Attic Insulation Calculator

How to Use This Calculator

Select your insulation material: blown cellulose for most attic projects (best value), blown fiberglass for humid climates or attics with moisture concerns. Enter the area to insulate in square feet (attic length times width for a rectangular attic; add sections for irregular shapes). Set your target R-value. For most US attics, R-38 is the minimum and R-49 to R-60 is ideal. If you have existing insulation, enter its current R-value; the calculator finds only what you need to add. The result shows: depth to add in inches, number of bags with a 10% waste factor, total final depth, and a cost estimate. A full R-value guide is on our main insulation calculator page.

How to Calculate Attic Insulation

The formula for blown insulation bags: Required Depth (inches) = (Target R-Value - Existing R-Value) ÷ R-Value per inch of material. Then Bags = (Area × Depth ÷ 12 × Installed Density) ÷ Bag Weight × 1.10 waste factor.

Attic Insulation Depth and R-Value

The required depth depends on your target R-value, existing insulation, and the R-value per inch of the selected material. An Attic R-Value Calculator like the one above can help determine how much additional insulation is needed when your attic already has an existing insulation layer.

R-value per inch by material:

Material	R-value per inch	Depth for R-38	Depth for R-49
Blown cellulose	3.7	10.3 in	13.2 in
Blown fiberglass	2.5	15.2 in	19.6 in

Worked example: 1,500 sq ft attic, R-38 target, existing R-11, blown cellulose. Additional R needed = 38 - 11 = 27. Depth = 27 ÷ 3.7 = 7.3 inches. Volume = 1,500 × (7.3 ÷ 12) = 912.5 cu ft. At 1.5 lb/cu ft, that is 1,369 lbs of cellulose. At 30 lbs per bag: 45.6 bags × 1.10 = 50.2, so order 51 bags.

Recommended R-Value by Climate Zone

Zone	Typical states / regions	Code minimum	Recommended
1–2	South FL, Gulf Coast, HI	R-30	R-38 to R-49
3	Southeast, West Coast	R-30	R-38 to R-60
4	Mid-Atlantic, Pacific NW	R-38	R-38 to R-60
5	Midwest, Great Plains	R-49	R-49 to R-60
6–7	Northern states, Mountain West	R-49	R-49 to R-60
8	Alaska	R-49	R-60

Attic Insulation Tips

Air seal before you insulate. Attic air leaks lose more heat than the insulation layer itself in many older homes. Before blowing insulation, seal around every penetration: recessed light fixtures, plumbing stacks, wire chases, and the tops of interior partition walls. Use acoustic sealant or spray foam. An hour of air sealing before insulating will double the effectiveness of your insulation upgrade.

Install proper attic ventilation before insulating. Insulation that covers soffit vents will cause moisture to accumulate in the attic, leading to mold and rot. Install baffles (cardboard or foam insulation dams) at every rafter bay along the eaves to keep a ventilation channel open between the insulation and the roof deck. Most building codes require 1 inch of clearance minimum.

Rent the blowing machine when you buy the bags. Most hardware stores that sell blown insulation offer free or low-cost machine rental with a minimum bag purchase of 10 to 20 bags. The machine is pneumatic and runs on a standard 110V outlet. The hose needs to reach all areas of your attic; pick up an extension hose if your attic is large.

What to Buy

For blown cellulose, look for bags labeled "R-value per bag" with a coverage chart printed on the packaging. Coverage varies by brand and bag weight (typically 25 to 30 lbs). Always use the bag's own coverage chart for your target R-value rather than a generic formula, because installed density varies between products.

For blown fiberglass, bags typically weigh 19 to 25 lbs each. Coverage per bag is lower than cellulose at equivalent R-values. Major brands (Owens Corning, CertainTeed) include coverage charts on every bag that tell you exactly how many bags per 1,000 sq ft at each R-value. Verify the chart matches your target R-value and buy 10% more than the chart suggests to account for irregular attic shapes and waste.

