ASHRAE 62.1 and 62.2 Ventilation Requirements
Minimum ventilation rates for commercial and residential indoor air quality — the two standards that decide how much outdoor air a building actually needs, and the math that backs it up.
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In This Guide
62.1 vs 62.2: Which One Applies
ASHRAE Standard 62 comes in two flavors, and the first job on any ventilation call is knowing which one governs the building in front of you. Both exist for the same reason: to define the minimum outdoor airflow needed to keep carbon dioxide, moisture, VOCs, and odors at levels that most occupants find acceptable.
- ASHRAE 62.1 — Ventilation for Acceptable Indoor Air Quality. Covers commercial and institutional buildings: offices, schools, retail, restaurants, healthcare (with 170 layered on top), and anything that isn't low-rise residential.
- ASHRAE 62.2 — Ventilation and Acceptable Indoor Air Quality in Residential Buildings. Covers single-family homes and low-rise dwelling units. This is the one behind the whole-house fan requirement you see on tight new construction.
Why it matters now: Homes and buildings are being built tighter than ever for energy efficiency. A house that used to leak enough air through gaps to ventilate itself no longer does. Standard 62 fills that gap with intentional, measured ventilation instead of accidental infiltration.
62.1 Ventilation Rate Procedure
The most widely used compliance path in 62.1 is the Ventilation Rate Procedure (VRP). It sets the breathing-zone outdoor airflow for each space based on two things: how many people are in it, and how big it is. The core equation is:
Breathing-Zone Outdoor Airflow
- Vbz = outdoor airflow required in the breathing zone (CFM)
- Rp = outdoor air rate per person (CFM/person)
- Pz = zone population (number of people)
- Ra = outdoor air rate per unit area (CFM/ft²)
- Az = zone floor area (ft²)
The Rp term handles contaminants people generate directly — CO₂, bioeffluents, odor. The Ra term handles contaminants the building gives off — off-gassing from carpet, furniture, and finishes — which happens whether anyone is in the room or not. Table 6.2.2.1 in the standard lists both values for every occupancy category. A few common ones:
| Occupancy | Rp (CFM/person) | Ra (CFM/ft²) | Default Density (per 1000 ft²) |
|---|---|---|---|
| Office space | 5 | 0.06 | 5 |
| Classroom (age 9+) | 10 | 0.12 | 35 |
| Conference room | 5 | 0.06 | 50 |
| Retail sales floor | 7.5 | 0.12 | 15 |
| Restaurant dining | 7.5 | 0.18 | 70 |
Vbz is not the end of the story. To get the outdoor air the air handler must pull in, you divide by the zone air distribution effectiveness (Ez) to get the zone outdoor airflow: Voz = Vbz ÷ Ez. Ez is 1.0 for ceiling supply of cool air (the usual case), but drops to 0.8 when you supply warm air from a ceiling diffuser more than 15°F above room temperature, because that warm air stratifies near the ceiling and short-circuits the breathing zone.
Caution on multi-zone systems: When one air handler serves several zones, you can't just add up the Voz values. The system-level calculation uses a system ventilation efficiency (Ev) that accounts for the worst-case (critical) zone — the zone with the highest ratio of outdoor air to supply air. This is why a VAV system often needs more total outdoor air than the sum of its parts suggests.
Worked Example: Office Space
Problem:
A single-zone rooftop unit serves a 4,000 ft² open office with 30 people. Supply is cool air from ceiling diffusers. How much outdoor air does 62.1 require?
Step 1: Pull the rates for office space
Rp = 5 CFM/person, Ra = 0.06 CFM/ft²
Step 2: Calculate the people component
Rp × Pz = 5 × 30 = 150 CFM
Step 3: Calculate the area component
Ra × Az = 0.06 × 4,000 = 240 CFM
Step 4: Add for breathing-zone airflow
Vbz = 150 + 240 = 390 CFM
Step 5: Divide by Ez (1.0 for cool ceiling supply)
Voz = 390 ÷ 1.0 = 390 CFM outdoor air
What this means in the field
On a 3-ton (about 1,200 CFM) rooftop unit, 390 CFM is roughly a 32% outdoor-air fraction. Set the economizer/fresh-air damper minimum position to deliver that at design supply airflow, then verify it with a traverse or CO₂ readings. If the space runs warm-air heating from the ceiling, recalculate with Ez = 0.8 — that bumps Voz to about 488 CFM.
