Insulation Calculator
How many batt packs of insulation you need
A 40 by 8 ft wall needs 9 packs of batt insulation at 40 square feet a pack, with 5% for offcuts. Enter your own area and the coverage printed on the bag to estimate the packs to buy, and use the R-value chart by climate zone for the level to aim for.
How much insulation do you need?
Batt insulation coverage is printed on every pack, commonly around 40 ft², though it varies by thickness and width, so check the bag. Measure the wall or ceiling face area, and do not deduct for studs: batts fill the bays between them and the offcuts around windows and doors generally cover the difference.
Match the batt width to your framing. A 15-inch batt suits studs at 16 inches on centre and a 23-inch batt suits 24 inches on centre, because the nominal spacing is measured centre to centre while the cavity between them is narrower. Buying the wrong width means cutting every single batt lengthways.
Batt insulation calculator: packs by wall size
Used as a wall insulation calculator at 40 ft² per pack, including the 5% added for offcuts.
| Wall | Area | With 5% | Packs |
|---|---|---|---|
| 20 × 8 ft | 160 ft² | 168 ft² | 5 |
| 40 × 8 ft | 320 ft² | 336 ft² | 9 |
| 60 × 8 ft | 480 ft² | 504 ft² | 13 |
| 80 × 8 ft | 640 ft² | 672 ft² | 17 |
| 100 × 9 ft | 900 ft² | 945 ft² | 24 |
A 40 by 8 ft wall framed at 16 inches on centre has 31 studs and 30 bays. Each 8 ft bay takes one 8 ft batt or two 4 ft ones, so the pack count and the bay count should land in the same neighbourhood — a quick sanity check before you buy.
R-value by climate zone
R-value measures resistance to heat flow, and higher is warmer. For the attic of an existing home, ENERGY STAR publishes the levels below, based on the 2021 International Energy Conservation Code. How much to add depends on whether the attic is empty or already has a few inches in it:
| Zone | Roughly | Attic uninsulated | Already has 3–4 in |
|---|---|---|---|
| 1 | South Florida, Hawaii | R-30 | R-25 |
| 2 | Gulf Coast, south Texas | R-49 | R-38 |
| 3 | Southern states | R-49 | R-38 |
| 4A, 4B | Mid-Atlantic, Missouri, Kansas | R-60 | R-49 |
| 4C, 5, 6 | Pacific Northwest coast, Midwest, New England | R-60 | R-49 |
| 7, 8 | Northern Minnesota, Alaska | R-60 | R-49 |
Walls get no row of their own because the cavity, not the climate, sets their limit: a 2×4 wall cavity is only 3.5 inches deep, and no amount of ambition fits R-38 into it. ENERGY STAR's advice for walls is instead to blow insulation into empty cavities, or add insulating sheathing when the siding comes off. Attics have unlimited depth, which is why they carry the heavy numbers and why they are almost always the best place to spend money first.
What actually fits in the cavity
| Framing | Cavity depth | Standard batt | High-density batt |
|---|---|---|---|
| 2×4 wall | 3.5 in | R-11 to R-13 | R-15 |
| 2×6 wall | 5.5 in | R-19 | R-21 |
| 2×8 floor/ceiling | 7.25 in | R-25 | — |
| 2×10 floor | 9.25 in | R-30 | — |
| Attic, unlimited depth | 12 to 20 in | R-38 to R-60 | — |
If your target exceeds what the cavity holds, the answer is not a thicker batt crammed in. It is a high-density batt, or continuous rigid foam sheathing on the outside of the framing, which has the additional benefit of covering the studs themselves.
Why your wall does not achieve its R-value
The number on the bag is the rating of the insulation alone, measured at full loft in a lab. The wall assembly performs worse, for three reasons worth knowing before you buy:
- Thermal bridging. Wood studs are roughly R-1 per inch, so a 3.5-inch stud is about R-4 against the R-13 beside it. Studs, plates and headers are typically 20 to 25% of a wall's area, which pulls a nominal R-13 wall down to an effective R-10 or so.
- Compression. Batts are rated at full thickness. Squashing an R-19 batt into a 3.5-inch cavity does not give you R-19; it gives you roughly R-13 and wastes the extra material.
