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Attic Insulation Calculator
Quick answer: Estimate the R-value already in your attic by multiplying its depth by a typical R per inch for that material, subtract it from your target, then divide the gap by the new product's R per inch. The result is how many settled inches to add. If you see vermiculite, stop and do not disturb it.
How it works
Existing R ≈ existing depth (in) × R per inch. R to add = target R − existing R. Inches to add = R to add ÷ new material's R per inch. Pick a target from the ENERGY STAR recommended R-values for your climate zone, or in Canada from Natural Resources Canada and your provincial code; see recommended R-value by zone. R per inch for old, compressed or wet insulation is often lower than new, so treat existing R as a rough estimate. Then use the product's bag chart or the blown-in calculator for bag counts.
Vermiculite warning: pebbly grey-brown or gold-silver granules may contain asbestos. Health Canada and the U.S. EPA advise not disturbing it; see attic insulation removal.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 1,000 sq ft attic, 4 in of old loose-fill fiberglass at an assumed R2.5/in, target R49, new loose-fill at an assumed R2.5/in. Existing ≈ R10. R to add = 39. Depth to add = 39 ÷ 2.5 = 15.6 in settled, total about 19.6 in.
FAQ
How do I measure existing depth?
Use a ruler in several spots away from the eaves and average them, without compressing the insulation.
Should I air seal first?
Usually yes; see air sealing first.
Does the target include existing insulation?
Yes. The ENERGY STAR table gives the total level to aim for.
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