Recommended R-values by climate zone
R-value measures how well insulation resists heat flowing through it, and the higher the number, the less heat gets through. ENERGY STAR publishes how much insulation is cost-effective to add to existing wood-framed homes in each US climate zone, based on the 2021 International Energy Conservation Code (Table R402.1.3). Find your zone on the map on the ENERGY STAR page. The Department of Energy’s consumer guide adds that zone 1 also covers Hawaii, Guam, Puerto Rico and the Virgin Islands, and that Alaska is in zone 7 except for some interior boroughs near and north of the Arctic Circle, which are in zone 8.
| Zone | Attic, uninsulated | Attic with 3 to 4 in | Floor | Basement or crawlspace wall |
|---|---|---|---|---|
| 1 | R-30 | R-25 | R-13 | none listed |
| 2 | R-49 | R-38 | R-13 | none listed |
| 3 | R-49 | R-38 | R-19 | R-5 sheathing or R-13 batt |
| 4A or 4B | R-60 | R-49 | R-19 | R-10 sheathing or R-13 batt |
| 4C | R-60 | R-49 | R-30 | R-15 sheathing or R-19 batt |
| 5 | R-60 | R-49 | R-30 | R-15 sheathing or R-19 batt |
| 6 | R-60 | R-49 | R-30 | R-15 sheathing or R-19 batt |
| 7 | R-60 | R-49 | R-38 | R-15 sheathing or R-19 batt |
| 8 | R-60 | R-49 | R-38 | R-15 sheathing or R-19 batt |
The attic columns list the R-value to add. For walls, ENERGY STAR recommends that whenever exterior siding comes off an uninsulated wood-frame wall, you drill holes in the sheathing and blow insulation into the empty cavity before the new siding goes on, and add R-5 insulative wall sheathing in zone 3 and R-5 to R-10 in zones 4 to 8. On an insulated 2x4 wall in zones 4 to 8, it recommends R-10 sheathing under the new siding. Use "My own R-value" for those.
How thick insulation needs to be
For materials sold by R-value per inch, the thickness is the target R-value divided by the R per inch. The U.S. Department of Energy’s guide to home insulation gives these values:
| Material | DOE figure | R per inch | Thickness for R-38 |
|---|---|---|---|
| Open-cell spray foam | R-12.6 at 3.5 in | 3.6 | 10.56 in |
| Closed-cell spray foam | R-6.5 at 1 in | 6.5 | 5.85 in |
| Expanded polystyrene (EPS) board | R-3.8 to 4.4 at 1 in | 3.8 to 4.4 | 8.64 to 10 in |
| Extruded polystyrene (XPS) board | R-5 at 1 in | 5 | 7.6 in |
| Polyisocyanurate board | R-6.5 at 1 in | 6.5 | 5.85 in |
For EPS the calculator uses the lower R-3.8 per inch, so the thickness is enough at either end of the range. Products differ, so check the R-value printed on the board or the spray foam installer’s data.
Batts come in fixed R-values. The calculator uses the Owens Corning EcoTouch line: R-11, R-13 and R-15 at 3 1/2 inches, R-21 at 5 1/2 inches, R-19 at 6 1/4 inches, R-22 at 6 3/4 inches, R-25 at 8 inches, R-30 at 9 1/2 inches, R-38 at 12 inches and R-49 at 14 inches, plus R-30C and R-38C cathedral ceiling batts at 8 1/4 and 10 1/4 inches. When no single batt reaches the target, it picks the two batts with the smallest total thickness. Wall batts come 93 inches long and the thicker floor and ceiling batts 48 inches. Each batt fills one framing bay, so it covers the spacing times its length.
Blown-in insulation is sold by the bag, and every bag carries a chart of bags per 1,000 square feet and minimum thickness for each R-value. The calculator uses the AttiCat fiberglass chart from Owens Corning (27.5 lb bags) and the Sanctuary cellulose chart from Greenfiber (25 lb bags, loose-fill, without the framing adjustment). For a target between two rows it uses the next higher row. Both makers state that you must install at least the listed number of bags and reach the minimum thickness to get the R-value. Greenfiber adds that job conditions and equipment can change coverage by as much as 10 percent.
