A commercial attic is insulated on the ceiling of the space below, to R-38, when the attic stays outside the building. The product is blown-in. Batts are for cavities.
- Code
- C402.1.3, attic and other
- Zone 1 target
- R-38
- Product
- Blown fiberglass or cellulose
- Not this job
- Above-deck roofing insulation
The commercial attic number
Palm Beach County and Broward County are Climate Zone 1. Table C402.1.3 of the 2023 Florida Building Code, Energy Conservation, lists attic and other roofs at R-38. That row covers an office and a Group R building, such as an apartment house. R-38 is the prescriptive target. It is not an upgrade over a lower commercial number.
A house in the same county is on a different table. The residential ceiling minimum is R-30. Do not mix the two. A commercial attic is quoted as a commercial attic.
What goes in the attic
The usual product is blown fiberglass or blown cellulose on the attic floor, after the leaks in the ceiling are sealed. New fiberglass is about R-2.5 per inch. New cellulose is about R-3.6 per inch. From a bare lid, R-38 is about 15 inches of fiberglass or about 10.5 inches of cellulose.
Batts are the wrong product for an open commercial attic floor. They gap at the joists and they cannot be installed to an even R-38 across a wide bay. Batts belong in a stud cavity. That is a different service.
Depth markers face the access, at least one for every 300 square feet, marked in inches. That is the commercial energy code rule for blown ceiling insulation. You should be able to read the depth from the opening.
What this is not
Insulation entirely above a flat roof deck is R-20 continuous for most occupancies and R-25 continuous for Group R. That is a roofing assembly. We do not install it.
We do not blow into a ceiling that is the return-air plenum. Loose fill in that airstream ends up in the unit. If the boundary is a lid above the plenum, the blow goes on that lid. If there is no lid, say so before the crew is booked.