Insulation Calculator - Batts, Blown-In Bags, R-Value & Cost

How to use it

1. Measure the attic floor: length by width, or type the square footage straight in if you already know it.
2. Push a ruler down through the existing insulation until it hits the ceiling drywall and read the depth. Do it in three or four places and use the shallowest, because that is the part that leaks.
3. Pick what the existing material looks like. Grey shredded paper is cellulose, pale fluff is fiberglass, gritty gray-brown is rock wool. Loose pebbles like popcorn are vermiculite, and that answer stops the tool for a good reason.
4. Choose your climate zone, or type your own R target if your utility rebate or local code names one.
5. Choose blown fiberglass or blown cellulose, then check the coverage figure against the chart printed on the bag and correct it if it differs. This is the single input that changes the bag count most.
6. Read the headline: the bag count for the gap, with the from-scratch answer beside it and the difference in bags and dollars.
7. Add the eave run and rafter spacing so the baffles get counted. They go in before the insulation, not after.
8. Switch to the walls tab for batt packages, or the foam tab to size a rim joist run in board feet and kits.
9. Edit any price line to your own store to turn the estimate into your estimate.

At a glance

• 1,200 sq ft attic, zone 5, 6 in of old cellulose: 33 bags of blown fiberglass to close the R-41 gap, against 48 bags from bare joists. Measuring first saved 15 bags and about $780.
• The same attic, taken as empty: 48 bags, 24 in of new fiberglass and about $2,658 of materials. The R-19 already up there was worth $780 and nobody was going to tell you.
• How deep is R-30?: 12 in of blown fiberglass, 8.6 in of blown cellulose, 8.8 in of fiberglass batt, or 4.6 in of closed cell foam.
• Garage conversion, three 2x6 walls: 365 sq ft after a door and two windows come out. Six packs of 15 in R-21, 41.8 sq ft left over, about $372.
• 160 ft of rim joist at 2 in closed cell: 123 sq ft, 247 board feet, R-13 achieved, one 600 board foot kit at about $749 with 263 board feet spare.
• R-19 batts pushed into a 2x4 wall: About R-13 once compressed, a third below the label. R-15 is the batt that actually fits 3.5 inches.

How the math works

Existing R is the depth you measured times the R per inch of the material you found, and this tool uses the low end of each published band because insulation that has sat in an attic for twenty years is dusty, compacted and has lost some loft. R to add is the target minus that, floored at zero. Depth to add is R to add divided by the R per inch of the new material, and for cellulose the blower has to leave more than that on the joists so it settles onto the number. A bag is modelled by its yield in R times square feet, which is what a printed coverage chart actually encodes: a bag whose chart reads 50 sq ft at R-30 carries 1,500 R-sq-ft, so at R-40.8 the same bag covers 1,500 divided by 40.8, or 36.8 sq ft. Bags equal the attic area times R to add, divided by that yield, rounded up. The naive answer beside it uses the full target instead of the gap. Batt packages are the net wall area divided by the square feet in a package, rounded up. Board feet are area times thickness in inches, and kits are board feet divided by a derated kit yield, because a rated kit does not deliver its rating in a cold crawlspace.

1. Measure the depth you have, in more than one place

   Take a rigid ruler or a length of scrap wood marked in inches, push it straight down through the existing insulation until it stops on the ceiling drywall, and read where the top of the material sits. Do it near the middle, near an eave and beside the hatch. Those three numbers will not match, because insulation settles unevenly and somebody has crawled on it, and the number to use is the shallowest one. The thin spot is where the heat leaves.

2. Turn that depth into the R you already own

   Multiply the depth by the R per inch of whatever you found. Grey shredded paper is cellulose at about 3.2 per inch settled, pale yellow or white fluff is blown fiberglass at about 2.2, gritty gray-brown is rock wool at about 3.0, and blankets between the joists are batts at about 3.1. So six inches of settled cellulose is about R-19 and six inches of blown fiberglass is about R-13. This is an estimate from a depth, not a measurement of performance, and it is deliberately taken at the bottom of each published band.

