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The Great Ventilation Battle Shifts to RESNET

ASHRAE Standard 62.2 For Residential Ventilation

Just because I haven’t written about the jockeying over ventilation rates and strategies with the ASHRAE 62.2 committee doesn’t mean nothing’s going on. If you’ve been following the battle over this issue, you know that Joe Lstiburek, PhD, PE, got fed up with ASHRAE and introduced his own ventilation “standard” in 2013. Then the “Great Ventilation Debate” happened in Detroit, and he got back on the ASHRAE 62.2 committee. Stuff happened and now RESNET just put an interesting amendment out for public comment. Follow that?

Just because I haven’t written about the jockeying over ventilation rates and strategies with the ASHRAE 62.2 committee doesn’t mean nothing’s going on. If you’ve been following the battle over this issue, you know that Joe Lstiburek, PhD, PE, got fed up with ASHRAE and introduced his own ventilation “standard” in 2013. Then the “Great Ventilation Debate” happened in Detroit, and he got back on the ASHRAE 62.2 committee. Stuff happened and now RESNET just put an interesting amendment out for public comment. Follow that?

A brief history of the recent battle

Let’s go over the recent history quickly because we want to get to the current state of affairs, right? (Plus I’m on an airplane that will be landing in an hour and then I’ve got to go watch the fireworks with my stepson. No, really. It’s the 4th of July.) So here’s my quick chronology.

  1. The 2010 version of the residential ventilation standard, ASHRAE 62.2, gets adopted for programs and codes. The simplified version of the rate is 1 cfm per 100 square feet of conditioned floor area (CFA) plus 7.5 cfm per person. (The number of people isn’t the actual number of people; it’s the number of bedrooms plus one.) Everyone’s happy, mostly.
  2. The 2013 version of ASHRAE 62.2 changes the first part of the calculation. It goes from 1 cfm per 100 square feet of CFA to 3 cfm per 100 square feet. The result is that the total rate nearly doubles. The natives become restless.
  3. Dr. Joe introduces BSC-01, his ventilation “standard.” (I put that in quotes because it’s not really a standard in the same sense that 62.2 is. There’s no consensus process or public comment behind it. Now that you know that, I’m going to drop the quotes, but you can do air quotes when you read it if you like.) It basically goes back to the 2010 rates of 62.2 but also includes credits to adjust the rate based on the ventilation strategy. The revolution has begun.
  4. At the 2014 Affordable Comfort conference in Detroit, the Great Ventilation Debate resulted in Dr. Joe becoming a member of the 62.2 committee again. Everyone sings Kumbaya.

 Got it? That’s where things were last time I wrote about this issue.

The 62.2 committee discusses rates

In 2014 I started going to the ASHRAE 62.2 committee meetings. The June meeting that year was in Seattle. Joe Lstiburek and Max Sherman spoke to each other, as you can see below, but not much happened at that meeting.

No, the first big move came at the 2015 meeting in Atlanta. There the committee voted on proposals to lower the ventilation rate and give a credit to balanced ventilation. The vote at the meeting was in favor of those proposals, but when absent committee members voted later, the proposals failed.

At the 2016 meeting in St. Louis a week and a half ago, the committee voted again on lowering the rate. They wanted to take the square footage part of the equation from 3 cfm per 100 square feet of CFA to 2.2 cfm/ 100 sf.

Those in favor of higher rates made various arguments to support their reasoning, but none had any hard evidence behind them. Max Sherman made what I thought was the worst argument: He quoted the first ASHRAE president as saying, “Ventilation is next to Godliness.” What he didn’t say was that in the 19th century, medical doctors often advised people to keep their homes closed up because they thought fresh air made people sick, so the big battle then was just to get any ventilation at all.

When the committee voted, the result was 10 in favor of lowering the rate and 8 opposed. The motion failed because it needed a 2/3 majority.

Following that discussion, the committee considered a proposal to give a credit to ventilation systems that distributed the air throughout a home. The vote was similar. A majority voted for the credit, but not a 2/3 majority. It failed, too.

