How this calculator works
Insulation is bought to cover an area to a target R-value. The calculator sizes it from your wall, attic and ceiling areas, takes out the openings, and divides by the area one bag or roll covers:
area = sum of (length × width) for each wall / ceiling
net area = area - openings (windows, doors)
bags = ceil( net area × wastage ÷ coverage per bag )
batts = ceil( net area × wastage ÷ (bay width × batt length) )
Pick batts / rolls or blown-in and a target R-value, and the default coverage per bag updates to a typical value for that choice - then override it from your own product’s label. Batt and roll bags carry a roughly fixed face area; a blown-in bag has a fixed yield, so a higher R-value (deeper fill) covers less area per bag. Framing spacing sets the batt width, which turns the covered area into a count of individual batts.
How much does insulation cost?
Insulation is sold by the bag, and the price swings widely by product and R-value, which is why this calculator estimates cost from your numbers rather than a national average that drifts out of date. In the Cost estimate section, pick how you are pricing the job:
- Buy the materials (DIY) - enter your price per bag and the calculator multiplies it by the bag count it already worked out (wastage included), then shows a cost per square foot / metre you can compare against an installed quote.
- Installed price - enter a single installer’s rate per square foot (or metre). A fitted quote bundles the material and labour into that one rate, so the cost is simply your net area × the rate.
materials = bags to buy × price per bag
installed = net area × installed rate per sq ft/m²
The estimate covers the insulation material only. Vapour barrier, fixings and labour aren’t included, and blown-in usually needs a blower (often a free loan with a bulk purchase) on top. Leave the price blank to skip the cost. Cost figures are indicative - set the price to a real quote for your area.
Worked example
This example follows the unit system you pick in the calculator above.
A 6 × 2.4 m wall with one 1.2 × 1.2 m window, R2.5 batts on 600 mm centres, 10% wastage:
- Wall area: 6 × 2.4 = 14.4 m².
- Window deducted: 1.2 × 1.2 = 1.44 m², so 12.96 m² net.
- With 10% wastage: 12.96 × 1.10 = 14.26 m² to cover.
- An R2.5 batt pack covers ~9.6 m²: 14.26 ÷ 9.6 = 1.5 → 2 packs.
- As individual 0.58 × 1.16 m batts: 14.26 ÷ 0.67 = 21.3 → 22 batts.
Attic, walls and where insulation goes
Different parts of the house take different R-values, so insulate them as separate areas:
- Attic / ceiling - the biggest payback and the highest R-values (R-30 to R-60). Open attics are usually blown-in; accessible joist bays take batts. See how much attic insulation do I need.
- Exterior walls - R-13 to R-15 in a 2×4 wall, R-19 to R-21 in a 2×6 wall. Batts cut to the stud bay are the norm.
- Floors and crawlspaces - typically R-19 to R-30 between the joists.
- Garages and sheds - insulate the walls and ceiling you want to condition; the door is the weak point.
Already know your square footage - “about 1,200 sq ft of attic”? Skip the room math and enter a single area equal to that figure, or use:
bags = ceil( square footage × wastage ÷ coverage per bag )
For 1,200 sq ft of R-30 blown-in at ~60 sq ft per bag with 5% wastage: 1,200 × 1.05 ÷ 60 = 21 bags.
R-value, cavity depth and where it goes
The higher the R-value, the deeper the insulation has to sit to reach it. This table is the typical fibreglass depth for each target - check your product, since high-density batts and blown-in fill hit the same R-value in less or more depth:
RSI is the metric R-value, and in Australia, New Zealand and the UK it’s the figure printed on the pack - so on those regional pages the calculator shows the metric scale (R1.5 to R6.0) directly rather than the imperial one above. Depth is R-value × thermal conductivity, so a dense “high-density” batt reaches the same R-value in less thickness than a standard one - an R2.0 HD wall batt is about 75 mm where a standard R2.0 is ~90 mm - which is why the metric depths show as a range. The coverage figures the calculator seeds are typical bag values; the thickness and coverage on your own pack are always the numbers to trust. For the full breakdown, see the insulation R-value chart.
Batts, blown-in and spray foam
- Batts and rolls - fibreglass or mineral wool, sized to 16 or 24 inch bays. The calculator’s batt mode counts both the bags and the individual batts.
- Blown-in (loose fill) - fibreglass or cellulose, poured or blown to fill. Sold to a coverage chart at each settled R-value; read the area for your target and enter it as the coverage. Blown-in vs batt insulation compares the two.
- Spray foam - sold and quoted by the board foot (1 sq ft at 1 inch thick), and it air-seals as it insulates. It is a different unit from this calculator, so size it from the kit’s stated board-foot coverage.
Tips for buying insulation
- Measure each wall, attic and ceiling separately and add the openings - large windows and doors quickly add up to a bag you do not need to buy.
- Match the batt width to your framing: 15 inch batts for 16 inch OC studs, 23 inch for 24 inch OC. A batt that is too wide has to be cut to every bay.
- Do not compress a thick batt into a shallow cavity to make it fit - you lose R-value. Use a batt rated for the depth you have, or fur the wall out.
- Blown-in is sold to a coverage chart on the bag at each settled R-value and density - read the area for your target R and enter it as the coverage above.
- Fill the gaps and avoid compressing around wires and pipes; the rated R-value assumes the batt sits full and even in the bay.
R-values and standards where you are
In Australia, the batts you buy are labelled in metric R-values, commonly R2.0 to R6.0 - ceiling batts are often R5.0 or R6.0 and walls R2.0 to R2.5. This calculator's target R-values use that same metric scale, so just pick the figure printed on your batts.
For the recommended levels by climate zone, see the Australian Government's YourHome guidance, which follows the National Construction Code minimums by climate zone.
