Enter your area and target R-value to calculate the number of bags of blown-in insulation or batt rolls needed, with cost estimate.
Area to Insulate (sq ft)
Target R-Value
Attic — climate zones 4–5 (most of US)
Insulation Type
Price per Bag
Waste Factor
10%Results — R-38
Enter area to calculate
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Try Suparate FreeAir seal before you insulate — always
Insulation slows heat transfer but does not stop air movement. Seal all attic penetrations — recessed lights, top plates, plumbing and electrical penetrations — with canned spray foam or acoustical caulk before blowing in any insulation. Air bypasses account for up to 40% of a home's heat loss even when insulation R-values look adequate on paper.
Follow the bag count chart, not a rule of thumb
The coverage chart printed on the blown-in bag is calibrated to the specific product density and is the number required to meet labeled R-values. Blown-in fiberglass compresses over time, so under-blowing is the most common installation mistake. Set attic depth rulers (R-value markers) at multiple locations so you can verify installed thickness before the job is called done.
Protect yourself — fiberglass is no joke
Always wear an N95 or P100 respirator, safety glasses, long sleeves, and gloves when handling fiberglass insulation. Fibers lodge in lungs and skin and the irritation may not start for hours after exposure. Work with the wind at your back when blowing insulation and take regular breaks in clean air.
The U.S. Department of Energy recommends R-38 to R-60 for attics depending on your climate zone. Zones 1–3 (southern states like Florida and Texas) call for R-30 to R-49; zones 4–5 (mid-Atlantic, midwest) need R-38 to R-60; zones 6–8 (northern states and Alaska) require R-49 to R-60. Check the DOE's zip-code climate zone map to find your specific recommendation before purchasing material.
Divide your attic square footage by the coverage rate per bag for your target R-value — this is printed on the bag's coverage chart and varies by product. As a reference, Owens Corning AttiCat fiberglass covers about 10.5 sq ft per bag at R-38; a 1,500 sq ft attic needs roughly 143 bags, plus 10% waste = 158 bags. Always buy to the chart on your specific bag, not generic estimates, as coverage rates differ between manufacturers.
Blown-in loose-fill insulation (fiberglass or cellulose) is pneumatically installed with a hose, filling gaps and voids around pipes, wires, and framing members better than batts can. Batt insulation comes in pre-cut blankets sized for standard stud and joist spacing and installs without special equipment. Blown-in is the preferred method for existing attics and retrofit projects; batts are faster and less expensive for new construction with open, accessible stud cavities.
Calculate total wall area by multiplying the perimeter by the wall height, then subtract door and window openings (a standard door is about 21 sq ft, a standard window about 15 sq ft). For 2×4 stud walls, use R-11 to R-15 batts (3.5" cavity); for 2×6 walls, use R-19 to R-21 batts (5.5" cavity). Divide the net wall area by the coverage per roll or bag, then add 10% for waste around outlets, pipes, and cutting.
Yes — topping up attic insulation is one of the highest-return energy upgrades available. Use unfaced batts or loose fill when adding over existing material — never install faced (vapor barrier) batts over existing insulation, as the trapped vapor barrier can cause moisture problems and mold. Before adding anything, verify that the existing insulation is dry and free of mold, and seal all air-bypasses (recessed lights, top plates) first for maximum effectiveness.