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Building Science

Ventilation Requirements for Weatherized Homes

Is the ASHRAE 62.2 ventilation standard too complex and expensive?

Image 1 of 2
The fight over mechanical ventilation and ASHRAE 62.2.
Image Credit: The Cosmopolitan of Las Vegas, from, Creative Commons license
The fight over mechanical ventilation and ASHRAE 62.2.
Image Credit: The Cosmopolitan of Las Vegas, from, Creative Commons license
The ASHRAE 62.2 ventilation standard for low-rise residential buildings is at the center of a debate over training weatherization professionals and adding to the cost of weatherizing homes.
Image Credit: Energy Vanguard

I went to school with Cajuns in south Louisiana, and fights were a big deal. They happened frequently, and when they did, a small crowd would gather. The noise would grow quickly and soon everyone in the schoolyard would run over to where the fight was happening. One day in sixth grade, we exploited this tendency and staged a fight between two shoes at recess. Sure enough — our tight circle of boys banging two shoes on the ground and making a lot of noise brought the whole school to us.

Today, I’m making noise for a fight that broke out over the topic of residential ventilation. This is a real fight among two heavyweights in the home performance industry: John Krigger of Saturn Resource Management and Paul Raymer of Heyoka Solutions.

Don’t worry; it’s not a Jon Stewart/Tucker Carlson kind of fight or something you might see on the Jerry Springer show. These guys respect each other and even teach classes together. It’s a very real debate, though, about fixing homes, making them tighter, and adding mechanical ventilation when necessary.

In this corner…

Krigger opened the battle recently with a roundhouse punch aimed at ASHRAE and the latest versions of their residential ventilation standard. “ASHRAE 62.2-2010 is a confusing jumble of requirements and choices that has wasted vast amounts of time and money in training personnel and coping with confusion,” he wrote. Krigger doesn’t like it that so much time and money is being spent in the home performance and weatherization industry to train people on this standard. Now the requirements ares about to change again with the 2013 update, “which will be different and lead to another cycle of confusion and wasted resources,” according to Krigger. He believes there’s no reason to move away from the 62-89 version because the older version is much simpler.

In addition, he claims that the assumptions on which they’ve based the changes in the standard are questionable; he lists four problems at the end of his article. I’ll let you go and read them and decide for yourself, but I will mention one here. He wrote that, “There are millions of respiratory problems relating to a lack of ventilation.” When I posted a link to the article on LinkedIn, Bobby Rhett, an industrial hygienist in the Washington, D.C. area, replied, “We may not always have the source of asthma pinned, but there is evidence that improving ventilation improves asthma outcomes.”

In the other corner…

Raymer responded with a jab, aiming to connect quickly and thus take some of the impact out of Krigger’s blow. His article, titled, Don’t Blame the ASHRAE 62.2-2010 Standard, disputed Krigger’s claim that ASHRAE moved away from simplicity when they revised the standard from their first version, 62-89. That early version, he wrote, was 26 pages long, while the 2010 standard is only 14. “The ASHRAE 62-89 Standard is no longer supported by ASHRAE partly because of its complexity,” Raymer explained.

He then went into some of the details of the two versions of the standard, offering a nice contrast between how they really work. One important point that many in the home performance and weatherization field probably don’t know is that, “there is nothing in the 62-89 Standard about calculating a Building Airflow Standard, Building Tightness Limit, Minimum Ventilation Level, etc.” Those terms come from the programs that reference the standard.

How many rounds will it go?

In case you don’t know who they are, both Krigger and Raymer are heavyweights in this field. Krigger is the author of the book Residential Energy, which is used, I imagine, in nearly every building analyst and home energy rater class in the U.S.

Raymer is the author of the Residential Ventilation Handbook, which covers the whole topic extensively. He’s also on the ASHRAE committee that decides what goes into the 62.2 standard and issues the new version every three years.

How we ventilate homes is a big deal and far from settled. ASHRAE 62.2 is leading the way, but as you can tell from this fight, there is pushback from some of the people who have to apply the standard in the field. The Building Performance Institute (BPI) was supposed to drop 62-89 and adopt 62.2-2010 this month but released a statement on 28 December announcing they’d postponed that change.

Both of these guys make good points, but I have to side with Paul on this issue overall. I think BPI should have dropped 62-89 long ago, and to keep postponing it only makes it worse, as more people get trained on an obsolete standard that was created in the 1980s. John definitely makes some good points, and I think we can do a better job of considering the training aspect of applying standards. Part of that falls on the organizations that adopt the standard, too.

