Ventilation Rate Guide — ACH Requirements, CO2 as Proxy, and the Airflow Calculations That Determine Whether Your Home Is Under-Ventilated

Your Energy-Efficient Home Is Airtight — and That Airtightness Is Making You Sick

Modern construction pursues airtightness. Spray foam insulation, house wrap, triple-pane windows, weatherstripping — every energy efficiency improvement reduces the rate at which indoor air is replaced by outdoor air. This saves energy. It also means that every pollutant generated indoors — CO2 from breathing, moisture from cooking and bathing, VOCs from furniture and cleaning products, PM2.5 from cooking, radon from soil, allergens from biological sources — accumulates faster and reaches higher concentrations than in older, leakier buildings.

The solution is not to choose between energy efficiency and air quality. It is to seal the building envelope tightly AND provide controlled, filtered ventilation. This is the fundamental principle behind ASHRAE 62.2 (residential ventilation standard) and the concept of “build tight, ventilate right.”

Yet most existing homes have neither tight envelopes nor adequate ventilation. They rely on air leakage through cracks, gaps, and building defects for their fresh air — which provides uncontrolled, unfiltered, weather-dependent, and unmeasured ventilation that may or may not be sufficient. The only way to know whether your home’s ventilation is adequate is to measure it — and the most practical residential measurement is CO2 concentration as a ventilation proxy.

Ventilation standards — what the codes require

Standard	Application	Required ventilation rate	CO2 proxy equivalent	Key provision
ASHRAE 62.2-2022	Residential (US)	Continuous: 0.35 ACH or 7.5 cfm/person + 3 cfm per 100 sq ft of floor area (whichever is greater)	Approximately 700-1000 ppm CO2 steady-state	Applies to all dwelling units; specifies both whole-building and local exhaust
ASHRAE 62.1-2022	Commercial buildings	5-20 cfm/person (varies by occupancy type) + area-based component	700-800 ppm above outdoor CO2 (~1100-1200 ppm total)	Demand-controlled ventilation allowed (CO2-based)
IRC 2021 (International Residential Code)	New residential construction (US)	Mechanical ventilation required; references ASHRAE 62.2	Not specified (defers to ASHRAE)	Requires mechanical ventilation in tightly built homes (≤5 ACH50)
Part F (UK Building Regulations)	Residential (UK)	Whole-dwelling: extract rates by room + background ventilators; continuous mechanical: 0.3-0.5 L/s per m²	Not specified	Requires trickle ventilators in all habitable rooms
Passivhaus	Ultra-low energy buildings	Continuous mechanical ventilation with heat recovery; minimum 30 m³/h per person	800-1000 ppm CO2 target	MVHR (mechanical ventilation with heat recovery) is mandatory
WELL Building Standard	Premium building certification	ASHRAE 62.1 + 30% above minimum	<800 ppm CO2 (absolute, not delta)	Continuous CO2 monitoring required in occupied spaces

ACH requirements by room type and use case

Room / Use case	Minimum ACH (code)	Recommended ACH	CO2 target (ppm)	Primary ventilation concern	Method
Bedroom (occupied, sleeping)	0.35 (ASHRAE 62.2)	0.5-1.0	<1000	CO2 accumulation; moisture from breathing	Window cracking; continuous exhaust; MVHR supply
Living room (occupied)	0.35	0.5-1.0	<1000	CO2; general IAQ maintenance	Natural ventilation (windows); continuous mechanical
Kitchen (during cooking)	Local exhaust: 100 cfm intermittent or 25 cfm continuous	100-400 cfm (range hood during cooking)	N/A (exhaust-driven, not CO2-based)	PM2.5, combustion gases (gas stove), cooking VOCs, moisture	Exterior-vented range hood; makeup air if >300 cfm exhaust
Bathroom (during/after use)	50 cfm intermittent or 20 cfm continuous (ASHRAE 62.2)	50-80 cfm during use; 20 min post-use	N/A (exhaust-driven)	Moisture (mold prevention); odor	Exhaust fan with timer or humidity sensor
Laundry room	50 cfm intermittent	50-100 cfm during dryer operation	N/A	Moisture; dryer lint (if indoor-vented — which should never be the case for gas dryers)	Exhaust fan; dryer must vent to exterior
Home office (single occupant)	0.35 ACH or 7.5 cfm/person	15-25 cfm/person (for cognitive performance)	<800 (productivity target)	CO2 cognitive effects; VOCs from office equipment	Window; continuous supply; demand-controlled
Home gym / exercise room	0.35 (code minimum)	2-4 ACH (high metabolic rate)	<1200 (higher CO2 generation during exercise)	CO2 (3-8x resting generation during exercise); moisture; body odor	High-capacity exhaust or supply; windows
Basement (finished, occupied)	0.35 ACH	0.5-1.0 ACH + radon consideration	<1000 (plus radon mitigation if applicable)	Radon; moisture; limited natural ventilation	Mechanical supply or exhaust; radon mitigation system
Garage (attached)	Isolated from living space; no specific ventilation rate for garage itself	Maintain negative pressure relative to house	N/A	CO, VOCs from vehicles and stored chemicals must not migrate to living space	Exhaust fan; air-seal garage-house interface; CO detector

