How Much Insulation Do I Need? Coverage per Pack Explained

Measure the area you are insulating in square metres, divide it by the coverage printed on the pack, then add 5 to 10 per cent for offcuts and laps. The catch is that coverage per pack drops as the R-value goes up, so the same 100 m² ceiling needs far more packs at R6.0 than at R3.5. Always work from the pack coverage, not from a batt count.

This is the question we get most often, and the reason it causes trouble is that people estimate off the last job. Two packs of the same brand and the same batt size can cover very different areas if the R-value is different.

Step 1: measure the area

Ceilings. Length times width of the area under the roof, room by room, added together. Do not deduct for downlights or manholes. You will need the offcuts.

Walls. Wall length times ceiling height, then take out large window and door openings. Small windows are not worth deducting because the offcuts around them go to waste anyway.

Roofs with a pitch. Roofing blanket follows the roof plane, not the floor plan. A pitched roof has more surface area than the building footprint. Measure along the slope, or take the plan area and add for the pitch.

Those are the plan-to-slope factors, rounded. On a low pitch shed the difference is nothing. On a 30 degree roof it is an extra 16 per cent of material, which is more than most people's waste allowance.

Step 2: match the batt width to your framing

Batts are made to friction fit between framing members. Two widths cover most Australian construction:

• 430 mm batts for framing at 450 mm centres
• 580 mm batts for framing at 600 mm centres

Check the actual spacing in the roof before ordering. It is a five minute job with a tape and it is the difference between batts that stay put and batts that need cutting all day. On older houses the centres are often inconsistent, so measure in a few places rather than assuming.

Step 3: read the real coverage figure

Here is the pattern, taken from our own ceiling insulation range:

Look at the 580 mm rows. R3.5 gives 6.7 m² per pack and R6.0 gives 4.0 m². A higher R-value means a thicker batt, fewer batts fit in a compressed pack, and coverage falls by a third. Estimate an R6.0 job using R3.5 numbers and you will be roughly 40 per cent short.

The same job also changes from 30 packs to 50, which changes the delivery, the storage space and how long it takes to carry into the roof.

Step 4: do the sum

Worked example, ceiling. A 180 m² ceiling, framing at 600 mm centres, specified at R4.1.

1. Coverage per pack for the 580 mm R4.1 batt: 6.7 m²
2. Add 10 per cent for offcuts and awkward areas: 180 x 1.1 = 198 m²
3. 198 ÷ 6.7 = 29.6
4. Order 30 packs

Worked example, walls. A house with 96 linear metres of external wall at 2.55 m ceiling height, less 18 m² of windows and doors.

1. 96 x 2.55 = 244.8 m² gross
2. Less 18 m² of large openings = 226.8 m²
3. Add 10 per cent = 249.5 m²
4. Divide by the pack coverage of the wall batt you have chosen and round up

Roll products and blanket

Roll products work the same way, with the roll coverage in place of the pack coverage. A 500 m² warehouse roof at R2.3 using a roofing blanket supplied in 1200 mm x 10 m rolls covering 12 m² each: 500 x 1.1 = 550, divided by 12 = 45.8, so 46 rolls.

Safety mesh is calculated differently again, because the side lap eats into the roll width rather than the length. That calculation is in our roof safety mesh guide.

How much waste to allow

Rounding always goes up. Half a pack short at the end of a job costs a second delivery.

Mistakes that cost money

• Estimating on floor area for a pitched roof. The roof plane is bigger than the slab, by up to 16 per cent at 30 degrees.
• Assuming pack coverage is constant across R-values. It is not, as the table above shows.
• Ordering the wrong batt width. 430 mm batts in 600 mm centres leaves a gap on every run, and gaps destroy performance. NCC 2022 Housing Provisions clause 13.2.2(1) requires insulation to abut or overlap adjoining insulation and form a continuous barrier.
• Compressing to fit. Squashing a thick batt into a shallow space removes the air that does the insulating. If the space is too shallow, change the product, not the fit.
• Forgetting the extras. Roll systems need laps and tape. Commercial roofs need safety mesh and spacers. Those go on the same order.
• Ordering to the exact number. Damaged packs, mis-cuts and one awkward corner all come out of the same allowance.

Measuring on site

A few habits that make the take off reliable:

• Work room by room and write the numbers down rather than adding in your head.
• Measure the framing centres in at least three places, at both ends and in the middle of the run.
• Note the joist depth as well as the spacing. It tells you what thickness will actually fit.
• Photograph anything unusual: bulkheads, ducts, transformers, downlights, exhaust penetrations.
• Check access before delivery. A 50 pack order needs somewhere dry to sit and a way to get it up.

A note on R-value choice

This article is about quantity, not selection. The R-value you need comes from the climate zone, the building class and the rest of the construction, and it is set by the specification or the energy rating for the building rather than by preference. If you are not sure what the job calls for, send us the plans or the energy report and we will read the target off it.

Frequently asked questions

Sources

Quantities here are a guide for ordering. Confirm coverage against the pack label of the product you buy, and confirm the R-value against the specification or energy report for your building.