3. Pick a target, and know what kind of number it is

   ENERGY STAR and the Department of Energy publish recommended attic levels by climate zone: R-30 in zone 1, R-49 through zones 2 and 3, and R-60 from zone 4 up, each published as a band rather than a single figure. That is guidance for an existing house, not a code requirement, and your local energy code, your utility rebate program and your own payback maths can all name a different number. Type your own if you have one.

4. Buy the gap, not the target

   R to add is the target minus what you already have. On a 1,200 sq ft attic in zone 5 with six inches of settled cellulose, that is R-60 minus R-19, so R-41. That single subtraction is the difference between 33 bags and 48 bags of blown fiberglass, about $780, and it is the reason a manufacturer's calculator will never lead with it.

5. Turn the gap into a depth, and into bags

   Depth to add is R to add divided by the R per inch of the new material: R-41 of blown fiberglass at 2.5 per inch is 16.3 in on top of what is there. Bags come off the coverage chart, and the honest way to read that chart is as a yield. A bag that covers 50 sq ft at R-30 holds 1,500 R-sq-ft of material, so at R-41 it covers 36.8 sq ft, and 1,200 sq ft takes 33 bags. Type your own bag's numbers in: brands differ by a third, and so does the answer.

6. Add the settling allowance if you are blowing cellulose

   Cellulose settles roughly 13 to 20 percent in the months after it goes down, and the R per inch on the bag is the settled figure. So the installed depth has to be higher than the finished depth: at 15 percent settling, a 14 in finished layer goes down at 16.5 in. Blowing to the finished depth is exactly how an attic ends up a full R-value short a year later, and it is why the bags carry two thickness columns.

7. Count the baffles before you count the bags

   A baffle is a cardboard or foam channel stapled between the rafters at the eave that holds a gap open so the soffit vents keep feeding air up under the roof deck. Deep insulation at the eave will bury those vents and stop the roof breathing, and a roof that cannot breathe gets frost on the sheathing in January and mold in April. The count is the eave run in inches divided by the rafter spacing: 80 ft of eave at 16 in on center is 60 baffles, and they cost less than two bags.

8. Price it as a ledger, then compare it to a quote

   Bags, baffles, a hatch dam and a pack of depth markers are the whole material list for a DIY top-up, and the blower is usually free for 24 hours if you buy ten bags or more, which is a threshold worth confirming at your own store. Against that sits the installed price, which runs roughly $1.50 to $3.50 per square foot of attic for blown-in work. On a 1,200 sq ft attic that is $1,800 to $4,200 against about $1,878 of materials, so the DIY saving is real but it is a day in a hot attic wearing a respirator.

Worked scenarios, start to finish

Real jobs with the numbers carried all the way through, so you can see how the estimate is built and check your own figures against them.

The judgement calls a pro makes

Decisions the tool cannot make for you, and how someone who does this for a living thinks them through.

Air seal before you insulate, always

Insulation slows heat moving through a material. It does nothing about heat riding on air moving through a hole, and a typical attic has a lot of holes: the gap around every top plate, every wire and pipe penetration, the chimney chase, the bathroom fan housing, the plumbing stack and the hatch itself. Sealing those with canned foam and fire-rated caulk costs a fraction of a bag of insulation and returns more per dollar than any depth you can blow. Do it while you can still see the ceiling plane, because once eighteen inches of loose fill is down you will never find them again.

Whether to remove the old insulation

Almost always no. New loose fill goes straight over old, the old material keeps every R it had, and removal costs roughly $1 to $2 a square foot to throw away insulation you already paid for. There are four reasons to remove: it is wet or has been wet, it is moldy, it is contaminated with rodent droppings, or it is vermiculite. The last of those is not a removal decision at all, it is a testing decision, and nothing else should happen until a lab has looked at it.

Blown-in or batts for an attic top-up

Blown-in wins in almost every existing attic, because a loose fill flows around the wiring, the bracing and the odd angles that a batt has to be cut around, and every gap you leave costs more R than the batt is worth. Batts win in three cases: a very small attic where a blower rental is not worth it, an attic with no safe machine access, or an attic where you want a second layer laid at right angles over an existing one and you want to see exactly where it lands. If you do lay batts over existing insulation, use unfaced, because a second vapor retarder in the middle of the assembly traps moisture.

