Insulation: R-values, attic depth and air sealing
Insulation is specified by R-value, the resistance to heat flow, and attic insulation is topped up to a target R-value rather than bought by the square foot alone. Going from R-11 to R-49 with blown insulation at R-2.5 per inch means adding about 15.2 inches, which over 1,000 square feet is 1,267 cubic feet of loose fill. The bag label tells you how many bags that is.
This page gathers our insulation calculators and guides: attic depth and R-value, how to measure what is there, spray foam, attic ventilation (insulation and venting go together) and the air sealing that makes insulation work.
Blown insulation to add, by starting and target R-value
Inches to add at R-2.5 per inch (typical loose fiberglass). Worked out by the attic insulation calculator; cellulose has a higher R per inch, so it needs less depth.
| Existing | To R-30 | To R-38 | To R-49 | To R-60 |
|---|---|---|---|---|
| None | 12.0 | 15.2 | 19.6 | 24.0 |
| R-11 | 7.6 | 10.8 | 15.2 | 19.6 |
| R-19 | 4.4 | 7.6 | 12.0 | 16.4 |
| R-30 | 0 | 3.2 | 7.6 | 12.0 |
Attic insulation
Measure the depth you have in a few places, identify the type, and the calculator works out the depth to add for your target. Most of the US is recommended R-38 to R-60 in the attic; the guide gives the climate zones.
Spray foam
Spray foam is sold in board feet: one square foot one inch thick. A 40 ft wall 8 ft high at 3 inches is 960 board feet, or 1,056 with 10% waste. Closed-cell foam also acts as an air and vapor barrier.
Ventilation and attic fans
Adding insulation without keeping soffit vents clear traps moisture. Check the vent area for your attic floor and fit baffles at the eaves before you blow in more.
Air sealing and weatherstripping
Gaps around doors, windows and attic hatches leak more heat than thin insulation. Weatherstripping is cheap and quick, and the calculator counts the feet and the door sweeps.
Every calculator for this topic
Guides
- How to Measure Attic Insulation Depth and Work Out the R-ValueMeasure attic insulation with a ruler in several spots, multiply the inches by the R per inch of the material, and see how much more to add.
- What Size Attic Fan Do I Need? CFM by Attic Size and IntakePlan 0.7 CFM per square foot of attic floor, or 0.805 for a dark or steep roof. A 1,500 ft² attic needs 1,050 CFM plus 504 in² of soffit intake.
- How Many Board Feet of Spray Foam Do I Need? Kits ChartA wall 40 × 8 ft at 3 in needs 960 board feet, or 2 kits of 600. Charts of board feet and kits by area and thickness.
- How Much Weatherstripping Do I Need for Doors and Windows?Two doors and ten windows take about 223 ft of weatherstripping (14 rolls of 17 ft). Feet needed by door and window size, and the best types.
- How Much Insulation Should Be in the Attic? R-Value and InchesMost US attics should be R-38 to R-60. On a bare attic floor, R-49 is about 19.6 in of blown fiberglass or 14.0 in of cellulose. Depth chart by R-value.
- How Much Attic Ventilation Do I Need? The 1:150 and 1:300 RulesHow to calculate attic ventilation in square inches of net free area, split between intake and exhaust, with a chart for common attic sizes and ridge vents.
Other guides that use these calculators: How Much Soffit Do I Need? Vinyl Panels and Channel · How Much Ice Melt Do I Need? Pounds per Driveway, Walk or Winter · Roof Replacement Materials List: Shingles, Underlayment and Vents
Answers for common sizes
Already know the size? These pages give the worked answer for one common size each, with the inputs filled in so you can adjust them.
Project planners
Planning the whole job rather than one material? These step-by-step planners put the calculators above in the order you will need them.
Frequently asked questions
How much attic insulation do I need?
R-38 to R-60 in most of the US, depending on climate zone. In blown fiberglass at R-2.5 per inch that is about 15 to 24 inches in total.
Can I add new insulation on top of old?
Yes, as long as the old insulation is dry and not moldy. Do not put a vapor barrier between the layers.
How deep is R-49 blown insulation?
About 19.6 inches of loose fiberglass at R-2.5 per inch. Cellulose needs less.
What is a board foot of spray foam?
One square foot, one inch thick. Kits are sold by the board feet they cover.
Related topics
Estimates only. Quantities depend on your materials, site and supplier, so check coverage and sizes on the label. Follow local building codes for anything structural. How we calculate.