R-value measures how well insulation resists heat. ENERGY STAR publishes the levels that pay off in each climate zone, from the warm South to the far North, and each material reaches them at a different depth: loose fiberglass needs the most, closed-cell spray foam the least.
How to use this calculator
- Measure the attic floor or the floor over the crawl space, length and width.
- Pick where the insulation goes. If the attic already has a few inches, choose that option: the target is then the amount to add.
- Pick your climate zone. The Department of Energy's climate zone map shows it by county.
- Choose the insulation. For blown-in fiberglass you also get a bag count from one maker's coverage chart.
Worked examples
An empty attic in a cold climate
Size: 40 × 30 ft · Where: Attic floor, no insulation now · Climate zone: Zone 7 or 8
- Recommended R-value
- R-60
- Bags for R-60
- 38
- Settled depth for R-60
- 20.5 in
- Area
- 1,200 sq ft
- Volume of insulation
- 2,050 cubic feet
Topping up an attic
Size: 50 × 28 ft · Where: Attic floor, 3 to 4 in already there · Climate zone: Zone 4A or 4B
- Recommended R-value
- R-49
- Bags for R-49
- 35
- Settled depth for R-49
- 17.0 in
- Area
- 1,400 sq ft
- Volume of insulation
- 1,983 cubic feet
Cellulose in a mild climate
Size: 36 × 26 ft · Where: Attic floor, no insulation now · Climate zone: Zone 3 · Insulation: Blown-in cellulose
- Recommended R-value
- R-49
- Depth for that R-value
- 13.2 in
- Area
- 936 sq ft
- Volume of insulation
- 1,033 cubic feet
Floor over a crawl space
Size: 40 × 25 ft · Where: Floor over a crawl space or garage · Climate zone: Zone 4C, 5 or 6 · Insulation: Fiberglass batts or rolls
- Recommended R-value
- R-30
- Depth for that R-value
- 9.4 in
- Area
- 1,000 sq ft
- Volume of insulation
- 781 cubic feet
Recommended R-values by climate zone
ENERGY STAR's table, based on the model energy code, asks for R-30 in an uninsulated attic in the warmest zone and R-60 in the colder zones. If the attic already has a few inches of insulation, add R-25 to R-49 on top.
Floors over unheated space get less: from R-13 in the South to R-38 in the coldest zones.
R-value per inch of common insulation
- Blown fiberglass: about R-2.2 per inch, so it takes the most depth.
- Fiberglass batts: about R-3.2 per inch.
- Blown cellulose: about R-3.7 per inch.
- Open-cell spray foam: about R-3.6 per inch.
- Closed-cell spray foam: about R-6.1 per inch, the most per inch of these.
Before you add insulation
- Seal the gaps around pipes, wires, chimneys and the attic hatch first: air leaks carry more heat than thin insulation does.
- Keep soffit vents open with baffles so the attic can still breathe.
- Keep insulation the required distance from recessed lights unless they are rated for contact, and from chimneys and flues.
Common questions
How much insulation do I need in my attic?
It depends on your climate zone. ENERGY STAR recommends R-30 in the warmest zone and R-60 in most of the country's colder zones. Pick your zone above to see the depth for your insulation.
How many bags of blown-in insulation do I need?
Divide the attic area by a thousand and multiply by the bag count on the coverage chart for your R-value. For R-38, one maker's chart calls for 19 bags per 1,000 sq ft; the calculator does the math for your attic.
How deep should blown-in insulation be?
Deep enough to reach the R-value on the bag chart. For one maker's blown fiberglass, R-49 takes about 17 inches settled. Cellulose reaches the same R-value with less depth.
Can I put new insulation over old insulation?
Yes, as long as the old insulation is dry and has no mold. Lay new batts across the old ones without a facing, or blow loose fill on top.