| R-value | Fiberglass min. thickness | Fiberglass bags per 1,000 sq ft | Cellulose installed / settled | Cellulose bags per 1,000 sq ft |
|---|---|---|---|---|
| R-13 | 5 in | 5.9 | 3.9 / 3.6 in | 15.2 |
| R-19 | 7 in | 9 | 5.7 / 5.3 in | 22.6 |
| R-22 | 8 in | 10.5 | 6.6 / 6.1 in | 26.3 |
| R-26 | 9.5 in | 12.6 | - | - |
| R-30 | 10.75 in | 14.6 | 8.9 / 8.3 in | 38.6 |
| R-38 | 13.5 in | 19 | 11.2 / 10.4 in | 52.6 |
| R-44 | 15.5 in | 22.4 | 12.9 / 12 in | 63.5 |
| R-49 | 17 in | 25 | 14.3 / 13.3 in | 70.8 |
| R-60 | 20.5 in | 31.5 | 17.3 / 16.1 in | 86 |
Worked example
An uninsulated attic of 1,200 square feet in zone 5 should get R-60. Blown-in fiberglass needs at least 20.5 in and 1.2 × 31.5 = 37.8, so 38 bags. Blown-in cellulose goes in at 17.3 in, settles to 16.1 in and takes 1.2 × 86 = 103.2, so 104 bags. With batts, the thinnest pair is R-49 plus R-11, for a total thickness of 17.5 in. Between joists 16 inches on center, a 48 inch R-49 batt covers 16 × 48 ÷ 144 = 5.33 square feet, so you need 225 of them, plus 117 R-11 batts 93 inches long. The Department of Energy pairs adding insulation with reducing air leaks. Owens Corning says not to insulate on top of or within 3 inches of recessed lights unless they are rated for contact with insulation (IC), and to keep insulation out of the air spaces around metal flues, chimneys and fireplaces. The BTU calculator helps size heating and cooling for a room, and the square footage calculator works out the area of rooms of any shape.
How to use the insulation calculator
- 1Pick your climate zone and where you insulate. The calculator fills in the R-value ENERGY STAR recommends adding. Change it or choose "My own R-value" for anything else.
- 2Enter the area to insulate in square feet or square meters.
- 3Choose the material and, for batts, the framing spacing.
- 4Read the thickness and the number of batts, bags or boards, then copy the result or a link.
Sources
- ENERGY STAR: Recommended home insulation R-values (based on the 2021 IECC)
- U.S. Department of Energy: Guide to Home Insulation (R-values of spray foam and foam boards)
- U.S. Department of Energy: Consumer Guide to Home Insulation (2021, climate zone notes for Alaska and zone 1)
- Owens Corning: EcoTouch PINK Fiberglas Insulation product data sheet (batt R-values, sizes and thickness)
- Owens Corning: AttiCat PINK Blown-In Insulation product data sheet (coverage chart)
- Greenfiber: Sanctuary cellulose insulation fact sheet (attic coverage chart)
Frequently asked questions
What R-value do I need for my attic?
ENERGY STAR recommends adding R-30 to an uninsulated attic in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8. If the attic already has 3 to 4 inches of insulation, it recommends adding R-25 in zone 1, R-38 in zones 2 and 3, and R-49 in zones 4 to 8. The guidance is based on the 2021 International Energy Conservation Code.
How thick is R-38 insulation?
It depends on the material. An Owens Corning R-38 fiberglass batt is 12 inches thick. Blown-in fiberglass (AttiCat, 27.5 lb bags) needs at least 13.5 inches, and Greenfiber Sanctuary blown-in cellulose 11.2 inches when installed, settling to 10.4 inches. With closed-cell spray foam at R-6.5 per inch, it takes about 5.8 inches.
How many bags of blown-in insulation do I need?
Use the coverage chart on the bag. For R-49, Owens Corning lists 25 bags of AttiCat fiberglass per 1,000 square feet, and Greenfiber lists 70.8 bags of Sanctuary cellulose. A 1,200 square foot attic at R-60 takes 38 bags of AttiCat or 104 bags of Sanctuary. Both makers say you must install the full number of bags and reach the minimum thickness to get the labeled R-value.
Can I add new insulation on top of old insulation?
Yes, that is what the ENERGY STAR attic column for existing 3 to 4 inches of insulation is for: it lists how much R-value to add on top. Check that the old insulation is dry first. Owens Corning says insulation that has become wet should be inspected and, if contaminated, removed and replaced.
Does compressing insulation lower its R-value?
Yes. Owens Corning notes that its R-19 batt, which is 6 1/4 inches thick, delivers R-18 when installed in a 5.5 inch deep cavity. Pick a batt made for the depth of your framing, like the R-21 batt, which Owens Corning makes 5 1/2 inches thick.
Which climate zone am I in?
ENERGY STAR shows the zones on a map of the United States next to its table of recommended R-values, so look up where you live on that map. The Department of Energy’s insulation guide adds that Hawaii, Guam, Puerto Rico and the Virgin Islands are in zone 1 and most of Alaska is in zone 7.