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Walls & DrywallInsulation Calculator — How Much Insulation Do I Need?
Enter the length and height of the wall or ceiling area to insulate to get the square footage to buy, plus a 5% allowance. Match that figure to the coverage printed on each bag of batts.
How this calculator works
We multiply the area (length × height) and add 5% for trimming and fit. Insulation bags are labeled by the square feet they cover at a given R-value, so divide this figure by your bag’s coverage to count bags.
What changes how much you need
- Bag coverage drops as R-value rises — an R-13 bag covers far more area than an R-30 bag of the same size.
- For walls, match the batt width to your stud spacing (16" or 24" on center).
- Don’t compress batts to fit — compressed insulation loses R-value.
- Subtract large openings (windows, doors, garage doors) for a tighter estimate.
Insulation to buy for common areas
Before subtracting windows and doors. Divide the figure to buy by the square feet printed on your bag to count bags.
| Area | Size | Area | To buy (+5%) |
|---|---|---|---|
| One wall | 12 × 8 ft | 96 sq ft | 101 sq ft |
| All four walls, 12×12 room | 48 × 8 ft | 384 sq ft | 403 sq ft |
| 2-car garage walls | 88 × 8 ft | 704 sq ft | 739 sq ft |
| Attic floor | 30 × 40 ft | 1,200 sq ft | 1,260 sq ft |
| Attic floor | 40 × 50 ft | 2,000 sq ft | 2,100 sq ft |
Recommended R-values for existing homes (ENERGY STAR)
For adding insulation to existing wood-framed homes. Find your zone on the ENERGY STAR or DOE climate-zone map; new construction follows your local energy code.
| Climate zone | Uninsulated attic | Attic with 3–4 in already | Floor |
|---|---|---|---|
| 1 | R-30 | R-25 | R-13 |
| 2 | R-49 | R-38 | R-13 |
| 3 | R-49 | R-38 | R-19 |
| 4A and 4B | R-60 | R-49 | R-19 |
| 4C, 5, and 6 | R-60 | R-49 | R-30 |
| 7 and 8 | R-60 | R-49 | R-38 |
Frequently asked questions
How much insulation do I need?
Measure the wall or ceiling area (length × height) and add about 5%. Then divide by the square-foot coverage printed on each bag of batts, which depends on the R-value.
How many square feet does a bag of insulation cover?
It varies by R-value — a bag of R-13 might cover ~100 sq ft while the same-size R-30 bag covers far less. Always read the bag’s labeled coverage.
What R-value do I need?
It depends on your climate zone and where the insulation goes. For attics in existing homes, ENERGY STAR recommends R-30 in the warmest zone and R-49 to R-60 almost everywhere else (see the table above). Walls are limited by stud depth: 2×4 walls take R-13 or R-15 batts, 2×6 walls R-19 or R-21. Check your local energy code for new work.
How much insulation do I need for a 12×12 room?
Four 12-ft walls at 8 ft tall are 384 sq ft, or about 403 sq ft with the 5% allowance, before subtracting windows and doors. The ceiling is another 144 sq ft (151 with the allowance) and usually takes a higher R-value, so buy it separately.
How many bags of insulation do I need?
Divide the square feet from this calculator by the coverage printed on the bag and round up. If your bag covers 75 sq ft, 403 sq ft of wall needs 403 ÷ 75 = 5.4, so 6 bags. Coverage per bag falls as the R-value rises, so compare bags at the R-value you are actually buying.
Can I add new insulation over old insulation in the attic?
Usually, as long as the old insulation is dry and free of mold or pest damage. Use unfaced batts or loose-fill on top — a second kraft or foil facing can trap moisture between the layers — and lay the new batts across the joists so they cover the gaps.