Meanwhile, at the code hearings…

While Lstiburek was trying to get his ideas adopted into the ASHRAE 62.2 standard, he was simultaneously working to get them adopted into the building code. Last year, the International Code Council (ICC) voted to adopt the BSC-01/ASHRAE 62.2-2010 ventilation rate: 1 cfm per 100 square feet of CFA + 7.5 cfm x (number of bedrooms + 1).

This ventilation rate is now codified in the International Residential Code (IRC). Now things are getting interesting.

RESNET’s sticky situation

Last week, RESNET sent out a notice of a proposed amendment to the home energy rating standard. The changes mostly have to do with how the ventilation rate is calculated for the reference home, but there are a few other updates in there, too. If you download the proposed amendment, you’ll see that references to the 62.2-2013 rate are struck through and replaced with 0.01 x CFA + 7.5 x (Nbr+1) cfm.

I asked Dr. Joe about the amendment, and he sent this response in an email:

RESNET is currently in conflict with the IRC on ventilation. RESNET by referencing ASHRAE is setting a higher ventilation rate than the code. RESNET is in the ratings business, not in the business of setting ventilation rates. If other programs such as Energy Star or LEED want to set a ventilation rate in order to participate in their program, that is up to Energy Star or LEED. And RESNET should rate those houses accordingly. But in terms of rating standard newly constructed houses, RESNET should follow the code.

If RESNET does not adopt this amendment then there is a high likelihood that the proposed IECC code change allowing RESNET to provide an alternative path for code compliance will lose at the Final Action Hearings in Kansas City in October unless the proposed code change is modified to bring it into compliance with the IRC. Building officials do not like third party groups setting important code changes such as ventilation rates that the Building Officials do not get to approve as part of their code process.

Additionally, the current language penalizes balanced ventilation in favor of exhaust ventilation. This is a bad precedent. Better ventilation systems are penalized in favor of poor ventilation systems. RESNET raters are aware of this as are builders and both are gaming the ratings system. Balanced system get rating point hits as compared to exhaust only systems.

He’s right, and there’s an important point in here that often trips people up. I got confused about it myself when I was new to the home energy rating field. A HERS rating is just an analysis. There are no thresholds you have to meet to “pass” anything. It’s supposed to be an objective look at the energy use of a house compared to a “reference home.”

That’s what Lstiburek is saying in the first paragraph above. The reference home, by definition, is supposed to be based on the code. Now that the code and ASHRAE 62.2 have parted ways, RESNET has to make a choice. Do they want to stick with the building code, as they should? Or will they follow a ventilation standard that’s not being used? I’m in favor of sticking with the code.

This is an important issue. Let your voice be heard. Go to the RESNET page for this amendment and post your comments.


Nota bene. This evening (7/5/16) I’ll be starting a six-day, silent meditation retreat, so I won’t be commenting here except possibly a bit before I go into silence. Your comments will still be able to be published, however, as Andy Bell will be taking care of that. Be nice! I look forward to reading the discussion when I emerge from the retreat.


Related Articles

Lstiburek Has New Ventilation Standard—Resistance May Not Be Futile (July 2013)

Interview with Dr. Joe Lstiburek — The Ventilation Debate Continues (July 2013)

ASHRAE 62.2 Committee Chair Predicts Confusion and Frustration from BSC-01 (August 2013)

The Ventilation Debate Continues: Interview with Dr. Iain Walker (March 2014)

The Great Ventilation Debate – Live at Affordable Comfort! (May 2014)


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This Post Has 40 Comments

  1. High integrity post Allison.
    High integrity post Allison. Seems this might put you at odds with how your bread is buttered. Hattip.

    “Better ventilation systems are penalized in favor of poor ventilation systems.”

    And since better ventilation systems are already at a cost disadvantage, piling disadvantages like this would be a clear sign RESNET doesn’t prioritize putting the consumer’s interests first.

