An Update on the Residential Ventilation Debate

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ASHRAE 62.2 standard for residential ventilation

It's been a while since I've written about what I had been calling "The Great Ventilation Debate" back when Joe Lstiburek was battling the ASHRAE 62.2 residential ventilation committee.  The 62.2 committee meets in person twice a year at the two ASHRAE conferences and they just met last Friday and Saturday in Houston, Texas.  A few things have happened over the past few years so let me give you a brief update.

Ventilation rates

Back in 2013 when Dr. Lstiburek introduced what he called his ventilation standard, BSC-01, the 62.2 committee had just removed the default infiltration credit for homes, which resulted in higher ventilation for most new homes that had to meet strict airtightness codes.  Lstiburek's rates were based on the 2010 version of the 62.2 standard, with credits for reducing the rates further for ventilation systems that were balanced, distributed, mixed, or some combination of those three. 

After the big debate at the 2014 Affordable Comfort conference in Detroit, Lstiburek rejoined the 62.2 committee.  In 2015, they had proposals to adopt something very similar to BSC-01, but they didn't pass.  The following year, a new proposal to lower the ventilation rates (but not as much) in the 62.2 standard was voted on...and it, too, failed. 

So, the rates specified in the 62.2 standard are pretty much the same as they've been since the 2013 version of the standard dropped the default infiltration credit.  A home needs 7.5 cfm per person (with number of people defined as number of bedrooms plus one) and 3 cfm per 100 square feet of conditioned floor area. 

(7.5 cfm/person) + (3 cfm/100 sf)

That's the total before adjustments.  The amount you have to add can be less if you do a blower door test and take the infiltration credit (not to be confused with the default infiltration credit mentioned above).  It may also change if you're doing intermittent ventilation instead of continuous.

Ventilation in the International Residential Code

Where things have gotten interesting is that the International Code Council has been taking a different path.  The 2018 International Residential Code adopted a ventilation rate requirement that's essentially the same as the 2010 version of the 62.2 standard:

(7.5 cfm/person) + (1 cfm/100 sf)

The 2021 code goes a step further in the direction of Lstiburek's BSC-01.  They've agreed to allow balanced ventilation systems (where the amount of air exhausted and the amount of air introduced balance each other out) to operate at 30% lower than the rate you'd get from the equation above.

Unvented gas heaters

The issue of unvented gas space heaters was added to the scope of the standard with the 2013 version.  Every meeting since then has included time wasted on...errr, I mean, lots of discussion of what, if anything, to do about these appliances.  The industry folks come and make their case.  They have some friends on the committee who support them.  The committee came close in 2016 to approving a motion that would have said a house with unvented gas heaters cannot be in compliance with the standard. 

The meeting in Houston didn't really have much discussion about unvented gas appliances.  The industry folks are trying to get ASHRAE to fund a study of the emissions from these devices but several committee members I spoke with are opposed to that because the industry already has data that they won't release.

The committee has taken one action, though.  They've put a limit on the capacity of an unvented gas heater based on the volume of air available to it.  According to commenter RoyC (see below), "The current change to the standard limits the rated heating capacity of unvented heaters (Btu/hr) to 0.71 times the volume (cu.ft.) of the 'open' space where it is located.  Thus, a 6000 Btu/hr furnace (which is about the smallest available) requires a space volume of about 8500 cu.ft., which is roughly an area of about 1000 sq.ft with 8.5 ft ceilings."

(Note:  This does not apply to ranges and ovens, which are covered by the kitchen ventilation requirements.)

Filtration credit

Another issue they've been working on for the past few years is a credit to allow reduction of the ventilation rate if a house meets some filtration criteria.  The idea is that by filtering the air more you won't need as much outdoor air to remove or dilute the contaminants.  It's a great idea in theory but they've had guests come to the past two meetings and point out some of the problems with doing this.

Most recently, Professor Jeff Siegel, an indoor air quality researcher from the University of Toronto, gave a presentation at the Houston meeting about filtration efficiency and ended with a word of caution.  A filter credit, he said, could work only with a performance-based approach and with consumer education about filter installation and changing.  Without these measures, extra filtration with less ventilation could lead to worse indoor air quality.  We'll have to wait and see if this idea goes anywhere.

