How much insulation do I need?

What determines the answer

1. Area — the square footage you're covering
2. Existing insulation — what's already up there, in R-value
3. Target R-value — set by your climate zone
4. Material — determines R-value per inch, and therefore depth
5. Coverage — how much area one bag or package covers at your target
6. Waste — the allowance that stops you running short mid-job

Miss any one of these and the number will be wrong. Most online calculators skip existing insulation entirely, which is why they routinely tell people to buy nearly twice what they need.

A worked example

A 40 ft × 31 ft attic in Denver, with about 8 inches of old loose-fill fiberglass, doing it with blown cellulose.

Why step 2 matters most

We use R-2.3 per inch for the existing fiberglass rather than the nominal 2.5, because insulation that has sat in an attic for twenty years has settled and been walked on. Assuming it performs as new would leave you short.

Step 2 is also the step most calculators skip entirely. Subtracting what you already have is the difference between 48 bags and 68.

Measuring your space

Attic: measure the floor, not the roofline. If you can't get in safely, the footprint of the rooms below is close enough — attics rarely extend past the exterior walls.

Existing depth: push a ruler straight down to the ceiling drywall in several spots and take the typical value, not the deepest. Insulation is rarely even.

Walls: length × height, minus doors and windows. Count cavities rather than relying on area alone if you're buying batts.

Why the waste allowance exists

Loose fill settles and spreads unevenly, so 10% covers the gap between theoretical and actual coverage. Batts need more — 12% — because cutting around obstructions genuinely wastes material.

Then round up to whole packages. You can't buy 47.3 bags. Running two bags short at 4pm on a Sunday means the job doesn't finish, so the allowance is worth having.

Keep receipts. Most retailers take back unopened bags, so over-buying by two is a much cheaper mistake than under-buying by two.

Common mistakes

• Ignoring existing insulation. The most expensive error, and the most common.
• Using nominal R-values for old material. Settled insulation underperforms its original rating.
• Buying by square footage alone. Coverage depends on target depth. "Covers 40 sq ft" is meaningless without the R-value it applies at.
• Forgetting the attic hatch. An uninsulated hatch undoes a surprising share of the work.
• Skipping baffles. Blowing insulation into the eaves blocks soffit vents and causes moisture problems and ice dams.
• Insulating before air sealing. Once the insulation is down you can't reach the gaps underneath.

Related

• What R-value do I need?
• R-value chart
• What will it cost?