    Hopefully they’ll see and avoid the decision that accelerates their journey to irrelevance.

  2. Think about it this way.
    Think about it this way.

    The difference between a home’s prescribed continuous ventilation rate in ASHRAE 62.2 and a balanced, filtered and distributed system under the BSC method is up to 90 CFM. Doesn’t seem like that big a deal, right? A couple of crappy bath fans or maybe one good one, but here’s what it means in terms I can relate to…

    90 CFM X 60 min X 24 hours X 365 days = 47,304,000 Cubic Feet of air!

    The Empire State Building is about 37,000,000 cubic feet so every home in America is supposed to heat/cool the volume of the ESB every 9-1/2 months?

    Mind you this is the INCREASE over the 60 or so CFM the BSC advanced design provides for.

    I’m a proponent of “ventilation times three” for all air sealed homes.
    1- Ventilate during construction. This will exhaust most VOCs and moisture from the building products and packaging.
    2- Ventilate shortly after construction. High volume ventilation during and for 24-48 hours after construction continues exhaust of most VOCs and moisture
    3- Ventilate forever thereafter. This is the phase we’re talking about here. Continuous, balanced, filtered and distributed ventilation in an air-sealed house is essential and using an ERV allows substantial energy savings, plus they’re nicely packaged systems and make for easy installation.

    In my talks with building professionals I ask them to hold three fingers together and blow on them then raise that hand in a Boy Scout sign. It’s a reminder to Ventilate Times Three……Scout’s Honor! Promise!

    My admonition to our industry is to never consider building an air tighten house or building until there’s a ventilation plan in place. An HVAC contractor who doesn’t pay close attention to the V in HVAC is a HAC. Avoid him like the scourge to our industry he is, or teach him to ventilate responsibly.

  3. Being a non-pro in the
    Being a non-pro in the industry I can’t stop pondering a few things. Ventilation seems to be all the rage but how about purification/filtration? We have survived in the space station for ~16yrs without any ERV/HRV or what have you…..just filtration.
    My brand new <1 yr old Energy Star house has been a mold factory this winter due to high humidity. People entering the house says it still has “the new home smell”, well that would indicate that the VOC’s are lingering around still. So it would seem that we have no filtration and no ventilation (well there is a 4″ duct from the outside to the return on the air-handler but that only brings in air when the unit is running, which was not very often in the fall/winter/spring time in Atlanta…specially when the unit is over-sized….<10 minutes run time is common).

    Having spent the extra money on a Energy Star house I was expecting a better product but now my confidence in any of these regulations and certifications are shot. Instead I am looking at having to spend X amount of dollars to get the house fixed, as my builder has no clue to how to fix it and the HVAC installer when they came out recommended to close the 4″ fresh air duct……

    As the code calls for tighter and tighter homes, and builders seems uneducated, and HVAC contractors ignorant it seems to me, from the point of a consumer, there is a storm brewing. There will be tons of people with insufficient ventilation and or filtration and they may or may not even know about it. And the industry is playing with peoples health.

    I have not seen any discussions about what are acceptable levels of VOC’s/mold/mites and so on, that anybody have measured this vs. various ventilation strategies. Is any of this based upon empirical data at all or just opinion?

    I will stop now, but reading the article I don’t think this issue will be resolved any time soon. In the mean time I will have to keep studying and try to figure out what the best solution is for my home but a new HVAC system is needed, ducts need re-balancing (master bedroom is like an icebox now in summertime), an ERV or whole home de-humidifier and a proper filtration system would be nice instead of a 1 inch media filter….donations are accepted…..
    Thanks for the many great articles no your website Mr Bailes, they have been very helpful!!!

  4. Add to the mix the ANSI
    Add to the mix the ANSI/RESNET/ICC 380-2016 Standard for Testing Airtightness of Building Enclosures which for most cases requires the interior basement door be open for blower door tests. I know Rick Karg of the ASHRAE committee likes this new standard. I like it as well for it’s clarity of language, but I see issues when applied to older housing stock.