ASHRAE and the real world

As I discussed in my other article this week, while the committee was getting lost in the weeds, the conference hotel was blowing unconditioned, high-humidity air into the hall near the meeting room. Yeah, that's a commercial building and I'm writing about a residential standard here, but the dissonance there was really interesting.

Oh, and that thing about hyperbolic cotangents.  I'm all for bringing up arcane mathematical functions in the proper context.  I've got degrees in physics and used to use that stuff all the time.  But when someone gets up in front of the committee and brags about how this is the hardest math he's ever used for 62.2 and probably the only ASHRAE standard that may rely on hyperbolic cotangents, it's more than the cotangents that were hyperbolic.  Plus, you gotta be careful doing that because it's easy to dislocate your shoulder when you're patting yourself on the back so vigorously.

I've been going to the ASHRAE 62.2 meetings since 2014.  I'm going to look for better ways to use my time in the future.

 

Related Articles

The Great Ventilation Debate

Unvented Gas Appliance Industry Falls Flat at ASHRAE Meeting

4 Ways to Do Balanced Ventilation

ASHRAE Conferences — Where Theory Meets the Real World

 

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Comments

Do you know if there has been testing on contaminate levels once residents move in to see how well the ventilation systems really work?

Allison
Bailes

Carol, yes, there's been some research on contaminants in occupied houses.  Professor Siegel's research on filtration efficiency, which he reported on at the 62.2 meeting in Houston, is one of the studies in that area.  He found a big difference between lab results and what he's found in real, occupied houses.   I know there are others but will have to look them up.  Sounds like a good topic for a future article.

Having read through the other comments regarding humid climates, I'm interested in thoughts regarding dry/heating climates. Albuquerque is a temperate climate and many builders use continuous vent exhaust only systems in tight homes. We use ERVs to help maintain humidity in the homes. I'm also interested in the air quality aspect of the ventilation systems. Bathrooms and utility rooms tend to have higher concentration of homeowner products with high VOC contents (laundry detergents, cleaning chemicals, colognes, perfumes, etc). Has anyone looked into spot ventilation along with continuous ventilation to help with steamy showers, odors and VOCs in these locations?

Allison, the latest unvented combustion heater change that was approved by Standards at Houston did not have a ventilation option, only a room volume limitation for these heaters. There was a ventilation option in the previous version, but the unvented heater manufacturers protested, so it was taken out. The current change to the standard limits the rated heating capacity of unvented heaters (Btu/hr) to 0.71 times the volume (cu.ft.) of the "open" space where it is located. Thus, a 6000 Btu/hr furnace (which is about the smallest available) requires a space volume of about 8500 cu.ft., which is roughly an area of about 1000 sq.ft with 8.5 ft ceilings.

Allison
Bailes

Thanks, RoyC.  I've updated the article with your correction.  I knew I was going to mess that up since I didn't write it down in Houston.

I just rebuilt my house south of very hot and humid Houston, Texas after Hurricane Harvey. It has all the neat things an Allison Bailes article would like to see in it including a nice ERV providing balanced fresh air. My question to you is, do these ASHRAE guys go out and actually do field work to see if their crazy calculations have anything to do with reality? Case in point....

My ERV was setup exactly to 62.2 standards. Even with a whole house dehumidifier we could not get the indoor RH below 65 - 70%. I went in the attic and unplugged the ERV. In 24 hours it was down to 44% in the house. Since then we have adjusted a few things and I now get fresh air and an indoor RH between 46 and 53% in swampy Climate Zone 2. I'm not a fan of 62.2!!

Allison
Bailes

Dave, yes, some of them really do field research.  Paul Francisco, who just finished his term as chair of the committee, reported on a field study of garage ventilation and IAQ inside the house about a year ago.  There are a lot of smart, experienced people on the committee but the problem seems to be the sausage-making process that is a consensus-based standard, especially one that affects companies that make products prescribed — or limited — by the standard.

I'd love to hear more about your ERV and why it may have had such an impact on your relative humidity.  Sounds like a good topic for an article.

