By Michael Moore, P.E., Newport Ventures, Inc.,


Where virtually no market existed a decade ago, the burgeoning practice of building tight, energy efficient homes has created a new market for outdoor air ventilation systems to provide minimum acceptable indoor air quality (IAQ). Though likely none of us grew up in a house with an outdoor ventilation system, all new dwelling units built to the requirements of the 2012 or later version of the International Residential Code (IRC) now require an outdoor air ventilation system, also known as a “whole house mechanical ventilation” or “dwelling unit ventilation” system. While outdoor air systems and local exhaust systems form the backbone of the model code’s strategy to provide acceptable IAQ in single family and multifamily dwelling units, efficient filtration of indoor air has been largely overlooked as an IAQ strategy.

The International Mechanical Code (IMC), applicable to many multifamily dwelling units, specifically exempts any air filters installed in dwelling units from listing or labeling requirements (605.2). The IRC, generally applicable to single family units and town-homes, does not even mention air filters. Above-code programs have been more aggressive at recognizing the importance of filters in providing IAQ but they represent a small share of the total market. LEED v4 BD+C, applicable to high rise dwelling units, incentivizes participants to install ≥ MERV 13 or equivalent filters in outdoor air systems but is silent on recirculation air handlers (EQ Credit: Enhanced Indoor Air Quality Strategies). LEED v4 for Homes, applicable to single family and multifamily dwelling units, requires ≥ MERV 8 or equivalent on ducted recirculating space conditioning systems and ≥ MERV 6 on “mechanically supplied outdoor air for systems with 10 feet of ductwork or more” (EQ Prerequisite: Air Filtering); no incentives are provided for installing higher performance filters.

General lack of traction for efficient filters in codes and above code programs can partially be attributed to their treatment within ASHRAE 62.2, which serves as a precedent for IAQ measures in other construction rule sets. Outdoor air and exhaust requirements for dwelling units complying with the IRC, IMC, and LEED can all be traced back to ASHRAE 62.2, as can LEED v4 for homes’ filtration requirement for outdoor air. Prior to 2016 addendum k, ASHRAE 62.2’s treatment of filtration was confined to mandating filters for the purpose of protecting ducted heating and cooling equipment from fouling or becoming a source of pollutants. With only minor modifications over the past several iterations, ASHRAE 62.2’s filtration efficiency language still reads:

“6.7 Minimum Filtration. Mechanical systems that supply air to an occupiable space through ductwork exceeding 10 ft (3 m) in length and through a thermal conditioning component, except evaporative coolers, shall be provided with a filter having a designated minimum efficiency of MERV 6 or better when tested in accordance with ANSI/ASHRAE Standard 52.2… or a minimum Particle Size Efficiency of 50% in the 3.0 to 10 ?m range in accordance with AHRI Standard 680… The system shall be designed such that all recirculated and mechanically supplied outdoor air is filtered before passing through the thermal conditioning components.”


The minimum time-averaged flow rate, Qƒr required for compliance with Section 4.1.4 is determined as a function of the filtration factor using the following equation:



New 62.2-2016 incentive for high efficiency filters

With the publication of addendum k to ASHRAE 62.2-2016 in June of 2016, ASHRAE 62.2 now recognizes and incentivizes the role of high efficiency filtration in improving a dwelling unit’s IAQ. Addendum k introduces a new Section 4.1.4 called “Ventilation-rate Reduction for Particle Filtration.” This optional section permits users to reduce the total ventilation rate by 20% when using a high-efficiency filter in a recirculated air distribution system that meets some minimum criteria. The concept of trading off outdoor air for filtered air was a subject of extensive debate within the committee, but ultimately, the committee voted in favor of this motion based on research1 that pegged PM2.5 as the primary pollutant of concern in dwelling units. Because outdoor PM2.5 levels vary based on location and proximity to transportation corridors, the most fail-proof means of reducing PM2.5 exposure across all dwelling units is filtration of indoor air. While not mandatory, Section 4.1.4 can be especially attractive to users of the standard located in hot-humid climate zones, where practitioners have voiced concerns about potentially negative effects of high outdoor air ventilation rates, such as energy costs and an increase in indoor air humidity.

