Insulation Calculator
How deep and how many bags to hit that R-value?
How this is calculated
- Depth = target R-value ÷ R per inch
- Bags = ⌈area ÷ coverage per bag⌉, using the label coverage at the calculated depth
- The bag's coverage chart (sq ft per bag at given depth) is the source of truth — manufacturers print it on every bag
Common questions
How many inches of insulation for R-38?
It depends on the material: blown cellulose (≈3.1 R/in) needs about 12.3 in; blown fiberglass (≈2.5 R/in) about 15.2 in; rock wool ≈ 11.2 in. The bag label has the exact number for that product — always trust it over generic tables.
What R-value should my attic have?
Most US climate zones call for R-38 to R-60 in attics — R-49 is the common target in snow country, R-38 in moderate zones. Walls are typically R-13 to R-21 (2×4 vs 2×6 framing). ENERGY STAR and your local building department publish zone maps.
How many bags of blown insulation do I need?
Every bag has a coverage chart: sq ft covered per bag at each depth. For a 480 sq ft attic at R-38 with cellulose (12.3 in deep), a typical bag covering ~40 sq ft at that depth means about 12 bags. Always buy one extra — depth settles and low spots happen.
Can I add new insulation over old?
Yes, and it is one of the highest-ROI home upgrades — but first: air-seal the ceiling penetrations (can lights, top plates), confirm the old insulation is not vermiculite (pre-1990 houses — get it tested for asbestos before disturbing), and keep baffles clear at the eaves so soffit vents breathe. Unfaced or blown over existing batts is fine; never trap a vapor barrier between layers.
Batts or blown-in for an attic?
Open attic with joists: blown-in wins on speed and coverage (it fills around wires and junction boxes perfectly). Short attic access or sloped ceilings: batts. Many attics use both — batts between the joists, blown on top to reach the target depth. Wear a mask and goggles either way; fiberglass is not skin food.