How Much Fresh Air Does Your Home Need?
The latest version of the ASHRAE 62.2 residential ventilation standard calls for bigger fans — and Joe Lstiburek is ticked off
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. ) has had a residential ventilation standard since 2003, when ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. (“Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings”) was first approved for publication. (For more information on providing fresh air for homes, see Designing a Good Ventilation System.)
Until recently, the basic ventilation rate formula established by ASHRAE 62.2 back in 2003 — 7.5 cfm per person plus 1 cfm per 100 square feet — has remained unchanged. (The standard assumes that the number of occupants in a home equals the number of bedrooms plus one.)
However, the latest (2013) version of ASHRAE 62.2 includes a significant change in the decade-old ventilation formula. Under the new formula, high-performance homes will need to be ventilated at a higher rate, namely 7.5 cfm per person plus 3 cfm per 100 square feet. This means that for a tightly built 2,400-square-foot home with 3 bedrooms, the minimum airflow rate of the ventilation equipment has jumped 89%, from 54 cfm to 102 cfm.
This new formula has been criticized by some building scientists, especially those who have long argued that even the older formula — the one specifying a lower ventilation rate — was probably too high. The most prominent critic of the new formula is Joseph Lstiburek, a principal of the Building Science Corporation in Massachusetts and one of the original members of the ASHRAE committee that developed the 62.2 standard.
One of the most prominent defenders of the new formula is Max Sherman, a senior scientist at Lawrence Berkeley National Laboratory and the former chairperson of the ASHRAE 62.2 committee. (Sherman is also a Holladay Distinguished Fellow, an ASHRAE honorific named after my grandfather.)
Getting rid of the infiltration credit
The 2010 version (and earlier versions) of ASHRAE 62.2 asserted that the (old) formula used to determine the minimum airflow rate of ventilation equipment was based on the assumption that all homes deserve an “infiltration credit” of 2 cfm per 100 square feet. The logic behind this infiltration credit irked many people in the building science community, since infiltration rates vary widely from house to house, from season to season, from climate to climate, and from day to day (depending on wind speed and temperature); clearly, these facts undermine the justification for providing an across-the-board “infiltration credit” to all homes.
As it turns out, the “infiltration credit” language in the ASHRAE standard wasn’t really based on technical grounds; rather, it was a political compromise necessary to get all members of the committee to agree to the original formula (7.5 cfm per person plus 1 cfm per 100 square feet).
The 2013 version of ASHRAE 62.2 has eliminated the infiltration credit. That decision wasn’t particularly controversial; most observers are happy to bid the infiltration credit adieu. However, the decision to increase the air flow required for ventilation fans from 1 cfm per 100 square feet to 3 cfm per 100 square feet — in effect, requiring fans to supply all of the air that was (incorrectly) assumed to be provided by infiltration — has upset many observers.
“My proposal was to eliminate the infiltration credit without changing the ventilation rate,” Lstiburek told me. “At the [June 2006] ASHRAE conference in Quebec City, I was outmaneuvered and tricked by Max Sherman. I supported 7.5 cfm per person plus .01 cfm per square foot of floor area. That was the number. But Max said that the research committee wouldn’t accept it. He said, ‘Let’s put in an infiltration credit that we will ignore. It will be a place-holder. We’ll put in a credit that no one will use.’ I said, ‘I don’t want any credit, and infiltration can’t be relied upon.’ But Max said, ‘We just want to use this formula to size the fan.’ Since then, I have supported dropping the infiltration credit because people were using the credit the wrong way. They were using it to say, ‘You are building your houses too tight — you need more infiltration.’ That was a horrible result. There should be no infiltration credit at all. It’s good they got rid of it, but they shouldn’t have bumped the ventilation rate. That was never intended at the Quebec City conference.”
Even the old formula was too high, Lstiburek asserts
Even back in 2006, when ASHRAE 62.2 still specified the lower ventilation rate, Lstiburek asserted that the minimum ventilation rates specified by ASHRAE 62.2 were too high.
In an article I wrote about the controversy for the January 2006 issue of Energy Design Update (“The Great Ventilation Rate Debate”), I quoted Lstiburek’s criticism of the 62.2 formula. “Higher ventilation rates lead to higher moisture levels in humid climates,” Lstiburek said. Lstiburek advised builders in the South who were enrolled in the Building America program to set up their ventilation systems to operate at a lower rate than the minimum rate specified by 62.2.
