Attic Insulation Depth Calculator
Enter a target R-value and your insulation material and this tool returns the depth in inches you have to install — because R divided by the material's R-per-inch is the whole story.
Calculator
Reaching R-49 with blown cellulose (~3.5 r/in) takes about 14.0 in (R ÷ R-per-inch). DOE recommends roughly R-38 to R-60 in attics by climate zone.
Insulation is rated by its R-value — its resistance to heat flow. The higher the R-value, the slower heat escapes your house in winter or leaks in during summer. Every insulation product also has an R-per-inch: how much R-value one inch of it contributes. Put those two facts together and the depth you must install is a single division. This calculator does it for the five materials you actually find at the top of an attic, so you can translate a target like "R-49" into "14 inches of blown cellulose" before you rent the machine or buy the batts.
Why does the same R-value need different depths? Because a fluffy blown fiberglass fills more volume per unit of resistance than dense cellulose or rigid foam. That is not a quality judgment — it is why the tool asks which material you are using. Pick the wrong one and you can end up a third short or a third over.
Formula
depth (in) = target R ÷ R-per-inch
R-per-inch values used here are typical, stable industry figures: fiberglass batt ≈ 3.2, blown fiberglass ≈ 2.5, blown cellulose ≈ 3.5, open-cell spray foam ≈ 3.7, closed-cell spray foam ≈ 6.5. They do not drift over time, so the answer stays correct year after year. Your actual product label may vary slightly — treat these as planning typicals and confirm with the bag or datasheet.
Worked example
Say you want to reach R-49 (a common cold-climate attic target) with blown cellulose at 3.5 R per inch:
49 ÷ 3.5 = 14.0 inches
Switch to blown fiberglass at 2.5 R per inch and the same R-49 needs:
49 ÷ 2.5 = 19.6 inches
Same resistance, five and a half inches more loft — which matters when your rafters or storage platform limit the depth you can pile in.
Practical notes
Two practical cautions. First, settling: loose-fill materials settle over the first months, so blow to the labeled installed thickness, not the just-poured height — the bag rates coverage at settled depth. Second, keep the eaves clear: never bury the soffit vents. Use baffles at the eaves so the depth you calculated here does not choke the ventilation path (see the attic ventilation calculator).
This tool sizes the insulation layer itself. To confirm the R-value of a stack of different materials, use the assembly R-value calculator; to count bags of blown-in product, use the blown-in bags calculator.
Frequently asked questions
How many inches of insulation is R-49?
It depends on the material. In blown cellulose (about 3.5 R per inch) R-49 is roughly 14 inches; in blown fiberglass (about 2.5 R per inch) it is about 19.6 inches; in fiberglass batts (3.2 R per inch) about 15.3 inches. Divide 49 by the R-per-inch of your product.
What R-value should my attic be?
The U.S. Department of Energy suggests roughly R-38 to R-60 for attics, depending on your climate zone — higher in cold northern zones, lower in the warm south. Check the DOE zone map for your area; local energy codes may set their own minimum.
Does adding insulation on top of old insulation work?
Yes. R-values add, so new blown-in over existing batts increases the total. Use the assembly R-value calculator to add the layers. Do not compress the old material, and do not add a second vapor retarder on top — that can trap moisture.
Is more insulation always better?
There are diminishing returns: going from R-13 to R-38 saves far more than R-38 to R-60, because each added inch reduces a smaller share of the remaining heat loss. Hitting your zone's recommended R-value captures most of the benefit; ventilation and air-sealing matter just as much beyond that.