How to measure an attic
Measure the attic floor, not the roof above it. That is usually the footprint of the house below, less anything that is not insulated, such as a garage or a porch. Walk the joists with care, or measure from below in the rooms. Only step on the joists or on a walk board, since a foot through the ceiling is an expensive way to find out where it is thin.
Then push a ruler down through the insulation in several places, right to the ceiling below, and take the average. We treat what is there as about R-3.2 per inch, from a maker's figure for fiberglass batts. If yours is old, flattened or was ever wet, it is doing less than that, so enter a smaller depth.
Bag charts cover the net area. Owens Corning's professional chart adds bags for the eaves depending on the roof pitch, so a long low roof will take a little more than this.
A worked example
This one is printed in Greenfiber's instructions: you want R-38 over 1,200 square feet, and you have about R-19 in place now. These lines come from the same formula the calculator above runs.
1,200 sq ft, R-38 wanted, R-19 in place
- R-value to add: 38 − 19 = 19
- SANCTUARY row: R-19, 44.2 sq ft a bag
- Bags: 1200 ÷ 44.2 = 27.15, so 28 to buy
- Blow to 5.7 in, settling to 5.3 in
Greenfiber prints 27 bags, rounding down. The same chart calls the bag count a minimum, so we round up to 28. To compare products, keep this page open with the insulation calculator, which handles rolls and batts, and use the home energy savings calculator to see whether the job pays for itself.
Bags for 1,000 square feet
These come straight from each maker's printed chart, rounded up to whole bags. Do not compare the two bag counts directly, since these are different bags. Compare the price of covering the same area at the same R-value.
| R-value added | AttiCat bags | AttiCat depth | SANCTUARY bags | SANCTUARY depth |
|---|---|---|---|---|
| R-30 | 15 | 10.50 in | 39 | 8.9 in |
| R-38 | 20 | 13.25 in | 53 | 11.2 in |
| R-49 | 26 | 16.50 in | 71 | 14.3 in |
| R-60 | 32 | 19.75 in | 87 | 17.3 in |
Mistakes that cost you R-value
- Subtracting twice. ENERGY STAR's second column is an amount to add. If you take off what is there from that, you end up short.
- Depth without bags. Owens Corning warns that without both the bags and the minimum thickness, the insulation gives a lower R-value.
- Blocking the vents. Insulation pushed against the soffits stops the attic from breathing. Fit baffles first.
- Forgetting to air seal. Leaks around lights, pipes and hatches are hard to find once they are under a foot of insulation.
- Settled depth as a target. Cellulose goes in thicker than it ends up, at about 90 percent. Blow to the installed figure.
Questions
How many bags of blown-in insulation do I need for 1,000 square feet?
To add R-49 over 1,000 square feet takes 26 bags of AttiCat fiberglass, whose chart prints 25.3, or 71 bags of 25 lb SANCTUARY cellulose. Bag sizes and products differ that widely, so check the chart on the bag you are buying.
How many inches of blown insulation is R-38?
R-38 takes 13.25 inches of AttiCat fiberglass, by its chart, or about 11.2 inches of SANCTUARY cellulose installed, settling to 10.4. Blow to the installed depth, and count the bags as well, since the R-value relies on both.
How many inches of blown insulation is R-49?
R-49 takes 16.5 inches of AttiCat fiberglass, or 14.3 inches of SANCTUARY cellulose installed, settling to 13.3. Cellulose settles, while fiberglass barely does, which is why charts list two depths for one and effectively one for the other.
What R-value should my attic have?
R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8, for an uninsulated attic by ENERGY STAR's table. If you have 3 to 4 inches already, its table lists an amount to add instead: R-25, R-38 and R-49 for those groups.
Why are there so many more bags of cellulose?
Because the bags are different. A SANCTUARY bag is 25 pounds and covers 14.1 square feet at R-49, while an AttiCat bag covers 39.5. It says nothing about which is better, so compare the price per square foot at the same R-value.
Do I need to air seal first?
Yes, seal gaps around lights, pipes and the attic hatch before blowing insulation, because it is much harder to reach them later. Keep the soffit vents clear with baffles, so the attic can still breathe.
