Recommended R-values
ENERGY STAR publishes recommended insulation levels for existing wood-framed homes, based on the 2021 International Energy Conservation Code:
| Zone | Attic, uninsulated | Attic, 3–4 in existing | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 4C, 5 and 6 | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
How the calculation works
R to add = recommended R − existing R
Depth = R to add ÷ R-value per inch of your product
Bags = area ÷ coverage per bag at that R-value (from the bag chart)
R-value per inch and bag coverage differ between products (fiberglass batts, blown fiberglass, cellulose, spray foam), so the calculator uses the figures printed on the product you choose rather than generic averages. Blown insulation bags include a chart of bags per 1,000 sq ft at each R-value.
Worked example
An uninsulated attic in zone 4A (target R60), with a product rated R-3.2 per inch: add R60 at 60 ÷ 3.2 = 18.75 inches. If the bag chart says one bag covers 40 sq ft at R60, a 1,000 sq ft attic needs 25 bags.
Seal air leaks before insulating, keep insulation clear of recessed lights not rated for contact and away from flues, and maintain soffit ventilation.
Frequently asked questions
What R-value should my attic have?
ENERGY STAR recommends R30 to R60 for uninsulated attics depending on climate zone: R30 in zone 1, R49 in zones 2–3, and R60 in zones 4 and colder.
Can I add new insulation on top of old?
Usually yes, if the old insulation is dry and not moldy. Use unfaced insulation on top of existing so you do not trap moisture with a second vapor retarder.