Ventilation Strategy: Build Tight, Ventilate Right

Homeowner Summary

"Build tight, ventilate right" is the foundational principle of modern building science. It means you should seal your home's envelope as tightly as possible to control energy loss, moisture, and comfort -- and then provide exactly the right amount of fresh air through controlled mechanical ventilation. This is the opposite of the old approach, which relied on a leaky building to provide "natural ventilation" through cracks and gaps (an uncontrolled, wasteful, and often inadequate strategy).

A well-sealed home without mechanical ventilation will develop poor indoor air quality. CO2 levels rise, humidity builds, odors linger, and pollutants from cooking, cleaning, building materials, and human metabolism accumulate. These problems gave birth to the term "sick building syndrome" in the 1970s-80s when buildings were first tightened for energy efficiency without adding compensating ventilation. The lesson was learned: you cannot have one without the other.

Modern ventilation systems bring in fresh outdoor air at a controlled rate, typically calculated per ASHRAE Standard 62.2 based on floor area and number of bedrooms. The three main approaches are exhaust-only (simplest, least expensive), supply-only (better control of incoming air), and balanced (best performance, highest cost). Balanced systems using energy recovery ventilators (ERVs) or heat recovery ventilators (HRVs) are the gold standard because they exchange heat (and sometimes moisture) between outgoing stale air and incoming fresh air, recovering 70-85% of the energy that would otherwise be lost.

How It Works

All ventilation systems achieve the same goal -- replacing stale indoor air with fresh outdoor air -- but they approach it differently:

Exhaust-only ventilation uses one or more exhaust fans (typically a bathroom fan or inline fan) running continuously at a low speed to pull stale air out of the house. Replacement air enters passively through gaps in the building envelope. This is the simplest and least expensive approach. Advantages: low cost ($100-$300 installed), simple controls, removes moisture and odors from bathrooms and kitchens. Disadvantages: you cannot filter or condition the incoming air (it enters through random gaps), the system slightly depressurizes the house (which can cause backdrafting of combustion appliances in very tight homes), and it may pull humid outdoor air through wall cavities in hot-humid climates, causing condensation. Best suited for cold and mixed climates (Zones 4-8) where the depressurization draws cold, dry air inward (lower moisture risk).

Supply-only ventilation uses a fan to push fresh outdoor air into the home, typically through the HVAC duct system via a fresh-air intake duct connected to the return plenum. Stale air exits passively through exhaust vents and envelope leakage. Advantages: incoming air can be filtered and tempered (mixed with return air before reaching living spaces), the slight positive pressure prevents uncontrolled infiltration and moisture intrusion, good for hot-humid climates. Disadvantages: higher cost than exhaust-only ($300-$700), requires the HVAC fan to run (energy penalty unless using an ECM blower), does not directly exhaust moisture or odors from bathrooms (still needs spot exhaust fans).

Balanced ventilation uses separate supply and exhaust fans (or a single balanced unit) to bring in fresh air and exhaust stale air in roughly equal amounts. The building pressure is neutral -- no pressurization or depressurization. Most balanced systems use an energy recovery ventilator (ERV) or heat recovery ventilator (HRV):

• HRV (Heat Recovery Ventilator): transfers heat between the outgoing and incoming air streams through a heat exchanger core. In winter, it warms incoming cold air using heat from the outgoing warm air, recovering 70-85% of the heat energy. In summer, it pre-cools incoming hot air. Does not transfer moisture. Best for cold climates (Zones 5-8) where winter indoor humidity is adequate or high.
• ERV (Energy Recovery Ventilator): transfers both heat and moisture between the air streams through an enthalpy core. In winter, it retains indoor humidity (beneficial in dry, cold climates). In summer, it rejects outdoor humidity (beneficial in humid climates). Slightly lower sensible heat recovery than an HRV but better total energy recovery. Best for mixed and humid climates (Zones 1-4) and cold climates with dry indoor conditions.

Both ERVs and HRVs include filters on both air streams, should be ducted to deliver fresh air to bedrooms and living areas and exhaust from bathrooms and kitchens, and run continuously at a low speed with boost capability for high-pollutant events (cooking, showering, cleaning).

Calculating ventilation requirements per ASHRAE 62.2-2022:

Continuous whole-building ventilation rate (CFM) = 0.03 x floor area (sq ft) + 7.5 x (number of bedrooms + 1)

Example: a 2,000 sq ft home with 3 bedrooms = 0.03 x 2,000 + 7.5 x 4 = 60 + 30 = 90 CFM continuous ventilation.

