Understanding your result
The headline is the number of packs, with the batt size for your framing, and the line under it gives the net area, the batts and the bays. The tiles repeat the batts and bays and give the coverage per pack used.
Two figures come from two areas on purpose. Packs are counted from the wall area, because the maker’s coverage per pack is per square foot of framed wall with the studs included. Batts are counted from the cavity area, the wall less the studs, because that is what the batts fill; it is the number to check against the delivery and to plan the cutting around outlets.
Insulation is part of most sound assemblies, and the soundproofing assembly calculator shows what it is worth in each. For a new wall, the partition wall calculator includes the same insulation count with the framing and board; for a ceiling on channel, the resilient channel calculator counts the channel.
How we calculate this
Coverage per pack is Rockwool’s published figure for Safe’n’Sound 3 in mineral wool: 59.7 sq ft for twelve 15-1/4 × 47 in batts and 60.1 sq ft for eight 23 × 47 in batts. If you use another product, type its pack coverage in and the calculator uses it. The cavity share is the spacing less the 1-1/2 in stud, over the spacing. Framing dimensions are on the standard sizes table, and the STC values of insulated assemblies on the sound and fire assemblies page.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Product | 3 in acoustic mineral wool for 2 × 4 framing | Rockwool Safe’n’Sound product data |
| Pack coverage | 59.7 sq ft (16 in framing), 60.1 sq ft (24 in) | Rockwool published coverage |
| Batt sizes | 15-1/4 × 47 in (12 per pack), 23 × 47 in (8 per pack) | Rockwool published sizes |
| Stud thickness | 1-1/2 in | Standard dimensional lumber |
| STC effect | About 33–35 empty, 34–39 insulated (single studs) | GA-600 and published lab summaries |
| Waste | 5% | Cuts around boxes and pipes |
Assumptions last reviewed October 8, 2026.
The calculator counts 3 in batts. Joists are deeper than studs, and a ceiling can take thicker batts or two layers; enter the coverage of the product you buy. It does not count thermal insulation for exterior walls, which is governed by the energy code and needs its own R-value. Where insulation sits among the ways to block sound is in the soundproofing basics guide; ceilings, where insulation between joists does real work against footfall noise when combined with clips, are in the ceilings guide.
Two worked examples
A 24 × 8 ft wall with a door and a window, 16 in studs
- Net area: 192 − 35 = 157 sq ft
- Packs: 157 × 1.05 ÷ 59.7 = 2.8, rounded up to 3 packs
- Cavity: 157 × 14.5 ÷ 16 = 142 sq ft; batts 142 ÷ 4.98 × 1.05 → 31 batts in 18 bays
Three packs hold 36 batts; the spare five fill the short bays above the door and below the window.
A 20 × 14 ft ceiling between floors, 24 in joists
- Area: 280 sq ft
- Packs: 280 × 1.05 ÷ 60.1 = 4.9, rounded up to 5 packs of 23 in batts
- Batts: 263 sq ft of cavity ÷ 7.51 → 37 batts
Between floors, the batts lie on the ceiling board or are pushed up against the subfloor. With isolation clips and a second layer, they help with both voices and footsteps.
Where to find your inputs
Size. Length and height of the wall, or length and width of the ceiling.
Openings. Doors and windows in the wall at their rough sizes; outlets and small boxes are cut around and not subtracted.
Spacing. Centre to centre on the studs or joists; it decides the batt width.
Other products. The coverage per pack printed on the bag, if you are not buying 3 in mineral wool.
Common mistakes
- Compressing batts. Squashed insulation does not help more. Cut to fit, do not stuff.
- Wrong width. Batts for 24 in framing fall out of 16 in bays and vice versa.
- Gaps around boxes. Split the batt around wires and pack behind boxes; voids leak sound.
- Insulation alone. It is a few points. Combine it with mass, damping or channel.
- Vapour retarder in an interior wall. Interior partitions do not need one; use unfaced batts.
- No protection when cutting. Gloves, sleeves, eye protection and a mask.
Questions people ask
- How much sound insulation do I need for a wall?
- One pack of 3 in mineral wool for 2 × 4 walls covers about 60 sq ft of framed wall (Rockwool lists 59.7 sq ft for 15-1/4 in batts and 60.1 sq ft for 23 in). A 12 × 8 ft wall is 96 sq ft, two packs.
- Does insulation soundproof a wall?
- It helps by a few points. Published tests put a single-stud wall at about STC 33 to 35 empty and 34 to 39 insulated. Insulation absorbs sound in the cavity; the large gains come from more mass, damping and decoupling the two sides.
- Mineral wool or fibreglass for sound?
- Both work in a stud cavity, and tests show similar results at the same thickness. Mineral wool is denser, holds its shape, friction-fits between studs without sagging and is easier to cut cleanly, which is why it is the usual choice for sound walls.
- Should sound insulation fill the whole cavity?
- Fill it without compressing it. In a 2 × 4 wall, 3 in or 3-1/2 in batts fill most of the 3-1/2 in cavity; a full fill helps a little more than a partial one, and packing it tight helps nothing.
- What width of batts for 16 in studs?
- Batts for 16 in framing are about 15 to 15-1/4 in wide so they friction-fit between studs 14-1/2 in apart; for 24 in framing they are about 23 in wide. Match the batt width to the framing.
- Can I add insulation to an existing wall?
- Only by opening it or by blowing loose fill through holes, which works for thermal insulation and helps a little for sound. For a wall you are opening anyway, batts are the simple answer; for a closed wall, a second layer of board with damping compound often helps more.