How many inches of insulation do I need for R-38?

About 12 inches of fibreglass batt, 11 inches of blown cellulose, 6½ inches of rigid polyiso, or 6 inches of closed-cell spray foam. Divide your target R by the material's R-per-inch to get the thickness.

What R-value do I need in my attic?

Roughly R-30 to R-49 in the warm South and R-49 to R-60 in the cold North. It depends on your climate zone, which is a map rather than a calculation — look up your county before you set the target.

Can I compress a batt to fit a smaller cavity?

You can physically, but you lose the R-value. Fibreglass insulates through the still air between the fibres, so compressing an R-19 batt into a 2x4 wall gives you about R-13. Buy the batt sized for the cavity you have.

Can I add new insulation over old?

Yes, and the R-values add. Blow loose fill straight over existing batts, or lay unfaced batts perpendicular across the joists. Never add a second faced batt — the extra vapour barrier traps moisture between the layers.

How many bags of blown-in insulation do I need?

It depends on the target R, because coverage per bag falls as depth rises. As a working figure, one bag covers about 24 square feet at R-27 or about 33 square feet at R-19. The coverage table printed on the bag is the number to trust.

Is spray foam worth the extra cost?

Where depth is limited or air sealing matters most — cathedral ceilings, rim joists, knee walls, hard-to-seal cavities — yes. In an open attic where you can pile depth on for free, cellulose or fibreglass gets you the same R for a fraction of the price.

Insulation Calculator

Enter the area, the insulation type and the R-value you are aiming for, and this calculator returns how much material to order and the R-value that will actually fit in your cavity depth. If you are topping up existing insulation, it adds what is already there.

Area to insulate(sq ft)

Insulation type

Target R-value(R)

R-13 to R-21 for walls, R-30 to R-60 for attics, depending on climate zone.

Stud or joist spacing

Cavity depth available(in)

A 2x4 wall is 3.5 in, a 2x6 is 5.5 in. An open attic is whatever you can pile up.

Existing insulation R-value(R)

Leave at 0 for a bare cavity. Old attic batts are often R-11 to R-19.

Batts to order

109

about 9.7 sq ft of coverage per batt

Area covered: 1,000sq ft
R-value achievedtarget met: R-38.0
Thickness required3.2 R per inch: 11.9in
Thickness that fits: 11.9in
R-value already in place: R-0
Estimated material costmaterial only, no labour or air sealing: $798

What this figure includes

Quantities come from the area you enter, the published R-per-inch of the product type, and the cavity depth available. It assumes full-depth, uncompressed installation with no gaps. It does not model air sealing, vapour retarders, baffles at the eaves, knee walls, rim joists, thermal bridging through the framing, or the settling that loose fill undergoes over its first year. Blown-in coverage is a working average; the bag itself prints the coverage table your inspector will read. Costs are material only.

How this is calculated

The formula

Insulation is one calculation repeated:

Thickness needed (in) = target R ÷ R per inch of the material

R achieved = existing R + (inches that actually fit × R per inch)

That second line matters more than the first. You can want R-38 all you like; if you have a 5½ inch cavity and fibreglass batt at R-3.2 an inch, you are getting R-17.6 and no amount of stuffing changes it.

R-per-inch for common products:

Fibreglass batt: about 3.2 per inch (R-13 in a 2x4, R-19 to R-21 in a 2x6)
Blown cellulose or loose fibreglass: about 3.4 per inch settled
Rigid polyiso board: about 6.0 per inch
Closed-cell spray foam: about 6.5 per inch
Open-cell spray foam: about 3.7 per inch

Higher R-per-inch is what you pay for. Foam board costs several times what fibreglass does for the same square foot, and buys you the same R in less than half the depth. When depth is free — an open attic — fibreglass and cellulose win on cost every time. When depth is constrained — a cathedral ceiling, a 2x4 wall, the back of a knee wall — the expensive product is the only one that gets you there.

A worked example

An unfinished attic floor, 1,000 sq ft, joists at 24 in on centre, aiming for R-38 with blown-in cellulose over existing R-11 batts you are leaving in place.