Lstiburek elaborated on this theme in a 2006 e-mail to Max Sherman: “I disagree with your argument that Building America assumes great risks if it does not endorse the 62.2 rates. … ASHRAE Standard 62.2 has far more of a credibility problem than Building America. In 62.2, there is no requirement for distribution of ventilation air, and the Standard’s acceptance of exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. , creating infiltration air change where the building enclosure is the filter, is a joke to anyone serious about health risks. … As a professional engineer I recommend that my clients install systems capable of meeting 62.2 rates, but that the systems should initially be operated at 60% of the 62.2 rates. This assumes that occupants will be educated — that is, informed of their ability to vary the rates upwards or downwards based on their lifestyle, needs, and desires.”
Sherman responded, “A builder would be incredibly foolish not to design the [HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building.] system to be able to handle the latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. from an occupant who chooses to run the [ventilation] system at 62.2 rates. … I am all for improving the standard. … I would help you fix the standard; … but that takes effort and time. It also may not come out the way you think.”
Sherman’s prediction has come true. The formula has been changed — but not in the direction that Lstiburek hoped. The ASHRAE 62.2 standard now requires higher rather than lower ventilation rates.
In a recent article, “Unintended Consequences Suck,”, Lstiburek wrote, “In my opinion there was no justification for the original flow rates except for the worst performing systems such as exhaust-only — just a bunch of folks making wild-ass guesses. Now we take these unjustified rates and make them even more unjustified.”
When I interviewed Sherman recently, he told me, “He [Lstiburek] has been banging that drum for years — about the 62.2 rate being too high. He’s a Don Quixote in some ways. We don’t yet have a full health basis for deciding what ventilation rates should be — the science is definitely still out — so it comes down to making some reasonable technical judgments.” (For more information on the connection between residential ventilation rates and occupant health, see Ventilation Rates and Human Health.)
ASHRAE 62.2 includes other ways to calculate the ventilation rate
Defenders of the new ASHRAE formula point out that the newest version of the standard provides an alternate method of calculating the minimum ventilation rate — a method that may restore at least some of the infiltration credit.
The alternate calculation method requires that the home be tested with a blower door. Once that’s done, and you have a cfm50 result from the blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas., you can get an infiltration credit that may reduce the need for mechanical ventilation. You have to use the following formula:
Infiltration credit = 0.052 • cfm50 • (story factor) • (weather factor)
The story factor is the number of stories raised to the 0.4 power. So, for a one-story building, the story factor is 1; for a 1 1/2 story building, the story factor is 1.18; for a 2-story building, the story factor is 1.32; for a 2 1/2 story building, the story factor is 1.44; and for a 3-story building, the story factor is 1.55.
What’s that you say? You’re not sure what your weather factor is? You'll have to look it up from a list of 1,100 locations.
Is it logical to take account of infiltration?
If the math necessary to calculate the infiltration credit is making your eyes glaze over, you will probably be receptive to Lstiburek’s analysis.
“It’s all bullsh*t,” Lstiburek told me. “I say, don’t measure infiltration. Just put in the fan. Max says we can rely on infiltration. But it’s based on a house of cards. Everyone knows that infiltration can’t be determined instantaneously. He argues it can be averaged over the year. The whole argument for infiltration is based on the idea of average annual exposure.”
However, Sherman defends the usefulness of calculations based on average annual exposure. “The ventilation rate of the 62.2 standard is not intended to protect you against CO or backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney.,” Sherman told me. “We have thought about all of the acute contaminants that we can. We have exhaust ventilation requirements for source control. There are depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. limits if you have atmospherically vented combustion equipment. The standard is not intended to cover high polluting events or cigarette smoking. What’s left over are the chronic contaminants. You can’t really have a dilution rate that will cover acute and intense contaminant exposures. Those events can’t be addressed with ventilation that operates 24/7.”
The “local ventilation deficit”
Once you’ve mastered all of the steps necessary to calculate the infiltration credit, you’re ready to tackle the “local ventilation deficit.”
To understand the local ventilation deficit, it’s first necessary to discuss ASHRAE 62.2 requirements for spot ventilation. The standard requires exhaust fans in bathrooms and kitchens to meet the following requriements:
- Every kitchen needs an exhaust fan (either a 100-cfm on-demand fan or a continuous fan rated at 5 air changes per hour, based on the volume of the kitchen).
- Every bathroom that has a tub or shower needs an exhaust fan (either a 50-cfm on-demand fan or a 20-cfm continuous fan).
These required exhaust fans for bathrooms and kitchens are (in most cases) independent of the fan or fans that are required to meet the needs of a whole-house ventilation system.
