Attic insulation & R-value calculator

Attic insulation questions

What R-value does a Florida attic need?

The 2023 Florida Building Code, Energy Conservation prescriptive table lists R-30 for ceilings in climate zones 1 and 2, which cover all of Florida. R-38 is the usual recommendation for comfort and cooling bills. The table applies to new construction and additions; verify what applies to an existing home with your city's building department.

How do I know what R-value I have now?

Measure the depth at a few spots and multiply by the value per inch: about R-2.5 for blown fiberglass, R-3.5 for blown cellulose and R-3.2 for fiberglass batts. Old, settled or compressed insulation is on the low side of those.

Fiberglass or cellulose for blowing in?

Cellulose gives more R per inch and fills around obstructions well; fiberglass is lighter, does not settle as much and is the common choice over existing fiberglass. Either can be blown over what is there as long as the old insulation is dry.

Does a radiant barrier help in Florida?

In a hot, sunny climate with ducts in the attic it can cut the heat radiating down from the roof deck. It has to face an air space, so it is stapled under the rafters or applied to the roof deck, never laid over the insulation.

How much attic ventilation do I need?

The usual rule is 1 square foot of net free vent area for every 150 square feet of attic floor, or 1 per 300 with a vapor retarder on the ceiling side or balanced intake and exhaust. About half goes low at the soffits and half high at the ridge. Product labels list each vent's net free area.