    Will the ASHRAE Committee be referencing ANSI 380 in the next version of 62.2?

  5. The money spent on the air
    The money spent on the air system in the space lab makes it unfeasible when, for most people, what is right outside is adequate to do the job.
    As to research, there is ample to make a reasonable decision. Sundell 2011 for one. CARB study in 2009 by Bud Offermann is another.
    As to your solution, install an ERV, supply bedrooms, LR, DR, etc. with Fresh Air and pull exhaust from bathrooms and kitchen. If you have a basement, supply and exhaust it also. My preference is to run on high all the time because getting sick not only sucks, it is very costly. You can read my testimonial page at to get first hand info from my customers. Over 500 systems installed over the last 10 years.

  6. I recently was asked to give
    I recently was asked to give a ventilation presentation to the code officers in our beautiful State of Maine. Since the subject is so controversial, I decided to look at the history. What I found was fascinating. The debate is not new! Who knew? I didn’t, until I did. The first standard set in 1895 was largely influenced by a doctor, J Billings who 2 years before wrote a paper titled “Ventilation and Heating” Apparently he convinced the engineers that the spread of disease, specifically TB, was reduced by increasing ventilation and fought for a minimum of 30 cfm p/p and recommended 60! Yes, in old leaky buildings and he stated “This required mechanical ventilation and placed responsibility for system design and construction on the engineers”. Of course, over a little time, the engineers decided that this was not good for energy. Hmmmmm, 100 plus years later and we are still there. Kinda crazy at this point since we have the technology to do this very cost effectively. Newest units out, here in Maine would cost less than $200 a year to run at healthy levels. What is that you say? Research, oh, and by the way, Dr. Billings was close, real close, says 53 cfm per person (Sundell 2011) or an air change every hour. Not opinion.

  7. Mac & Terje, neither this
    Mac & Terje, neither this article nor the debate in general is questioning the need for ventilation, but rather what the minimum rate should be and whether/how to address the bias against balanced ventilation solutions.

    Allison is spot-on regarding HERS being a yardstick, not a threshold or standard. The issue here is what the subject home is being compared with (e.g., how to model ventilation in the reference home). That said, there is one threshold related to ventilation that can significantly impact the HERS Index. There’s no credit for a tight envelope unless the subject home has a ventilation system that meets the underlying threshold standard. Just what that standard should be is what’s being debated here.

    As for source control, that’s a great objective but it in no way eliminates the need for ventilation in a tight home. How do you address excess CO2 with filtration and source control?

    Admittedly, C02 isn’t likely to be an issue in a large home with few occupants even if the envelope is tight. But what happens when you have a family of 5 living in an 1,200 ft2 starter home that’s ‘toilet-flushing’ tight?

  8. How does one break into a
    How does one break into a career in this field? Is the money good? I really have a passion for energy efficiency and construction stuff, but I’m an engineer in an unrelated field. Would I need to go back to school? Auburn is near me and has a building science program. What are the job titles in this field?

  9. Kurt,
    Thanks for your feedback and link to your website. Glad to see that you have been able to help so many people out. Regarding the ERV, will it be able to pull humidity out of the house in winter time? My concern is that the ERV will provide the fresh air from the outside, but not be able to help out on the humidity issue at the same time as it is still is humid outside in winter Atlanta and no AC running helping out pulling humidity levels down.

  10. David,
    So my house is certified, I have a Energy Start report so someone supposedly reviewed and measured my house. I got a copy of my manual J and it says the air handler will pull in 83CFM of fresh air from the outside to the return plenum of the air handler. It is a 4 inch duct. So this sound all good…? But to my understanding this only happens when the air handler is actually running. It was not running often in the fall, winter or spring. So on paper we might comply with the intent of the requirements, 83CFM is probably OK for a ~2000sq.ft. house. But between the builder, the HVAC contractor and the person creating our report, nobody seem to have picked up on the fact that there is no ventilation when the system is off. Maybe my understanding is insufficient? I personally favor balanced ventilation if i could choose. The discussion seems to me to be a somewhat moot point if the ventilation ration cannot be checked/recorded by measurements. If a balanced system is used, the HVAC contractor and builder has less chance of messing up like in my house.

