Home & Construction
How much insulation do you need? Get batts, rolls, or blown-in bags for your area and target R-value — and the depth to install.
Insulation is sold by coverage area, so the count is simple — the part people get wrong is the depth needed for their target R-value:
Packages = area (sq ft) ÷ coverage per package, rounded up
Depth (inches) = target R-value ÷ R-value per inch for the material
The coverage number matters. For batts and rolls, coverage is printed on the package and depends on the batt size and cavity width — 40 sq ft per roll is typical. For blown-in insulation, coverage per bag shrinks as the target R-value rises, because you're blowing the same bag deeper. Always read the coverage chart on the bag for your chosen R-value and enter that figure above; 50 sq ft per bag is a mid-range starting point for cellulose around R-38.
General R-value guidance (broad US rules of thumb — local code and climate zone govern the real requirement):
| Where | Typical target | Notes |
|---|---|---|
| 2×4 exterior wall | R-13 to R-15 | Limited by cavity depth (3.5 in) |
| 2×6 exterior wall | R-19 to R-21 | 5.5 in cavity |
| Attic — mild / southern | R-30 to R-38 | Warmer climate zones |
| Attic — most of the US | R-38 to R-49 | Common code range |
| Attic — cold / northern | R-49 to R-60 | Cheapest heating fuel is insulation |
| Floors over unheated space | R-19 to R-30 | Match to cavity depth |
| Basement walls | R-10 to R-15 | Often rigid foam or framed batts |
This table is general guidance only, not a code determination — check your local energy code or an insulation contractor for the requirement in your area.
Can I just add new insulation over old? Usually yes for attics, as long as the old material is dry and not moldy — the R-values add together. Enter the additional R you want to add as the target, not the combined total.
Why does blown-in settle? Loose-fill settles a few percent over the first years, which slightly reduces R-value; manufacturers' bag charts already account for settled density when you install to the chart's stated depth and bag count.
Batts or blown-in for an attic? Blown-in fills around trusses, pipes, and wiring with fewer gaps and rents with a free blower at most home centers; batts are cleaner for walls and small, simple areas. Either hits the same R target at the right depth.
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