Insulation Calculator
Work out how many bags or batts of insulation you need for walls, attics and floors.
What you need
27
Bags needed — Rounded up, waste allowance included.
- Area to cover
- 1,000.0 sq ft
- Area including waste allowance
- 1,050.0 sq ft
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
How to use the insulation calculator
Insulation is sold by the bag or bundle, each covering a stated area at a stated R-value — and the coverage drops sharply as R-value rises. Pick the product you are using and enter your area to get the count.
What this calculator assumes
Every estimate depends on figures that vary by supplier and conditions. These are the ones used here, so you can sanity-check the result against your own quote.
- Coverage per bag is taken from your entry above — always check the actual packaging, because it varies by product.
- A 5% waste allowance is applied by default. Increase it for diagonal layouts, busy patterns or rooms with lots of corners.
- Bags are rounded up to whole units.
- Coverage figures are typical manufacturer values and vary by brand — always work from the bag in front of you.
- Blown-in coverage assumes the stated installed depth. Settling reduces R-value over time, so install slightly deep rather than exactly to the line.
- For batts, the area is the cavity face area. Batt width must match your stud or joist spacing (16 or 24 inch centres).
- Recommended attic R-value depends on climate zone — commonly R-38 to R-60 in cold zones and R-30 to R-49 in mild ones. Check your local code.
- Air sealing before insulating matters more than the last few R-points. Insulation slows heat transfer; it does not stop air leaks.
Frequently asked questions
How much insulation do I need for my attic?
Most US climate zones call for R-38 to R-60 in the attic. At R-38, one bag of blown cellulose or fibreglass covers around 32 square feet, so a 1,000 square foot attic needs roughly 32 bags. Check the bag — coverage varies by brand and by target depth.
What R-value do I need?
It depends on climate zone and where the insulation is going. Broadly: attics R-38 to R-60, 2x4 walls R-13 to R-15, 2x6 walls R-19 to R-21, floors over unheated space R-25 to R-30. Your local building department publishes the figures that actually apply to you.
Blown-in or batts?
Blown-in wins in an open attic: it fills irregular spaces and covers the top plates and joist bays that batts bridge over. Batts win in accessible framed cavities — walls, floors, rim joists — where you can cut and fit them tight. Many jobs use both.
Can I put new insulation over old?
Usually yes, and it is the cheapest way to raise attic R-value. Use unfaced insulation over the top — a second vapour barrier mid-assembly can trap moisture. Do not compress the existing layer, and keep insulation clear of recessed lights unless they are rated for contact.
How much extra should I buy?
The default here is 5%. Buying a little extra from the same batch is cheap insurance — colours and lots shift between production runs, so a mid-project top-up rarely matches. Keep the offcuts for future repairs.
How do I handle an L-shaped or irregular room?
Split it into rectangles, work out each one separately and add the results. Measure at the widest point of each section and ignore small alcoves unless you are covering them too.
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