Contractors or weatherization workers who are retrofitting a ventilation system in an existing home sometimes encounter kitchens and bathrooms that lack exhaust fans. It’s possible to install a whole-house ventilation system that complies with ASHRAE 62.2 in such a home, as long as the ventilation fan has a minimum airflow rating that is large enough to make up for the “local ventilation deficit.”
The local ventilation deficit is a quantity that describes the effect of the missing bath or kitchen fans. To calculate the local ventilation deficit, you add up all of the deficits — that is, the cfm ratings of the exhaust fans that should have been installed in the kitchen and bathrooms. This number can be reduced by 20 cfm per room if the room in question has an operable window. (You aren’t allowed to get credit for more than one window per room.) Then you divide the sum by 4. That is your local ventilation deficit; the number must be added to the minimum airflow rating of the whole-house ventilation fan.
Does every house need a bigger fan?
Lstiburek is unhappy that the new formula will result in builders installing larger ventilation fans than ever before. Sherman responds that, since most houses are leaky, most houses can take advantage of the infiltration credit — which should bring the size of the ventilation fan back down to the size it used to be under the older formula.
However, there are a few flies in the ointment:
- Most new homes never get tested with a blower door. If you don’t have blower-door results, you can’t take the infiltration credit, and so you’ll need to install a bigger fan.
- Most leaky homes ignore ASHRAE 62.2. The only builders who are paying attention to ASHRAE 62.2 at this point (outside of California, where ASHRAE 62.2 is referenced in the Title 24 energy code) are builders of tight, energy-efficient homes. If you are building a tight, energy-efficient home, the latest version of ASHRAE 62.2 will require you to install a bigger fan than the older formula did — because these homes are too tight to claim an infiltration credit.
Sherman acknowledges these points. “High performance homes will have to put in an a bigger fan,” he told me.
Supply-only systems and balanced systems work better than exhaust systems
ASHRAE 62.2 is agnostic when it comes to the debate over exhaust-only, supply-only, or balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). systems. It allows ventilation systems designers to choose any method, and it requires the same minimum airflow rates no matter which type of equipment is installed.
For years, Lstiburek has criticised the standard's agnosticism. He thinks that supply-only ventilation systems and balanced ventilation systems should be given credit for the fact that they provide better distribution of ventilation air than exhaust-only systems.
In “Unintended Consequences Suck,”, Lstiburek wrote, “This is part where I am going to try to be constructive and make some suggestions on fixing Standard 62.2. Lower the rates for ventilation systems that have good ventilation effectiveness. This will encourage good systems and discourage bad systems. Heresy, I know. Here is my take on exhaust-only ventilation. Extracting air out of one location with an exhaust fan does not help other rooms and does not address the quality or location of the air being drawn into the building. What is the ‘effectiveness’ of the ventilation in a secondary bedroom with an exhaust fan in the master bedroom? With a single-point exhaust you need about three times the flow rate compared to supply air ducted to a central air handler that distributes the air an provides mixing to get similar ventilation ‘effectiveness.’ Yet Standard 62.2 treats them the same. To get exhaust-only ventilation to work minimally well you need to provide mixing and distribution with the house central air handler, and even then you don’t get to filter the air before you bring it in, you don’t get to pick where the air comes from and you certainly don’t get to precondition it.”
Elsewhere in the same article, Lstiburek wrote, “A fully ducted and balanced ventilation system, such as an HRV or ERV with spot ventilation via intermittent exhaust fans in bathrooms and the kitchen in a house, apartment or condominium, is significantly more effective than single-point exhaust. Why not reduce the ventilation flow rates for these systems by 50 percent in Standard 62.2 as compared to the ventilation flow rate for a single-point exhaust fan? Consider something similar for supply systems. Supply systems are much better than exhaust systems and should be rewarded for being so.”
According to Paul Francisco, director of the Weatherization Training Center at the University of Illinois and vice-chair of the ASHRAE 62.2 committee, the committee has considered and discussed the question of distribution effectiveness. "The analyses clearly showed that whether distribution was desired depended on what the assumptions were for where the contaminants were," said Francisco. "With one set Joe was right. With another set Max was right. And we just don't know the right answer for where the majority of contaminants are. So, without a compelling argument that most homes are better off one way or another, there was not a sufficiently strong majority to go one way or the other."
On the horizon: an increased need for dehumidifiers
During the winter, a high ventilation rate tends to result in dry indoor conditions. During the summer, however — especially in hot, humid climates — a high ventilation rate raises the indoor humidity level.