Which material to blow

Cellulose is cheaper per R, takes about a third less depth for the same R, and is denser, which slows air moving through the layer and quietens the house. It settles, it is heavy, and it will absorb water if the roof ever leaks. Fiberglass is lighter, does not settle, does not hold water, and covers a lot more square feet per bag, but it needs five or six more inches to reach the same R and it will drift in a windy attic if it is shallow. In an unvented or a very tight attic, weight and depth decide it. In a leaky old roof, water behaviour decides it.

IC rated or non-IC recessed lights

A non-IC recessed light must have a three inch clearance to insulation on all sides, which means a dam around every fixture and a hole in your insulated ceiling at every one. An IC rated fixture can be buried, and an IC-AT fixture is also air tight. The right move in an attic with a dozen old cans is usually to replace them with air-tight LED retrofits before you insulate, which costs less than the dams and closes a dozen air leaks in the same afternoon. If you cannot, build the dams from rigid mineral wool or metal flashing, never from cardboard.

Whether to stop short of the target

The returns fall off. Going from R-19 to R-38 cuts conductive heat loss through the ceiling roughly in half. Going from R-38 to R-60 cuts it by less than another 40 percent of what is left, on a much smaller number. If the budget is fixed, R-38 plus a serious air sealing session almost always beats R-60 with the leaks still open. Type the R you can afford into the target box and see the bag count fall; that is a legitimate way to use this tool, and pretending otherwise is how people talk themselves out of doing anything at all.

DIY kit or a professional foam contractor

A two-component kit is genuinely good at rim joists, band joists, crawlspace hatches and awkward gaps. It is a bad answer for a whole attic or a whole roof deck: those need supplied-air respirators, controlled substrate temperature, a re-entry period and someone who can hear when the mix has gone off ratio. Off-ratio foam smells, never fully cures and comes out again with a demolition bill. The honest test is the size of the job against the contractor's minimum call-out charge, usually $500 to $1,500, not the per board foot rate.

Common mistakes, and the fix

• × Burying the soffit vents under the new insulation at the eave.

  ✓ Fit baffles first, one per rafter bay along every vented eave. They hold a channel open so the soffit keeps feeding air up under the deck. Sixty baffles cost less than two bags of insulation and they are the difference between an insulated roof and a moldy one.

• × Buying by depth: measuring the inches you want and dividing by the inches per bag.

  ✓ Buy by the coverage chart. A bag is a fixed amount of material, so its coverage falls as the R you are adding rises, and the chart on the bag prints that relationship in a table. Read the row for the R you are adding, not the row somebody quoted you.

• × Assuming the coverage figure in an online calculator matches the bag you are holding.

  ✓ Correct it. Bag sizes, densities and coverages differ by a third between brands and can change between production runs. The FTC R-value Rule puts a coverage chart on every bag sold in the United States precisely so you can check, and every coverage figure on this page is an editable field for the same reason.

• × Compressing an R-19 batt into a 2x4 wall to get more R.

  ✓ It gives you about R-13, a third below the label, plus a bulge that stops the drywall sitting flat. R-15 is a high density batt built to fill 3.5 inches exactly, and R-21 is the one that fills a 2x6. Match the batt to the cavity, not to the number you would like.

• × Blowing cellulose to its finished depth and walking away.

  ✓ Cellulose settles 13 to 20 percent, and the R per inch on the bag is the settled figure. Blow it to the installed depth printed in the other column on the chart, roughly 15 percent higher, so it finishes at the depth that carries the R you paid for.

• × Laying faced batts over insulation that already has a facing.

  ✓ Use unfaced. Two vapor retarders in one assembly trap moisture between them and there is no way for it to dry in either direction. If you cannot get unfaced, slash the facing thoroughly, though buying the right product is a better answer than vandalizing the wrong one.

• × Insulating the attic floor and forgetting the hatch.

  ✓ The hatch is a two by three foot hole in an otherwise insulated ceiling, and warm air pours out of it. Build a dam around the opening so the loose fill cannot spill in, glue rigid board to the back of the hatch panel to match the surrounding R, and weatherstrip the frame.