Paul’s basically right, though, that we need to go with the newer version. Yes, it needs work, but the three-year cycle is getting us there.

This is an important battle in the Building Science Fight Club. It’s not just a bunch of sixth graders banging shoes on the ground to get attention. We need to ventilate homes to improve indoor air quality, and we need to be able to do it cost-effectively. Debate and discussion are healthy, and we need to keep it going. I’d love to see John and Paul do an evening session at the RESNET conference this year and continue this debate.

Blower doors and mechanical ventilation

One problem here is that weatherization and home performance crews are trying to use blower doors to determine if they need to add mechanical ventilation. Joe Lstiburek wrote about this issue in his article Just Right and Airtight. After opening with praise for using a blower door to help make homes more airtight and to measure leakage, he then wrote:

But then they think that a blower door actually is a precise measuring tool for how air will leak across the building during service. Wrong. Even more serious an issue is to then take the leap that using a wrong assumption about the results of an approximate measurement can be used to decide that mechanical ventilation is not needed. Bad, very bad, and potentially deadly.

I wrote about the problems with ACHnat here recently, and that’s what Joe’s referring to above. What he says they do at Building Science Corporation is, “To me, the ventilate right part is easy: put in a ventilation system and pick a rate.” In footnote 4 of that article, he describes that their method is to install a system that’s capable of providing 1.5 times the ventilation rate recommended by the latest version of ASHRAE 62.2.

This is an important debate because we want homes to be have less air leakage. We also want them to have good indoor air quality, and that means mechanical ventilation. Krigger makes some good points about all the resources going into training for ASHRAE 62.2, but the time has come to drop the Building Airflow Standard and stop pretending that blower doors can tell us how much mechanical ventilation a home needs.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. GBA Editor
    Martin Holladay | | #1

    We don't have all the answers yet
    Thanks for this blog. I'm a fan of ASHRAE 62.2. I think the committee is doing good work; I think we need a ventilation standard; and I think that it makes sense for builders of new homes to include ventilation equipment that can ventilate at the ASHRAE 62.2 rate.

    That said, there is a lot we don't know. Many ventilation experts who have served on the ASHRAE 62.2 committee understand the extent of our ignorance on ventilation issues. The data don't yet exist to answer our questions; the studies have not yet been performed. Here's my list of what we don't know.

    1. We don't know if there is any connection between residential ventilation rates and occupant health.

    2. We don't know how to predict the right ventilation rate to match a family's lifestyle.

    3. We don't know whether the energy penalty suffered by families who ventilate at the ASHRAE 62.2 rate is money well spent or money down the drain.

    Here's a list of my hunches:

    1. Many homes operate well, and have healthy occupants, even when ventilated at rates that are under those recommended by ASHRAE 62.2.

    2. Most indoor air contaminant issues are lifestyle and behavioral issues, not ventilation issues. The most egregious example of this type of contaminant is tobacco smoke, but others come to mind: insecticides, strong cleaning products, and chemicals used for indoor hobbies. We can't design a ventilation standard for these issues. (The ASHRAE 62.2 standard acknowledges this.)

  2. GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Martin Holladay
    Martin, yes, I agree there's plenty of uncertainty left. Another contaminant source not covered by ASHRAE 62.2 is ventless gas fireplaces, the topic of my article last week.

    Still, if we're doing work on homes to make them tighter, I don't think we can walk away from the ventilation issue. We also need to acknowledge what Lstiburek emphatically pointed out in the article I referenced above: Blower doors cannot determine whether you need ventilation or not.

    I also agree with Joe that we don't need everyone ventilating their home at the ASHRAE 62.2 rate. We just need to give them systems capable of ventilating that much (or more) and let them decide how they use it. (See his footnote 4.) For all the homes with healthy people and low ventilation rates, it takes only one weatherized home with mechanical ventilation that makes people sick to fire up all the people who say we shouldn't make homes too tight.

  3. stuccofirst | | #3

    managing IAQ
    I think the question "what is it that makes people sick" needs to be defined. If its mechanical heating, then separate the mechanical room and use only sealed combustion. If its VOC's and Formaldehyde, then use materials devoid of these toxins. Simply requiring more ventilation to improve IAQ doesn't address the sources of the problem in the first place. Build a healthy home, and build a tight home. The ASHRAE approach seems to encourage the "Build an unhealthy home, and make sure you ventilate it a bunch so the occupants don't get sick".

