PreConstruct AIA bag of Comfortbatt R14 and a bag of R24 look the same on the truck. One covers about twice the wall of the other.
Bag coverage is set by the size of the batts and how many are in the bag, and both change with the R-value and the stud spacing. Count bags off the wrong line and a 2×6 wall comes up short by half. This post gives the Canadian bag sizes from ROCKWOOL's own residential product guide, what the "covers" figure on the bag actually measures, two ways to count, and a worked wall.
Comfortbatt for wood studs
These are the wall sizes from the Canadian guide (May 2025). R12 and R14 fill a 2×4 wall, 3½ inches thick. R20, R22 and R24 fill a 2×6 wall, 5½ inches thick.
| Batt | Stud spacing | Batts a bag | Batt size | Covers a bag |
|---|---|---|---|---|
| R12, 3½ in | 16 in | 12 | 47 × 15¼ in | 59.7 ft² (5.55 m²) |
| R12, 3½ in | 24 in | 8 | 47 × 23 in | 60.1 ft² (5.58 m²) |
| R14, 3½ in | 16 in | 12 | 47 × 15¼ in | 59.7 ft² (5.55 m²) |
| R14, 3½ in | 24 in | 8 | 47 × 23 in | 60.1 ft² (5.58 m²) |
| R20, 5½ in | 16 in | 8 | 47 × 15¼ in | 39.8 ft² (3.70 m²) |
| R20, 5½ in | 24 in | 5 | 47 × 23 in | 37.5 ft² (3.48 m²) |
| R22, 5½ in | 16 in | 8 | 47 × 15¼ in | 39.8 ft² (3.70 m²) |
| R22, 5½ in | 24 in | 5 | 47 × 23 in | 37.5 ft² (3.48 m²) |
| R24, 5½ in | 16 in | 6 | 47 × 15 in | 29.4 ft² (2.73 m²) |
| R24, 5½ in | 24 in | 4 | 47 × 22¾ in | 29.7 ft² (2.76 m²) |
One correction to the printed guide: it gives the 24-inch R12 and R14 bags as 61.1 ft². Eight batts of 47 × 23 inches come to 60.1 ft², which is also what its own metric figure, 5.58 m², works out to. The table above uses 60.1.
R24 is the line people trip on. It's the same 5½ inches as R22, but there are 6 batts in a bag instead of 8, so a bag covers 29.4 ft² instead of 39.8. A wall counted with R22's coverage and ordered in R24 is about a quarter short.
Comfortbatt for steel studs
The steel stud batts are wider and an inch longer, sized for a C-stud's channel rather than a wood stud's face.
| Batt | Stud spacing | Batts a bag | Batt size | Covers a bag |
|---|---|---|---|---|
| R10, 2½ in | 16 in | 16 | 48 × 16¼ in | 86.7 ft² (8.05 m²) |
| R14, 3½ in | 16 in | 12 | 48 × 16¼ in | 65.0 ft² (6.04 m²) |
| R14, 3½ in | 24 in | 8 | 48 × 24¼ in | 64.7 ft² (6.01 m²) |
| R22.5 or R24, 6 in | 16 in | 8 | 48 × 16¼ in | 43.3 ft² (4.03 m²) |
| R22.5 or R24, 6 in | 24 in | 5 | 48 × 24¼ in | 40.4 ft² (3.75 m²) |
The guide lists the 6-inch steel batt at both R22.5 and R24 at the same size and count. Which R-value the wall needs is on the drawings. The metric figures here are worked from the batt sizes; two of the guide's printed metric figures for steel batts don't match their own dimensions.
Don't put wood stud batts in steel studs or the other way round. A 15¼-inch batt in a 16-inch steel bay leaves a gap down the channel, and a 16¼-inch batt is too wide for a 14½-inch wood cavity.
What "covers" on the bag means
The square feet on the bag is the batts' own face area. Twelve batts of 47 × 15¼ inches is 8,601 square inches, which is 59.7 ft². It isn't the area of wall the bag fills.
Each batt is cut a little wider than the space it goes into, so it presses in and stays put. Between 2×4s at 16 inches the cavity is 14½ inches wide, and the batt is 15¼. One batt per bay fills a strip of wall 16 inches wide, stud included. So a bag of 59.7 ft² fills about 62.6 ft² of 16-inch wall (59.7 × 16 ÷ 15¼).
That means dividing the gross wall area by the figure on the bag leaves you about 5 per cent over. For most jobs that's the waste allowance already in the count, so don't add a second one on top without a reason, like a lot of short bays.
Two ways to count bags
By area, the usual way to order:
- Wall length × height, for each wall.
- Take off the doors and windows.
- Divide by the bag's coverage at your stud spacing, and round up.
By bays, as a check on walls with odd heights or lots of short bays:
- Count the cavities: wall length in inches ÷ stud spacing.
- Batts per cavity: cavity height ÷ batt length (47 inches for wood, 48 for steel), rounded up. An 8-foot wall on 92⅝-inch studs takes two batts a bay. A 9-foot wall on 104⅝-inch studs takes three, with one cut.
- Total batts ÷ batts in a bag, rounded up.
The two should land within a bag of each other. If the bay count comes out well above the area count, the wall's height is wasting part of a batt in every bay, and the bay count is the one to order from.
Worked example: a 40-foot 2×6 wall
An exterior wall 40 feet long and 8 feet high, 2×6 studs at 16 inches, R22 on the drawings. It has one door 3 feet × 6 feet 8 inches and two windows 3 × 4 feet.
By area:
- Wall: 40 × 8 = 320 ft²
- Door: 3 × 6.67 = 20.0 ft²
- Windows: 2 × (3 × 4) = 24.0 ft²
- Net: 320 − 44 = 276 ft² (25.6 m²)
- Bags: 276 ÷ 39.8 = 6.9, so 7 bags (in metric, 25.6 ÷ 3.70 = 6.9, the same 7)
By bays, as the check:
- Cavities: 480 inches ÷ 16 = 30
- Batts: 30 × 2 = 60, before openings
- The door and windows take out 44 ft² ÷ 4.98 ft² a batt ≈ 9 batts, so about 51 batts
- Bags: 51 ÷ 8 = 6.4, so 7 bags
Both counts say 7. If the drawings called for R24 on the same wall, it would be 276 ÷ 29.4 = 9.4, so 10 bags. Same wall, same thickness, three more bags.
What the bag count doesn't tell you
The R-value isn't something to pick from a bag size. It comes from the drawings and the energy compliance path for the permit: in Ontario, SB-12 for houses and SB-10 for other buildings. Count bags for the batt the drawings name.
Fibreglass and other brands have different batt sizes and different bag counts, so the coverage above is only for Comfortbatt. Whatever the brand, the method is the same: check whether the bag figure is batt area, and use the coverage printed on the bag you're actually buying.
To run the numbers on your own walls, the insulation calculator takes the length, height, openings and the coverage on your bag. If the batt and spacing come from a partition schedule, the guide to reading a partition schedule shows where to find them.