Blown-In Insulation R-Value: R-49, Depth & What It Means (2026)

R-value is the number that decides how much insulation your attic needs, and it is simpler than it looks. R-value measures how well a material resists heat flow: a higher number means slower heat loss, a warmer upstairs, and a furnace that runs less. Illinois is climate zone 5, and the recommended attic target is R-49 to R-60. Most older homes in Waukegan and Lake County sit at R-15 or lower, which is why the upstairs runs cold in January and ice dams show up after a snowfall. This guide explains what R-value means, the depth each material needs to reach R-49, and how to measure the R-value your attic already has.

What R-Value Means

R-value is resistance to heat flow, and it adds up. Two layers of R-19 make R-38. Six inches of fiberglass at 2.5 R-value per inch make R-15. The attic floor is the biggest heat-loss surface in most homes, because heat rises, and the ceiling is the last barrier before the cold attic. Every point of R-value slows that heat a little more, but the benefit shrinks as the number grows. Going from R-15 to R-49 cuts the heat loss through the attic floor by about two-thirds, while going from R-49 to R-60 cuts it by a smaller amount again. That is why R-49 is the standard target.

R-value per inch is the number that separates the materials. Blown fiberglass is 2.5 per inch, cellulose is 3.5 per inch, and mineral wool is 3.0 per inch. Those three numbers decide the depth: divide the target R-value by the R-value per inch, and you get the inches. For R-49, fiberglass needs 19.6 inches, cellulose needs 14.0 inches, and mineral wool needs 16.3 inches. For R-38, the depths are 15.2, 10.9, and 12.7 inches. The same arithmetic works for a top-up: if the attic already has R-15, the missing 34 points need 9.7 inches of cellulose.

Depth to R-38 and R-49 by Material

Material	R-value per inch	Depth to R-38	Depth to R-49	Bags per 1,500 sq ft at R-49
Blown fiberglass	2.5	15.2 in	19.6 in	40
Cellulose	3.5	10.9 in	14.0 in	50
Mineral wool	3.0	12.7 in	16.3 in	46

The depth is the final settled depth, measured from the ceiling drywall to the top of the insulation. Cellulose settles a little in the first year, so the crew should blow it slightly deep and check the settled depth afterward. Fiberglass and mineral wool do not settle, so the depth on the day of the install is the depth you keep. In a low attic, the depth matters twice: the insulation must fit under the roof deck without blocking the soffit vents, and the baffles at the eaves hold it back. If the full R-49 depth will not fit, the crew can use R-38 in the tight bays and R-49 in the open middle, or switch to cellulose.

How to Measure Your Current R-Value

Go into the attic with a tape measure and a flashlight, and measure the insulation depth at five or six spots: the middle, each corner, and near the hatch. Use the shallowest reading, not the average, because insulation settles unevenly and the thin spots are where the heat escapes. Multiply the inches by the R-value per inch: 2.5 for fiberglass, 3.5 for cellulose, 3.0 for mineral wool. Six inches of fiberglass is R-15; ten inches of cellulose is R-35.

If you do not know what the old material is, look at it. Fiberglass is pink, yellow, or white and fluffy. Cellulose is grey and looks like shredded paper. Mineral wool is brown or grey and feels dense and springy. If you truly cannot tell, use 2.5 per inch, which is the most conservative figure, and the blown-in insulation calculator will ask for a little more material than you strictly need. If you cannot get into the attic safely, a free inspection covers the measurement in ten minutes. Do not step on the drywall: walk only on the joists or on boards laid across them.

R-49 or R-38: Which Target?

R-49 is the recommended target for climate zone 5, and it is the default in the calculator. R-38 is the older target that many homes were built to, and it is still a big improvement over R-15. On a 1,500 square foot attic with no insulation, R-38 needs 15.2 inches of fiberglass and 32 bags, against 19.6 inches and 40 bags for R-49. The material saves about $280 to $400, and the installed range is the same $1,500 to $3,000. The saving is real but small, and the attic gives up 11 points of R-value for it.

There are two situations where R-38 is a reasonable choice. The first is a tight budget where the choice is R-38 now or nothing until next year: R-38 now is far better than R-15 for another winter. The second is an attic with a low roof pitch where the full 19.6 inches of fiberglass will not fit under the roof deck at the eaves without blocking the soffit vents. In that case the crew can use R-38 in the tight bays and R-49 in the open middle, or switch to cellulose.

What R-Value Does Not Do

R-value is not the whole story, and a high R-value does not fix a leaky attic. Insulation slows heat moving through the attic floor, but it does not stop air leaking through gaps around pipes, wires, recessed lights, and the attic hatch. Air sealing has to happen first, and it usually costs a few hundred dollars. A leaky attic with R-49 insulation performs worse than a sealed attic with R-38, because the air bypasses the insulation entirely. R-value also does not ventilate: the attic still needs soffit and ridge vents to keep the roof deck cold, or ice dams will form even with thick insulation. And insulation does not stop water: a roof leak must be fixed before any insulation goes in, because wet insulation is worse than thin insulation.

Think of it as a chain: air sealing first, insulation second, ventilation third. Each link makes the next one work better, and skipping one weakens the others. A quote that names the target R-value, the material, and the bag count, and that includes air sealing and a vent check, is the quote to trust. Check it against the typical installed range in the blown-in insulation calculator.

Related reading: blown-in insulation calculator, plus our guides on how much blown-in insulation do I need and cellulose vs fiberglass blown-in insulation, and the roofing calculators hub.

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