• × Blowing insulation over a knob-and-tube wiring circuit that is still live.

  ✓ Stop and get it looked at. Knob-and-tube was designed to shed its heat into open air and burying it is a fire risk that most insurers and many local codes specifically exclude. The same caution applies to any wiring with cracked or crumbling insulation. It is a small electrical job against a large one.

Material and unit specifics

R-value, and R per inch

R-value is resistance to heat flow through an assembly, so it is cumulative: R-19 already down plus R-41 blown on top is R-60, and layers simply add. R per inch is the rate a material delivers it, and it is a band rather than a fixed number because density, temperature and age all move it. Blown fiberglass is 2.2 to 2.7, blown cellulose 3.2 to 3.7, fiberglass batt 3.1 to 3.7, rock wool 3.0 to 3.3, open cell foam 3.5 to 3.7, closed cell foam 6.0 to 7.0, EPS 3.8 to 4.2, XPS 5.0 and polyiso 6.0 to 6.5.

The coverage chart, and why the law puts one on every bag

The FTC R-value Rule requires every bag of loose fill insulation sold in the United States to carry a coverage chart: a table of R-value against maximum coverage per bag, minimum thickness, minimum settled thickness and minimum weight per square foot. It exists because a bag holds a fixed amount of material, so the deeper you lay it the less floor it covers, and without the table nobody could compare two brands. Read the row for the R you are adding. It is the most authoritative number in this whole calculation and it is printed six inches from your hand.

Blown fiberglass

Loose glass fiber fluffed through a blower, usually in 25 to 33 lb bags. It does not settle, does not absorb water, and covers more square feet per bag than anything else, which means fewer bags and fewer trips. It has the lowest R per inch of anything you would put in an attic, so it takes the most depth: R-60 is about 24 inches. In a shallow layer it can be moved around by wind coming in the soffits, which is one more argument for depth.

Blown cellulose

Milled recycled paper treated with borate for fire and pest resistance, usually in 19 to 30 lb bags. Higher R per inch than blown fiberglass, denser, better at slowing air moving through the layer, and normally cheaper per R. It settles 13 to 20 percent, it weighs roughly twice as much per square foot at the same R, and it will hold water if the roof leaks, which slows the drying of everything under it.

Rock wool and mineral wool

Spun stone or steel slag. As loose fill it is the gray-brown gritty material in houses built before about 1960 and it is worth about 3.0 per inch. As batts it is the modern premium option: it holds its shape, it is water shedding rather than absorbing, it is a genuinely good sound break and it does not burn. It costs more than fiberglass and it is noticeably heavier to handle.

Batt sizes, and which one fits which stud

Batts come 15 in wide for 16 in on center framing and 23 in wide for 24 in on center, both sized to friction fit between studs. R-13 and R-15 are 3.5 in and fill a 2x4. R-21 is 5.5 in and fills a 2x6. R-19 is 6.25 in, which fits nothing standard: in a 2x6 it compresses to about R-18, and in a 2x4 to about R-13. R-30 is 9.5 in and belongs in a 2x10 floor bay.

Faced, unfaced, and which way the paper goes

The kraft paper or foil on a faced batt is a vapor retarder, and it goes toward the side that is warm in winter, which in almost all of the United States is the room side. Unfaced batts have no retarder and are the right choice over existing insulation, in a wall that already has a vapor barrier, and in hot humid climates where the vapor drive runs the other way. The one rule that never bends is that an assembly gets one vapor retarder, never two.

Board feet and spray foam kits

Spray foam is sold by the board foot: one square foot at one inch thick. So 100 sq ft at 2 in is 200 board feet, and thickness multiplies. DIY kits come in roughly 200 and 600 board foot classes and are almost always closed cell. The rating is a laboratory figure, and a cold substrate, overspray and the residue left in the tanks mean 10 to 25 percent under the label is the honest planning yield.