- Gaps and voids. A batt that leaves a small gap at the edge loses far more performance than the gap's area suggests, because air moves through it. Cut around wiring and boxes rather than compressing the batt over them, and split batts around pipes so insulation sits on both sides.
Air sealing before insulating is consistently the highest-return step, and it is cheap. Caulk and foam the gaps at top and bottom plates, around penetrations and at rim joists first; insulation slows heat but does almost nothing to stop moving air.
Batts, rolls or blown-in
| Type | Best for | Watch out for |
|---|---|---|
| Batts | Open stud and joist bays | Fit quality decides performance |
| Rolls | Long uninterrupted runs, attic floors | More cutting on obstructed runs |
| Blown-in loose fill | Attics, existing closed walls | Settles over time; needs a machine |
| Rigid foam board | Continuous exterior, basement walls | Must be fire-rated or covered |
| Spray foam | Rim joists, awkward cavities, air sealing | Cost, and usually a pro job |
For an existing finished wall, batts are not an option without opening it up, so dense-packed blown-in through small holes is the usual retrofit. For an attic you can walk into, blown-in is faster and fills awkward corners better than batts do.
What the insulation count assumes
Packs are area × (1 + waste) ÷ coverage per pack, rounded up, using 40 ft² per pack and a 5% default waste allowance for offcuts.
- Pack coverage varies with batt thickness, width and length: real packs run from about 30 to over 100 ft². 40 ft² is a common mid-range figure, and the bag is the authority. Divide your total area by the printed coverage if yours differs.
- Face area, not cavity area. No deduction is made for framing, because offcuts around windows, doors and short bays generally absorb the difference.
- R-value recommendations are ENERGY STAR's attic levels for retrofitting existing wood-framed homes, which follow the 2021 IECC. Local building codes may require more and take precedence.
- Cavity R-values are typical fibreglass batt figures. Mineral wool runs slightly higher per inch and rigid foam higher still.
- Not modelled: thermal bridging, air leakage, and the effective assembly R-value, all of which make a real wall perform below its nominal rating.
Reviewed September 2026.
Both unit systems. Metric mode measures in metres and m². Any coverage you enter yourself is read as m².
Sources, checked
- Recommended Home Insulation R-Values, ENERGY STAR. Every attic figure in the climate-zone table.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Insulation Calculator: frequently asked questions
How do I know how much insulation to buy?
Measure the wall or ceiling face area and divide by the coverage printed on the pack, commonly around 40 ft². A 40 × 8 ft wall is 320 ft², or 9 packs with 5% added for offcuts. Do not deduct for studs.
What R-value do I need?
For walls, R-13 to R-21 almost everywhere, limited by cavity depth rather than climate. For an empty attic, ENERGY STAR recommends adding R-30 in the far south, R-49 in zones 2 and 3 and R-60 from zone 4 north, or R-25 to R-49 on top of 3 to 4 inches already there. Check your local code, which may exceed these.
What size batt fits 16 inch stud spacing?
A 15-inch wide batt. For 24-inch on-centre framing use a 23-inch batt. The nominal spacing is measured centre to centre, so the actual cavity is narrower than the number suggests.
What R-value fits in a 2x4 wall?
R-13 as a standard batt or R-15 as a high-density one, since the cavity is only 3.5 inches deep. A 2×6 wall takes R-19 or R-21. To exceed that you need continuous rigid foam on the outside of the framing.
Does compressing insulation reduce its R-value?
Yes. Batts are rated at full loft, so forcing an R-19 batt into a 3.5-inch cavity yields roughly R-13, not R-19. You pay for the higher rating and do not get it. Buy the batt sized for the cavity.
Why doesn't my wall achieve its rated R-value?
Thermal bridging through the framing. Studs are about R-1 per inch and make up 20 to 25% of a wall's area, so a nominal R-13 wall performs closer to R-10. Gaps and compression lower it further.
Should I air seal before insulating?
Yes, and it is usually the better value of the two. Insulation slows heat conduction but does very little to stop air movement. Caulk and foam gaps at plates, penetrations and rim joists first.