  4. user-723121 | | #4

    Is ASHRAE 62.2 too much?
    When I attended the 2nd Annual Passive House Conference I posed this question to a very renowned building expert from Canada. Is ASHRAE 62.2 more ventilation than is needed and he said yes. I agree we should have extra ventilation capacity for tight homes with the ability to control the rate. Excessive fresh air really dries out homes in a cold climate, somewhere there is a balance.

  5. RobFisher | | #5

    Common sense?
    I do not know all of the details of ASHRAE 62.2 then vs now, but it seems to me that a fact based standard, one that answers Martins questions above, would be most appropriate. And perhaps Lstiburek's range, .6-1.5 of current ASHRAE standard, is a good place to start, lacking hard facts. We expect building occupants to be able to adjust the temperature and to use range and bathroom fans appropriately. Is it too much to expect them to understand ventilation or to teach them how to use it?

    Also can't we just use CO2 sensors to control ventilation?

  6. watercop | | #6

    I would like to see
    I would like to see CO2-driven, or otherwise sensor-controlled ventilation providing no more than necessary outside air.

    I have a bro-in-law who builds passive houses in Philly and dutifully supplies robust ventilation systems. The vent fans seem to run way more and way faster than necessary, almost completely defeating the laborious adherence to the PH envelope air tightness standard.

    We live in a 3000-ish SF house whose blower door infiltration rate (1600CFM50) translates roughly to 3 ACH 50. Applying the verboten ACHnach ratio guesstimate yields a rate fairly consistent with ASHRAE's 7.5 CFM / person + 0.01 CFM / SF.

    Guess what? we are healthy and comfy, and the last five electricity bills (which include well and septic pumps) have been sub $85.

    Will my next house have an ERV or similar whole house ventilation system...yeah, probably. Will I use it much? Maybe...maybe not.

  7. Expert Member
    Dana Dorsett | | #7

    Sensor controls DO exist.
    Ventilation control via de-humidistat has been used successfully on PassiveHouses in cold climates, but that wouldn't necessarily work for everyone, or even all the time. (It's a control strategy that works in the summertime for most of the eastern US, when outdoor dew points can go well north of 70F at times.) Indoor air pollutants come in many flavors other than CO2 or H2O, and there's probably never going to be a "one size fits all" approach that works for every home or climate.

  8. user-943732 | | #8

    Blower doors cannot determine whether you need ventilation?
    I posted much of this comment on Allison's blog post on a similar topic, but thought I'd include it here as well.

    You wrote: "Blower doors cannot determine whether you need ventilation or not." This statement is, strictly speaking, true. But so is the statement "the presence of ventilation cannot determine whether you need ventilation or not". Because indoor air quality is not simply a function of the amount of ventilation installed or the amount of natural infiltration or the pollutant source strength or occupancy types and patterns -- it is a combination of these factors. We can't pretend that installing a 50 CFM exhaust fan in a bathroom and running it all the time will ensure good indoor air quality. It becomes even sillier to think that you should spend limited resources on adding fans to homes that are very leaky -- yet that is what you seem to be saying..

    Many low income weatherization homes start at 20 ACH50 or higher and after air sealing and insulation work is completed they are still pretty leaky -- maybe at 12 ACH50 or more. Should the weatherization agency use $800 of its limited funds to install a fan in that fairly leaky house and increase their energy costs? What sort of impact on health do you think that fan will have?

    The bottom line is that the estimated infiltration credit is important to consider in ventilation and the argument has (rightly) been about how much credit to provide. It's ridiculous to suggest that every home needs a mechanical ventilation system -- even if you can't get the blower door up to 50pa? The title of this post ("A Blower Door Can't Tell You How Much Mechanical Ventilation You Need") is just plain wrong.

    Also, you and Joe are both wrong about how well we can estimate natural infiltration from blower door measurements. The correlation between blower door measurements and energy usage is quite strong. If you don't believe we can estimate infiltration from blower door tests then you should also agree that we can't estimate solar gain from window SHGC values and you should even start to wonder if we can estimate heating loads across assemblies from R values. All of these conversions from material properties to actual building performance require estimates and simplifications. The accuracy of these estimates for infiltration aren't a whole lot worse than many other estimates we make in modeling a home.

    I'm also surprised how you (and Joe) are so adamant about mechanical ventilation but then think we can rely on occupants to set their own ventilation rates. That would be a good idea if radon, asbestos, CO, and other pollutants were all smelly -- but they aren't.