  11. Terje, what you have is a
    Terje, what you have is a central fan integrated ventilation system (CFIS), which is beyond the scope of this article. Here’s an earlier article Allison wrote on CFIS:

  12. David,
    Thank you so much for that link. That is my system. It does not work for my house in my climate (Atlanta)…..not at all…..sorry if I’m getting this discussion off topic.

  13. WOW !!! I love this !! Is
    WOW !!! I love this !! Is everyone here right NO !! Is everyone wrong NO !! What we have is a group of well spoken highly educated building science people that understand there is a problem. However the problem can’t be solved by a baseline standard without more data. As I see it the unknowns are many the biggest unknown is the occupant/occupants, pet/pets. There is no one size fits all answer that will work. A tight home will save energy however at what cost. Setting a ventilation rate is simply a best guess. Will this ever be solved NO !! We all agree to disagree. So let the thinkers think and let the builders build and not kill each other with a problem that has no real world fix. So any of the 62.2’s or the BSC’s will work at some level. What are we doing trying to fix the unfixable?

  14. As long as we are talking
    As long as we are talking about these issues, we are making progress. It can take a long time to get to an agreement and see real progress. It’s easy to sit here and judge the other side thinking that they are just getting on the way. Energy efficiency is becoming a real issue, we need to do everything we can to improve it.

  15. David, you are right when
    David, you are right when system not running no ventilation is occurring. But the code I believe is based on total air changes a day and the CFM rate is just a way to calculate to get there. So if your house needs a 60 CFM rate per 62.2 and your system brings in 80 CFM like the Broan system then you don’t have to run 24/7.

  16. IIRC you could add a timer on
    IIRC you could add a timer on the air handler or bath fans and your system shouldn’t be oversized.

  17. I think the folks at BSC
    I think the folks at BSC would disagree and instead argue there are other reasons why a balanced system gets short thrift. Perhaps the HVAC industry and builders think it’s too expensive and too difficult to implement correctly.

  18. @Chris, the residential
    @Chris, the residential building code is currently based on ASHRAE 62-2, which was originally specified in terms of air changes. However, for the last decade 62.2 has based on floor area and number of occupants (defined as # of bedrooms +1). Volume or air changes do no enter into the calculation.

    There’s a table in ASHRAE 62.2 for adjusting the rate for non-continuous ventilation. Depending on the duty cycle, it’s not necessarily a straight conversion as you suggest.

  19. Until IAQ measurements for
    Until IAQ measurements for each room are part of the input to a real-time ventilation control algorithm, the ventilation debates will continue. The RIVEC project from LBNL goes a long ways towards smarter ventilation strategies, but is designed for Zero Energy Ready Homes (ZERH), not for the existing housing stock. It wouldn’t be much of a stretch to tweak the design for older homes, though.

    I predict the cheap 62.2 solutions will eventually fall to the wayside as their shortcomings become more known with the increased use of indoor air monitors. A bath fan on a timer won’t have much impact on CO2 levels in a closed bedroom at the other end of the house. The ventilation debate won’t move forward until we have some real IAQ numbers to discuss.

  20. Yeah, seat belts and air bags
    Yeah, seat belts and air bags are expensive in cars too. We should just leave them out.

    We build homes to last over a hundred years, right? Why are we quibbling over a few thousand dollars, and why are we willing to sacrifice the health of the house and the occupants?

    Every code-built building/house requires mechanical ventilation in most of the US. Period. The only way to do it correctly is to engineer and properly install a balanced, filtered and distributed ventilation system. If you want to eventually return the cost of the system through energy savings, use an ERV.