Rigid board: EPS, XPS and polyiso

Rigid foam belongs on continuous surfaces rather than between framing: basement walls, rim joists and exterior sheathing, where it also breaks the thermal bridge through the studs. Polyiso is the highest R per inch at room temperature but loses R as it gets cold, XPS is the water resistant middle, and EPS is the cheapest per R and stable over time. If you are putting continuous foam on the outside of a wall, the panel and trim takeoff belongs on the siding calculator, not here.

Vermiculite, and why this tool stops

Vermiculite loose fill looks like gray-brown popcorn or mica flakes and pours freely. The great majority of what was sold in the United States came from one Montana mine whose ore was contaminated with asbestos, and the EPA's standing advice is to treat any vermiculite in an attic as if it contains asbestos until a lab says otherwise. Do not disturb it, do not blow over it and do not store anything up there. Have it sampled by an accredited lab first. That is why this calculator refuses to print a bag count for an attic that has it.

Reading your result

Read the two bag counts together, because the contrast is the point of the page. The large number is what closes the gap between what you have and what you are aiming for. The dimmed number beside it is what the same attic costs if you pretend it is empty, which is what every manufacturer's calculator quotes you. The difference is not a discount, it is R-value you already own. Under that, the depth line matters as much as the bag count: it tells you how deep the material finishes, and whether it will bury your joists, which decides whether you need depth markers and walk boards. The estimated existing R is exactly that, an estimate from a depth, taken at the bottom of each published band on the assumption that twenty year old insulation has lost some loft. If your attic has clearly compressed material, cut the R per inch further; if it went in last year and still looks lofty, raise it. The cost ledger is a planning ledger and never a quote: every line is editable, the defaults sit mid-band for typical US retail, and the installed range printed beside it is what a contractor would charge for the same attic. If a quote sits inside that range the price is ordinary, and the conversation should be about air sealing and baffles instead.

Limitations

• This is planning guidance, not an energy audit. It gives you quantities and a depth, not a heat loss model, a payback period or a load calculation. If you want to know what a top-up will do to your bill, a blower door test and a real audit will tell you and this will not.
• R-value targets here are ENERGY STAR and DOE guidance for existing homes, published as bands. They are not code. Your local energy code, your permit office and any rebate program you are claiming under may all name different numbers, and where they do, theirs govern.
• Estimated existing R is derived from a depth and a published R per inch, deliberately at the low end of each band. Real material varies with density, compaction, moisture and age, and a depth taken in one spot will not describe an attic that somebody has crawled across.
• Cathedral ceilings, knee walls, skylight shafts, dormer cheeks and the awkward bays behind a chimney are all simplified to floor area here. They genuinely need more material per square foot and often a different assembly, so treat this as the flat part of the job.
• Coverage, yield, weight and price presets are typical figures for planning and every one of them is editable. The chart printed on the bag in your hand is the authority, and it will differ between brands and sometimes between production runs of the same brand.
• Spray foam kit yields are rated in laboratory conditions. Real yield depends on substrate temperature, tank temperature, technique and how much foam ends up on your sleeves. The derate here is a planning allowance, not a guarantee.
• This page counts thermal insulation for attics and cavities. Exterior cladding, house wrap and continuous exterior foam panels belong on the siding calculator, the drywall that closes the wall belongs on the drywall calculator, and shingles and ridge vents belong on the roofing calculator.
• Asbestos, knob-and-tube wiring, active roof leaks, mold and rodent contamination are all stop-and-get-help conditions, not quantities. If you find any of them, the next step is a professional, not a purchase.

Frequently asked questions

How many bags of blown insulation do I need for 1,000 sq ft?

It depends entirely on the R you are adding, not on the square footage alone, because a bag covers less floor the deeper you lay it. For 1,000 sq ft of bare attic to R-49, that is about 33 bags of blown fiberglass or about 61 bags of cellulose. If the attic already has six inches of cellulose in it, worth roughly R-19, you only need the R-30 gap and that falls to about 21 bags of fiberglass. Read the coverage chart on the bag for the row matching the R you are adding, then divide your area by that figure.

How many inches is R-38?

