Insulation Calculator

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and R-value, which is why this calculator estimates cost from your numbers rather than a national average that drifts out of date.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 13 × 8 ft wall with one 4 × 4 ft window, R-15 batts on 16 in OC studs, 10% wastage:

1. Wall area: 13 × 8 = 104 sq ft.
2. Window deducted: 4 × 4 = 16 sq ft, so 88 sq ft net.
3. With 10% wastage: 88 × 1.10 = 96.8 sq ft to cover.
4. An R-15 batt bag covers ~88 sq ft: 96.8 ÷ 88 = 1.1 → 2 bags.
5. As individual 15 × 92 in batts (9.6 sq ft each): 96.8 ÷ 9.6 = 10.1 → 11 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.

Starting from a square-footage figure

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 the United States, insulation is rated in imperial R-values - the same scale this calculator uses. Exterior walls are commonly R-13 to R-21 and attics R-30 to R-60.

For the recommended levels by climate zone, see the US Department of Energy's insulation recommendations.