The ACH Formula
Air changes per hour is the number of times the air in a room is completely replaced in one hour. The math is trivial but important: ACH = CFM × 60 ÷ volume. A 100 CFM fan exhausting a 1,000 cu ft bathroom gives 6 ACH — the whole room of air is replaced every 10 minutes. Higher ACH means more dilution of contaminants (CO2, humidity, odors, pathogens) but also more energy spent heating or cooling fresh air.
Typical Target ACH by Space
Bedrooms and living rooms: 3-4 ACH (ASHRAE 62.2 residential). Bathrooms: 6-8 ACH continuous or 50 CFM intermittent during use. Kitchens: 10+ ACH with the range hood on during cooking (vented to outside). Laundry rooms: 8 ACH. Offices: 4-6 ACH. Classrooms: 6 ACH (higher because of student CO2 production). Gyms: 8-10 ACH. Restaurants and bars: 10 ACH. Hospital isolation rooms: 12+ ACH with negative pressure.
Infiltration vs Ventilation
Infiltration is unplanned air leakage through the building envelope — wall penetrations, windows, outlets, rim joists. Old houses have 1-2 ACH of natural infiltration; modern code-compliant houses are 0.25-0.5 ACH; passive houses are under 0.1 ACH. Ventilation is intentional air supply from fans, vents, or windows. ASHRAE 62.2 sets the minimum total outdoor air (natural infiltration + mechanical ventilation) for healthy indoor air.
CFM vs ACH
CFM (airflow) is the physical rate of air movement. ACH (air change rate) is CFM relative to room volume. A 100 CFM fan gives 6 ACH in a small bathroom but only 1 ACH in a large open living room. Engineers use both: ACH for dilution of contaminants relative to space, and CFM per person for load-based fresh air sizing.
Recommended ACH by Room Type
Industry-standard outdoor-air change rates from ASHRAE 62.1/62.2 and industry best practices:
| Space | Recommended ACH | Notes |
|---|---|---|
| Bedroom | 3-4 | ASHRAE 62.2 residential minimum |
| Living room | 3-4 | Same as bedroom |
| Bathroom | 6-8 | Continuous; or 50 CFM intermittent during use |
| Kitchen | 10-15 | Range hood vented outside, 100 CFM min |
| Laundry room | 8-10 | Moisture control |
| Office | 4-6 | ASHRAE 62.1 per-person |
| Classroom | 6-10 | High CO2 from students |
| Gym / exercise | 8-12 | Sweat and body heat |
| Restaurant dining | 8-12 | Food odors |
| Restaurant kitchen | 20-40 | Heavy cooking + makeup air |
| Hospital operating room | 20+ | FGI Guidelines |
| Hospital isolation room | 12+ | Negative pressure, CDC |
Frequently Asked Questions
How many air changes should my basement get?
Basement conditioned space: 0.35 ACH minimum outdoor air. Unconditioned crawl space: 1 CFM per 50 sq ft of ground area per IRC.
Does ACH50 equal natural ACH?
No. ACH50 is a pressure-test result at 50 Pa. Divide ACH50 by the LBL N-factor (typically 15-20 depending on climate and stories) to estimate natural ACH.
What ACH is recommended for COVID-19?
CDC recommends 6 ACH minimum (4 outdoor + equivalent filtration from MERV 13+ recirculated air) in occupied indoor spaces to reduce transmission risk.
Can I use a bathroom fan to hit the ACH target?
Yes for the bathroom. For whole-house ACH, you need a continuous ERV/HRV or a low-speed exhaust fan running 24/7.
How do I measure actual room volume?
Length × width × average ceiling height. For a 12 × 15 ft room with 8 ft ceilings, volume = 1,440 cu ft.
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