Insulation is bought by the bag and judged by its R-value, and the two are joined by the chart printed on the bag. Blown insulation only reaches the R-value its chart promises when both the number of bags and the depth are there — Owens Corning and Greenfiber each say so in as many words — so this page gives both, and never one without the other.
- How much to add comes from ENERGY STAR’s table for existing homes, which is based on the 2021 International Energy Conservation Code: R-30 to R-60 in an attic with none, R-25 to R-49 on top of the 3 to 4 inches many attics already have, and R-13 to R-38 under a floor, by climate zone.
- Blown fiberglass is counted from Owens Corning’s AttiCat chart and blown cellulose from Greenfiber’s Sanctuary Defense card: the bags per 1,000 square feet, the depth to blow it to and, for cellulose, the depth it settles to. When a chart has no row for the R-value asked for, the page uses its next row up and says so.
- Walls that are already built can be filled from outside by drilling holes and blowing cellulose into the cavities, as ENERGY STAR suggests whenever the siding comes off. Greenfiber’s dense-pack chart gives the bags by the stud size.
- Batts sit between the framing, so each one covers its length times the framing’s spacing; the page counts pieces that way, and bags at the coverage printed on yours.
The code asks for something the bags do not show: in a blown attic, depth markers — one for every 300 square feet, marked with the blown depth and facing the hatch. The page counts those too.
The roof calculator hands the attic under a roof straight to this page, and the calculator’s rooms arrive as an area.