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62.2 Whole-House Ventilation
Standard 62.2 takes a simpler, single-number approach for dwellings. Instead of per-space calculations, it sets one whole-house mechanical ventilation rate based on the home's conditioned floor area and the number of bedrooms (a stand-in for occupancy). The current formula is:
Whole-House Ventilation Rate
- Qtot = required total ventilation rate (CFM)
- Afloor = conditioned floor area (ft²)
- Nbr = number of bedrooms (count as 1 if it's a studio)
Take a typical 2,000 ft², 3-bedroom home: Qtot = 0.03 × 2,000 + 7.5 × (3 + 1) = 60 + 30 = 90 CFM of continuous whole-house ventilation. That's the target a supply fan, exhaust fan, or balanced ERV/HRV has to hit. If the system runs intermittently rather than continuously, 62.2 lets you size up the fan and use a runtime fraction to deliver the same daily air volume.
Three ways to hit Qtot
- Exhaust-only: a quiet continuous bath fan pulls stale air out; make-up air enters through envelope leakage. Cheapest, but can depressurize a tight home.
- Supply-only: fresh air ducted to the return or a dedicated fan pressurizes the house. Better control over where air comes from.
- Balanced (HRV/ERV): equal supply and exhaust with heat/energy recovery. Best comfort and efficiency in cold or humid climates, highest cost.
Local Exhaust: Baths & Kitchens
Whole-house ventilation handles the background; local exhaust handles the point sources of moisture and cooking pollutants. 62.2 requires mechanical exhaust in every bathroom and kitchen, with these minimums:
| Location | Continuous | Intermittent (on demand) |
|---|---|---|
| Bathroom | 20 CFM | 50 CFM |
| Kitchen | 5 ACH (based on kitchen volume) | 100 CFM (vented range hood) |
The 50 CFM bath fan and 100 CFM vented range hood are the numbers to memorize for on-demand systems. Two things trip techs up here: first, a recirculating (ductless) range hood does not count — the air has to be exhausted to the outdoors. Second, fan ratings are at a specific static pressure; a fan rated 50 CFM at 0.1 iwc may only move 30 CFM through a long, restrictive duct run. Verify actual airflow with a flow hood, not the box label.
Combustion safety warning
Powerful exhaust in a tight house can backdraft atmospheric gas water heaters and furnaces, pulling carbon monoxide back down the flue. Any time you add or upsize exhaust in a home with natural-draft combustion appliances, perform a worst-case depressurization and spillage test. Balanced ventilation or sealed-combustion appliances avoid the problem entirely.
Field Tips & Common Mistakes
- Adopted code trumps the standard. The IMC and IRC reference 62.1/62.2 but sometimes with their own tables and amendments. Check what your AHJ actually adopted before you cite a number.
- Design airflow ≠ measured airflow. A minimum outdoor-air damper set at the wrong position, a stuck economizer, or a dirty filter all starve the fresh-air side. Ventilation compliance is only real if you verify it.
- CO₂ is a proxy, not the target. Demand-controlled ventilation modulates outdoor air off CO₂ (roughly 1,100 ppm often maps to the per-person rate), but the Ra area component must still run even when the space is empty.
- More is not always better. Over-ventilating wastes energy and can drive humidity problems in cooling climates. The standard sets a floor, not a recommendation to blow past it.
- Don't forget make-up air. Every CFM you exhaust has to come from somewhere. Large kitchen hoods in light-commercial jobs need engineered make-up air, or the building goes negative and doors won't close.
Quick sizing check
For a fast residential gut-check: whole-house rate is roughly 1 CFM per 100 ft² of floor area, plus about 7.5 CFM per bedroom-plus-one. For commercial, when you don't have a people count, use the default occupant density from the 62.1 table for that occupancy type and run the same Vbz math.
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