Mechanical Ventilation Requirements for Tight Houses Under IRC 2024

Quick Answer

IRC 2024 Section N1103.6 requires whole-house mechanical ventilation when tested air leakage is less than 5 ACH50. Because the new air leakage limit in zones 3 through 8 is 3 ACH50, virtually all new construction in those zones will need a mechanical ventilation system. IRC 2024 references ASHRAE 62.2-2022 for ventilation sizing.

Under IRC 2024, hRV and ERV systems are strongly preferred in cold and mixed climates. Balanced ventilation systems may qualify for an energy credit under the ERI compliance path.

What IRC 2024 Actually Requires

IRC 2024 N1103.6 states that when the tested air leakage of a dwelling is less than 5 ACH50, a whole-house mechanical ventilation system must be installed. Because IRC 2024 requires new construction in climate zones 3 through 8 to achieve no more than 3 ACH50 (per N1102.4.1.2), every code-compliant new home in those zones will trigger the mechanical ventilation requirement. Even in zones 1 and 2 where the 5 ACH50 limit remains, well-built homes often test below 5 ACH50 and therefore also trigger the requirement.

The ventilation system must be sized per ASHRAE 62.2-2022, “Ventilation and Acceptable Indoor Air Quality in Residential Buildings.” The 2022 edition introduced an airflow credit for infiltration, which can reduce the mechanical ventilation rate required compared to the 2016 edition. The basic formula: Qfan = 0.15 x CFA + 3.5 x (Nbr + 1), where CFA is conditioned floor area in square feet and Nbr is the number of bedrooms. For a 2,000 sq ft four-bedroom home, this is approximately 0.15 x 2,000 + 3.5 x 5 = 300 + 17.5 = 317.5 CFM, minus any infiltration credit. In practice, sized systems for typical homes run 50 to 100 CFM of continuous ventilation.

System type is not specified by the code as a requirement, but three categories are common: exhaust-only (a bathroom fan running continuously or intermittently on a timer), supply-only (a small duct bringing outdoor air to the return plenum), and balanced (an HRV or ERV that brings in fresh air while exhausting stale air through a heat or energy recovery core). Balanced systems with heat or energy recovery are specifically noted in IRC 2024 as qualifying for a credit on the ERI compliance path.

Why This Rule Exists

The relationship between air tightness and air quality is direct: as houses become tighter to meet blower door requirements, the natural infiltration that provided incidental ventilation in older homes is intentionally eliminated. Without mechanical ventilation, moisture from cooking, bathing, and breathing accumulates, creating conditions for mold, dust mite growth, and elevated VOC concentrations. The EPA identifies indoor air pollution as among the top five environmental health risks facing Americans. ASHRAE 62.2 was developed specifically to quantify the minimum ventilation needed to maintain acceptable indoor air quality in tight residential buildings. IRC 2024’s link to ASHRAE 62.2-2022 ensures that the ventilation rate is calculated on sound occupant-load and floor area criteria rather than intuition.

HRV vs ERV: Which One?

Both an HRV (Heat Recovery Ventilator) and an ERV (Energy Recovery Ventilator) bring in fresh outdoor air and exhaust stale indoor air through a core that transfers energy between the two airstreams, recovering 70 to 85 percent of the energy that would otherwise be thrown away. The difference is what they transfer: an HRV transfers only sensible heat, while an ERV transfers both heat and moisture.

In cold, dry climates (climate zones 6, 7, and 8 — Minnesota, Wisconsin, Montana, most of Canada), an ERV is generally the better choice. Tight homes in cold climates can become very dry in winter because the only moisture source is the occupants themselves; an ERV retains some of the humidity in the outgoing exhaust air and transfers it to the incoming fresh air, reducing or eliminating the need for supplemental humidification. An HRV in a very dry cold climate will allow indoor humidity to drop to uncomfortable levels.

In hot, humid climates (climate zones 1 and 2 — Florida, coastal Gulf states), an ERV is also preferred, but for the opposite reason: the ERV transfers moisture from the hot, humid incoming outdoor air to the outgoing exhaust air, removing latent load before it enters the conditioned space. An HRV in a humid climate allows that latent load to pass through, increasing the dehumidification burden on the air conditioner.

