without impact on R-value.
You don’t have to resolve that argument to order correctly. Blow to the initial installed thickness printed on the chart, never the settled one, since the allowance is already built into that first number.
How many rolls of R-13 for a 2x4 wall, and what changes on a 2x6
Batts and rolls are sold by coverage per package, so wall work is a straight division. A bag of R-13 kraft-faced batts at 15 in x 93 in holds 11 pieces and covers 106.56 sq ft, which is the size for studs at 16 in on center. The 23 in x 93 in version of the same bag covers 163.39 sq ft for framing at 24 in on center.
R-19 unfaced rolls come 15 in wide at 48.96 sq ft or 23 in wide at 75.07 sq ft, and a bag of seven R-21 kraft batts at 15 in x 93 in covers 67.81 sq ft. Set the product to blown cellulose and this works as a cellulose insulation calculator. Leave it on batts and it’s a batt insulation calculator. Only the package figure changes.
Worked through, a 40 ft run of wall at 8 ft high is 320 sq ft gross. Deduct two openings at the 18 sq ft each this calculator assumes and you have 284 sq ft net. Add the default 8 percent waste and you’re buying for 307 sq ft. Divide by 106.56 and you get 2.88, so three bags of R-13 for a 2x4 wall framed at 16 in on center. Frame the same wall at 24 in on center and 307 divided by 163.39 is 1.88, so two bags.
The mistake to avoid is buying by batt count. Eleven batts won’t fill 11 bays, because the last bay in every run needs cutting and the offcut rarely fits anywhere else.
Depth decides which R-value belongs in the wall. A 2x4 is 3.5 in deep, so R-13 or high-density R-15 fills it with no compression. A 2x6 is 5.5 in, which R-21 fits to the inch. An R-19 batt is 6.25 in and gets squeezed three-quarters of an inch, and NAIMA’s compressed-batt data puts that squashed batt at about R-18. So buying R-19 for a 2x6 costs you around R-3 against the R-21 that was made for the cavity.
If you’re ordering an R-value this tool has no preset for, take the coverage per bag from that product’s own label and divide your net area by it. Don’t assume batts of different R-values come in equal-sized bags.
R-value by climate zone: attic, wall, floor and crawl space targets
The 2021 IECC assigns climate zones by county, with a marine subzone down the Pacific coast. The prescriptive minimums follow from the zone. Ceilings run R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Wood-frame walls are R-13 in zones 1 and 2, R-20 or R-13 plus R-5 continuous in zone 3, and R-30 or R-20 plus R-5 continuous in zones 4 and up.
Floors over unconditioned space go R-13 in zones 1 and 2, R-19 in zones 3 and 4, R-30 in zones 5 and 6 and R-38 in zones 7 and 8. Crawl space walls start at nothing in zones 1 and 2, then R-5 continuous or R-13 in zone 3, R-10 continuous or R-13 in zone 4, and R-15 continuous or R-19 from zone 5 upward.
The 2024 edition moved the ceiling numbers back down. The Department of Energy’s determination analysis records that prescriptive ceiling insulation was cut from R-49 to R-38 in zones 2 and 3, and from R-60 to R-49 in zones 4 through 8, returning them to 2018 levels. So an identical attic in the same county can be legally compliant at R-49 or at R-60, depending on which code year your jurisdiction has adopted. Which is why any R-value calculator by climate zone gives you a band rather than a single number.
Ask your building department which edition is in force. Then use the lower figure as your floor and the higher as your target. The few extra bags cost far less than getting a hose back into that attic a second time.
The awkward parts of the envelope are worth setting targets for too. In a sealed, conditioned crawl space you insulate the perimeter walls to the crawl space figure for your zone rather than the floor above, and you keep a ground vapor barrier down underneath. The rim joist counts as part of the wall assembly it sits in, so aim for your zone’s wall R-value there.
Don’t forget the hatch. Under IECC R402.2.4 the attic hatch or pull-down stair has to be weatherstripped and insulated to the same level as the surrounding attic, with a wood-framed dam built around any opening where loose fill could otherwise pour into the house.
Common questions
What R-value do I need for my attic?
