Kitchen Ventilation & Make-Up Air in Idaho
Under IRC M1503 as adopted in Idaho, kitchen range hoods must exhaust to the exterior — recirculating-only is not code-compliant for new installs. Hoods over 400 CFM trigger a make-up air requirement per IRC M1503.6, adding $1,500–$5,000 in scope. Here is how to size the hood, route the duct, and decide whether a professional-style CFM is worth the make-up air adder.
Quick answer
Idaho follows IRC M1503 for kitchen range hood ventilation (per Idaho Code § 39-4109). Key requirements: exhaust must be ducted to the exterior (recirculating-only is not code-compliant for new installs); backdraft damper required; smooth-interior duct. IRC M1503.6 triggers a make-up air requirement above 400 CFM — professional-style hoods for 36"+ ranges commonly hit this. Sizing rule of thumb: 100 CFM per linear foot of range (wall-mount) or 150 CFM per foot (island). Right-sizing avoids the $1,500–$5,000 make-up air scope. Treasure Valley elevation (~2,700 ft) does not materially affect hood sizing.
Quick answer
Under the 2018 International Residential Code as adopted by Idaho Code § 39-4109, IRC M1503 governs kitchen range hood ventilation. Key requirements: exhaust must be ducted to the exterior (not recirculating filters only for permitted new installs); duct material must be smooth interior (galvanized steel is standard); duct sizing must match manufacturer specifications; and a backdraft damper is required to prevent outside air infiltration when the hood is off.
| Range Width | Wall-Mount CFM | Island CFM | Triggers Make-Up Air? |
|---|---|---|---|
| 24 inches (compact) | 240 CFM | 300 CFM | No |
| 30 inches (standard) | 300 CFM | 375 CFM | No |
| 36 inches | 360 CFM | 450 CFM | Yes on island (>400) |
| 48 inches (pro) | 480 CFM | 600 CFM | Yes |
| 60 inches (pro with griddle) | 600 CFM | 750 CFM | Yes (larger) |
Sizing rule: 100 CFM per linear foot wall-mount, 150 CFM per linear foot island. Manufacturer ratings are at sea level; Treasure Valley elevation reduces effective capacity 5–10% but is within the sizing margin.
What are the Idaho code requirements for kitchen range hood ventilation?
Under the 2018 International Residential Code as adopted by Idaho Code § 39-4109, IRC M1503 governs kitchen range hood ventilation. Key requirements: exhaust must be ducted to the exterior (not recirculating filters only for permitted new installs); duct material must be smooth interior (galvanized steel is standard); duct sizing must match manufacturer specifications; and a backdraft damper is required to prevent outside air infiltration when the hood is off. The most consequential rule for larger hoods is IRC M1503.6 — any hood exhausting more than 400 CFM triggers a make-up air requirement (see next FAQ). For Treasure Valley new construction and remodels, city inspectors verify duct routing and CFM sizing at rough mechanical inspection.
When do I need a make-up air system for my kitchen hood in Idaho?
IRC M1503.6 requires make-up air whenever the range hood exhausts more than 400 CFM. That threshold is easy to hit — professional-style hoods over 36-inch ranges (Wolf, Viking, Thermador) commonly run 600–1200 CFM. Make-up air replaces the air the hood pulls out of the house; without it, the negative pressure can backdraft a fireplace or gas water heater, or simply prevent the hood from moving its rated CFM. A make-up air system typically involves a motorized damper tied to the hood switch, an insulated intake duct, and (in Idaho winters) a heater or tempering coil so the incoming winter air does not chill the house. Add $1,500–$5,000 for the make-up air scope on top of the hood itself.
What CFM range hood do I actually need for a Treasure Valley kitchen?
Rule of thumb: 100 CFM per linear foot of range for wall-mount hoods, 150 CFM per linear foot for island hoods (which lose capture efficiency to open-side airflow). A standard 30-inch range needs 300 CFM wall-mount or 375 CFM island; a 36-inch range needs 360 CFM wall or 450 CFM island; a 48-inch professional range with a griddle needs 480 CFM wall or 600 CFM island. Most Treasure Valley cooking styles do not need the 1000+ CFM commercial-style hoods that trigger heavy make-up air requirements. Iron Crest sizes hoods to actual cooking needs, not to the biggest number the appliance dealer will sell — right-sizing keeps the make-up air scope out of the project or keeps it small.
Does Treasure Valley elevation affect range hood sizing?
Slightly. Boise sits at about 2,700 feet elevation; Meridian is similar. Air is thinner at elevation, so the same fan moves slightly less mass of air. In practice, this rarely changes hood selection — the CFM ratings from major manufacturers are calibrated at sea level, and the 5–10% capacity loss at Treasure Valley elevation is within the tolerance already built into the 100–150 CFM/linear-foot sizing rules. Higher-elevation Idaho markets (McCall at 5,000 feet, Sun Valley at 5,800 feet) can see more meaningful capacity loss and may need one hood size up. Iron Crest does not work at those elevations — our sizing guidance is Treasure Valley-specific.
Can I use a recirculating range hood in an Idaho remodel?
Not for new permit-required installations under the 2018 IRC as adopted in Idaho. Range hood exhaust must be ducted to the exterior for new work — a recirculating hood with only carbon filters does not satisfy the code. There are narrow exceptions for like-for-like replacement of an existing recirculating hood that was legally installed under earlier code, but any new remodel scope that requires a permit will require exterior ducting. Iron Crest routes exterior ductwork on every permitted kitchen — through the roof (most common on single-story), through a soffit and out an exterior wall, or straight through an exterior wall if the range is on an exterior wall.
How does the range hood duct routing get planned in an Idaho remodel?
Duct routing is a design decision made early, not late. The shortest routing with the fewest 90-degree elbows delivers the rated CFM; long routes with multiple elbows can lose 30–50% of rated performance. Common Treasure Valley routings: straight up through the roof (best for single-story ranch homes — 8–12 feet of straight duct, one termination); through a soffit to an exterior wall (common in two-story homes where the roof route is blocked by second-floor framing); through an exterior wall directly (best when the range is against an exterior wall). Iron Crest walks the routing at initial scoping — the duct decision affects cabinet layout (soffit above the hood) and any structural framing needed.
Right-Sized Ventilation Keeps Make-Up Air Out of the Budget
Iron Crest Remodel (RCE-6681702) sizes range hoods to actual cooking needs and routes ducting for max efficiency. Free estimates. 3-year workmanship warranty.