About 15.2 inches of blown fiberglass, 10.9 inches of blown cellulose, 11.2 inches of fiberglass batt, 12.3 inches of rock wool, or 5.8 inches of closed cell spray foam. Divide the R you want by the material's R per inch: blown fiberglass runs about 2.5, blown cellulose about 3.5, fiberglass batt about 3.4, rock wool about 3.2 and closed cell foam about 6.5. The converter on this page does it for any R and any material, both directions.

Can I put new insulation over old insulation?

Yes, and in almost every attic you should. The R-values add, so six inches of old cellulose at about R-19 plus sixteen inches of new fiberglass at R-41 gives you R-60. Removing the old material costs roughly $1 to $2 a square foot and throws away R you already own. There are four exceptions: insulation that is wet or has been wet, insulation that is moldy, insulation contaminated by rodents, and vermiculite, which needs testing for asbestos before anything at all happens. If you are laying batts over existing insulation, use unfaced batts so you do not create a second vapor retarder in the middle of the assembly.

What R-value do I need for my attic?

ENERGY STAR and the Department of Energy recommend by climate zone: R-30 to R-49 in zone 1, R-30 to R-60 in zones 2 and 3, R-38 to R-60 in zone 4, and R-49 to R-60 in zones 5 through 8. Roughly, that means R-30 for Hawaii and southern Florida, R-49 for the South and southern California, and R-60 from the mid-Atlantic and the Midwest northward. These are recommendations for an existing home rather than a code requirement, so confirm the number against your local energy code and any rebate program you are claiming under, because both can ask for something different.

Should I use faced or unfaced insulation for an attic top-up?

Unfaced, every time. The kraft or foil facing is a vapor retarder, and the existing insulation underneath, or the ceiling below it, is already handling that job. Adding a second retarder above the first traps moisture between them with no way to dry, which is how attic sheathing gets stained. Faced batts belong in a new wall or a new ceiling where there is no retarder yet, with the paper facing the side that is warm in winter. Loose fill, whether fiberglass or cellulose, has no facing at all, which is one more reason it suits a top-up.

How much does it cost to insulate an attic?

For a DIY blown-in top-up, budget roughly $1.30 to $1.80 per square foot of attic in materials at typical US retail, so about $1,600 to $2,200 for a 1,200 sq ft attic, including baffles, a hatch dam and depth markers. The blower rental is usually free for 24 hours if you buy ten bags or more, though the threshold varies by store. Installed by a contractor, the same work runs roughly $1.50 to $3.50 per square foot, or $1,800 to $4,200 on that attic. The gap is a day in a hot attic wearing a respirator, and it narrows sharply if the attic is hard to access.

Is cellulose or fiberglass better for blowing into an attic?

Cellulose is cheaper per R, needs about a third less depth for the same R, and is dense enough to slow air moving through the layer, which helps in a windy or leaky attic. Fiberglass does not settle, does not absorb water if the roof ever leaks, weighs about half as much on the ceiling, and covers far more square feet per bag, so it is fewer bags and fewer trips. Choose cellulose when depth or budget is tight, fiberglass when ceiling weight or water is the concern. Both are fine products and neither is a mistake.

Do I need baffles before I insulate my attic?

If your roof has soffit vents, yes, and they go in before any insulation does. A baffle is a channel stapled between the rafters at the eave that keeps a gap open above the insulation so air can move from the soffit up under the roof deck. Deep insulation at the eave buries the vents, the roof stops breathing, and you get frost on the sheathing in winter and mold after it thaws. Count one per rafter bay along the vented eaves: 80 feet of eave at 16 inches on center is 60 baffles, which costs less than two bags of insulation.

How do I know if my attic insulation is vermiculite?

Vermiculite looks like loose gray-brown pebbles, similar to popcorn or shiny mica flakes, and it pours rather than clumps, unlike the fibers of fiberglass or the shredded paper of cellulose. If that is what you find, stop. Most vermiculite sold in the United States came from one Montana mine whose ore was contaminated with asbestos, and the EPA advises treating any of it as if it contains asbestos until a lab says otherwise. Do not disturb it, do not blow over it, do not use the attic for storage, and have it sampled by an accredited asbestos lab. This calculator will not print a bag count for an attic that has it, because the first step is a test and not a purchase.