Additional requirements include local exhaust for kitchens (100 CFM intermittent or 25 CFM continuous) and bathrooms (50 CFM intermittent or 20 CFM continuous).

Maintenance Guide

DIY (Homeowner)

• Clean or replace ERV/HRV filters every 3-6 months (most units have washable filters; check manufacturer instructions)
• Clean the ERV/HRV core annually: remove the heat/enthalpy exchanger core (slides out on most units), vacuum loose debris, wash in mild soapy water, rinse, and air dry completely before reinstalling
• Clean exhaust fan grilles and intake screens every 6 months; accumulated dust and debris reduce airflow significantly
• Verify the ERV/HRV is actually running: check the control panel; some systems get turned off and never restarted
• Test bathroom exhaust fans with a tissue: hold a single ply of tissue near the fan grille while it is running -- it should hold the tissue firmly; if not, the fan may be dirty, the duct may be blocked, or the fan motor may be failing
• Check outdoor intake and exhaust hoods: ensure screens are clear of debris, insect nests, or snow/ice blockage
• Do not obstruct supply air grilles or return air grilles with furniture, curtains, or stored items
• Run the kitchen range hood when cooking: this is local exhaust ventilation and dramatically reduces cooking-related pollutants

Professional

• Measure actual ventilation airflow rates (CFM) at all supply and exhaust points using a flow hood or anemometer; compare to design specifications and ASHRAE 62.2 requirements
• Inspect ERV/HRV condensate drain and trap for blockage (common cause of unit shutdown or water damage)
• Verify ERV/HRV defrost control operation (in cold climates, the core can freeze; defrost cycles must function correctly)
• Check ductwork connections for leaks and verify insulation on outdoor air ducts to prevent condensation
• Balance supply and exhaust airflows in balanced systems (within 10% of each other to avoid pressurization/depressurization)
• Test bathroom exhaust fan sone levels and airflow per HVI certification (fans degrade over time; motor bearings wear)
• Verify kitchen range hood captures effectively (smoke pencil test across the front edge of the cooktop)
• Commission or recommission ventilation controls after any HVAC modifications

Warning Signs

• Persistent stuffiness or stale air, especially in bedrooms after sleeping
• CO2 levels above 1,000 PPM (measured with a CO2 monitor; normal outdoor is 420 PPM)
• Lingering cooking odors hours after cooking
• Condensation on interior window surfaces (excessive humidity from inadequate ventilation)
• Mold growth in bathrooms, on exterior walls, or in closets
• Allergy symptoms or headaches that improve when leaving the home
• Bathroom exhaust fan no longer audibly running or moving air weakly
• ERV/HRV display showing error codes or unit not operating
• Strong chemical or "new" smell from building materials or furnishings that does not dissipate (VOC accumulation)
• Respiratory issues in occupants that doctors cannot attribute to other causes

When to Replace vs Repair

• Repair exhaust fans with reduced airflow: clean grille and duct, check for duct obstruction, verify damper operation. If airflow is still below rated CFM, replace the fan motor or entire fan unit
• Replace bathroom exhaust fans after 10-15 years; motor bearings wear and airflow degrades below useful levels. Modern fans are quieter (0.3-1.0 sone vs 3-4 sone for older units) and more energy-efficient (DC motors)
• Replace ERV/HRV at 15-20 years or when the heat exchanger core shows visible deterioration, the unit cannot maintain rated recovery efficiency, or the motors consume excessive energy. Cores on some units can be replaced independently ($300-$600) to extend unit life
• Upgrade from exhaust-only to balanced (ERV/HRV) when performing significant air-sealing work, when the home reaches 3 ACH50 or tighter, or when indoor air quality testing shows persistent issues
• Do not decommission ventilation because of perceived energy cost -- the energy penalty of proper ventilation is 3-8% of total HVAC energy; the health and moisture risks of inadequate ventilation far exceed this cost