Under the 2021 IECC it’s R-30 in zone 1, R-49 in zones 2–3 and R-60 in zones 4–8. The 2024 edition relaxes these to R-38/R-49, so whichever code year your area has adopted decides.
How thick is R-38 or R-49 in inches?
It depends on the material. Blown to R-38 sits at roughly 10–14 in, and R-49 at about 14.5–18.5 in initial depth, depending on the product. An R-38 batt is around 12 in.
How many bags of blown-in per 1,000 sq ft?
On the Owens Corning AttiCat chart, about 15 bags to R-30, 19 to R-38, 25 to R-49 and 32 to R-60. Cellulose needs far more bags for the same R-value, and it varies by brand and by blower. Go by the chart on the bag you’re buying.
Do I remove old insulation first?
Usually not. Top it up unless it’s wet, moldy or contaminated. Subtract what’s already there (existing depth × R-per-inch) from your target and buy the difference.
Batts or blown-in for an attic?
Blown-in usually wins on an open, flat attic. It’s faster, cheaper and fills the gaps around joists. Batts win in walls and anywhere you need to walk or store things.
How much does insulation cost?
In the US as of 2026, blown-in attic jobs commonly land at about $1–2.50 per sq ft installed. DIY blown fiberglass runs roughly $0.60–1.00 per sq ft, usually with free blower rental once you’re over about 20 bags.
How many bags of blown in insulation do I need for a 1,500 sq ft attic?
Multiply the chart’s bags-per-1,000 figure by 1.5. To reach R-49 that’s about 33 bags of Owens Corning ProCat, 38 bags of AttiCat or CertainTeed InsulSafe SP, or roughly 101 bags of 30 lb Greenfiber cellulose. The gap between products is bigger than any rounding you’d do, so confirm the number against the bag you buy.
How many square feet does a bag or a roll of insulation cover?
For blown material, read the maximum net coverage column. At R-49 that’s 46.4 sq ft a bag for ProCat, 39.7 for InsulSafe SP and 14.9 for 30 lb cellulose. For batts, a 15 in x 93 in R-13 bag covers 106.56 sq ft, the 23 in version 163.39 sq ft, and R-19 unfaced rolls cover 48.96 or 75.07 sq ft depending on width.
Do I need baffles and soffit vents before I blow an attic?
Yes, wherever there are eave vents. IRC R806.3 requires a clear 1 in air space between the insulation and the roof sheathing at the vent, so fit a baffle in every vented rafter bay before the hose comes out. Attic ventilation itself is 1 sq ft of net free area per 150 sq ft of attic floor, halved to 1 per 300 where venting is properly split low and high. Canada runs it the other way round. NBC 9.19.1.2 sets 1 per 300 as the default and tightens to 1 per 150 only where the slope is under 1 in 6 or the roof is framed with roof joists, and NBC 9.19.1.3 wants at least 63 mm (2-1/2 in) of clear space above the insulation rather than the IRC’s 1 in.
How many board feet of spray foam do I need?
A board foot is 1 sq ft at 1 in thick, so board feet equal area multiplied by thickness in inches. The rim joist band on a 40 x 25 ft house built with 2x10 joists is about 100 sq ft, and at 3 in that’s 300 board feet. DuPont’s Froth-Pak insulation kits are sold on a theoretical yield of 210 or 650 board feet, with a 12 board foot sealant kit for small jobs. DuPont notes that yield is worked out in perfect lab conditions at 1 in thick, so buy above what you calculate.
Should I use faced or unfaced batts?
Kraft facing is a Class II vapor retarder. The IRC requires a Class I or II vapor retarder on the interior side of frame walls in zones 5, 6, 7, 8 and Marine 4, so faced batts with the paper to the warm side suit those. In hot, humid zones an interior vapor barrier can trap moisture, so go unfaced. Never add a second facing over existing faced insulation.
How do I insulate a rim joist or a crawl space?
Code counts the rim joist as part of the wall, so target your zone’s wall R-value there. Rigid or closed-cell foam, cut and sealed into each joist bay, is the usual answer. A bare batt lets humid indoor air reach cold wood and condense. In a sealed crawl space, insulate the perimeter walls rather than the floor above, and keep a ground vapor barrier in place.