    Many homes have air quality problems and so may be considered under-ventilated (some would argue over-sourced). Some of these might fail ASHRAE 62.2 and some would pass 62.2. Adding a fan will marginally reduce some pollutant concentrations in homes -- but if you have high radon or smokers or CO or other pollutant sources, that marginal change in dilution won't usually move you from unsafe to safe.
    The impact of a fan on pollutant concentrations can be quite large in houses that are very tight but becomes smaller and smaller as the house gets leakier.and leakier. I don't think anyone knows the magic line (and it would differ in every home), but i do know that changes to the infiltration credit calculations can lead to significant increases in retrofit costs and reductions in energy savings that could render building shell work no longer cost-effective (from an energy savings perspective) in many homes. You can certainly try to make health impact arguments based on sketchy data, but those arguments may not work as well when it comes to getting utilities to invest in efficiency as a resource.

    I'm not saying we shouldn't have mechanical ventilation in new homes or tight homes or even most homes. I'm saying that we can use blower door measurements to identify homes that are still pretty leaky and not spend money installing fans that have little impact at a fairly high cost.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Michael Blasnik
    Excellent points. I'd like to comment on one of your points: your skepticism that it's a good idea to "rely on occupants to set their own ventilation rates."

    Here's my response: whether or not it's a good idea, it's a fact that occupants control the mechanical equipment in a home. It's the job of the builder to build a good home, with all the equipment it needs, and to provide instructions to the occupants on how to operate the equipment. But once the occupants take possession of the home's keys, we can't control where they set their thermostat or how many hours a day they run their ventilation equipment.

    That might drive you crazy -- but it's a fact.

    Back in the 1970s, during the energy crisis, colleges used to put cages around thermostats so students wouldn't mess with them. That really worked, didn't it? No one ever pried those cages off the wall...

    I used to work for a nonprofit housing developer, and I often visited subsidized low-income apartments. Even when the ventilation fans were hard-wired to run 24 hours a day, occupants found ways to disable them if they (rightly or wrongly) concluded that the "frigging fan is wasting too much energy."

  10. user-943732 | | #10

    response to Martin
    Actually, I'm not really against the idea of letting occupants adjust their ventilation rates within a range -- which i think is what Allison and Joe advocate. I was just trying to point out that if you believe IAQ problems are so severe (potentially deadly) that we must invest $1000 in every home in the country for mechanical ventilation (even leaky homes), then how can it be OK to let occupants reduce the ventilation rate to levels far below the guidelines that have been set? Do you believe in the guidelines or not?

  11. GBA Editor
    Martin Holladay | | #11

    Response to Michael Blasnik
    It would be interesting to hear Joe's response, but we may not get that pleasure.

    I can imagine his response, however -- something along the lines of, "In this country, people have always been free to be stupid."

  12. Joe Lstiburek | | #12

    Response to Michael Blasnik
    The correlation between a blower door number and average infiltration over a year is pretty good if there are no large pressure induced effects by mechanical systems. As such a blower door number can be used to estimate yearly energy usage after large pressure effects are "fixed'.

    There is a huge leap required to take this "average yearly infiltration" information and use it as a proxy for determining acceptable ventilation rates in houses for heath reasons. To do so requires a belief that average annual exposure is the appropriate metric for health reasons. I believe that that is incorrect and that the appropriate exposure metric should be on the order of hours not a year. If you accept this then a blower door is not the approach to take for reasons that are quite obvious since you don't know the size of the hole, the location of the hole or the pressure difference acting across the hole. Therefore the air change rate from hour to hour or day to day does not correlate to a blower door number. Our tracer gas work and many others has shown this repeatedly.

    We recommend "sizing" systems at 1.5 times the ASHRAE rate to provide a large "reserve" capacity. We "commission" them at 0.5 times the ASHRAE rate and give control to the occupant. In our experience, the occupant is more intelligent on setting acceptable ventilation rates specific to them than the members of the ASHRAE 62 committees.

  13. GBA Editor
    Allison A. Bailes III, PhD | | #13

    Responses to Michael and Joe
    Michael B.: I think the title of the article is pretty good actually. I agree that we might be able to use a blower door in some cases to see if a home is too leaky to bother with ventilation...yet. I think Joe addressed your comments about correlating blower door numbers with natural infiltration. Yeah, we might be able to get a decent correlation over a longer time average, but ventilation requirements apply over a short time scale of hours.

    Joseph L.: Thanks for jumping in here. Your statement that "the air change rate from hour to hour or day to day does not correlate to a blower door number" is the main point here. There's no sane way to accept an infiltration credit of 11.8 cfm, for example, once you understand that.