Signs You Need More Attic Insulation

Ice dams forming on the roof edge in winter, a sign heat is escaping through the attic and melting snow unevenly
Uneven room temperatures, especially upstairs rooms that run hot in summer or cold in winter
Rising heating or cooling bills without a clear cause
Visible ceiling joists; if you can see the tops of the joists above the insulation line, you likely have less than R-19
Cold floors in rooms directly below an unheated attic
Icicles hanging from the eaves after a snowfall, another heat-loss indicator

Frequently Asked Questions

How many bags of blown insulation do I need for a 1,200 sq ft attic? +

For a 1,200 sq ft attic to R-38 using blown cellulose: approximately 57 bags (30 lb bags). To R-49: approximately 73 bags. To R-60: approximately 89 bags. Blown fiberglass requires 40 to 50% more bags than cellulose for the same R-value because it is less dense. Use the calculator with your specific target to get an exact count with the waste factor included.

What R-value should my attic have? +

The Department of Energy recommends R-38 to R-60 for most attic applications in the continental US. Climate zones 1 and 2 (Southern Florida, Gulf Coast): R-30 minimum. Zones 3 and 4 (Southeast, Pacific Northwest, Mid-Atlantic): R-38 to R-49. Zones 5 through 7 (Midwest, Mountain states, Northern states): R-49 to R-60. Zone 8 (Alaska): R-60 minimum. Most homes built before 1990 have R-11 to R-19 in the attic. Upgrading to R-38 or R-49 is typically the single highest-return energy improvement a homeowner can make.

How deep is R-38 of blown cellulose insulation? +

Blown cellulose has an R-value of approximately 3.7 per inch. R-38 requires about 10.3 inches of depth. R-49 requires about 13.2 inches. R-60 requires about 16.2 inches. These depths assume no existing insulation. If you have existing insulation, use the calculator's existing R-value field to calculate only the additional depth needed.

Should I use blown cellulose or blown fiberglass in my attic? +

Blown cellulose (3.7 R per inch) is denser, covers more area per bag, and costs less per bag than blown fiberglass (2.5 R per inch). Cellulose is the better value for most attic projects. Blown fiberglass is lighter, faster to install, and does not absorb moisture, making it a practical choice in humid climates or attics with any history of water infiltration. Both materials can be installed as DIY projects with a rented blowing machine.

Can I blow insulation over existing insulation? +

Yes. Blown cellulose and blown fiberglass can be applied directly over existing fiberglass batts without removing them. This is the standard approach when topping up an under-insulated attic. If existing batts have a vapor retarder facing (kraft paper), it should face the warm side of the house (downward in most US climates). Enter your existing R-value in the calculator to find how much additional material you need.

How long does it take to insulate an attic with blown insulation? +

A 1,000 to 1,500 sq ft attic typically takes 4 to 6 hours for a two-person team. One person loads bags into the blowing machine; the other directs the hose in the attic. The blowing machine itself is straightforward: most hardware stores provide an operating guide when you rent it with a minimum bag purchase. Wear a respirator, safety glasses, and long sleeves; cellulose and fiberglass particles are irritating to airways and skin.

Does adding attic insulation really reduce energy bills? +

Yes, reliably for under-insulated homes. Going from R-11 to R-38 in an attic typically reduces heating and cooling costs by 15 to 25% depending on climate and heating system. The payback period for DIY attic insulation is often 2 to 5 years, making it one of the best return-on-investment home improvements available. Professional installation extends the payback period to 5 to 10 years but is still typically cost-effective over a home's lifetime.

How do I calculate attic insulation cost? +

Calculate the required number of bags using the calculator above, then multiply the quantity by the current price per bag. Add potential costs for machine rental, delivery, air sealing materials, or professional installation. Comparing DIY material costs against a few contractor quotes is the fastest way to see whether renting a blowing machine is worth your time.