Blown-In Insulation Calculator: Bags, Depth & Cost (2026)
Work out how many bags of blown-in insulation your attic needs, how deep it should be, and what it should cost in Waukegan and Lake County. Enter your attic size and the insulation already in place, and the calculator does the rest.
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Most attics in Waukegan, Gurnee, and the rest of Lake County are under-insulated. The average older home here has six inches of old fiberglass in the attic floor, which is about R-15, while Illinois climate zone 5 calls for R-49. That gap is why upstairs rooms run cold in January, why the furnace runs longer than it should, and why ice dams show up on the eaves after a snowfall. Blown-in insulation is the fastest way to close the gap, because a crew can blow it over the old layer in a day without tearing anything out. The hard part is knowing how much you need and what a fair price looks like, and that is what this page is for. The calculator below takes your attic floor area, the depth and material of the insulation already there, the target R-value, and the material you want, and it returns the inches to add, the bag count, the material cost, and the installed cost range. Every number comes from one set of published manufacturer coverage figures, so the calculator, the worked examples, and the tables on this page all agree with each other.
Blown-In Insulation Calculator
Enter your attic floor size, what insulation is already there, and the material you want. The calculator works out the inches to add, the bags, the material cost, and the installed cost range, and then checks any quote against the typical installed range.
| Item | Amount |
|---|---|
| Attic floor area | — |
| Existing R-value | — |
| Target R-value | — |
| Inches to add | — |
| Final insulation depth | — |
| Bags needed | — |
| Material cost | — |
| Installed cost range | — |
| Installed cost per sq ft | — |
Ranges only. The final quote comes after an inspection of the attic, the access, and the existing insulation. Bag counts come from the Owens Corning AttiCat coverage chart for blown fiberglass and the Greenfiber coverage chart for cellulose, with mineral wool loose-fill at 33 sq ft per bag at R-49.
How the Calculation Works
The calculation has four steps, and each one is simple on its own. First, the attic floor area is the length times the width, or the direct area you enter. Second, the R-value already in place is the existing depth times the R-value per inch of the existing material. Third, the R-value still needed is the target minus the existing R-value, floored at zero. Fourth, the inches to add is the needed R-value divided by the R-value per inch of the new material, and the bag count is the area times the needed R-value divided by the square-foot R-value each bag delivers. The bag count is always rounded up, because you cannot buy part of a bag and a short attic is worse than a spare bag.
The coverage figures come from published manufacturer charts. The Owens Corning AttiCat coverage chart puts blown fiberglass at 37.5 square feet per bag at R-49, which is 1,837.5 square-foot R-value per bag. The Greenfiber coverage chart puts cellulose at 30 square feet per bag at R-49, which is 1,470 square-foot R-value per bag. Mineral wool loose-fill is rated at 33 square feet per bag at R-49, or 1,617 square-foot R-value per bag. The R-value per inch figures are 2.5 for blown fiberglass, 3.5 for cellulose, and 3.0 for mineral wool. Those are the only coverage numbers used on this page, in the calculator and in every table and example below, so the numbers never contradict each other.
| Material | R-value per inch | Coverage at R-49 (sq ft per bag) | Bags per 1,000 sq ft at R-49 | Bag price range | Installed $ per sq ft |
|---|---|---|---|---|---|
| Blown fiberglass | 2.5 | 37.5 | 26.7 | $35 to $50 | $1.00 to $2.00 |
| Cellulose | 3.5 | 30 | 33.3 | $11 to $16 | $1.20 to $2.40 |
| Mineral wool | 3.0 | 33 | 30.3 | $45 to $65 | $1.50 to $3.00 |
Two things in that table surprise most homeowners. The first is that cellulose needs more bags than fiberglass for the same attic, 33.3 bags per 1,000 square feet against 26.7, yet the material cost is far lower, because a bag of cellulose costs $11 to $16 against $35 to $50 for fiberglass. The second is that the installed price per square foot is a range, not a single number, and it moves with the attic. A wide-open attic with a big hatch and a straight run from the truck is at the low end. A low-slope attic with tight corners, a small scuttle hole, and a long hose run is at the high end. The calculator shows the full range and a likely midpoint, and the fairness checker below it tells you where any quote you receive sits against that range.
One more number matters before you measure: the existing depth. Go up into the attic with a tape measure and a flashlight, and measure the insulation depth at five or six spots, including the corners and the middle. Use the shallowest reading, not the average, because insulation settles unevenly and the thin spots are where the heat escapes. If the old layer is a mix of batts and blown-in material, measure the blown-in depth and note the batts separately. If you cannot get into the attic safely, a free inspection covers it, and the measurement takes ten minutes.
