How Much Insulation for a 1,800 Sq Ft Attic?

A 1,800 sq ft attic going from R-11 to R-49 needs about 15.2 inches of blown fiberglass, which is 2,280 cubic feet of insulation. The tables cover other starting R-values, targets and insulation types.

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Insulation to add

15.2 inches

R-38 more to reach R-49

Volume of insulation: 2,280 ft³
R-value to add: 38 R
Attic floor area: 1,800 ft²

Loose-fill insulation settles and its R-value per inch varies with the product, so use the coverage chart on the bag for the exact bag count. Air-seal gaps and bypasses before insulating, keep insulation off the soffit vents (use baffles), and keep it clear of recessed lights unless they are IC-rated.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Insulation depth to add, by current and target R-value

Depths are for blown fiberglass at R-2.5 per inch, in a 1,800 sq ft attic. Loose-fill settles, so add a small allowance, and keep it clear of the soffit vents with baffles.

You have now	To R-30	To R-38	To R-49	To R-60
None (bare floor)	12.0 in	15.2 in	19.6 in	24.0 in
R-11	7.6 in	10.8 in	15.2 in	19.6 in
R-19	4.4 in	7.6 in	12.0 in	16.4 in
R-30	none needed	3.2 in	7.6 in	12.0 in

Which insulation, going from R-11 to R-49

The R-value you add is the same, but the thickness and the volume change with the type of insulation. Spray foam is much thinner for the same R-value but costs more.

Type	R per inch	Depth to add	Volume to buy
Blown fiberglass	R-2.5	15.2 in	2,280 cu ft
Fiberglass batts	R-3.2	11.9 in	1,781 cu ft
Blown cellulose	R-3.5	10.9 in	1,629 cu ft
Open-cell spray foam	R-3.7	10.3 in	1,541 cu ft
Closed-cell spray foam	R-6.5	5.8 in	877 cu ft

Use the attic insulation calculator for your own attic, and read how much attic insulation you need for climate-zone targets. Seal air leaks first, and check attic ventilation before you add more.

Want to change anything else? The calculator above is the full Attic Insulation Calculator: edit any number and the results update instantly.

Tools & materials for this project

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Blown-in fiberglass insulation Fiberglass batts (R-30)Attic baffles Spray foam sealant Respirator mask

Frequently asked questions

How many inches of insulation does a 1,800 sq ft attic need?

From R-11 to R-49 with blown fiberglass (R-2.5 per inch) you add about 15.2 inches. From a bare floor it is 19.6 inches. The depth is the same for any attic size: the area only changes how much material you buy.

How much insulation do I need to buy for a 1,800 sq ft attic?

About 2,280 cubic feet of blown fiberglass to go from R-11 to R-49. Check the coverage chart on the bag: it lists bags needed at each depth, and settling means you should buy a few extra.

Is R-38 enough for a 1,800 sq ft attic?

R-38 is a common recommendation for milder climates, and from R-11 it means adding 10.8 inches of blown fiberglass (1,620 cu ft). Cold climates are better at R-49 to R-60. Check your climate zone in the calculator.

Related sizes and guides

Related sizes

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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