    Don’t over-size the system. It’s wasteful. BSC has it right. Use their “standard” and don’t let a heating and cooling HAC dictate how Ventilation is to be installed unless they can explain the differences between ASHRAE 62.2 and BSC system design guidance, and assure the homeowner/GC/rater/engineer/architect their system will be properly sized, installed and commissioned.

    Make America Think Again,

  21. There are two related but
    There are two related but distinct issues regarding ventilation rate: i) how much ventilation is needed under various circumstances and ii) how much ventilation capability should be installed in the building. It is the former that influences energy use, comfort, and health. Codes can only specify the later one. Ideally, the later should exceed the former with some capability for control. Heating and cooling present the same two issues and we rely on thermostats for the control. Demand controlled ventilation seems to be uncommon in residential applications. (I guess that isn’t surprising since controllable ventilation itself is still relatively uncommon.) If we had more demand controlled systems with decent algorithms, then monitoring the duty cycle patterns of those systems would provide very useful data for specifying how much capability to install. Furthermore, oversizing of the ventilation system would be less problematic.

  22. David says that what you have
    David says that what you have is a CFIS. I think you actually only have part of a CFIS, but could add the remaining parts if you desire. CFIS requires:
    i) a fresh air intake to your air handler (which you have)
    ii) a control which ensures that your air handler fan runs at least a specified amount even when there is no heat or cooling call (as mentioned by Kris)
    iii) a controllable damper in the fresh air intake, and
    iv) a control that closes the damper when your air handler is running more than a specified amount due to heating and cooling calls.
    For CFIS to be energy efficient, your air handler must be appropriately sized and have an ECM.

  23. 1+ what Reid said. Another
    1+ what Reid said. Another common deficiency in CFIS type systems is a lack of commissioning (especially measuring the flow rate).

    BTW, smart CFIS controls are available from AprilAire (8126A) and AirCyler (g2, etc.)

  24. In the CFIS system how do you
    In the CFIS system how do you control filtration of incoming air in the following situation; When a bathroom fan or two or my dryer is running and the dampers are open (I only have a mechanical damper but lets assume they are all there and open), air is flowing into the return of the air handler. Will it flow up through the filter, through the coil, through the blower and through the supply ducts (that normally split up into smaller ducts) or will the air just flow straight into my livingspace through the 12 inch return duct totally unfiltered….as it is the path of least resistance. So I might as well just open a window and let the pollen in….

  25. Terje,

    If your furnace fan is running, the incoming outdoor air will be pulled through your filter. If your furnace fan is not running, it will backflow through the return air grilles. If the damper is closed tightly, then air will come in through whatever leaks you have in your envelope, just like with an exhaust only system. Depending on the location of those leaks, it may or may not get filtered by the insulation.

  26. It seems to me that a very
    It seems to me that a very important element is being left out in the discussion here regarding the 2010 vs. 2013 ventilation calculations. When the infiltration credit is taken into account, the ventilation requirement ends up being very close. The 2010 version assumes infiltration, which is why the required mechanical ventilation comes in lower. Then, to take additional credit for actual measured infiltration, there are quite a few mathematical hoops to jump through before determining how much infiltration credit you can take which is basically half of the excess above the already included natural infiltration. In the 2013 version there is not a built-in infiltration credit. Infiltration is supposed to be measured and natural infiltration may then be used toward satisfying the ventilation needs. When you do the math and incorporate the measured infiltration rate, the required mechanical ventilation for 2010 and 2013 ends up very close.

    I believe this is a step in the right direction. Let’s test and use facts rather than assumptions to make these important decisions.

  27. Sharla, that’s a good point,
    Sharla, that’s a good point, but the issue here isn’t really what the rates are. RESNET has to decide whether they follow the IRC or ASHRAE 62.2-2013. Since the reference home is supposed to be based on the code. That’s what they should do. The standard as currently written also is biased in favor of the least effective ventilation strategy: exhaust only.

    Regarding the infiltration credit, the rate is significantly higher for new homes that meet the 2015 IECC (3 ACH50 for most of the country, 5 ACH50 for the rest).

