Sizing blown insulation bags by R value
Why bag coverage drops as your R value climbs, how the waste allowance shapes the count, and how to read an attic coverage chart.
Coverage per bag is an R value figure
A bag of loose fill insulation does not have a single coverage number. The bag label prints a chart that lists square feet per bag at each R value, alongside the minimum thickness and the maximum bags per thousand square feet. As the target R value rises, you blow the material thicker, so a bag stretches over less floor. That is why this tool asks you to enter coverage per bag rather than assuming one: you read the row for the R value your climate calls for and type that number in.
How the bag count is calculated
The engine takes your area, either entered directly or found from length times width, and multiplies it by one plus the waste percent. It then divides that padded area by the coverage per bag and rounds up, because insulation is sold whole. A 40 by 30 foot attic is 1200 square feet, which at 5% waste becomes 1260, and at 40 square feet per bag that is 31.5 bags rounded up to 32. A small guard prevents an area that lands exactly on a whole bag from being pushed to the next one.
Choosing a realistic waste percent
Loose fill settles after it is blown, and it never lands in a perfectly even blanket, so a modest waste allowance keeps you from coming up short. Five percent is a sensible default for an open attic. If the space is cut up by trusses, ducts, wiring and knee walls, or if you are fitting batts around framing, raise the percent so the count covers the trimming and the harder to reach corners. It is easier to return an unopened bag than to make a second trip.
Length by width versus a known area
For a simple rectangular attic, the length by width mode is quickest: measure the two sides in feet and let the tool multiply them. For an L shaped or irregular space, work out the square footage yourself by splitting it into rectangles and adding them, then switch to Known area and enter the total. Either way the coverage and waste steps that follow are identical, so use whichever input you already have on hand.