Worked Examples
Three real attics, worked through the same calculation the calculator runs, so you can check the tool against hand math. All three target R-49, the Illinois climate zone 5 recommendation.
Example 1: Empty 1,500 sq ft attic, blown fiberglass
A two-story home in Waukegan with a 50 by 30 foot attic floor and no insulation at all. The attic floor is 1,500 square feet, the existing R-value is zero, and the full R-49 is still needed. At 2.5 R-value per inch, fiberglass needs 19.6 inches to reach R-49. The bag count is 1,500 times 49 divided by 1,837.5, which is exactly 40 bags. The material costs 40 bags at $35 to $50, which is $1,400 to $2,000. Installed, the range is 1,500 square feet at $1.00 to $2.00 per square foot, which is $1,500 to $3,000, with a likely cost near $2,250. This is the default the calculator opens with, so you can change any input and watch the numbers move.
Example 2: 1,200 sq ft attic with 6 inches of old fiberglass, cellulose top-up
A ranch in Gurnee with a 40 by 30 foot attic and 6 inches of old blown fiberglass. Six inches at 2.5 R-value per inch is R-15 already in place, so the attic needs 34 more R-value to reach R-49. Cellulose at 3.5 R-value per inch needs 9.7 inches to add those 34 points, bringing the final depth to 15.7 inches. The bag count is 1,200 times 34 divided by 1,470, which is 27.8, rounded up to 28 bags. The material costs 28 bags at $11 to $16, which is $308 to $448. Installed, the range is 1,200 square feet at $1.20 to $2.40 per square foot, which is $1,440 to $2,880, with a likely cost near $2,160. Notice how much cheaper the material is than Example 1 even though the bag count is close: cellulose bags cost a third of fiberglass bags.
Example 3: 2,400 sq ft attic with 10 inches of old cellulose, mineral wool top-up
A large two-story home in Libertyville with a 60 by 40 foot attic and 10 inches of old cellulose. Ten inches at 3.5 R-value per inch is R-35 already in place, so only 14 more R-value is needed. Mineral wool at 3.0 R-value per inch needs 4.7 inches to add, bringing the final depth to 14.7 inches. The bag count is 2,400 times 14 divided by 1,617, which is 20.8, rounded up to 21 bags. The material costs 21 bags at $45 to $65, which is $945 to $1,365. Installed, the range is 2,400 square feet at $1.50 to $3.00 per square foot, which is $3,600 to $7,200, with a likely cost near $5,400. This is the expensive end of the scale: a big attic, a premium material, and a wide installed range because large attics vary more in access and hose runs.
| Example | Attic | Existing | Material | Inches to add | Bags | Material cost | Installed range | Likely cost |
|---|---|---|---|---|---|---|---|---|
| 1 | 1,500 sq ft | None | Fiberglass | 19.6 in | 40 | $1,400 to $2,000 | $1,500 to $3,000 | $2,250 |
| 2 | 1,200 sq ft | 6 in fiberglass (R-15) | Cellulose | 9.7 in | 28 | $308 to $448 | $1,440 to $2,880 | $2,160 |
| 3 | 2,400 sq ft | 10 in cellulose (R-35) | Mineral wool | 4.7 in | 21 | $945 to $1,365 | $3,600 to $7,200 | $5,400 |
R-49 or R-38: Which Target for Illinois?
Illinois sits in climate zone 5, and the U.S. Department of Energy recommends R-49 to R-60 for attic insulation in zone 5. R-49 is the standard target for a top-up in Waukegan and Lake County, 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 or R-19, but it leaves the attic below the current recommendation. 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 because the installed price is per square foot, not per bag. The saving is real but small, and the attic gives up 11 points of R-value for it. Over a Chicago-area winter, that missing R-value shows up as a colder upstairs and a furnace that cycles more often.
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, which reaches R-49 in 14.0 inches. What does not make sense is paying for R-38 when the attic has room for R-49 and the budget covers the extra eight bags. The calculator lets you switch the target and compare the two in seconds, and the difference is usually smaller than homeowners expect.
Which Material Should You Pick?
All three blown-in materials work. The choice is about price, settling, and what the attic needs. The table below compares them on the same attic, 1,500 square feet with no insulation, to R-49, so the differences are the materials themselves and nothing else.
| Material | Inches to R-49 | Bags (1,500 sq ft) | Material cost | Installed range | Likely cost |
|---|---|---|---|---|---|
| Blown fiberglass | 19.6 in | 40 | $1,400 to $2,000 | $1,500 to $3,000 | $2,250 |
| Cellulose | 14.0 in | 50 | $550 to $800 | $1,800 to $3,600 | $2,700 |
| Mineral wool | 16.3 in | 46 | $2,070 to $2,990 | $2,250 to $4,500 | $3,375 |
Blown fiberglass is the default for most jobs. It does not settle, it does not absorb water easily, and the installed cost is the lowest of the three because crews blow it fast and the bag count is the smallest. Its weakness is the depth: 19.6 inches to reach R-49 is a lot of material to fit under a low roof, and it is the easiest material to leave thin spots in if the crew rushes. Ask the crew to measure the depth in several places before they pack up, and to add bags where it is thin.