  14. user-943732 | | #14

    Response to Joe Lstiburek
    Hi Joe-

    As much as I hate to disagree with you, I've got a few issues with your reply.

    Joe wrote " The correlation between a blower door number and average infiltration over a year is pretty good if there are no large pressure induced effects by mechanical systems."

    Yes -- but in the leaky homes I'm talking about, this isn't an issue.

    Joe wrote "There is a huge leap required to take this "average yearly infiltration" information and use it as a proxy for determining acceptable ventilation rates in houses for heath reasons. To do so requires a belief that average annual exposure is the appropriate metric for health reasons. I believe that that is incorrect and that the appropriate exposure metric should be on the order of hours not a year. "

    I agree that the annual average infiltration rate doesn't have the same impact on IAQ as a constant air change rate would. Varying driving forces means varying pollutant levels for a given source strength. But I disagree that hours is the right timeframe for assessing this -- I think something more like a day would be a fine measure. One hour concentrations really matter mostly for acute poisoning such as CO -- but I think CO detectors are better suited for these events than bathroom exhaust fans. In fact, exhaust ventilation might actually increase the likelihood of such an event occurring in the first place.

    For many pollutants what we care about is the number of hours per year that concentrations exceed certain levels (although for some pollutants such as radon we really do care mostly about an annual average). This distribution of concentrations will be strongly correlated with the blower door result (for a given home and source strength). Adding a fan can shift this distribution of pollutant concentrations by a lot for tight homes but has very little impact on leaky homes.

    Also, if you really care about a few hours of low air change rates then you should not allow a single bath fan to meet the standard -- air change rates in bedrooms with closed doors at night are often very low.

    Joe wrote [some of my responses in brackets]: "If you accept this [MB: I don't] then a blower door is not the approach to take for reasons that are quite obvious since you don't know the size of the hole [MB: yes, that's what the blower door measures] , the location of the hole [MB: same is true for exhaust ventilation - if make-up air is your reason for concern here] or the pressure difference acting across the hole. Therefore the air change rate from hour to hour or day to day does not correlate to a blower door number. Our tracer gas work and many others has shown this repeatedly. "

    I disagree. Yes the air change rate varies from hour to hour and day to day but it is still correlated with the blower door number. There will be far more hours of very low air change rates in tight houses than leaky houses.

    Joe wrote: "We recommend "sizing" systems at 1.5 times the ASHRAE rate to provide a large "reserve" capacity. We "commission" them at 0.5 times the ASHRAE rate and give control to the occupant. In our experience, the occupant is more intelligent on setting acceptable ventilation rates specific to them than the members of the ASHRAE 62 committees. "

    That sounds good (and the last part is catchy) and I actually agree with the general approach -- but mostly because I think 62.2 over-ventilates most homes and that odors and other things detectable by occupants are pretty well correlated with IAQ concerns. But I don't see how you can like this approach but think having a few hours of low air change rates is unacceptable. If you are really worried about CO poisoning or the many other pollutants that don't smell, then how can you think the occupants are better at determining ventilation rates than the standard committee? Do most occupants monitor multiple pollutants? Is it safe to rely on the correlation of odors with other pollutants? If so, can't we just have people open their windows when it's smelly or stuffy during mild weather with low driving forces?

    I think if there are resources available in a weatherization program to spend on improving occupant health, safety, and IAQ (beyond basic combustion safety testing, smoke alarms and CO alarms) that those would be better spent on radon testing and mitigation in high radon areas and other efforts at reducing pollutant source strength than on adding fans to homes that are quite leaky. But, just to be clear, I'm not against adding fans to homes that are fairly tight. It's just a matter of where we draw the line.

  15. user-943732 | | #15

    A study by ECW
    I just wanted to be sure people were aware of this study by ECW

    where they measured CO2 and humidity as indicators of IAQ in weatherization homes that had fans installed under 62.2 (with infiltration credit and actual occupancy not BR+1) and then they turned the fans on and off every week or so to assess impacts. These were generally tight homes. Some of the key findings:

    "Poor indoor air quality in the absence of mechanical ventilation was generally correlated with the factors used in ASHRAE 62.2 to determine the need for mechanical ventilation. Generally, tight homes and homes with higher occupancy density had higher humidity and CO 2 levels. This suggests the current ASHRAE 62.2 based protocol does help provide useful screening in determining the need for, and amount of, mechanical ventilation based on
    measured air leakage and occupancy level. "

    So, they found that blower door measured tightness was well correlated with IAQ and was a valuable tool for determining whether a fan was needed.

