Insulation Calculator
Work out how many bags of batt insulation a wall or attic needs. The tool deducts the framing — batts go between the studs, not over them — and divides by the coverage printed on your bag.
How to use this calculator
- Add up the walls being insulated — usually exterior walls only — and enter the total length and the height. For an attic, it is simply length times width of the floor.
- Set the framing spacing. It picks the width of batt you buy (15 in vs 23 in) and the deduction for the studs or joists themselves, which occupy 6–10% of the area.
- Read the coverage off the actual bag. It varies with R-value and width — an R-13 bag often covers around 106 sq ft, an R-30 bag closer to 58. The default 100 is a placeholder, not a constant.
- Buy one spare bag for odd cavities, rips and the strip above the last full course.
The formula
net area = gross area × framing factor (0.90 at 16 in centres,
0.94 at 24 in)
bags = ceiling( net area ÷ coverage per bag )
Typical R-values by climate (US)
| Climate | Attic | 2x4 walls | 2x6 walls | Floors |
|---|---|---|---|---|
| Hot (Deep South) | R-30–49 | R-13 | R-19 | R-13 |
| Mixed (Mid-Atlantic, lower Midwest) | R-38–49 | R-13–15 | R-19–21 | R-19–25 |
| Cold (Upper Midwest, Northeast) | R-49–60 | R-13–15 + sheathing | R-19–21 | R-25–30 |
Typical ranges in the spirit of the DOE recommendations — the enforceable number is your local energy code, which goes by IECC climate zone. Attic first: it is the cheapest R per dollar and the biggest loss surface.
Worked example
Insulating 40 ft of exterior wall, 8 ft high, one door and two windows, studs at 16 in, bags covering 100 sq ft:
- Wall surface: 40 × 8 = 320 sq ft
- Less openings: 21 + 30 = 51 → 269 sq ft
- Framing factor at 16 in: 269 × 0.90 = 242 sq ft of cavity
- Bags: 242 ÷ 100 = 2.42 → 3 bags
The framing deduction matters more than it looks: skipping it here would still round to 3 bags, but on a whole-house job it is the difference between 27 bags and 30 — a trip back to the store either way if you guess wrong.
Batt sizes match framing
| Framing | Batt width | Batt R (full depth) |
|---|---|---|
| 2x4 at 16 in | 15 in | R-13 or R-15 |
| 2x4 at 24 in | 23 in | R-13 or R-15 |
| 2x6 at 16 in | 15 in | R-19 or R-21 |
| 2x6 at 24 in | 23 in | R-19 or R-21 |
| Attic joists, first layer | Match joists | R-30 / R-38 |
| Attic, second layer | Unfaced, laid across the joists | Whatever tops up the target |
Frequently asked questions
How many bags of insulation do I need?
Multiply the gross area by about 0.90 for framing at 16 inch centres (0.94 at 24), then divide by the coverage printed on your bag and round up. A 269 square foot wall needs about 242 square feet of batts — three bags at 100 square feet per bag.
What R-value should I use in walls?
What fits the framing: R-13 or R-15 in a 2x4 wall, R-19 or R-21 in a 2x6. Stuffing a thicker batt into a thinner wall compresses it and loses R-value. The enforceable minimum comes from your local energy code by climate zone.
What R-value do I need in the attic?
Typically R-30 to R-49 in warm climates and R-49 to R-60 in cold ones, per the usual DOE guidance — check your local code. The attic is the first place to spend insulation money: most heat loss goes up, and adding depth there is cheap.
Do I subtract the studs from the area?
Yes — batts go between the framing, and studs plus plates occupy roughly 10 percent of a wall at 16 inch centres. The calculator applies that factor. Bag coverage figures assume the batts fill cavities, not gross wall.
Can I put new insulation over old in the attic?
Yes, if the old material is dry and not compressed flat. Lay unfaced batts perpendicular across the joists on top of the existing layer. Never put a faced batt over old insulation — a vapour retarder in the middle of the stack traps moisture.