Cellulose is the value pick on material cost. A bag costs $11 to $16, so 50 bags for a 1,500 square foot attic run $550 to $800, less than half the fiberglass material cost. It also packs tighter into gaps and reaches R-49 in only 14.0 inches, which helps in low attics. The trade-off is settling: cellulose drops a little in the first year, so the crew should blow it slightly deep and confirm the settled depth afterward. It is made from recycled paper treated with borate, which gives it decent fire resistance, but it absorbs water if a leak sits on it, so any roof leak must be fixed first.
Mineral wool is the premium pick. It does not settle, it does not burn, and it resists moisture better than either of the other two. The material cost is the highest, $2,070 to $2,990 for 46 bags on the same 1,500 square foot attic, and the installed range runs $2,250 to $4,500. It makes sense where the attic has had moisture trouble, where fire resistance matters, or where the homeowner wants a material that will hold its depth for decades without a second visit. For most Waukegan attics, fiberglass or cellulose is the right call, and mineral wool is the upgrade.
What the Installed Cost Range Includes
The installed range on this page is the cost of the material, the labor to blow it, and the standard prep: protecting the hatch, laying baffles at the eaves so the new insulation does not block the soffit vents, and a depth check at the end. It does not include air sealing, and that is the most important thing to understand about any quote. Air sealing means caulking and foaming the gaps around pipes, wires, recessed lights, and the attic hatch before the insulation goes in. It is usually a few hundred dollars of work, and it is worth more than the insulation itself on a leaky attic, because insulation does not stop air. A quote that includes air sealing is worth more than a cheaper quote that skips it, even if the cheaper quote looks better on paper.
Other work that may or may not be in a quote: removing old insulation, which is needed only if the old layer is wet, moldy, or damaged; repairing a roof leak, which must happen before any insulation goes in; adding a raised attic hatch or walkway boards; and fixing ventilation, such as adding soffit vents or a ridge vent. None of those are part of the blown-in price, and a good quote lists them separately so you can see what you are paying for. If a quote is far below the range the calculator shows, the usual reason is that one of those items was left out. If a quote is far above the range, the usual reason is a steep roof, a tiny access hatch, or a long hose run from the truck, and the quote should say so in writing.
Insulation and Ventilation Work Together
Insulation keeps heat in the house. Ventilation keeps the roof deck cold. Both matter in a Chicago-area winter, because ice dams form when heat from the house melts snow on the roof and the meltwater refreezes at the cold eaves. Thick insulation cuts the heat reaching the roof, and soffit and ridge vents carry away what is left, so the roof deck stays near the outdoor temperature and the snow melts evenly. An attic with R-49 insulation and no ventilation can still grow ice dams. An attic with great ventilation and R-15 insulation can still grow ice dams. The two systems are a pair, and a quote that fixes only one of them is half a fix.
The practical rule is simple: when the crew blows insulation, they must leave the soffit vents open. The baffles at the eaves, sometimes called vent chutes, hold the new insulation back from the soffit so air can still flow from the eaves to the ridge. A crew that blows insulation straight over the soffit vents has made the attic worse, not better, and the fix is expensive once the insulation is in place. Ask the quote to name the baffles and the vent check explicitly. If your home already has ice dams, read the attic ventilation calculator to size the venting, and the attic ventilation ice dams guide to see how the two fixes fit together. The soffit calculator sizes the intake vents that feed the whole system.
How to Read a Quote
Use the fairness checker in the calculator. Enter the quote total, and it tells you whether the number sits below, inside, or above the typical installed range for your attic, your material, and your target R-value. A quote below the range is not automatically a bargain: check that it includes the full bag count, the air sealing, and the baffles. A quote inside the range is fair if it names the material, the bag count, and the target R-value in writing. A quote above the range is not automatically a rip-off: a steep attic, a small hatch, or added air sealing can justify it, but the quote should explain why in a line-by-line breakdown. What should worry you is a quote that names no material, no bag count, and no R-value, because there is nothing to check it against.