    "Operation of the mechanical ventilation improved indoor air quality in most homes where
    it was installed. Indoor humidity and CO 2 levels both showed statistically significant declines in most cases when the mechanical ventilation was enabled. The strongest predictor of the magnitude of the reduction was the amount of mechanical ventilation provided in relation to the estimated natural ventilation rate"

    In other words -- if the fan was large relative to the ACHnat estimated from the blower door it had a much bigger impact on IAQ. So the fan didn't help much in leaky homes.

    " CO 2 levels remained high in a number of homes, even when mechanical ventilation was enabled. Mean CO2 levels were often higher in bedrooms than in living rooms, which may be an indication of poor air mixing"

    So a bath exhaust fan doesn't necessarily lead to much improvement.

    They also concluded: "The data from this study suggests further limiting the installation of mechanical ventilation to homes where the estimated natural ventilation rate is less than 30 cfm per occupant would help
    mitigate the installation of mechanical ventilation where it is not needed and is unlikely to have much impact"

    In other words, you could use the blower door test result to decide that some homes don't need to have a fan added -- specifically based on the estimated natural ACH vs. occupancy.

    There's lots more interesting stuff -- highly recommend people read it if they are interested in this issue. Wisconsin weatherization treats lots of homes that are pretty tight.

  16. Joe Lstiburek | | #16

    Response to Michael Blasnik
    I think that using an exhaust ventilation systems such as an bathroom fan is a dumb idea for a controlled ventilation system.

    I think that you have no idea what the pollutants of importance are. I think that I might not know either.

    I think that leaky houses are not the issue.

    I think that you have no basis for asserting that a day long average is appropriate. That is your opinion. My opinion is different and equally as valid. No data exists. No epi studies have been done. I think I am better looking and therefore my view is better. In the absence of data this is all total speculation. I am willing to admit that. Are you?

    CO2 and humidity are not good proxies for good air quality. They are better than nothing. But not much better.

    The study you cited is total nonsense. No epi studies were done. None have in fact been done. This has all been guess work. No health studies exist. No baselines have ever been measured.

    RH and CO have almost no connection to health. The work has not been done. This is all total BS and I do not mean "building science". Cold climate work via surveys does not have any connection to Houston.

  17. user-943732 | | #17

    Response to Joe Lstiburek

    I agree with pretty much everything you just wrote. We don't really know nearly as much as we should know. All we can do is make out best guesses based on available science.

    The ECW study provides some data about IAQ -- but I agree it doesn't measure IAQ itself because we don't know how to do that. But it does track some proxies and the findings make general sense.

    Do you have hard data that shows we should spend $800 to install a fan in house that leaks 2000CFM50 (or 4000 CFM50?) because the health benefits are not only worth it but also greater than other ways those funds could be used? I would guess, based on your last post, that you don't think such evidence really exists.

    So why should anyone follow 62.2? Maybe because it represents the best guesses of the people on the committee and we should trust them? But the standard has changed multiple times and will change in the future so maybe the current version isn't really right? Hmmm.

    Let me back up a little and go after the whole hourly variation issue. For arguments sake, say I agree that we really care about hourly air change rates. So, how variable do we expect these hourly air change rates to be? Well, it seems like it would be a reasonable first cut to look at using an hourly infiltration model with hourly weather data (TMY3) to estimate how air change rates vary due to varying driving forces. I did a few quick calculations (using all 1,020 TMY3 stations).

    I assumed a 2000 sqft two story home with 3 occupants (83 CFM ventilation requirement) and calculated hourly infiltration for a few different leakage rates. Since you set your ventilation systems at half the ASHRAE target air flow, I thought it would make sense to assess how often we expect the hourly infiltration rate to be below this same 50% level. If concentration spikes are the real concern, then we might only really worry about hours when the infiltration rate is less than half this level -- implying double the concentrations.

    In the first scenario, I assumed the leakage rate was on the cusp of the infiltration credit -- 2000 CFM50 -- which is not far from a common post-retrofit value in many cold climate retrofit programs.

    The analysis estimates that the hourly infiltration rate will be less than the 50% target rate about 5.8% of the time -- although 90% of those hours occur when you could open windows (daily average between 65 and 75) and if people can adjust ventilation systems maybe they can open windows. The infiltration rate was below 25% of the target level in 0.7% of the hours. There are very few hours with extremely low infiltration rates as the stack and wind effects are rarely both near zero. Looking at milder climates, just 10% of the TMY3 stations show more than 10% of their hours below the 50% threshold -- but those climate would, appropriately, have smaller infiltration credits

    A house at 3000 CFM50 is estimated to have just 1.8% of the hours with infiltration less than 50% of the target rate and 0.3% of the hours below 25% of the target rate. Very low hourly infiltration rates are pretty rare in houses that leak 3000 CFM50.

