- Where it goes
- Attic floor, vented attic
- Code minimum
- R-30 in Zone 1A
- Installed standard
- R-38, even depth
- Markers
- One per 300 sq ft
The attic is outside
A vented attic is not part of the house. The thermal boundary is the ceiling plane: drywall, top plates, the hatch, and every wire that pokes through. Insulation sits on that plane. The roof deck above it is supposed to run hot and then dump that heat out the vents.
If the soffits are blocked, the attic gets hotter and the roof deck stays wetter after a rain. We set baffles at the eaves so the blow cannot close the intake. We do not stuff the soffit to gain a few inches of coverage.
R-30, R-38, and what is already up there
Zone 1A's prescriptive ceiling value is R-30. We install R-38 on a blown attic floor. Starting from a bare ceiling, that is about 15 inches of new blown fiberglass at R-2.5 per inch, or about 10.5 inches of new cellulose at R-3.6 per inch.
Most of these houses are not bare. Four inches of settled fiberglass is about R-9, figured at R-2.2 per inch for old fill. The new blow only has to close the gap. We measure what is there before we say how many bags.
Settled fiberglass is figured lower than new fiberglass because it has packed down. Using the new-material R-value on old fill is how a top-up comes up short.
The ceiling has to be tight first
Top plates, wire penetrations, plumbing stacks, can lights, open chases, and the hatch. Those are the leaks. Insulation laid over them does not seal them. Air from the house still reaches the attic, and in this climate that air is moist.
Non-IC recessed lights cannot be buried. An IC or IC-AT can, rated for insulation contact, can. Anything else gets a listed cover that keeps the clearance, or the fixture gets changed. Covering a hot can to hit a depth number is how fires start.
A bath fan has to terminate outdoors. A fan dumped into the attic is a moisture machine on the roof deck. We do not bury that and leave it.
Knee walls and bonus rooms
A knee wall is a short wall with attic on the back. Insulating only the studs leaves the floor behind the wall and the sloped ceiling uninsulated. The room still loses heat on those two surfaces.
The attic side of the knee wall needs an air barrier, not just batts shoved in the bay. The slope and the attic floor behind it are part of the same boundary. If that space should be inside the house, it is a roof-deck foam job, not a loose blow on the floor.
How you can check the work
The Florida energy code calls for depth markers on blown ceiling insulation, at least one for every 300 square feet, marked in inches, facing the attic opening. You should be able to read the depth from the hatch.
Look for an even field, baffles still open at the eaves, and the hatch weatherstripped. A mountain of insulation at the hatch and a bare spot at the eaves is not R-38. It is a pile.