In mixed climates (zones 4 and 5), either unit can work. An HRV is the simpler and generally less expensive option and performs well when the primary concern is winter heat recovery. An ERV adds value where summer humidity is also a significant load. When in doubt, consult the ACCA Manual J climate data for your jurisdiction and discuss with the HVAC designer.

Sizing rules for both are identical under ASHRAE 62.2-2022: the unit must deliver the calculated Qfan rate at the unit’s rated static pressure for the actual installed duct configuration. Most residential HRV and ERV units are rated at 0.1 to 0.4 inches of water column (iwc). If the installed duct run is long or uses small-diameter flex duct, the actual delivered airflow may be substantially less than the unit’s nameplate rating. Always verify delivered airflows with a measurement tool after installation, not just the nameplate specification.

What the Inspector Checks at Rough and Final

At rough mechanical inspection, the inspector looks for the ventilation duct rough-in. For an HRV or ERV, this includes the fresh air intake duct and exhaust duct penetrating the building envelope, typically with separate exterior terminations. For a supply-only system, the inspector checks for the outdoor air duct connected to the return plenum. For exhaust-only, the inspector verifies that the designated exhaust fan is wired to a timer or controller that will ensure continuous or timed intermittent operation.

At final, the inspector verifies the ventilation equipment is installed and operational. For HRV and ERV systems, the inspector checks that airflows are balanced between the supply and exhaust sides using measured airflow data — not an assumption of balance — and that the core is accessible for annual cleaning. The energy certificate near the electrical panel must document the ventilation system type, the design airflow rate in CFM, and the ASHRAE 62.2-2022 sizing calculation or a reference to the HERS rating that documents it.

Commissioning is a critical step that is often skipped. After installation, the contractor must measure supply and exhaust airflows using a flow capture hood, a calibrated anemometer, or a manufacturer-provided measuring tool. For a balanced system, supply and exhaust flows should be within 10 percent of each other. The measured flows must meet or exceed the ASHRAE 62.2-2022 required ventilation rate. Results should be recorded on a commissioning sheet and left with the homeowner and the energy certificate. In jurisdictions that require HERS ratings, the HERS rater independently verifies ventilation airflows as part of the rating process using calibrated instruments. Some jurisdictions require a commissioning report signed by the installer before the certificate of occupancy is issued.

What Contractors Need to Know

The easiest ventilation systems to install are exhaust-only (a bath fan on a timer controller like a Broan FV-BCSFK or equivalent). However, exhaust-only systems depressurize the house, which can cause backdrafting of combustion appliances in older homes and can draw unconditioned air from uncontrolled pathways. In cold climates with sealed combustion appliances, exhaust-only systems are often acceptable, but verify with the HVAC designer. HRV and ERV systems are the preferred solution in climate zones 5 through 8.

For duct sizing, HRV and ERV supply and exhaust ducts should be a minimum of 4 inches in diameter for units up to approximately 80 CFM, and 6 inches for larger units. Avoid flex duct runs longer than 5 feet for the exterior duct stubs; use smooth rigid metal duct for exterior penetrations to minimize static pressure losses and prevent condensation from accumulating in sagging flex. Keep total equivalent duct length on each side of the unit within the manufacturer’s rated maximum — typically 50 to 100 equivalent feet — or the unit will not deliver its rated airflow.

Exterior termination locations are critical for both safety and performance. The fresh air intake must not terminate into a garage, a vented crawlspace, an unconditioned attic, or within 10 feet of any exhaust air outlet, dryer vent, combustion appliance vent, or sanitary drain vent. The exhaust termination must not discharge into any of those same spaces. Both terminations must be fitted with manufacturer-approved weatherhoods that prevent rain, insects, and backdrafting but do not restrict airflow below the rated static pressure for the unit. Verify that the weatherhood free area is adequate; undersized grilles are a common cause of low measured airflows at commissioning.