Three questions to ask any contractor before you sign: first, how many bags are in the quote, and does the bag count match the coverage chart for the material and the target R-value? Second, is air sealing included, and which gaps will be sealed? Third, how will the crew keep the soffit vents open, and will they measure the finished depth in several places before they leave? A contractor who answers all three clearly is the one to hire. A contractor who cannot answer them is guessing, and the attic is not the place to guess.
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Related Guides
- How Much Blown-In Insulation Do I Need?
- Blown-In Insulation Cost
- Blown-In Insulation vs Batts
- Attic Insulation Cost
- Cellulose vs Fiberglass Blown-In Insulation
- Blown-In Insulation R-Value
- Signs You Need More Attic Insulation
- Attic Ventilation Calculator
- Soffit Calculator
- All Roofing Calculators
Frequently Asked Questions
How many bags of blown-in insulation do I need for a 1,500 sq ft attic?
It depends on the material and the target R-value. At R-49, a 1,500 sq ft attic with no insulation in place needs about 40 bags of blown fiberglass, about 50 bags of cellulose, or about 46 bags of mineral wool. If you already have insulation, the bag count falls: 6 inches of old fiberglass (R-15) leaves 34 R-value to add, which takes 28 bags of cellulose on a 1,200 sq ft attic. Enter your own attic size and existing depth in the calculator above for the exact bag count for your home.
How deep should blown-in insulation be in Illinois?
Illinois is climate zone 5, and the recommended attic target is R-49. That means about 19.6 inches of blown fiberglass, about 14.0 inches of cellulose, or about 16.3 inches of mineral wool when you start from nothing. The depth is the final depth after settling, so the crew should blow cellulose slightly deep and confirm the settled depth. R-38 is a lower target that takes about 15.2 inches of fiberglass, and it saves bags up front but leaves the attic below the zone 5 recommendation.
How much does blown-in insulation cost in Waukegan?
For a typical 1,500 sq ft attic in Waukegan or Lake County, blown fiberglass installed to R-49 runs about $1,500 to $3,000, with a likely cost near $2,250. Cellulose on the same attic runs about $1,800 to $3,600, and mineral wool about $2,250 to $4,500. The material alone is about $1,400 to $2,000 for 40 bags of fiberglass, or $550 to $800 for 50 bags of cellulose. Every total is a range, and the final quote comes after an inspection of the attic, the access, and the existing insulation.
Can you blow new insulation over old insulation?
Yes, in most cases. Blown-in insulation layers over old batts or old blown-in material as long as the old layer is dry, undamaged, and not hiding a leak. The calculator treats the old layer as a starting R-value and adds only what is missing. Wet, moldy, or compressed insulation should come out first, and any roof leak should be fixed before new insulation goes in, because insulation does not stop water.
Does blown-in insulation need air sealing first?
Yes, and this is the step most often skipped. Insulation slows heat moving through the attic floor; it does not stop air leaking through gaps around pipes, wires, light fixtures, and the attic hatch. Sealing those gaps before the crew blows insulation usually costs far less than the insulation itself and makes the R-value you pay for actually work. A quote that includes air sealing is worth more than a cheaper quote that skips it.
Which is better, blown fiberglass or cellulose?
Neither wins on every point. Fiberglass costs more per bag but needs fewer bags for the same R-value at high targets in some cases, and it does not settle. Cellulose costs much less per bag and covers more R-value per inch, but it settles a little after install and needs the depth checked afterward. Mineral wool costs the most per bag, does not settle, and resists fire and moisture. For a 1,500 sq ft attic to R-49, the installed ranges are about $1,500 to $3,000 for fiberglass, $1,800 to $3,600 for cellulose, and $2,250 to $4,500 for mineral wool.
How long does blown-in insulation last?
Blown-in insulation does not wear out on a schedule. Fiberglass and mineral wool hold their depth for decades once installed. Cellulose settles by a small amount in the first year, which is why the crew should blow it slightly deep and check the settled depth later. What shortens the life of any insulation is water from a leak, compression from stored boxes or foot traffic, and pest damage. If the depth checks out and the attic stays dry, the R-value stays close to what was installed.
Will blown-in insulation stop ice dams?
It helps a lot, but it is not the whole fix. Ice dams form when heat from the house melts snow on the roof, and that heat escapes through a thin attic floor and through air leaks. Bringing the attic to R-49 cuts the heat loss through the floor, and air sealing cuts the leaks. Ventilation matters too: soffit and ridge vents keep the roof deck cold so snow melts evenly. If ice dams are your main problem, read the attic ventilation ice dams guide as well, because insulation and ventilation work as a pair.
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Call (858) 399-8124 now for a free attic inspection in Waukegan and Lake County. We will measure your insulation, check your ventilation, and give you a written quote with the bag count and the target R-value.