Zone 3 has hot summers and mild winters. The code asks for an R-49 attic, R-20 walls (or R-13 plus R-5 foam) and R-10 foam along the slab edge, 2 ft deep.
| Attic / ceiling | R-49 (2018: R-38) |
|---|---|
| Wood-frame walls | R-20, or 13+5ci, or R-15ci (2018: R-20, or 13+5ci) |
| Mass walls (block, brick) | R-8 exterior / R-13 interior |
| Floors over unheated space | R-19 |
| Basement walls | R-5ci or R-13 |
| Slab edge | R-10, 2 ft deep (2018: None required) |
| Crawl space walls | R-5ci or R-13 |
| Windows: heat loss (max U-factor) | U-0.30 (2018: U-0.32) |
| Windows: sun heat let in (max SHGC) | 0.25 |
How to read this: "13+5ci" means R-13 batts between the studs plus R-5 foam board over the outside of the wall ("ci" is continuous insulation). For windows, a lower U-factor keeps more heat in, and a lower SHGC lets less of the sun's heat through. Both numbers are on the window's NFRC sticker.
ENERGY STAR attic target: R-49 for an uninsulated attic, or add R-38 if you already have 3–4 inches. After air sealing, topping up the attic is usually the best-value energy upgrade in a house.