How it works
Pick where you are insulating and your climate zone, and the calculator looks up a recommended R-value, subtracts what is already there and counts the batts needed to make up the difference.
Attic targets follow the ENERGY STAR table based on the 2021 IECC: R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8 (lower end of each range). Wall cavities are limited by their depth: R-13 to R-15 fits a 2 × 4 wall and R-19 to R-21 a 2 × 6 wall; colder zones call for R-20 or cavity plus continuous foam.
Batt coverage assumes each batt fills one framing bay: 15 or 23 in wide to fit 16 or 24 in spacing, 93 in long for wall batts and 48 in long for thick attic batts. When layering in an attic, run the second layer of unfaced batts perpendicular to the first.
Formula
R to add = recommended R − existing R. Layers = ⌈R to add ÷ batt R⌉. Batts = area × layers ÷ (spacing × batt length ÷ 144) × 1.05.
Questions people ask
What R-value do I need in my attic?
ENERGY STAR recommends R-30 to R-49 in zone 1, R-49 to R-60 in zones 2 and 3 and R-60 in zones 4 to 8 for an uninsulated attic. If you already have 3 to 4 inches, add R-38 to R-49 in colder zones.
Can I put new insulation on top of old?
Yes, as long as the old insulation is dry and not moldy. Use unfaced batts or loose fill on top so you do not trap moisture behind a second vapor retarder.
How much R-value per inch does fiberglass have?
Roughly R-3 to R-4 per inch for fiberglass batts, R-2.5 to R-3 for blown fiberglass and about R-3.5 for blown cellulose. Closed-cell spray foam is about R-6 to R-7 per inch.
Should batts have a vapor retarder facing?
In cold climates (zones 5 and up), kraft-faced batts go with the paper toward the heated side of the wall. In attics over existing insulation and in hot-humid climates, use unfaced batts.