If the latest version of ASHRAE 62.2 results in higher ventilation rates, Lstiburek predicts that many homes in the South will need to install dehumidifiers.
“What is discouraging is that we are encouraging poor ventilation systems and approaches through our standards rather than encouraging good ventilation systems,” Lstiburek wrote. “We actually are penalizing good systems. It actually gets worse as we end up over ventilating houses with these poor systems, wasting energy and creating part-load humidity problems. This just sucks. If this keeps up we are going to see lots of folks in the dehumidifier business.”
Wasting all of the energy saved by decades of energy efficiency improvements
In Lstiburek's view, increasing ventilation rates in hot, humid climates will lead to higher energy bills — due in part to the need for more fan energy, but more importantly, due to the fact that many homes will now need to install a dehumidifier to handle the increased moisture load.
Lstiburek wrote, “Did the folks on the committee that voted for these rates know they were also voting for dehumidification? Interesting question. The comment I got from one luminary is that it is the job of the engineer to figure this out, not the job of the committee. I mentioned the energy penalty as well. The response from another luminary is that the Standard 62.2 is not supposed to consider energy — and it does not consider it. The Standard 62.2 committee has managed in the last set of changes to waste all of the energy the model building codes have saved in the past two decades of encouraging the construction of energy efficient buildings, due to the energy penalty from excessively high ventilation rates.”
Sherman responded, “The cause of the dehumidification need isn’t ventilation. It’s that with a better envelope, the sensible loadThis is the heat content of just the air; it does not include the heat content required to remove (condense) moisture from air. Sensible heat is measured by dry bulb temperature. Latent heat is measured by wet bulb temperature. is lower, so the AC don’t run as much. It makes no sense to blame the need for dehumidification on ventilation. Just because you need dehumidification is no reason not to ventilate. We ventilate to protect the occupants from indoor contaminants.”
Lstiburek advises builders to ignore ASHRAE 62.2
According to Lstiburek, the makeup of the ASHRAE 62.2 committee is tilted in favor of fan manufacturers. “One third of the people on the ASHRAE 62.2 committee are manufacturers, and one third are government-regulation-leaning activists,” Lstiburek told me. “There are very few engineers on the committee. The engineers are always outvoted 2 to 1. Of course, the manufacturers want bigger equipment. The manufacturers want to sell exhaust fans.”
Paul Francisco disputes Lstiburek's analysis. "It is simply not correct to imply that the engineers on the 62.2 committee had this pushed past them by other interests," said Francisco. "There was a single vote against the change in committee, from a trade organization rep, and no comments in public review."
Sherman also disputes Lstiburek's conclusions. “I think it is a pretty balanced committee," said Sherman. "But maybe only one third of the committee members agree with him [Lstiburek], and that’s why he thinks the committee is unbalanced.”
Lstiburek has abandoned any hope of working with the ASHRAE 62.2 committee. “At this point, my job is to ridicule them,” he told me. “ASHRAE 62.2 will be relegated to the dustbin of history. ASHRAE is not the only group in the standards business. There are all kinds of really stupid ASHRAE standards, and there also some good ones. I say, if it is a stupid standard, don’t use it. Maybe LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. or Building America could develop their own standard.” (On August 7, 2013, Lstiburek released his own proposed ventilation standard: "Ventilation for New Low-Rise Residential Buildings.")
Lstiburek is ready to oppose any building code changes that might require builders to follow ASHRAE 62.2. In his article, “Unintended Consequences Suck,” Lstiburek predicted, “Standard 62.2 will not be adopted by most jurisdictions for multifamily construction and [the standard] will sit on the shelf like most standards that make themselves irrelevant because they overreach. Standard 62.2 is currently being ignored by the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. committee for this reason.”
Sherman’s response: “Joe wants to fight this. Because he lost at the standard level, he will fight it at the code adoption level.” Sherman continued, “Joe has opted to not participate in the process anymore, but instead to bang his drum from the outside. He’s saying that he knows more than the 20 people on the committee.”
Sherman made a telling comment at the July 2012 Building America Technical Update Meeting in Denver, Colorado. According to the official meeting minutes, Sherman said, “ASHRAE 62 is the only national consensus standard document there is. Follow 62.2, resistance is futile. Makes no sense for someone to suggest guidance that is contrary to 62.2.”
Lstiburek predicts that code officials will ignore 62.2. Sherman predicts that we will all learn to follow 62.2, because “resistance is futile.” Time will tell which prediction is most accurate.
Martin Holladay’s previous blog: “New Green Building Products.”
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