  28. That’s true. Thanks Allison
    That’s true. Thanks Allison for bringing the point back to the main discussion in your post. I got caught up in reading all the comments and went off track. I agree that RESNET needs to follow code and also agree with Mr. Lstiburek that the EEPs need to consider and implement requirements for their specific programs if they differ; even if that does mean a bit more work for us QA types…

  29. someone asked me (off-list)
    someone asked me (off-list) what I meant in my initial comment in this thread by ‘toilet flushing’ tight. It just means a home so tight that the toilet flushes when you slam the door :-/

    (I believe attribution goes to John Tooley of Advanced Energy)

  30. Just an live in
    Just an live in mixed-humid climate zone 3 so an ERV (and HRV for that matter) will not perform like a dehumidifier in the summer. That’s what your air conditioner is for.

  31. This is such an interesting
    This is such an interesting topic, I had to post my two cents worth. Sorry for not addressing the RESNET conundrum. It has always been my opinion that a balanced ventilation system with “continuous” fan operation is superior to any other. I would like to address Terje’s issue.

    It is doubtful that the 4″ OA duct is actually moving 83cfm, especially if it’s flex-duct. It probably should have been 6″ and equipped with a motorized damper. Terje is correct that the system is “ineffective” unless the fan is running.
    Because you cannot condition or clean the air, unless you are moving the air! A timer or thermostat with a “circulate” mode will turn the fan on for 15 minutes, if no heat/cool demands occur within each hour.

    The HRV/ERV is a must and a stand alone DeHumidifier is obviously needed and should be interlocked into the system as well.

    Install a 2″ MERV 11 air filter, if the pressure drop does not result in high system ESP. Finally, one or two good quality UV lamps that energize whenever the fan is running, is a must have in any home.

    Sorry for the long response…
    John Brown
    Chief Engineer
    EWC Controls

  32. (Sorry, my previous reply was
    (Sorry, my previous reply was poorly edited. Hopefully Allison will delete.)

    @Terje, ERV’s are not dehumidifiers. There’s a lot of confusion about this, even among building and energy professionals.

    The difference between an ERV and an HRV is that the former limits the amount of moisture transferred through the heat exchange core. When the dew point is higher outside than inside (typical in summer), it reduces the amount of moisture introduced via ventilation. However, ventilation still adds moisture to the house, just not as much as with an HRV or non-recovery ventilator.

    When outside air is drier than inside air as is often the case in winter, the opposite occurs: The ERV reduces the amount of moisture ejected in the exhaust air, thus acting to keep more moisture inside. This is considered a benefit since HRV’s and non-recovery ventilation methods tend to overdry the air in winter. This is also the reason why ERV’s should not be used as the primary exhaust for high use bathrooms. In that case, the ERV can lead to excess indoor humidity.

    To address your specific question… During periods of mild wet weather with a high outside dewpoint, ventilation (and/or infiltration) can lead to unacceptable indoor RH levels since the the air conditioner doesn’t operate long enough, or at all. There are several mechanical design strategies for addressing this issue, not necessarily requiring a dehumidifier.

  33. So how does the balanced
    So how does the balanced system handle a “unbalanced load” say the dryer and/or bathroom fan is running? As fa as I understand, none of the units available on the market knows when the dryer is running. Should not in this scenario the supply fan in the ERV/HRV be running to keep up with the demand from the dryer and/or bathroom fan? It would seem that for the system to be truly balanced all devices pushing air in and out of the building envelope should communicate with each other. Or is local supply of air for the dryer a better option? Or do like the Europeans, use a condensing dryer…?

  34. Terje, you answered your own
    Terje, you answered your own question in the end. And it’s a very good question. Yes, clothes dryers and exhaust fans create a pressure imbalance in a home.

    ERVs and HRVs are called balanced ventilation systems because, if they’re commissioned properly, they exhaust and supply equal amounts of air. They’re not called neutral pressure ventilation systems because there are other factors that affect the pressure in a house (stack effect, wind, and other mechanical systems).