Ventilation system comparison

System type	How it works	Ventilation rate control	Energy cost	Air filtration	Humidity management	Installation cost	Best for
Natural ventilation (windows)	Pressure difference (wind, stack effect) drives air exchange	None — weather-dependent, occupant-dependent	$0	None	None (whatever outdoor humidity is)	$0	Mild climates; moderate outdoor air quality; occupants willing to manage windows
Exhaust-only (single-point)	Exhaust fan(s) in bathroom/kitchen depressurize house; outdoor air infiltrates through leaks	Fan speed (continuous or intermittent)	$20-60/year	None on incoming air (enters through cracks)	None	$100-500	Existing homes; supplementing natural ventilation; bathroom/kitchen moisture removal
Exhaust-only (whole-house)	Central exhaust fan (often modified bath fan) runs continuously	Adjustable fan speed	$30-80/year	None on incoming air	None	$200-800	Code compliance in existing homes; cold climates (avoids icing supply ductwork)
Supply-only	Fan brings outdoor air in (often through HVAC duct); house positively pressurized	Damper or fan speed	$30-80/year	Yes — incoming air can be filtered (MERV 13+)	Can dehumidify if run through HVAC cooling coil	$200-800	Hot-humid climates (positive pressure prevents humid infiltration); homes needing filtered supply
Balanced (HRV — Heat Recovery Ventilator)	Equal exhaust and supply; heat exchanger transfers heat between streams	Adjustable; can be CO2-controlled	$50-150/year (fan energy)	Yes — filters on supply and sometimes exhaust	Recovers sensible heat (70-90% efficiency)	$1,500-5,000 installed	Cold climates (recovers heating energy); tight homes; best overall IAQ control
Balanced (ERV — Energy Recovery Ventilator)	Same as HRV but also transfers moisture between airstreams	Same	$50-150/year	Yes	Recovers both heat AND moisture (reduces winter dryness; reduces summer humidity load)	$1,500-5,500 installed	Mixed/humid climates (moisture recovery in winter prevents over-drying; reduces summer dehumidification load)
Demand-controlled (CO2-based)	Ventilation rate adjusts based on CO2 sensor reading	Automatic — increases when CO2 rises, reduces when space unoccupied	Optimized (only ventilates when needed)	Depends on base system	Depends on base system	+$200-500 on top of base system (sensor + controller)	Occupied spaces with variable occupancy; energy optimization; commercial/premium residential

HRV vs ERV — the climate-based decision

Climate	Winter outdoor air	Summer outdoor air	HRV or ERV?	Why
Cold, dry winters (Minneapolis, Calgary, Tromsø)	Cold and dry	Warm, moderate humidity	ERV	ERV retains indoor moisture in winter (prevents over-drying to 15-20% RH that HRV causes in extreme cold)
Cold, humid winters (Seattle, UK, NZ)	Cool and humid	Mild, moderate humidity	HRV	HRV does not transfer moisture — prevents bringing outdoor humidity in during damp winters
Hot, humid summers (Houston, Singapore, Bangkok)	Mild (if applicable)	Hot and humid	ERV	ERV removes some moisture from incoming supply air, reducing cooling load
Hot, dry summers (Phoenix, Riyadh)	Mild	Hot and dry	HRV (or ERV with limited benefit)	Humidity is not the issue; HRV recovers cooling energy from exhaust stream
Mixed climate (New York, Tokyo, Sydney)	Cold-cool	Warm-hot, variable humidity	ERV	Versatility — moisture recovery benefits both winter (dryness prevention) and summer (humidity reduction)