For systems that connect the HRV or ERV supply to the central air handler return plenum, install a motorized damper on the outdoor air connection that is interlocked with the HRV or ERV fan. The damper must open only when the ventilation unit is running, preventing the air handler blower from driving uncontrolled airflow through the ventilation duct when the ventilation unit is off. Without this interlock, a high-static air handler can pull or push air through the ventilation duct at rates far exceeding the design ventilation rate, wasting energy and potentially causing comfort problems.

Dedicated supply-and-exhaust systems (HRV or ERV with their own duct distribution network separate from the central HVAC system) avoid the interlock issue entirely. These systems deliver conditioned fresh air directly to bedrooms and living areas and exhaust from bathrooms and kitchen, providing the most controlled and predictable ventilation. They require more duct work than a plenum-connected system but eliminate the variable airflow problem caused by air handler cycling. For exhaust-only systems serving as the whole-house ventilation strategy, the exhaust fan must be rated for continuous duty, not just intermittent use; standard bath fans not rated for continuous operation will fail prematurely when run 24 hours a day or on a frequent duty cycle. Look for fans rated for continuous operation in the product specifications before specifying them for whole-house ventilation duty.

What Homeowners Get Wrong

The most common question: “Can I just open a window instead of installing a ventilation system?” The code does not allow a window-opening strategy to substitute for mechanical ventilation in tight new construction. Windows are not a controlled, continuous ventilation source; they depend on occupant behavior and weather conditions and cannot meet the ASHRAE 62.2 airflow targets reliably. Another misconception: “Bath fans are already required by the code, so doesn’t that count?” Bathroom exhaust fans required under Section M1507 are local exhaust fans intended to remove moisture from bathrooms during and after use. They do not operate continuously and are not designed or sized as whole-house ventilation. A separate, controlled whole-house ventilation strategy is required. A third question: “Won’t mechanical ventilation just waste the energy we gained from tighter construction?” An HRV or ERV recovers 70 to 85 percent of the heat from the exhausted air, so the net ventilation energy penalty in cold climates is small — far less than the energy gained from the tighter envelope.

State and Local Amendments

California’s Title 24 Part 6 requires whole-house mechanical ventilation per ASHRAE 62.2 in all new construction and specifies that balanced ventilation must be used in certain high-performance construction categories. Washington State’s 2021 WSEC requires whole-house ventilation and references ASHRAE 62.2 for sizing. New York’s stretch energy code explicitly requires balanced ventilation in new construction in most downstate areas. Massachusetts stretch code requires an HRV or ERV as the preferred ventilation strategy in climate zone 5 and 6 new construction. Minnesota has adopted ventilation requirements ahead of the IRC that reference ASHRAE 62.2 and require a minimum 10-minute run cycle per hour for exhaust-only systems. Canada’s National Building Code has required HRV in new construction in cold climate zones for many years, providing established market infrastructure that U.S. builders in cold climates are now catching up to.

Common Violations Found at Inspection

• No whole-house ventilation system installed in a new home that tested at 2.8 ACH50, far below the 5 ACH50 trigger threshold; the omission is discovered at final when the energy certificate lists ventilation as required but not present.
• Exhaust-only ventilation system installed without a timer controller, relying on occupants to manually run the bathroom fan; this does not meet the continuous or scheduled operation requirement of ASHRAE 62.2-2022.
• HRV installed but airflows not balanced; the exhaust side delivers more than the supply side, creating net depressurization of the conditioned space and reducing the energy recovery efficiency.
• Outdoor air intake termination located within 10 feet of the dryer vent exhaust, potentially drawing lint and exhaust gases into the fresh air stream.
• Supply-only ventilation duct connected to the return plenum without a motorized damper, allowing the HVAC blower to pressurize the house whenever the air handler runs even when ventilation is not desired.
• Ventilation system type and airflow rate not documented on the energy certificate, leaving an incomplete record that fails the final inspection checklist.
• ERV core not accessible for annual cleaning because the unit was installed in a tight mechanical room without adequate service clearance per the manufacturer’s installation instructions.
• ASHRAE 62.2 sizing calculation not available at inspection; the contractor estimated airflow rather than calculating per the standard formula, resulting in an undersized fan that does not meet the required ventilation rate.