Insulation Calculator
Find the insulation thickness for a target R-value and estimate fiberglass batts or bags of blown-in fiberglass or cellulose for an attic or wall.
Add this calculator to your website — get the free widget
How to Calculate
Thickness = target R-value ÷ R per inch. This tool uses R-3.1 per inch for fiberglass batts, R-2.5 for blown-in fiberglass and R-3.5 for blown-in cellulose, which are mid-range figures for common products.
For blown-in, bags are estimated from installed volume: one bag of loose-fill fiberglass is assumed to cover about 400 ft² at 1 in deep (40 ft² at 10 in), and one 25 lb bag of cellulose about 200 ft² at 1 in. Brands vary a lot, so check the coverage chart printed on the bag before buying.
For batts, each 15 × 93 in piece covers 9.69 ft² of a wall or ceiling with joists 16 in on center. The count assumes one layer; for R-38 and up in an attic you would usually lay a second layer across the first.
See our methodology for how every calculator on this site is built.
Worked Example: Topping Up an Attic to R-49
A 1,000 ft² attic floor, target R-49, using blown-in cellulose at R-3.5 per inch.
- Thickness: 49 ÷ 3.5 = 14 in
- Installed volume: 1,000 × 14 = 14,000 ft²·in (square feet at 1 inch deep)
- Bags: 14,000 ÷ 200 = 70 bags of cellulose
The 200 ft²-at-1-inch figure per bag is an assumption based on typical 25 lb cellulose bags. Every brand prints its own chart, and real coverage can be 10 to 20% either way, so read the bag before you buy 70 of them. If the attic already has 4 in of old fiberglass (roughly R-10 to R-12), you only need to add enough for the difference, but most installers blow the full depth over it anyway and call the old stuff a bonus.
The same R-49 in blown fiberglass at R-2.5 per inch would be 19.6 in deep. Make sure the attic baffles and the hatch dam are tall enough before you start.
Thickness by R-Value
| Target | Fiberglass batts (R-3.1/in) | Blown fiberglass (R-2.5/in) | Blown cellulose (R-3.5/in) |
|---|---|---|---|
| R-13 | 4.2 in | 5.2 in | 3.7 in |
| R-19 | 6.1 in | 7.6 in | 5.4 in |
| R-30 | 9.7 in | 12 in | 8.6 in |
| R-38 | 12.3 in | 15.2 in | 10.9 in |
| R-49 | 15.8 in | 19.6 in | 14 in |
Real batts come in fixed products (R-13 at 3.5 in, R-19 at 6.25 in, R-30 at about 9.5 in), so for attics above R-30 you lay two layers: one between the joists, a second across them.
Common Mistakes
Skipping air sealing
Insulation slows heat flow, but it doesn't stop air. Seal the top plates, wire and plumbing holes, and around the attic hatch with foam or caulk before adding insulation. It often does more than the extra R-value.
Burying the soffit vents
Blown insulation drifts right into the eaves and blocks airflow. Staple rafter baffles in every bay before you blow.
Covering recessed lights
Older can lights that are not IC-rated need clearance or a fire-rated cover. Check before you bury them.
Compressing batts
A batt squashed into a cavity shallower than its rated thickness loses R-value. Cut it to fit; don't stuff it.
Practical Tips
- Many home centers lend the blowing machine free with a minimum bag purchase.
- Nail depth markers to joists around the attic so you can see when you have hit the target thickness.
- Wear a fitted N95 or better, long sleeves and goggles. Cellulose dust and glass fibers are both miserable.
- Keep faced batts with the paper toward the heated side of the house.
Rough Cost
Bagged cellulose usually costs $12 to $18 a bag and blown fiberglass $35 to $50 a bag (fewer bags for the same area). Fiberglass batts run about $0.50 to $1.20 per square foot depending on R-value. Professional attic insulation to R-49 typically costs $1.50 to $3.50 per square foot.
Sources
Frequently Asked Questions
What R-value do I need in my attic?
The DOE recommends R-30 to R-49 in most of the southern US and R-49 to R-60 in colder northern climates. Walls are typically R-13 to R-21.
How thick is R-30 insulation?
About 9.7 in of fiberglass batts, 12 in of blown fiberglass or 8.6 in of cellulose.
Is blown-in or batt insulation better for an attic?
Blown-in fills around wires, trusses and odd framing with fewer gaps, which is why most attic upgrades use it. Batts are easier for open walls and floors.
Can I add new insulation on top of old insulation?
Yes, as long as the old material is dry and not moldy. Do not put faced batts on top; the vapor retarder would trap moisture between layers.
Does blown-in insulation settle?
Cellulose settles about 10 to 20% over time, so installers blow it a bit deeper than the target. Bag charts usually list both installed and settled thickness.