How deep should blown-in insulation be for R-49?
R-49 takes about 14" of blown cellulose or 19 5/8" of blown fiberglass, settled. Depths for every attic R-value, and why the bag's chart has the last word.
R-49 takes about 14" of blown cellulose, or 19 5/8" of blown fiberglass, measured settled.
Over a 1,000 sq ft attic that is 1167 cu ft of cellulose — R-49 sits inside the R-30 to R-60 range the Department of Energy recommends for attics.
How that is worked out
- Depth is the R-value you want divided by the R-value per inch of the material.
- Settled cellulose runs about R-3.5 an inch (published figures range 3.2–3.8), so R-49 ÷ 3.5 = 14.0".
- Loose fiberglass is fluffier, about R-2.5 an inch (2.2–2.9), so R-49 ÷ 2.5 = 19.6" — the same R-value needs more of it.
Settled depth of loose fill by R-value
| R-value | Cellulose | Fiberglass | Cellulose per 1,000 sq ft |
|---|---|---|---|
| R-19 | 5 7/16" | 7 5/8" | 452 cu ft |
| R-30 | 8 9/16" | 12" | 714 cu ft |
| R-38 | 10 7/8" | 15 3/16" | 905 cu ft |
| R-44 | 12 9/16" | 17 5/8" | 1048 cu ft |
| R-49 | 14" | 19 5/8" | 1167 cu ft |
| R-60 | 17 1/8" | 24" | 1429 cu ft |
Worth knowing
- R-per-inch is a property of the product, not a constant: the figures here are round numbers inside the published ranges. The bag you buy states its own, and its coverage chart — installed depth, settled depth and bags per 1,000 sq ft at each R-value — is the authority.
- Blow to the installed depth on the chart, not the settled one. Loose fill settles after it goes in, and the R-value is rated at the settled depth.
- Which R-value you want is set by your climate zone and the local energy code. The Department of Energy's range for attics runs from R-30 in the warmest zones to R-60 in the coldest.
- Seal the air leaks in the ceiling first — around light fixtures, the attic hatch, plumbing and wiring holes. Warm air moving through insulation carries the heat straight past it.
Work out your own numbers
- Insulation calculator
Bags of batts for a wall, floor or attic at 16" or 24" on centre, or the depth and bags of blown-in for a target R-value.
- Drywall
Sheets of 4x8, 4x10 or 4x12 for a room's walls and ceiling, less doors and windows, with the mud, tape and screws to finish them.
Related questions
- How many inches is R-49 blown insulation?
- About 14" of cellulose or 19 5/8" of fiberglass, settled, at typical R-values per inch. Check the coverage chart on the bag for the exact figure for that product.
- How many inches is R-38?
- About 10 7/8" of blown cellulose or 15 3/16" of blown fiberglass, settled.
- How many bags of blown-in insulation do I need?
- Read it off the bag. Every loose-fill bag carries a chart of how many square feet it covers at each R-value, because that depends on the density the maker tested at. Divide your attic area by that figure — the insulation calculator does it with the number from your bag.