Achieving compliance with Section 4.1.4 involves three components: distribution, an efficiency designation called the “filtration factor,” and a time-averaged airflow rate. From a distribution perspective, the filtered air is required to be “supplied to or returned from all rooms in the habitable space through air handling systems.” Most centrally ducted HVAC systems will comply. The filtration factor is 62.2’s method of normalizing filter efficiency across test methods. The higher the filtration factor, the lower the efficiency. The maximum filtration factor permitted for compliance with the section is 4.3, corresponding to a MERV 11 filter. Table 2 provides a summary of filtration factors for filters tested to different protocols.

Working through a quick example, let’s suppose the total fresh air ventilation rate required, Qtot, is 80 cfm. The builder could reduce this total ventilation rate by 20% (i.e., to Qtot = 64 cfm) if complying with Section 4.1.4. To achieve this, the builder first installs a fully ducted HVAC system (which was likely already standard practice). Now, suppose the builder installs a MERV 12 filter in the unit. From Table 2 (i.e., 62.2-2016 Table 4.3), the filtration factor for this filter, ffr, is 3. The minimum time-averaged recirculation air flow rate across the filter required to reduce Qtot to 64 cfm is calculated as Qfr = 3?80 = 240 cfm. Let’s further suppose the air handler is a single speed unit that moves 1200 cfm. Over the course of a day, the fan would need to operate a total of 4.8 hours (240/1200*24). During heating or cooling seasons, this run time might be achieved by the fan operation for heating or cooling cycles. If not, controls must be provided to ensure that the minimum run time is achieved on a daily basis. Additionally, Section requires at least 10% of Qfr to be provided in each 12-hour period.

Addendum k permits the user to take credit for the reduction in flow rate “during any period in which the requirements… are met.” In other words, a smart thermostat and outdoor air ventilation controller could theoretically modulate the ventilation flow rate on an hourly basis (or any time period selected). Hypothetically, if a dwelling unit’s HVAC system is right-sized and a smart control is configured to only take filtration credit when the air handler achieves the required recirculation run time based on heating or cooling cycles alone (i.e., no additional run time for filtration of recirculated air), this configuration would reduce the energy penalty associated with outdoor air ventilation during those times when it is most beneficial (i.e., the hottest and coldest days of the year). Who says you can’t have IAQ and energy savings too? Of course, taking the credit without the use of intelligent controls (e.g., setting the fan to run continuously or not accounting for differences in flow rates associated with variable speed operation) could result in tremendous energy penalties. For example, a central air handling unit that operates at 0.58 W/cfm (the minimum performance permitted in California) would cost ~$700 to run continuously on an annual basis (assuming $0.10/kWh, 3-ton equipment, 400 cfm/ton).

What’s next?

Unless referenced by another code or above-code program, this change in 62.2 will just remain an obscure, academic curiosity. But as proven out with whole house mechanical ventilation requirements, obscure references in 62.2 can be the mustard seeds that yield mountainous market transformation. Through U.S. DOE’s weatherization program, roughly 100,000 existing dwelling units are brought into compliance with 62.2 annually. Based on historic precedent, DOE’s weatherization program will likely transition to 62.2-2016 within the next 18 months. California is expected to adopt 62.2-2016 in their 2019 version of Title 24, so they will likely be the first state-wide new home test market for addendum k beginning in 2020. And in case you missed it, California Title 20 now requires air filters manufactured on or after July 1, 2016, to be labeled with their efficiency and pressure drop as a function of airflow, so assuming addendum k is adopted in CA, practitioners will literally have the required filtration efficiency information at their fingertips. The greatest opportunity for market transformation would be an industry-backed proposal to amend the model codes (i.e., the IRC and IMC) to include a filtration trade-off against the whole house mechanical ventilation rate. This would be welcomed by many builders in the south looking for ways to reduce their outdoor ventilation rates or for energy efficiency advocates who would like to see the credit implemented intelligently to reduce ventilation energy consumption without compromising IAQ. But that’s work for another day.


  1. Logue JM, Price PN, Sherman MH, and Singer BC. 2012. A Method to Estimate the Chronic Health Impact of Air Pollutants in U.S. Residences. Environmental Health Perspectives 120(2): 216-222.


Mike Moore, P.E., has over 15 years of experience in building consulting, with specializations in building energy use, ventilation, and indoor air quality. On behalf of manufacturer clients, Mike has authored several successful code change proposals to codes and standards bodies such as ASHRAE, the International Code Council, and various state-level code bodies including California, New York, Virginia and Washington. Mike currently serves on ASHRAE 62.2 as the Indoor Air Quality subcommittee chair.


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