Blown-In Insulation Calculator

Blown-In Insulation formula

R-value to add = Target R-value - Existing R-value
Depth (in) = R-value to add / R per inch  (cellulose 3.5, fiberglass 2.5)
Weight (lb) = Area (sq ft) x Depth (in) / 12 x settled density (lb per cu ft)
Bags = Weight / bag weight x (1 + waste %)

Worked example

A 25 x 32 ft attic with about R-11 of old fiberglass, brought up to R-49 with blown fiberglass: 25 x 32 = 800 sq ft. 49 - 11 = R-38 to add. 38 / 2.5 = 15.2 in of new material. 800 x 15.2 / 12 = 1,013.3 cubic feet. At 0.7 lb per cubic foot that is 709.3 lb, and 709.3 / 28 = 25.3 bags. With 5% waste, 26.6, so buy 27 bags.

The existing R-11 is about 4.4 in at R-2.5 per inch, so the finished depth is about 19.6 in. Mark that height on the rafters or on attic rulers before you start blowing.

Quick reference

How to calculate blown-in insulation

Measure the attic floor at the outside walls and multiply for square feet; do not subtract anything but a large stairwell. Decide the target R-value for your climate zone, subtract what is already there, and divide by the R per inch of the product to get the settled depth to add. Every bag lists a coverage chart with the number of bags per 1,000 sq ft at each R-value; the calculator reproduces that chart from the settled density and bag weight, so compare the two and use the larger number.

Blown-in works over existing batts or old loose fill as long as the old material is dry. Wet, moldy or rodent-damaged insulation should be removed first. Vermiculite may contain asbestos and must not be disturbed without testing.

Before you blow

Air seal the attic floor first: foam or caulk around top plates, wiring holes, plumbing stacks, the chimney chase and recessed lights, and build a dam around the hatch. Install baffles at every rafter bay so the new insulation cannot block the soffit vents, and keep insulation 3 in away from non-IC rated recessed lights and flue pipes.

Staple attic rulers to the trusses every few feet and blow to the installed depth on the bag chart, which is thicker than the settled depth. Work from the far corners back to the hatch with one person feeding the hopper. Wear a respirator, eye protection and long sleeves, and work on cool mornings in summer.

Key facts

• The US Department of Energy recommends R-38 to R-60 in attics for most of the country, and R-49 or more in climate zones 4 and colder.
• Blown cellulose is about R-3.2 to R-3.8 per inch and settles to roughly 1.5 to 1.6 lb per cubic foot; blown fiberglass is about R-2.2 to R-2.7 per inch at 0.5 to 0.8 lb per cubic foot.
• The IRC requires attic rulers or depth markers about every 300 sq ft so the installed depth can be checked, and a card stating the installed R-value near the attic access.
• Most big box stores lend the blowing machine free for the day when you buy 10 or more bags, which is why bag counts are rounded up to whole bags.
• Cellulose can settle up to 20 percent from its blown depth, so the bag chart lists a thicker installed depth than the settled depth that delivers the rated R-value.

Frequently asked questions

Sources and references

• •Sanctuary blow-in cellulose insulation fact sheet with attic coverage chart (25 lb bags) (Greenfiber)
• •AttiCat PINK blown-in fiberglass insulation product data sheet (bags per 1,000 sq ft by R-value) (Owens Corning)
• •Recommended Home Insulation R-Values by climate zone (ENERGY STAR)
• •Chapter 11 Energy Efficiency (insulation requirements, blown insulation depth markers), 2021 IRC (ICC)