    But if the house was 1000 CFM50 (fairly tight, but not extreme) then the estimate is that 43% of the time the infiltration rate will be less than the 50% rate and 5.8% of the time if will be less than the 25% rate. There is a huge difference here compared to the leakier homes. Infiltration rates less than half the ASHRAE threshold would be very common.

    I think mechanical ventilation would be a good idea for the 1000 CFM50 house. But I don't think you can say the same for the house at 3000 CFM50 and you could certainly argue not at 2000 CFM50 either.

    This modeling exercise was meant to show that driving forces don't go to zero very often and homes that are fairly leaky will have very few hours with infiltration rates lower than the rate that Joe is setting in homes. The definition of fairly leaky should vary with climate -- as the infiltration credit does.

    You can quibble with many of the details, but I think the overall point is pretty solid -- blower door test results can and should be used to help decide whether and how much ventilation is needed in homes. Leaky homes usually don't need added fans and wouldn't benefit much from them anyway (especially the types of fans people are using). The debate should be over defining how leaky and the infiltration credit is a good idea -- but it may need tweaking.

    sorry this is so long...

  18. GBA Editor
    Allison A. Bailes III, PhD | | #18

    Response to Michael Blasnik
    Leave it to you, Michael, to do "a few quick calculations (using all 1,020 TMY3 stations)." I do agree that it's pointless to add mechanical ventilation to really leaky homes because the intentional air is swamped by the infiltration air. Then the question becomes, what is the threshold where we need to add MV. And is a blower door result all we need to set a threshold?

    I think the kind of modeling you just did could be a good start to setting a threshold, but we need more information as well. Does the house sit on top of a moldy crawl space or basement? Is there an attached garage? Are there combustion appliances inside the building enclosure? We shouldn't give credit for infiltration without knowing something about the kind of air infiltrating into the house.

  19. user-943732 | | #19

    Response to Allison Bailes
    Allison- yes, all good points. The last thng you want to see is people leaving holes between the house and garage so that the house doesn't end up "too tight" (I've actually seen this). The same potential problem also affects exhaust only ventilation -- there's no guarantee that the fresh air is fresh. There aren't any easy answers that aren't expensive, especially for retrofit. It would be great if we had those large scale health studies to help guide the choices.

    p.s. computers are fast, running hourly infiltration calcs for 1020 weather stations takes just a couple of minutes.

  20. Joe Lstiburek | | #20

    Response to Michael Blasnik
    I think the modeling is wrong. You have to guess at the distribution of the openings and pressures. So your percentages are bogus in my view.

    Having said that the real question is why spend $500 to $800 dollars on a blower door test? That is the waste.

    It appears to be necessary to appease the Cult of the Blower Door.

    Go back to my original ASHRAE article or my post on my website for the basis for this view.

    I think ASHRAE 62.2 makes no sense for existing homes.

    I think that old existing homes are not the problem.

  21. GBA Editor
    Allison A. Bailes III, PhD | | #21

    Response to Joe Lstiburek
    Joe L.: In your article that I linked to above (Just Right and Airtight), you wrote, "They use a blower door to help measure leakage, and this is also great." When you talk about the 'Cult of the Blower Door' in your most recent comment above, I assume you're referring to the tendency of some people to overstate what they can do, not that they shouldn't be used. It also looks like you're saying that when you're working on a tight budget, it's better to eliminate the blower door test than the mechanical ventilation system. Right?

  22. user-943732 | | #22

    response to Joe
    Joe wrote:"I think the modeling is wrong. You have to guess at the distribution of the openings and pressures. So your percentages are bogus in my view."

    I certainly agree that the modeling was more an exercise than super precise -- but the general results are probably not that far off -- in very leaky homes you will rarely have infiltration rates near zero because the lack of temperature difference and wind isn't that common in most climates. The stack effect is actually pretty well known and the variations in the distribution of openings has a fairly small effect on stack infiltration. In general, the lack of knowledge of leak location is an overblown source of uncertainty. The real problem with infiltration modeling is not knowing the site-specific wind.

    Joe said:" Having said that the real question is why spend $500 to $800 dollars on a blower door test? That is the waste."