Pro Detail

Specifications & Sizing

• ASHRAE 62.2-2022 whole-building rate: Q (CFM) = 0.03 x A_floor + 7.5 x (N_bedrooms + 1). Adjusted for infiltration credit per Section 4.1.3 if blower door test results are available (homes leakier than the default get a ventilation reduction credit).
• Local exhaust (ASHRAE 62.2): kitchen 100 CFM intermittent / 25 CFM continuous; bathrooms 50 CFM intermittent / 20 CFM continuous
• ERV/HRV sizing: total unit CFM must meet whole-building requirement plus any distribution losses. Typical residential units range from 70 to 200 CFM. Select based on ASHRAE calculation, not home square footage alone.
• Sensible Recovery Efficiency (SRE): HRVs typically achieve 70-85% at rated flow; ERVs 60-80%. Look for HVI-certified ratings (Home Ventilating Institute).
• Total Recovery Efficiency (TRE): ERVs typically achieve 55-75% total (sensible + latent) recovery. Most relevant metric for humid climates.
• Duct sizing: ventilation ducts must be sized for low-velocity, low-noise operation. Typical: 6-inch round for 50-80 CFM, 8-inch round for 100-150 CFM. Insulated flex or rigid metal; outdoor air ducts must be insulated to prevent condensation (minimum R-8).
• Controls: timer-based (fixed schedule), occupancy-sensor-based, or demand-controlled (CO2 or humidity sensor triggers boost speed). Demand-controlled is most energy-efficient but most expensive.
• Sound rating: background ventilation should be below 1.0 sone (near-silent). Boost mode acceptable up to 3.0 sone. HVI certification includes sound ratings.
• Filtration: ERV/HRV intake filters should be MERV 8 minimum; MERV 13 for areas with wildfire smoke or high outdoor PM2.5. Some units accept only proprietary filters.

Common Failure Modes

| Failure Mode | Cause | Impact | Prevention | |-------------|-------|--------|------------| | ERV/HRV core freeze-up | Sustained cold temps without proper defrost cycle | Unit shuts down or ice damages core | Verify defrost mode operation; pre-heater coil in extreme climates | | Condensate drain blockage | Biological growth, debris | Water overflow, unit shutdown, water damage | Annual drain cleaning; algae tablets in drain pan | | Dirty filters restricting airflow | Deferred maintenance | Reduced ventilation rate; motor strain | 3-6 month filter cleaning/replacement schedule | | Exhaust fan duct disconnection | Poor installation, flex duct sag | Fan runs but air dumps into attic; moisture damage | Rigid duct or properly supported flex; verify at installation | | Ventilation system turned off | Occupant turns off due to noise or perceived energy waste | Indoor air quality degrades; humidity problems | Education; controls that prevent full shutdown; IAQ monitoring | | Unbalanced ERV/HRV airflows | Duct layout, filter differential, damper drift | Pressurization or depressurization; moisture issues | Annual balancing check; matched duct runs | | Intake near exhaust or contaminant source | Poor placement of outdoor hoods | Short-circuiting (exhausted air re-enters) | Minimum 6 ft (1.8 m) separation; away from dryer/furnace exhaust |

Diagnostic Procedures

1. Ventilation rate measurement: use a powered flow hood (Alnor, TEC) or anemometer traverse to measure actual CFM at each supply and exhaust point. Sum all points and compare to ASHRAE 62.2 requirement for the home. If total measured ventilation is below 80% of the requirement, investigate duct restrictions, filter condition, and fan speed settings.
2. CO2 decay test: seal the home (close windows and doors), allow CO2 to build (occupants present for 2+ hours), then enable ventilation and monitor CO2 decay rate with a data-logging CO2 sensor. The time to reach steady-state outdoor CO2 levels indicates effective air change rate. Compare to calculated requirement.
3. ERV/HRV performance verification: measure inlet and outlet air temperatures on both supply and exhaust streams. Calculate Apparent Sensible Effectiveness (ASE) = (supply outlet temp - outdoor temp) / (exhaust inlet temp - outdoor temp). Should be within 10% of manufacturer's HVI-rated SRE. If significantly lower, the core may be dirty, bypassed, or failing.
4. Pressure balance measurement: with the ventilation system running and all other fans off, measure house pressure relative to outdoors using a digital manometer. A balanced system should produce less than 1 Pa of pressurization or depressurization. Greater than 3 Pa indicates significant imbalance requiring airflow adjustment.
5. Duct leakage assessment: pressurize or depressurize the ventilation ductwork and measure leakage; alternatively, compare fan-inlet CFM (measured at the unit) to point-of-delivery CFM (measured at grilles). More than 15% loss indicates duct leaks requiring repair.

Code & Compliance

• ASHRAE Standard 62.2-2022: mechanical ventilation requirements for residential buildings; adopted by reference in most energy codes and green building programs; defines whole-building rates, local exhaust rates, and filtration
• 2021 IECC R403.6: requires mechanical ventilation in new residential construction per ASHRAE 62.2 or equivalent; cannot rely on infiltration alone
• IRC M1505: residential mechanical ventilation requirements; exhaust must terminate outside; kitchen exhaust discharge rates
• IRC M1506.2: recirculating range hoods do not satisfy local exhaust requirements for kitchens
• Energy Star Certified Homes v3.1: requires ASHRAE 62.2 compliance plus additional commissioning and testing requirements
• LEED for Homes: requires ASHRAE 62.2 compliance; awards additional points for enhanced ventilation, filtration, and demand control
• Passive House (PHIUS/PHI): requires balanced ventilation with heat recovery; minimum 75% heat recovery efficiency; ductwork must be within the thermal envelope
• HVI Certification: Home Ventilating Institute testing and certification program for fans, ERVs, and HRVs; provides independently verified CFM, sone, and efficiency ratings

