For a typical 1,200 sq ft attic, you need enough insulation to cover 1,200 sq ft at your target R-value, plus about 5–10% extra for trimming and awkward corners. With R-30 batts that cover roughly 50 sq ft per bag, that’s around 25 to 26 bags. With blown-in insulation, you’d follow the coverage chart on the bag for your chosen R-value.
The quantity always comes down to three things: the area you’re insulating, the R-value (or thickness) you’re aiming for, and the coverage printed on the product you buy. Here’s how to work each one out.
Step 1: Measure the area
For an attic or loft floor, measure the length and width of the space and multiply. A 30 ft × 40 ft attic is 1,200 sq ft. In metric, a 9 m × 12 m loft is 108 m².
For walls, measure the length of each wall and multiply by its height. Then subtract large openings like doors and windows. A 12 ft wall that’s 8 ft high is 96 sq ft. Take off a 3 × 5 ft window (15 sq ft) and you’re left with 81 sq ft.
Don’t subtract the joists or studs. Batts are sized to fit between framing at standard spacing (16 or 24 inches in North America, 400 or 600 mm elsewhere), and the coverage on the bag already allows for that.
Step 2: Choose your R-value
R-value measures how well insulation resists heat flow. The higher the number, the better it insulates. Australia and New Zealand use the metric R-value (m²K/W), and Canada often quotes RSI. These aren’t the same as US R-values: multiply an RSI or metric R-value by 5.68 to compare it with a US figure.
| Where you live | Attic / loft / ceiling | Walls (new build) |
|---|---|---|
| USA, warm south | R-30 to R-49 | R-13 to R-15 (2×4) |
| USA, cold north | R-49 to R-60 | R-19 to R-21 (2×6) |
| Canada | About R-50 (RSI 8.8) or more in most regions | R-22 to R-24 is common |
| UK | 270 mm of mineral wool | Depends on wall type |
| Australia | R4.0 to R6.0 (metric) | R2.0 to R2.8 (metric) |
These are typical guidance levels from bodies like ENERGY STAR in the US and the Energy Saving Trust in the UK. Your local building code has the final say, so check it before you buy, especially for new builds and renovations that need sign-off.
Already have some insulation? Measure how deep it is. If your attic has 3 to 4 inches of old fibreglass (roughly R-10), you only need to add enough to reach your target. In the UK, if you have 100 mm between the joists, you’d usually lay another 170 mm across the top to reach 270 mm.
Step 3: Divide by the coverage on the bag
Every pack of insulation lists how much area it covers. This is the number that turns your measurement into a shopping list.
Bags needed = area to insulate ÷ coverage per bag, then round up.
Example 1: Batts in an attic (USA)
- Attic: 30 × 40 ft = 1,200 sq ft
- Target: R-30
- R-30 batts for 16-inch joists: about 50 sq ft per bag (check the label)
- 1,200 ÷ 50 = 24 bags
- Add about 5% for waste: 26 bags
Example 2: Loft rolls (UK)
- Loft: 9 × 12 m = 108 m²
- Plan: 100 mm between the joists, then 170 mm laid across them
- One 100 mm roll covers about 8 m², so 108 ÷ 8 = 13.5, round up to 14 rolls
- One 170 mm roll covers about 5.5 m², so 108 ÷ 5.5 = 19.6, round up to 20 rolls
- Roll sizes vary by brand, so always use the coverage printed on the pack you’re buying
Example 3: Blown-in insulation
Loose-fill bags come with a coverage chart. For each R-value, it tells you the depth to blow, and how many square feet one bag covers. As the R-value goes up, each bag covers less area.
If the chart says one bag covers 40 sq ft at R-38, a 1,200 sq ft attic needs 1,200 ÷ 40 = 30 bags. Many home stores lend the blowing machine for free when you buy a minimum number of bags.
How thick will it be?
People often ask how deep R-38 or R-49 is. It depends on the material:
| Material | Approximate R-value per inch | Depth for R-38 |
|---|---|---|
| Fibreglass batts | R-3.1 to R-3.4 | About 12 inches |
| Loose-fill fibreglass | R-2.2 to R-2.7 | About 14 to 17 inches |
| Loose-fill cellulose | R-3.2 to R-3.8 | About 10 to 12 inches |
| Mineral wool batts | R-3.3 to R-4.2 | About 10 to 11 inches |
Loose-fill settles a little over time, especially cellulose. The bag chart usually gives both an initial depth and a settled depth, so blow to the initial depth.
Common mistakes
- Squashing batts. Compressing insulation to fit a space lowers its R-value. If R-30 batts won’t fit under your floor boarding, use a thinner, denser product, or raise the boarding on loft legs.
- Covering the eaves. Leave a clear gap at the edges of the attic so air can flow in from the soffit vents. Baffles (rafter vents) hold the gap open.
- Burying light fittings. Recessed lights that aren’t rated for contact with insulation need clearance. Check the fitting, and use a cover or keep the gap the manufacturer specifies.
- Forgetting the hatch. An uninsulated loft hatch leaks a surprising amount of heat. Glue a piece of rigid foam board to the back of it and add a draught strip.
- Mixing up R-value systems. An Australian R4.0 ceiling batt is not the same as a US R-4. It’s closer to R-23 in US terms.
Safety first
Wear long sleeves, gloves, safety glasses and a dust mask (an N95 or FFP2 is a good choice) when handling fibreglass or mineral wool. Only walk on the joists or on boards laid across them, never on the ceiling plaster between them. In hot climates, work in the attic early in the morning, when it’s cooler.
Work it out in seconds
Our free Insulation Calculator does the maths for you. Enter your measurements in feet or metres and the coverage printed on your pack, choose one layer or two, and it gives you a shopping list with waste already included.
Quick answer: measure the area, choose the R-value your climate needs, divide by the coverage on the bag and round up. Add 5–10% for cutting and corners, and you’ll rarely have to make a second trip to the store.
Related: the DIY Project Budget & Materials Planner turns your measurements into quantities, costs and a shopping list.
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