    This would be true -- if a blower door test cost anywhere near that. A blower door test costs about $50 -- that's the incremental cost of adding a blower door test to a site visit from a contractor that was going to be there anyway (the cost is lower for weatherization agencies using in-house crews). The $50 is based on actual bids. The blower door test can also help lead to more effective air sealing.

    Joe Wrote: "It appears to be necessary to appease the Cult of the Blower Door."

    Well, if the test weren't pretty cheap and useful, i would agree. But I would agree that there are many cults in our field: the cult of the duct blaster (which is expensive and not very useful in my opinion), the cult of overly complex building energy modeling, and maybe the cult of complex combustion safety testing (which may have gone overboard)

    Joe wrote: "I think ASHRAE 62.2 makes no sense for existing homes.
    I think that old existing homes are not the problem."

    OK -- maybe we agree more than it seems. I completely agree that new homes need ventilation systems and many existing homes do too.

  23. GBA Editor
    Allison A. Bailes III, PhD | | #23

    Response to Michael Blasnik
    How precise do you think your results are? I doubt that it's at the level you indicated with your decimal (5.8% of hours with <50% ventilation met by infiltration, for example). Would you say 6% +/- 5%? +/- 10%? More?

    Also, as I pointed out above, I don't think it's tremendously useful to talk about infiltration without also looking at where that air is coming from. If it's from moldy crawl spaces or attached garages, it shouldn't count at all toward any kind of infiltration credit.

    Finally, if a contractor is charging $20 for a blower door test, they're making up for it somewhere else because they probably can't even carry the equipment from the truck to the house for that price.

  24. user-943732 | | #24

    cult of the blower door
    Speaking on behalf of the cult of the blower door, I would just like to apologize for the recent statements made by Brother Joseph. He has been under a lot of strain lately. We have dispatched re-education specialists to BSC to help him through this period of troubles.

    Bless you and your flow exponents

  25. user-943732 | | #25

    response to Allision
    Allison wrote:"How precise do you think your results are? I doubt that it's at the level you indicated with your decimal (5.8% of hours with <50% ventilation met by infiltration, for example). Would you say 6% +/- 5%? +/- 10%? More?"

    I didn't indicate any level of precision, I just tend to use 2 figures for most answers -- even if they are rough. The purpose of the results was for roughly comparing between homes of different leakiness -- and for that purpose it made sense not to round 5.8% vs 1.8% vs 0.3% (round to zero?). The difference between 43% and 5.8% is large even if uncertainty levels are pretty high.

    The exact numbers don't matter much. In fact, even if we knew the exact air change rates of homes we still wouldn't know in most cases which would would have bad air quality. Pollutant source strength tends to dominate when it comes to big IAQ problems -- until you get to very low air change rates. All we can try to do is try to affect the likelihood of problems -- we can't control them.

    I agree with your comments about where the air is coming from -- and the same problem is true if you add an exhaust fan. In fact, adding an exhaust fan may make it much worse than relying on natural infiltration as you depressurize the home and suck more air out of the garage or crawlspace or flue. In terms of the infiltration credit, I could see counting it as zero for homes with attached garages and maybe even homes over crawlspaces that haven't been isolated form the living space.

    In terms of blower door test costs -- the $20 figure was based on having a low income weatherization agency crew person include a blower door test during their visit. The equipment amortizes quickly and the actual labor time should be less than half an hour per job. You might know it, but there are weatherization agencies installing condensing furnaces for $2000 and sometimes less (including the cost of the furnace) and doing dense pack of walls for less than $1/sqft and doing air sealing for less than $500. The $50 figure is based on an incremental cost for performing a large number of energy audits with vs. without a blower door test. I'm sure it varies between contractors and bids. I'm not at all saying that you could hire someone to come to your home and perform a test for anything near that. But that would be a waste of time.

  26. Joe Lstiburek | | #26

    Response to Michael Blasnik
    I just woke up with a blower door in my bed..... I apparently got an offer I could not refuse.

  27. user-943732 | | #27

    Response to Joe
    Hey, that's what happens when you take sides against the family. But remember, it's just business, not personal.

  28. Tedkidd | | #28

    Got lucky, or got a horse head?
    "I just woke up with a blower door in my bed..... I apparently got an offer I could not refuse."

    I think Joe got drunk, then got lucky... Blower doors are HOTT!

  29. lutro | | #29

    A timely quote, perhaps...
    “The road to truth is long, and lined the entire way with annoying bastards.” -- Alexander Jablokov

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