Cost Guide

| Item | Cost Range | Notes | |------|-----------|-------| | Exhaust-only ventilation (continuous bath fan) | $100-$300 | Low-sone, DC motor bath fan on timer; simplest system | | Supply-only ventilation (fresh air duct to HVAC) | $300-$700 | Motorized damper + control + filter; requires HVAC fan | | HRV unit (residential, 70-150 CFM) | $800-$2,000 | Unit cost only; HVI-certified | | ERV unit (residential, 70-150 CFM) | $900-$2,500 | Unit cost only; HVI-certified | | ERV/HRV full installation (ducted) | $2,500-$6,000 | Unit + ductwork + controls + commissioning | | ERV/HRV core replacement | $300-$600 | Extends unit life 5-10 years | | Bathroom exhaust fan replacement | $150-$400 | Fan + installation; low-sone DC motor | | Kitchen range hood (ducted) | $300-$1,500 | Plus $200-$600 for duct installation if new | | Demand-controlled ventilation upgrade (CO2 sensor) | $200-$500 | Sensor + controller; retrofit to existing system | | Ventilation commissioning (flow measurement + balance) | $200-$400 | Should be included in new system installation |

Energy Impact

Mechanical ventilation costs energy, but far less than most homeowners expect. A properly sized ERV or HRV operating continuously at the ASHRAE 62.2 rate consumes 40-100 watts (equivalent to a light bulb), adding $35-$90 per year in electricity. The heat recovery function saves 70-85% of the energy that would be lost by simply exhausting warm air and replacing it with cold outdoor air, making the net energy cost minimal.

Exhaust-only ventilation has zero heat recovery, so the full energy cost of conditioning replacement air applies. In a cold climate, this can add $100-$200 per year in heating costs for a typical home. However, this is still far less than the energy lost through the uncontrolled air leakage that the ventilation system replaces (a tight home with mechanical ventilation uses significantly less total energy than a leaky home relying on natural ventilation).

Demand-controlled ventilation using CO2 or occupancy sensors reduces energy consumption by 20-40% compared to continuous operation by running at full speed only when needed. However, the ASHRAE 62.2 calculation already accounts for typical occupancy, so continuous low-speed operation is the baseline recommendation. Boost mode for cooking, showering, and cleaning supplements the base rate.

Shipshape Integration

SAM monitors ventilation performance through indoor air quality sensors and equipment runtime tracking to ensure continuous, adequate fresh air delivery:

• CO2 monitoring: Shipshape environmental sensors track CO2 levels as a direct indicator of ventilation adequacy. Sustained CO2 above 1,000 PPM triggers a "ventilation insufficient" alert. Above 1,500 PPM, SAM escalates to an urgent recommendation to check ventilation equipment operation and open windows as an immediate measure.
• Humidity correlation: SAM cross-references indoor humidity with ventilation system operation. Rising humidity despite ventilation running may indicate a failed ERV enthalpy core, duct disconnection, or undersized system. Falling humidity in winter may indicate the system is over-ventilating (HRV in a dry climate without humidity recovery).
• Equipment runtime tracking: SAM monitors ventilation system operation and alerts when the system has been off for more than 4 hours (excluding scheduled shutdown periods). Ventilation systems that are turned off and forgotten are a common and serious problem.
• Filter change reminders: based on manufacturer-specified intervals and adjusted for local air quality conditions (wildfire smoke, high pollen, construction nearby), SAM issues filter cleaning or replacement reminders.
• Seasonal optimization: SAM recommends ventilation strategy adjustments by season -- economizer mode (open windows) when outdoor conditions are favorable, boost ventilation during high-pollen or high-PM2.5 days, and humidity-aware ERV/HRV control during seasonal transitions.
• Home Health Score impact: ventilation adequacy is a core component of the indoor air quality score. Homes with verified mechanical ventilation meeting ASHRAE 62.2 requirements, good CO2 levels, and maintained equipment score highest. Homes with no mechanical ventilation and poor CO2 readings receive significant score reductions with specific, actionable recommendations.
• Dealer action triggers: ventilation alerts include CO2 trends, humidity data, and equipment runtime history, enabling technicians to diagnose whether the issue is equipment failure, duct disconnection, undersizing, or occupant override.