How Many Inches of Blown-In Insulation for R-38, R-49 and R-60?
October 19, 2026 · 7 min read
Exact depths for R-30, R-38, R-49 and R-60 in blown fiberglass and cellulose, bags per 1,000 sq ft, which R-value your climate zone needs, and how much to add over old insulation.
The quick answer
For blown-in fiberglass, Owens Corning's AttiCat coverage chart calls for 13.5 inches for R-38, 17 inches for R-49 and 20.5 inches for R-60. Blown cellulose is denser, so it needs less depth: Greenfiber's chart calls for installing 11.2, 14.3 and 17.3 inches, which settle to 10.4, 13.3 and 16.1 inches.
Most attics already have some insulation, so you usually only need to add the difference. Our attic insulation calculator works out how many inches to add on top of what you have, and how much it will save on your heating bill.
Depth and bags for each R-value
Blown-in insulation is rated by both depth and weight, so manufacturers publish a coverage chart for each product. These are the minimums from two widely sold DIY products:
| R-value | Fiberglass depth | Fiberglass bags | Cellulose installed | Cellulose settled | Cellulose bags |
|---|---|---|---|---|---|
| R-30 | 10.75″ | 14.6 | 8.9″ | 8.3″ | 38.6 |
| R-38 | 13.5″ | 19 | 11.2″ | 10.4″ | 52.6 |
| R-49 | 17″ | 25 | 14.3″ | 13.3″ | 70.8 |
| R-60 | 20.5″ | 31.5 | 17.3″ | 16.1″ | 86.0 |
Owens Corning says settling of AttiCat is negligible, so the installed and settled depths are the same (AttiCat application instructions). Greenfiber bases its R-values on the settled depth, which is 90% of the installed depth, so you blow it about 10% deeper than the final number (Greenfiber Do-It-Yourself Guide).
Other brands are close but not identical, so always follow the chart printed on the bag you buy. You have to meet both the depth and the number of bags. Blowing the right depth with too few bags means fluffier, less dense insulation and a lower R-value than you paid for.
Which R-value does your attic need?
ENERGY STAR publishes recommended attic R-values by climate zone for existing homes. Find your zone on the map on that page, then:
| Climate zone | Uninsulated attic | Attic with 3–4″ already |
|---|---|---|
| 1 (South Florida, Hawaii) | R-30 | R-25 |
| 2 and 3 (Gulf Coast, Southwest, Southeast) | R-49 | R-38 |
| 4 to 8 (mid-Atlantic, Midwest, Northeast, Pacific Northwest, mountain states, Alaska) | R-60 | R-49 |
In inches, ENERGY STAR's rule of thumb is a minimum of about 13–14 inches (R-38) in the southern US and 16–18 inches (R-49) in the north. If you can still see the tops of the ceiling joists, you almost certainly need more.
Adding insulation over what you already have
To work out how much to add, estimate the R-value you have now and subtract it from your target. ENERGY STAR says common fiberglass, cellulose and mineral wool insulation is roughly R-3 to R-3.5 per inch, so a quick check is to measure the depth in several places and multiply by 3. Old fiberglass batts about 3½ inches thick are typically rated R-11.
Worked example: a 1,000 sq ft attic with R-11 batts, upgraded to R-49. You need to add R-38. In our insulation calculator, that comes to about 10.6 inches of cellulose. For blown fiberglass, the calculator uses a typical R-2.5 per inch and gives 15.2 inches. Denser products like AttiCat reach R-38 in 13.5 inches, so go by the bag chart for the product you choose.
In bags, adding R-38 over 1,000 sq ft takes about 19 bags of AttiCat or 53 bags of Greenfiber cellulose. Measure your attic floor first. Multiply length by width for each rectangular section, or add the areas up in our square footage calculator. Then scale the bag count to your area.
Is R-60 worth it over R-49?
Each extra inch saves less than the one before, because heat loss falls with 1 ÷ R. Here's what our calculator shows for a 1,000 sq ft attic topped up to R-49 in a cold-winter climate:
| Starting point | Add | Annual savings | Simple payback |
|---|---|---|---|
| R-11 (3–4″ of old batts) | R-38 | $364 | 4.8 years |
| R-19 (about 6″) | R-30 | $166 | 10.5 years |
| R-30 (about 9–10″) | R-19 | $67 | 26 years |
The message is clear: the first top-up pays back fast, while adding more to an attic that's already reasonably insulated takes much longer to repay. If your attic is already at R-30 or more, sealing air leaks often does more for comfort and bills than another layer. Change the climate, energy price and cost to your own figures in the attic insulation calculator to see your payback.
Before you start blowing
- Seal air leaks first. Gaps around light fixtures, plumbing stacks, wiring holes and the attic hatch let warm, moist air into the attic. ENERGY STAR says sealing air leaks and adding insulation together can save up to 10% on annual energy bills. It's much harder to do once the insulation is down.
- Keep the soffit vents open. Staple baffles (rafter vents) between the rafters at the eaves so the new insulation doesn't block airflow into the attic.
- Respect clearances. Keep loose-fill insulation away from chimneys, flues and recessed lights that aren't rated for insulation contact (IC-rated). Follow the product instructions and your local code for required gaps and dams.
- Mark the depth. Staple depth rulers to the joists every few hundred square feet, so you can see from the hatch that you've reached the target depth everywhere.
- Count the bags. Divide your attic area by the coverage per bag before you go to the store, and use them all. Running out of bags is the most common reason a DIY job ends up below its target R-value.
Not sure whether fiberglass or cellulose is the better fit for your attic? See our comparison of insulation materials, R-values and costs.
Sources & further reading
- AttiCat Expanding Blown-In PINK FIBERGLAS Insulation: Application Instructions (coverage chart) — Owens Corning
- Greenfiber Do-It-Yourself Guide (Sanctuary attic coverage chart) — Greenfiber
- Recommended Home Insulation R-Values — ENERGY STAR (US EPA)
- How to Check Your Home's Attic Insulation Level — ENERGY STAR (US EPA)
- Seal and Insulate with ENERGY STAR — ENERGY STAR (US EPA)