This bag count is a planning estimate from two makers' coverage charts, and job conditions and blower settings can move real coverage by about 10 percent. Hit both the bags on the label and the minimum thickness, since the R-value depends on both. Air seal first, keep soffit vents clear, and check with your supplier about the blower.
Sources and review
- Recommended home insulation R-values, ENERGY STAR, no date shown, read November 2026.
- AttiCat expanding blown-in insulation coverage chart, Owens Corning, 2015.
- SANCTUARY cellulose DIY attic installation instructions, Greenfiber, 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
R-value to add
R to add = Target − Existing depth × 3.17
- 3.17
- R-19 for six inches of batt, from Greenfiber, or about 3.17 per inch
Worked through: 38 − 19 = 19
Bags
Bags = Attic area ÷ Square feet per bag, rounded up
- Square feet per bag
- From the printed chart row at or above the R-value to add
Worked through: 1200 ÷ 44.2 = 27.15, so 28 bags
The constants, and where they come from
TARGET_R_BY_ZONE
Published figureTotal R-value for an attic, by climate zone
- 1
- 30
- 2
- 49
- 3
- 49
- 4
- 60
- 5
- 60
- 6
- 60
- 7
- 60
- 8
- 60
Applies to: Existing wood-framed houses. It is cost guidance, not a code
Source: Recommended home insulation R-values, ENERGY STAR, no date on the page, read 2026-11.
ENERGY STAR's column for an uninsulated attic, used as a total to reach. Its other column, for 3 to 4 inches already there, is an amount to add, and subtracting from that would count your existing insulation twice.
EXISTING_R_PER_INCH
Published range, narrowedR-value per inch assumed for what is already in the attic
Applies to: Fiberglass batts in fair shape. Old, flattened or wet insulation does less
Source: SANCTUARY cellulose DIY attic installation instructions, DM-6.3-38 Rev F, Greenfiber, published 2026-05, read 2026-11.
From Greenfiber's statement that about six inches of fiberglass batt is R-19. It is a rough figure for an unknown material.
CHARTS
Published figurePrinted coverage: R-value, square feet per bag, and installed and settled inches, by product
- atticat
- r 30, sqFtPerBag 68.5, installedInches 10.5, settledInches 10.5, r 38, sqFtPerBag 51.8, installedInches 13.25, settledInches 13.25, r 49, sqFtPerBag 39.5, installedInches 16.5, settledInches 16.5, r 60, sqFtPerBag 31.4, installedInches 19.75, settledInches 19.75
- sanctuary
- r 19, sqFtPerBag 44.2, installedInches 5.7, settledInches 5.3, r 30, sqFtPerBag 25.9, installedInches 8.9, settledInches 8.3, r 38, sqFtPerBag 19, installedInches 11.2, settledInches 10.4, r 49, sqFtPerBag 14.1, installedInches 14.3, settledInches 13.3, r 60, sqFtPerBag 11.6, installedInches 17.3, settledInches 16.1
Applies to: AttiCat fiberglass and SANCTUARY 25 lb cellulose. Other products differ widely
Source: AttiCat expanding blown-in insulation coverage chart, Owens Corning, published 2015, read 2026-11. This source is about 11 years old, and it is the one the figure rests on.
Also from: SANCTUARY cellulose DIY attic installation instructions, DM-6.3-38 Rev F, Greenfiber, published 2026-05, read 2026-11.
Read from each maker's chart. Greenfiber says conditions can move actual coverage by as much as 10 percent, and that settled thickness is 90 percent of installed. Rows differ by product, so there is no single coverage figure.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the blown-in insulation calculator
- 1
Measure the attic floor, not the roof
- 2
Measure the depth of what is already up there
- 3
Find your climate zone
- 4
Choose the product you can buy
- 5
Read the bags, then mark the depth on rulers before you start
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- What is up there now is about R-3.2 per inch, from a maker's figure for fiberglass batts. Old or flattened insulation does less
- Bags come from a printed chart row, rounded up to the next R-value, and never interpolated
- The area is net. Maker charts add bags for the eaves, depending on roof pitch, and this does not
- The product charts differ widely, so match the row on your own bag
Pro tips
- Fix depth rulers to joists across the attic before you start
- Air seal first, because it is the job you cannot reach afterwards
- Compare price per square foot, not per bag