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Generator Hut

Insulation Calculator

Bags or rolls of insulation for walls and attics.

Your measurements

Set to 0 if you are only insulating a loft or attic floor.

Printed on the pack. Batts commonly cover 40–90 sq ft per bag.

Covers trimming around noggins, pipes and cables.

Insulation required

14bags

14 bags for 480 sq ft

Wall area60 ft perimeter × 8 ft high
480 sq ft
Total area
480 sq ft
With waste allowance+10%
528 sq ft
Bags or rollsat 40 sq ft each
14

R-value depends on the product and the cavity depth, not on the area, so it does not affect this quantity — but it does affect which product you buy. Compressing a thick batt into a shallow cavity loses most of the rating you paid for.

No deduction is made for studs and joists. The framing typically occupies 15% or so of a wall, and that is roughly cancelled out by the offcuts and the awkward bays, so the gross area is the practical figure.

A vapour barrier, if your climate and assembly need one, is a separate product. Which side it belongs on depends on your climate zone and getting it wrong can trap moisture inside the wall.

Enter the wall perimeter and height, or an attic floor area, along with the coverage printed on the pack. R-value does not change the quantity — it changes which product you buy — so this answers the how-many question and the notes cover the which-one question.

Why use this tool?

Walls and attic together

Wall area from perimeter and height, attic as its own field, so one calculation covers a whole job.

Any pack size

Batts commonly cover 40 to 90 square feet per bag depending on width and thickness. Enter what is on the pack.

No framing deduction, deliberately

Studs occupy roughly 15% of a wall, which is about what the offcuts and awkward bays waste. The two effectively cancel.

R-value explained

Rating depends on product and cavity depth, not area — and compressing a thick batt into a shallow cavity loses most of what you paid for.

How this insulation calculator works

Wall area is perimeter times height. Attic or ceiling area is entered directly, since a loft floor is measured as a floor rather than derived from a perimeter. The two are added, uplifted by the waste allowance, and divided by the pack coverage.

No deduction is made for studs and joists. Framing typically occupies around 15 per cent of a wall's face, but the offcuts, the narrow bays beside openings and the pieces trimmed around pipes and cables waste a similar amount — so working from the gross area is both simpler and about right.

R-value is deliberately absent from the arithmetic. It is a property of the product and the depth it sits at, not of the area, so it affects which pack you buy rather than how many. What it does affect is whether the batt fits: compressing an R-19 batt into a 2x4 cavity gives you neither R-19 nor good value.

How to use it

  1. Step 1: Measure the walls

    Perimeter of the walls being insulated and their height. Set both to zero if you are only doing a loft.

  2. Step 2: Add the attic area

    The floor area of the loft space, entered directly in square feet.

  3. Step 3: Read the pack coverage

    Printed on every bag or roll. It varies widely with batt width and thickness, so check rather than assume.

  4. Step 4: Match product to cavity

    Choose a batt that fits the cavity depth uncompressed. A thicker batt squeezed in performs worse than the correct one.

Example usage

A single room
A 60 foot perimeter at 8 feet is 480 square feet, 528 with waste. At 40 square feet per bag that is 14 bags.
A loft floor
Set the wall fields to zero and enter 900 square feet of attic. With 10% waste that is 990 square feet, or 25 bags at 40 square feet each.
A whole-house retrofit
Walls and loft together on one calculation. Enter both, and the total is what you order — though wall and loft products are usually different and bought separately.

Frequently asked questions

How much insulation do I need?

Divide the total wall and ceiling area by the coverage printed on the pack, adding around 10 per cent for trimming. A 480 square foot wall area at 40 square feet per bag needs about 14 bags.

Does R-value change how much I need?

No. R-value is a property of the product and the depth it occupies, not of the area, so it determines which product you buy rather than how many packs. It does determine whether a batt fits the cavity properly.

Should I subtract the studs?

In practice no. Framing occupies roughly 15 per cent of a wall face, and the offcuts plus the awkward bays around openings waste about the same, so the gross area works out close to right.

Can I compress insulation to fit?

You can physically, but you lose most of the benefit. A batt rated for a 6 inch cavity squeezed into a 4 inch one performs closer to the thinner product while costing more — buy the batt that matches the cavity.

Do I need a vapour barrier?

It depends on your climate zone and wall assembly, and which side it belongs on varies too. Getting it wrong can trap moisture inside the wall, so it is worth checking local practice rather than guessing.

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