Insulation Calculator
Bags and batts for the R-value you're actually aiming at.
Getting your insulation order right
Buying a bigger R-value can leave you worse off if the product does not have room to do its job.
Why a bigger R-value can perform worse
It is common to buy R-19 batts for a 2x4 wall on the reasoning that more is better. A 2x4 cavity is three and a half inches deep and an R-19 batt is around six. Pushed into that space it compresses, and compressing it removes the trapped air that the R-value depends on.
The result performs worse than an R-13 batt that fits the cavity properly, and you have paid more for it. The rule is to match the batt to the cavity depth, not to buy the largest number on the shelf.
This is also why gaps matter so much. Insulation works by stopping air moving, so a bay that is almost full but open at one edge loses a surprising amount of its rated performance.
Blown-in settles, and that is already in the number
Loose-fill insulation drops over the months after it goes in, and people reasonably assume that means buying extra to compensate. It does not.
Manufacturer coverage charts list two depths. Greenfiber’s cellulose card, for example, gives an applied thickness of 8.8 inches and a settled thickness of 7.9 inches for R-30. Both describe the same bag of material. What settles is the volume, not the mass, and the coverage figure is worked out from the mass. The quantity you need is already correct.
What people actually get wrong is the depth they blow to. Blow to your finished target and you will finish below it, because it has not settled yet. Blow to the applied figure and it settles down to the number you wanted.
So the rule is the opposite of what it sounds like. Do not buy more. Do go deeper than the mark.
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FAQFrequently asked questions
Measure the attic floor area, then decide the R-value you are aiming for — that depends on your climate zone, and colder zones want substantially more. Whether the attic is bare or already has a few inches in it also changes the target. The bag count follows from the depth that R-value needs, not from the area alone. Enter your dimensions and target above.
It depends on the depth you are blowing to and on which material you are using. One bag covers far fewer square feet at a high R-value than a low one, because coverage is a function of thickness. Material matters just as much: at R-30, a bag of blown fibreglass covers around 68 square feet while a bag of cellulose covers around 24. The calculator uses the manufacturer's figures for the material and target you select, and shows the per-bag coverage so you can check it against the chart on the bag.
It depends on your climate zone and which part of the building you are insulating. Energy Star's current guidance, based on the 2021 energy code, puts a bare attic at R-30 in the warmest zone and R-60 across most of the country. If your attic already has three or four inches in it, the recommended top-up target is lower. Your local building code sets the minimum, so check it before you buy.
Yes, and it is already accounted for in the quantity. Manufacturer charts list an applied thickness and a settled thickness for the same bag of material — what settles is the volume, not the mass, so you do not need to buy extra. What you do need to do is blow deeper than your finished target. Blow to the applied figure and it will settle to the depth you were aiming at.
Faced insulation has a paper or foil vapour retarder attached, unfaced does not. Which you want depends on your climate and on whether there is already a vapour retarder in the assembly. Getting it wrong can trap moisture inside a wall, so it is worth checking local practice rather than defaulting to one.
For walls, yes — insulation goes in the cavities, and an opening has no cavity. Enter the number of openings above and the calculator deducts a typical area for each. For a job with unusually large glazing, measure the openings and subtract the real figure instead.