Alabama's code minimums for zone 3
| Part | Minimum | Note |
|---|---|---|
| Ceiling | R-30 | Or R-20 air-impermeable insulation on the roof deck |
| Wood-frame walls | R-13 | Mandatory, not tradeable |
| Floor over a crawl space | R-19 | Rare in slab barndos |
| Slab edge | Not required | Alabama deleted the slab rule |
| Windows | U-0.35, SHGC 0.27 | Mandatory |
| Air leakage | 5 ACH50 or less | Blower-door test, mandatory |
| Duct leakage | 4 cfm per 100 sq ft or less | Rough-in or post-construction test |
From Ala. Admin. Code 305-2-4-.10 and its Appendix A, as published by the Alabama Energy and Residential Codes Board (supp. Sept 2017); edition confirmed by US DOE. Codes get updated, so ask your inspector which edition applies when you permit.
The R-20 roof-deck rule
Alabama's amendment says that where R-30 is required in the ceiling, air-impermeable insulation of R-20 applied to the underside of the roof deck counts as equal (R402.2.2.1). That is the clause that makes spray foam on a barndo roof work under code. At about R-6 per inch for closed-cell foam (DOE), R-20 is roughly 3.5 in; open-cell at about R-3.6 per inch would need about 5.5 in (our math). Wall insulation still has to reach R-13.
Closed-cell vs open-cell spray foam
| Option | Closed-cell | Open-cell |
|---|---|---|
| R-value per inch | About R-5 to 7 | About R-3.5 to 3.7 |
| Typical installed cost | $3.00–5.00 per sq ft | $1.50–3.50 per sq ft |
| Vapour retarder? | Yes | No |
| Best use in a barndo | Sprayed directly on steel roof and wall panels | Inside framed walls and ceilings, away from bare metal |
Cost and R-value from HomeGuide (Dec 2025); vapour behaviour from DOE Building America.
Closed-cell costs more but stops humid air reaching the cold metal, which is what causes dripping and rust (see steel vs wood frame). A common approach is closed-cell on the metal skin and cheaper insulation in interior framed walls.
Sizing the HVAC
A foam-sealed barndo leaks far less air than an old farmhouse, so the rule-of-thumb tonnage in many quotes is too big. An oversized system cools the air quickly and shuts off before it pulls moisture out, leaving the house cold and clammy. Ask the HVAC contractor for a room-by-room load calculation (ACCA Manual J) for your plan, and ask how the system handles humidity on mild, muggy days. Tall great rooms and lofts often need their own zone or a ducted mini-split.
Fresh air in a tight house
Tight construction keeps conditioned air in, so plan how fresh air gets in on purpose: kitchen and bath fans ducted outside, and a ventilation strategy your HVAC contractor can explain. If the shop shares the roof, keep its air separate from the house air; fumes from fuel, paint or welding shouldn't reach the bedrooms.
Getting through inspection
Madison County's inspections include an energy compliance step before final (county). Schedule the blower-door and duct tests with your builder so there is time to seal leaks before drywall and final. Keep the foam installer's certificate showing thickness and R-value. Permit steps are on our zoning and permits page, and finish-level prices in the cost guide.
Common questions
What insulation does Alabama require for a new home?
In climate zone 3, Alabama's code calls for R-30 ceilings (or R-20 foam on the roof deck), R-13 wood-frame walls and U-0.35 windows, all mandatory (Alabama code).
Is spray foam worth it in a barndominium?
Under bare metal, usually yes. Closed-cell foam is air-impermeable and a vapour retarder, which stops condensation on the steel (DOE). It costs about $3–5 per sq ft installed (HomeGuide).
Does a barndo need a blower-door test in Alabama?
Yes. The state code makes an air-leakage test mandatory, with a limit of 5 air changes per hour at 50 pascals (Alabama code R402.4.1.2).
What climate zone is Huntsville in?
Madison and Limestone counties are in IECC climate zone 3A, a warm, humid zone (ICC).
Sources & further reading
- Alabama Energy and Residential Codes, Ala. Admin. Code 305-2-4 (supp. Sept 2017; City of Auburn copy). Zone 3 minimums, R-20 roof deck, blower door, duct leakage
- US DOE Building Energy Codes Program: Alabama status. 2015 IECC, effective Oct 1, 2016
- International Code Council: 2024 IECC climate zone sample. Madison and Limestone: 3A
- HomeGuide: spray foam insulation cost (Dec 2025)
- US DOE Building America: spray foam guide
- Madison County: residential permitting and inspections (archived Jul 2026). What a permit needs; engineered post-frame
Editorial note: General planning information, not engineering or legal advice. Prices are estimates; confirm current rules with your local officials and a qualified pro.