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Insulation

Insulation Calculator

Turn attic square footage and a target R-value into a bag count taken straight from the manufacturer’s coverage chart — plus the depth you have to rake it to for that R-value to be real.

How are you measuring the attic?
ft
ft
Product

ENERGY STAR recommends, over an uninsulated attic: Zone 1 R30, Zone 2 R49, Zone 3 R49, Zone 4A / 4B R60, Zone 4C, 5, 6 R60, Zone 7 / 8 R60. Over 3–4 in of existing insulation the added figure drops (for example R49 in Zones 7–8). These are cost-effective retrofit recommendations, not code minimums — your permit office applies whatever IECC table your jurisdiction adopted.

%

Defaults to 0, unlike most calculators on this site. The manufacturer's bag count is already a stated minimum — "shall not be less than" — and rounding up to whole bags adds a margin on top. Add a percentage only if you want spare on the truck.

Buy

25

bags

Blow to 17 in for R-49

The working

Area: 40 × 25 = 1000.0 sq ft

R-49 requires 25 bags per 1,000 sq ft

1000.0 ÷ 1,000 × 25 = 25.00 bags

→ 25 whole bags

Check: 25 × 39.9 sq ft/bag = 997.5 sq ft

Install depth: not less than 17 in

Planning estimate only. Check the result against current product instructions, local requirements, and your own takeoff. See the calculator disclaimer.

Depth and bag count are one rule

The mistake that wastes the most money in an attic is blowing to a depth that looks right and assuming the R-value followed. It does not. Loose-fill only reaches its rated R-value at a given density, which means a specific weight of material spread over a specific area — and depth alone does not tell you the density. Blow 17 inches of AttiCat using 18 bags instead of 25 over 1,000 sq ft and you have a fluffy 17 inches that does not perform like R-49.

That is why Owens Corning’s install card states both numbers together and says you need to install both. This calculator reports both for the same reason: the hero number is the bag count, and the line under it is the depth that bag count has to be raked to.

Why the two published columns disagree

Every loose-fill coverage chart prints two figures that ought to be reciprocal: minimum bags per 1,000 sq ft, and maximum coverage per bag. They rarely reconcile exactly, because both are rounded for the label. AttiCat at R-22 lists 10.5 bags per 1,000 and 94.8 sq ft per bag — but 1,000 ÷ 94.8 is 10.549, not 10.5.

The gap is small and the direction matters. The manufacturers word the bag column as a floor: the number of bags "shall not be less than". So this tool counts from that column and shows the coverage cross-check in the working rather than hiding the discrepancy. If the two disagree enough to change your order, the calculator says so instead of picking silently.

What this does not do

It will not tell you the R-value of whatever is already in your attic. Converting a measured depth of existing insulation into an R-value requires knowing what the material is and how far it has settled, and a wrong assumption there compounds straight into the order. ENERGY STAR sidesteps the problem with a separate published column for attics that already have 3–4 inches, and so does this tool.

It also does not cover ventilation baffles, air sealing, or the clearance rules around recessed lights and chimneys — all of which come before insulation on a real job. Both manufacturers warn against insulating on top of or within 3 inches of non-IC-rated recessed fixtures. Air sealing first is what makes the insulation work; blowing over unsealed top plates and open chases buries the problem instead of fixing it.

Sources

Grouped by manufacturer or publisher, then by document. Expand a group for the cited document, its verified-on date, and every value taken from it — all of it stays in the HTML either way.

CertainTeed(1 document, 1 value)
Owens Corning(1 document, 1 value)
Other sources(1 document, 1 value)

Frequently asked questions

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

It depends entirely on the R-value you are blowing to, and the answer comes off the manufacturer’s own chart rather than a rule of thumb. At R-49, Owens Corning AttiCat requires a minimum of 25 bags per 1,000 sq ft, so a 1,000 sq ft attic is 25 bags. The same attic at R-30 is 14.6 bags, so 15. CertainTeed InsulSafe SP runs slightly further per bag because its bags are heavier — 23.5 bags per 1,000 at R-49.

What R-value should my attic be?

ENERGY STAR recommends R-30 in Climate Zone 1, R-49 in Zones 2 and 3, and R-60 in Zones 4 through 8 when adding to an uninsulated attic. If you already have 3–4 inches up there, the recommended addition drops to R-25 through R-49 depending on zone. Those are cost-effective retrofit recommendations based on the 2021 IECC, not code minimums — your building department applies whichever code edition your jurisdiction adopted.

How deep does blown-in insulation need to be?

Depth and bag count are two halves of the same requirement, and hitting one without the other does not give you the rated R-value. AttiCat needs 17 inches for R-49 and 20.5 inches for R-60. InsulSafe SP needs 18.25 and 21.75 inches for the same two. Owens Corning states it plainly: you must install both the minimum thickness and the prescribed number of bags. Rake to depth and check with a ruler stapled to a truss.

Why does this calculator use bags per 1,000 sq ft instead of coverage per bag?

Both figures are printed on the same chart, and they are not exact reciprocals because each is rounded independently. The manufacturers state bags per 1,000 sq ft as a floor — the number of bags "shall not be less than" — so that is the binding figure and the one this tool counts from. The working shows the coverage-per-bag cross-check underneath so you can see both, which is more than the bag label gives you.

Do I need to add a waste factor to blown-in insulation?

Not in the way you would for tile or drywall, which is why this calculator defaults its overage to 0 while most others on this site start at 10%. The published bag count is already a minimum rather than a target, and rounding up to whole bags adds margin on top. Contractors often still throw an extra bag on the truck rather than make a second trip, so the field is there and adjustable.

Can I just add insulation on top of what is already in the attic?

Usually yes, and ENERGY STAR publishes a separate column for exactly that case — over 3–4 inches of existing insulation you add less. What this calculator will not do is convert an arbitrary existing depth into an R-value, because that needs an R-per-inch figure for a material that may be settled, mixed, or unidentifiable. Guessing it would put a wrong number in front of you with no way to check it.