Indoor vs. Outdoor Ventilation Comparison

This diagram contrasts plume capture in enclosed indoor environments versus open-boundary outdoor environments. The physics are fundamentally different — indoor hoods benefit from four structural advantages that do not exist outdoors.

Reference readings

Representative values below are computed directly from the same capture-fraction module (physics/capture.mjs) driving the instrument above, at a fixed 48-inch island hood over a medium gas grill at a 30-inch rise with the wind across the hood face — this instrument’s only control is wind speed, since it isolates the outdoor wind-exposure effect from hood geometry; live values update as you move the control. The table is regenerated by scripts/generate-reference-tables.mjs, never typed by hand.

In still air this 48-inch island hood models at about 87% capture along the side-wind axis — short of the near-100% indoor baseline RB-004 describes, but for a geometric reason rather than an environmental one: 48 inches is 84% of the 57-inch width RB-002 recommends for this grill, the band RB-008 Table 3.10 calls marginal, “80–85% at best” [RB-008 §3.10]. Still air is the one indoor advantage this instrument grants the outdoor hood: an environment free of crosswind and ambient turbulence [RB-004 §2.1]. Introducing a 5 mph side wind, with no other change to the hood, drops modeled capture to about 51% — the bottom of the 50–65% first-pass capture RB-004 estimates for a UL 710-rated hood operated outdoors [RB-004 §3.3.2] — and 8 mph takes it to about 20%. By 12 mph — the top of the wind band RB-008 classifies as Exposed — modeled capture is about 2%; for that class RB-008 sets the CFM multiplier over bare plume mass flow at 5.75× without panels, versus 3.0× for a sheltered installation, a 92% increase in required exhaust for the same hood [RB-008 §2.2, §3.6].

The Four Indoor Advantages

1. Wall confinement. Indoor walls physically prevent the plume from escaping laterally. Any spillage from the hood is redirected back toward the capture zone by room boundaries. Outdoors, escaped plume disperses freely in three dimensions.

2. Ceiling redirection. When indoor plume rises past the hood, the ceiling acts as a horizontal barrier that redirects the rising gases back toward the hood inlet. Outdoors, there is no overhead barrier — the plume rises indefinitely or is carried away by wind.

3. Pressure-assisted capture. The enclosed room creates a slight negative pressure when the exhaust fan operates, drawing replacement air through room openings. This pressure differential actively assists plume capture. Outdoors, the infinite open boundary eliminates any pressure assist — the hood must rely entirely on its own suction.

4. No wind deflection. Indoor environments have near-zero ambient air velocity at the cooking station. Outdoors, even light breezes (3–5 mph) deflect the plume partially or fully outside the hood capture envelope — at a 30-inch rise a 5 mph wind moves the centerline about 11.7 inches [RB-006 §3.2].

The Capture Efficiency Gap

The combined effect of these four differences reduces outdoor capture efficiency from near-100% (indoor, properly sized hood) to approximately 50–70% (outdoor, same hood, same CFM) [RB-004 §4.3]. This is the central finding of RB-004 and the fundamental reason that indoor ventilation standards cannot be directly applied to outdoor cooking installations.

Implications for Hood Sizing

To achieve reliable outdoor capture, hoods must compensate through:

• Larger canopy dimensions — wider overhang to accommodate plume expansion and wind deflection (see RB-005)
• Higher exhaust rates — 1.7–2.5× the indoor exhaust-rate tables (RB-004 §3.4), reached through a K_CFM of 3.0 to 5.75 over the bare plume flow depending on wind exposure (see RB-008 §2.2)
• Physical wind shielding — side panels and rear walls that partially restore the confinement benefits of indoor environments (see RB-009)

Source Paper

• RB-004: Why Indoor Ventilation Assumptions Fail Outdoors — Complete analysis of all indoor assumptions and their outdoor failure modes