CO2-based ventilation assessment — practical guide

Measurement	Setup	What to measure	Good result	Action trigger	What the result means
Bedroom overnight test	Place CO2 monitor at head height near bed; close door and windows; sleep normally	CO2 at bedtime, 4-hour mark, and waking	Peak <1000 ppm	>1500 ppm at any point	>1500: significantly under-ventilated during sleep; cognitive and sleep quality impact likely
Whole-house occupied	Monitor in main living area during normal daytime occupancy	Sustained CO2 level while home is occupied	<800 ppm	>1000 ppm sustained	>1000 sustained: whole-house ventilation rate below ASHRAE 62.2
Whole-house unoccupied	Leave monitor running while everyone is out (4+ hours)	CO2 level when house is unoccupied	Returns to near outdoor (~425-450 ppm) within 2-3 hours	>600 ppm after 4 hours unoccupied	>600 ppm after 4 hours: very low air exchange rate; potential radon/moisture/VOC concern
Post-cooking kitchen	Monitor in kitchen; cook normally; note time to return to baseline	CO2 + PM2.5 during and after cooking	Returns to baseline within 30-60 min with range hood	>60 min to return to baseline	>60 min: range hood or kitchen ventilation inadequate
Room tightness comparison	Monitor in each room with door closed; note rates of rise	Rate of CO2 rise (ppm per minute)	—	—	Higher rise rate = tighter room = more ventilation needed when occupied

Ventilation rate calculation from CO2 decay

Method	What you need	Procedure	Formula	Accuracy
CO2 decay method	CO2 monitor; empty room; initial elevated CO2 (from occupancy or deliberate CO2 release)	1. Elevate CO2 to 1500-2000 ppm (occupy room, then leave). 2. Record CO2 every 5-10 min as it decays. 3. Plot natural log of (C(t) - C_outdoor) vs time. 4. Slope = air change rate	ACH = -slope of ln(C(t) - C_outdoor) vs time (in hours)	±15-25% (affected by mixing, leakage variability, wind)
Steady-state method	CO2 monitor; known occupancy; steady-state reading	1. Occupy room with known number of people. 2. Wait for CO2 to stabilize (1-3 hours). 3. Calculate ventilation rate from steady-state equation	Q (cfm) = n × CO2_generation_rate / (C_indoor - C_outdoor)	±20-30% (assumes well-mixed room, constant generation)

CO2 generation rate reference: An average adult at rest generates approximately 200 mL/min (0.007 cfm) of CO2. Light activity: ~300-400 mL/min. Moderate exercise: ~1000-2000 mL/min. These values are needed for steady-state ventilation calculations.

Ventilation improvement hierarchy — cheapest to most effective

Intervention	Ventilation improvement	Cost	Energy penalty	Filtration	Comfort
Open bedroom door at night	30-60% CO2 reduction (connects bedroom to whole-house volume)	$0	$0	None	Privacy/noise tradeoff
Open window (crack)	Variable; typically 0.5-2 ACH depending on wind and temperature differential	$0	Moderate (heating/cooling loss)	None (outdoor pollutants enter)	Noise, security, weather-dependent
Trickle vent installation	0.2-0.5 ACH per vent	$50-200 per vent	Low (small opening)	Optional filter element	Minimal — by design
Bathroom fan upgrade + timer	Improves local exhaust; indirect whole-house benefit	$100-400	$20-40/year	None on incoming air	Noise at high speeds
Whole-house exhaust fan (continuous)	0.35 ACH target	$200-600	$30-80/year	None on incoming air	Fan noise (can be very quiet at low speeds)
HVAC fan cycling with fresh air damper	0.2-0.5 ACH (intermittent)	$200-500 (damper + controller)	$50-200/year (fan energy)	MERV 13+ filter on HVAC system	Filtered supply; uses existing ductwork
ERV/HRV (balanced ventilation)	0.35-0.5 ACH controlled; adjustable	$1,500-5,500 installed	$50-150/year (fan energy, offset by heat recovery)	MERV 8-13 on supply	Best comfort — tempered, filtered supply air; minimal drafts
Demand-controlled ventilation (CO2-based)	Dynamic; ventilates when needed	+$200-500 on base system	Optimized	Depends on base system	Auto-adjusting; no occupant action needed