    I’ve never heard of anyone trying to add makeup air for dryers or exhaust fans with an ERV or HRV. The best way to do it is to have dedicated makeup air systems for them or to use condensing dryers that don’t exhaust air from the house.

    Here’s an article Joe Lstiburek wrote about that topic:

    First Deal with the Manure and Then Don’t Suck

  35. Adding to what Allison said..
    Adding to what Allison said… For most homes, make-up air systems aren’t necessary for intermittent exhaust fans. This can be demonstrated with a simple zonal pressure test (especially important if there’s an atmospheric combustion appliance on board!). In any case, the heat loss/gain associated with intermittent exhaust is sufficiently small that it can be ignored in all but the most efficient homes.

    Not so for commercial style range hoods that can operate at up to 1,200 CFM. Even half that is enough to suck soot from a chimney and overwhelm a right-sized heating/cooling system.

  36. What are the several
    What are the several mechanical design strategies, that don’t require a dehumidifierfor , that address humidity, in the humid south east, during periods of mild wet weather with a high outside dewpoint, for dehumidifying ventilation and indoor air?

  37. @Craig, you’re referring to
    @Craig, you’re referring to the last sentence in my initial comment. Glad you asked!

    First and foremost must be source control. In humid climates like you describe, a tight envelope** is essential. That may not be a mechanical design strategy per se. but as a mechanical designer, I fully embrace the house-as-a-system design approach. I work with my clients to ensure there’s a good air sealing plan in place. Likewise, below grade walls (including crawls) must be damp-proofed with roof and site drainage properly addressed.

    ** as an aside, the 2012/15 IECC mis-stepped in my opinion by setting the blower door threshold for climate zones 1A and 2A (hot humid) to 5 ACH50. These days, getting to 3 ACH50 isn’t that hard. I encourage my clients to go for 2 ACH50, and that the builder be contractually obligated to meet the agreed upon threshold.

    The other half of source control – applicable in any climate – is spot ventilation. Design it right and verify. And make sure the family understands the importance of using and maintaining those fans!

    And of course, the continuous ventilation system must be properly designed and commissioned. ERV’s are more appropriate in humid climates (again, source control).. just don’t use an ERV as primary exhaust for high-use baths.

    Mechanical dehumidification is not likely to be needed if these simple design strategies are implemented. But if and when necessary, there are better solutions to moisture removal than a stand-alone dehumidifier (which I consider a last resort band-aid for poor design and/or implementation). For example, some thermostats can remove moisture in part-load and no-load situations by minimally overshooting the cooling setpoint to meet the latent load. This works best with a communicating system with multi-stage compressor and VS air handler. Other options include the heat pipe (, or the Lennox Humiditrol, an add-on component that turns an A/C into dehumidifier ( It uses hot refrigerant to reheat the supply air when there’s no sensible load. It’s more efficient than a central ducted dehumidifier, and costs less

    And finally, to minimize the energy impact of dehumidification, it’s important the homeowner understand that maintaining RH at 50% or lower is not necessary for comfort. Up to 60% RH has minimal impact on comfort and even brief excursions above 60% are acceptable if the envelope (especially vapor control layer) is properly done.

  38. Kurt

    You are absolutely right- I’m thinking back to the energy crisis for 1975 and NESEA sponsoring the liberal thinking Dr Joe for lectures on ventilation. In those days commercial ventilation codes wanted 35 CFM per person unless you were dancing at a bar- then 55 CFM.
    I’m not sure there is a way to legislate common sense any more than interior clean air- but at least through most of these discussions the various topics get defined.

  39. Part of this “battle” will
    Part of this “battle” will play out within weeks. A code proposal by Joseph Lstiburek would require the lower, IRC ventilation rate in computing the Energy Rating Index. (RE166-16 public comment #1. I am the junior proponent.) If approved, ratings done for compliance with the IECC would require the lower ventilation rate even if RESNET 301 said otherwise.

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