R still needed: 38 − 11 = R-27
Thickness of cellulose: 27 ÷ 3.4 = 7.9 inches on top of what is there
Coverage: a bag covering 640 sq ft at R-1 covers 640 ÷ 27 = 23.7 sq ft at R-27
Bags: 1,000 ÷ 23.7 = 43 bags
Achieved: R-11 + R-27 = R-38, target met

Note that the joists are only 5½ or 7¼ inches deep. That is fine on an attic floor — the cellulose simply buries them, and burying them is good, because the wood itself is only about R-1.2 an inch and is a thermal bridge straight through your insulation.

Compressing a batt destroys it

This is the single most common installation error, and it is invisible once the drywall is on.

Fibreglass works by trapping still air. The glass fibres themselves conduct heat perfectly well; the R-value lives in the air pockets between them. Squash an R-19 batt intended for a 2x6 into a 3½ inch 2x4 cavity and you do not get R-19 in less space — you get about R-13, because you have crushed out the air that was doing the work. You have paid for R-19, used the labour to install it, and got the performance of the cheaper batt.

The same principle bites at the edges. A batt cut short leaves a gap; a batt jammed in leaves a bulge that the drywall then compresses. Cut to fit, split the batt around wiring rather than stuffing it behind, and let it fill the cavity to its full loft. An R-19 batt with a five percent gap performs about like R-14 — gaps hurt far more than the arithmetic suggests, because air moves through them.

Attic and wall targets are different, and both depend on climate

Attics get the highest numbers because heat rises and because the depth is free. Walls are limited by framing depth and by what it costs to build a thicker wall.

Current guidance runs roughly:

Attics: R-30 to R-49 in the warm South, R-49 to R-60 in the cold North
Walls: R-13 to R-15 in a 2x4, R-19 to R-21 in a 2x6, with continuous exterior foam adding R-5 or more on top in colder zones
Floors over unconditioned space: R-19 to R-30
Crawlspace walls: R-10 to R-19 where the crawl is sealed and conditioned

Those bands come from climate zone, which is a map, not a formula. Zone 1 is south Florida, zone 7 is northern Minnesota, and the recommended attic R roughly doubles between them. Look up your county's zone before you settle on a target — this calculator will happily build to whatever number you type, and typing the wrong one is the mistake it cannot catch for you.

Air sealing comes first

An unsealed attic with R-49 over it underperforms a properly sealed attic at R-30. Insulation slows conduction; it does almost nothing about air moving through gaps around can lights, top plates, plumbing stacks and the attic hatch. Seal those with foam and caulk before a single bag goes in, install baffles so the soffit vents stay clear, and only then blow the depth. It is the cheapest performance you will ever buy.

Cost figures last reviewed 9 Sept 2026 · Published R-per-inch figures for common insulation products, with US retail material prices normalised per square foot per R-1.

Frequently asked questions

How many inches of insulation do I need for R-38?: About 12 inches of fibreglass batt, 11 inches of blown cellulose, 6½ inches of rigid polyiso, or 6 inches of closed-cell spray foam. Divide your target R by the material's R-per-inch to get the thickness.
What R-value do I need in my attic?: Roughly R-30 to R-49 in the warm South and R-49 to R-60 in the cold North. It depends on your climate zone, which is a map rather than a calculation — look up your county before you set the target.
Can I compress a batt to fit a smaller cavity?: You can physically, but you lose the R-value. Fibreglass insulates through the still air between the fibres, so compressing an R-19 batt into a 2x4 wall gives you about R-13. Buy the batt sized for the cavity you have.
Can I add new insulation over old?: Yes, and the R-values add. Blow loose fill straight over existing batts, or lay unfaced batts perpendicular across the joists. Never add a second faced batt — the extra vapour barrier traps moisture between the layers.
How many bags of blown-in insulation do I need?: It depends on the target R, because coverage per bag falls as depth rises. As a working figure, one bag covers about 24 square feet at R-27 or about 33 square feet at R-19. The coverage table printed on the bag is the number to trust.
Is spray foam worth the extra cost?: Where depth is limited or air sealing matters most — cathedral ceilings, rim joists, knee walls, hard-to-seal cavities — yes. In an open attic where you can pile depth on for free, cellulose or fibreglass gets you the same R for a fraction of the price.