Loose-fill coverage is printed on the bag as square feet per bag at each R-value, and it drops steeply as R-value rises. R-60 covers half what R-30 does from the same bag.
Bags, not depth, is the measure
Blown insulation settles, and how much depends on the material and how hard it was blown. Two attics at the same installed depth can differ in R-value.
Manufacturers publish minimum bags per 1,000 square feet for each R-value, and that is the number that matters for warranty and for code inspection.
Seal before you insulate
Air leaks around top plates, wire penetrations, plumbing stacks and recessed lights move far more heat than the insulation stops.
Foam or caulk those first. Insulation blown over an unsealed attic floor is money spent on the smaller half of the problem.
Keep it out of the eaves and off the fixtures
Insulation pushed into the eaves blocks soffit ventilation. Baffles at each rafter bay keep the air channel open.
Older recessed lights that are not IC-rated must be kept clear by three inches — burying them is a genuine fire risk. Check the label inside the can before covering anything.
Cellulose or fibreglass
Both are blown, both work, and they behave differently. Cellulose is denser, fills around obstructions better, and settles more. Fibreglass is lighter, settles less, and is easier on the machine.
Cellulose is treated with borate for fire and pest resistance and holds that treatment indefinitely. Its density means it also slows air movement through the insulation, which fibreglass at low density does not.
Coverage per bag differs between them and between products, which is why the calculator asks for coverage rather than assuming. The number is on the bag.
Settling is already in the bag chart
Blown insulation settles after installation — cellulose noticeably, fibreglass less so. Manufacturers account for this, and the coverage chart on the bag is stated at the settled thickness.
That means you install to the initial thickness printed on the chart, which is greater than the final one, and you do not add extra on top for settling. Doing it twice is a real and common error.
Rulers stapled to the trusses before blowing are the way to check depth as you go, because judging thickness by eye across an attic is not possible.