What size attic insulation do I need in Georgia?
R-value first, inches second
R-value measures resistance to heat flow, and different materials reach the same R-value at different depths:
- Fiberglass batts: R-30 batts are roughly 9 to 10 inches thick and R-38 batts roughly 12 inches.
- Blown cellulose: about R-3.2 to 3.8 per inch, so R-38 is roughly 10 to 12 inches after settling.
- Blown fiberglass: lower per inch than cellulose, so R-38 often takes 13 inches or more.
- Spray foam against the roof deck: a different approach used for unvented attics, with much less thickness.
The bag label lists depth and coverage for each R-value. Use it instead of a rule of thumb, and staple up depth rulers so you can check later.
Georgia climate zones
Most of Georgia, including metro Atlanta, is in Climate Zone 3, the far south is in Zone 2, and the north Georgia mountains are in Zone 4. Requirements for new construction come from Georgia's adopted energy code with state amendments, so check with your building department for current numbers on new work or a major remodel.
Do not block the roof's airflow
More insulation is good right up until it buries the soffit vents. Before topping up, install baffles at the eaves so there is at least a 1 inch air channel above the insulation. Seal air leaks at light fixtures, top plates and the hatch first, since insulation slows heat but does not stop air. And make sure bath fan ducts are connected and insulated so you are not burying a moisture source. See roof ventilation basics and attic heat and shingles.