Makeup air — the overlooked requirement

Exhaust device	Exhaust rate	Makeup air needed?	What happens without it	Solution
Bathroom fan (50-80 cfm)	50-80 cfm	Usually not (house leakage provides sufficient makeup air)	Minimal negative pressure	Not needed for most homes
Range hood (100-200 cfm)	100-200 cfm	Sometimes — depends on house tightness	Slight negative pressure; may cause backdrafting in leaky combustion appliances	Crack window in kitchen
Range hood (300-600 cfm)	300-600 cfm	Yes — many building codes require makeup air above 300-400 cfm	Significant negative pressure; backdrafting of gas water heater/furnace (CO poisoning risk); doors difficult to open; whistling at air leaks	Dedicated makeup air system (heated/tempered supply); required by IMC 505.2 for >400 cfm
Professional range hood (600-1200 cfm)	600-1200 cfm	Absolutely required	Dangerous negative pressure; combustion appliance backdrafting; structural stress on building envelope	Professional makeup air unit (interlocked with range hood)
Whole-house fan (attic fan)	2000-5000 cfm	Yes — windows must be open (by design)	Cannot operate without open windows; fan stalls; motor damage	Open windows equivalent to exhaust area before activating

The makeup air safety issue: In homes with gas appliances (furnace, water heater, gas stove), operating a large exhaust fan without adequate makeup air creates negative pressure that can reverse the draft in flue pipes — pulling combustion gases (including carbon monoxide) back into the house instead of up the chimney. This “backdrafting” is a genuine safety hazard. If your kitchen range hood is rated above 300 cfm and you have gas appliances with atmospheric venting (not sealed combustion), you either need a makeup air system or should ensure the range hood does not operate at full power without compensating air supply.

How to apply this

Use the ingredient-checker tool to evaluate air fresheners, cleaning products, and other VOC sources in your home — reducing VOC generation reduces the ventilation rate required to maintain acceptable air quality, complementing mechanical ventilation improvements.

Measure your bedroom CO2 tonight. A CO2 monitor running overnight provides the most immediately actionable ventilation data. If your bedroom CO2 exceeds 1500 ppm, the simplest fix is opening the door — connecting the bedroom to the whole-house volume dramatically reduces overnight CO2 accumulation. If that is insufficient, crack a window or install a trickle vent.

Calculate your ASHRAE 62.2 requirement. Formula: continuous ventilation rate (cfm) = 0.03 × floor area (sq ft) + 7.5 × number of bedrooms + 7.5. For a 1,500 sq ft home with 3 bedrooms: 0.03 × 1500 + 7.5 × 3 + 7.5 = 45 + 22.5 + 7.5 = 75 cfm continuous. Compare this to your actual ventilation (measured by CO2 decay or blower door test).

If your home is tight and you have gas appliances, address combustion safety. Atmospheric-vented gas appliances in tight homes are a carbon monoxide risk, especially when exhaust fans operate. If your home was built or retrofitted for energy efficiency, ensure gas appliances are sealed-combustion (direct-vent) or that adequate makeup air is provided.

Choose ERV for most climates; HRV for humid-winter climates. ERV (Energy Recovery Ventilator) is the better choice for most North American climates because it balances both heat and moisture — preventing winter over-drying and summer humidity import. HRV is better only where winter outdoor air is already humid and you want to avoid bringing that moisture indoors.

Honest limitations

ASHRAE 62.2 ventilation rates are designed for general health — they do not account for individual sensitivity, specific pollutant loads (heavy cooking, chemical hobbies), or vulnerable populations (infants, elderly, immunocompromised) who may need higher rates. CO2 as a ventilation proxy assumes that human occupancy is the primary pollutant source — in homes with significant non-occupant sources (off-gassing furniture, attached garage, radon), CO2 may under-represent the ventilation need. The CO2 decay method for measuring ACH is affected by wind conditions, temperature differential, and mixing assumptions — it provides an estimate, not a precise measurement. HRV/ERV efficiency ratings are tested under standardized conditions (0°C outdoor, 22°C indoor, balanced flows) — real-world efficiency varies with temperature, flow imbalance, and frost formation. ERV moisture transfer data depends on the enthalpy wheel or membrane material — effectiveness varies between manufacturers and degrades with age and contamination. Makeup air calculations assume balanced pressure — actual house depressurization depends on envelope leakage, HVAC operation, and all exhaust devices operating simultaneously. The ventilation-energy tradeoff is real — in extreme climates, meeting ASHRAE 62.2 ventilation rates without heat recovery imposes a significant energy cost, which is why HRV/ERV systems are recommended for tight construction. Demand-controlled ventilation (CO2-based) saves energy but may under-ventilate for non-CO2 pollutants (VOCs, radon) during low-occupancy periods.