@Dave, does the ERV have an exhaust inlet in your bathroom? One thing that could explain what you describe is if the ERV is used as primary exhaust for a bathroom with a shower. Even in Houston, shower exhaust has more moisture (i.e., higher dew point) than the outside air, thus the ERV will dutifully transfer much of that moisture back into the incoming fresh air stream, thus spiking indoor RH.

The way an ERV media works is to keep as much water vapor as possible on the side of the media with the highest dew point. In cold climates, this helps maintain a more comfortable (higher) humidity level in winter. However, as we build tighter envelopes, this can backfire, especially if the ERV pulls it's exhaust air from a high use bathroom. In warm, humid climates, an ERV will greatly reduce (but not eliminate) the moisture loads associated with ventilation air, but not if it's used as the primary bath exhaust.

David, I agree with your comment that running bathroom exhaust through an ERV can "backfire" in hot/humid climates, but your comment that "The way an ERV media works is to keep as much water vapor as possible on the side of the media with the highest dew point." does not make sense to me. Did you really mean that? An ERV allows water vapor (but not air) to pass from one air stream with a higher dew point to another air stream with a lower dew point. Sometimes that is not desirable as you correctly indicate, but the water vapor does always flow from high to low absolute humidity, right?

@Roy, I was trying to explain what happens in simple terms, but my comment is correct as far as it goes. The difference in the partial vapor pressure between the incoming and outgoing air streams creates vapor drive that moves moisture from wetter air to drier air. In doing so, in effect, it's acting to keep moisture on the side with the highest vapor pressure.

In summer, the outdoor dew point will often be higher than the indoor dew point, so a high percentage of the water vapor in the fresh air stream (ERV ratings typically show 50%) will get transferred into the exhaust air stream, effectively keeping the moisture on the side with the highest partial vapor pressure. But since shower 'steam' will almost certainly be at a higher vapor pressure than outdoor air, the opposite occurs. Exhaust air moisture gets transferred over to the drier fresh air stream. Again, it's acting to keep moisture on the high side. Another way of looking at this is to say that the ERV recycles some portion of the shower moisture back into the house.

No. My ERV pulls in and vents directly to the outside.

I disagree, David, and this might be the first time that this has happened! The ERV membrane only allows water vapor to go from the more humid side to the less humid side. The fact that the less humid air stream may be carrying that transferred water vapor back to where it came from is just a misapplication of the ERV, not a property of the ERV membrane. Perhaps this is just semantics. Any English majors out there?

Allison
Bailes

OK, David, it took me a minute but now I see what you're saying.  Like RoyC, I also thought you had it wrong.  I figured you just used the wrong word, but now I see it.  When you say, "it's acting to keep moisture on the side with the highest vapor pressure," the confusion comes from the word "side."  In that quote, you're talking about the side of the building enclosure (indoors vs. outdoors), but I was thinking you were talking about the side of the ERV membrane.  I'll bet that's what threw RoyC, too.

Roy wrote: "The ERV membrane only allows water vapor to go from the more humid side to the less humid side."

Exactly

Roy continues... "The fact that the less humid air stream may be carrying that transferred water vapor back to where it came from is just a misapplication of the ERV, not a property of the ERV membrane."

No, not misapplication. What I said holds true in the 'normal' mode as I previously explained. Outside air is humid, say 65F dew point. Inside air is drier, say 53F dew point. ERV will act to keep outside moisture on the "side" with the highest dew point, which is the outside of the enclosure. As Allison noted, by 'sides' I'm referring to the enclosure, not the membrane / media. At least that's how a lay person would interpret my comment. I would never had written it that way in a technical forum.

Sorry for the confusion, folks.

Allison, David's original post said:
"The way an ERV media works is to keep as much water vapor as possible on the side of the media with the highest dew point."
I still claim that is incorrect. The MEDIA allows water vapor to pass through from high dew point to low dew point, the more the better as far as the manufacturer is concerned. A properly designed ventilation SYSTEM using an ERV does try to keep as much water vapor as possible outdoors during the cooling season and indoors during the heating season. But that is not what the above quote says. Another case of the FAKE MEDIA!

Allison
Bailes

RoyC, I didn't go back and look at the original comment, but you're right.  His original statement was about what happens inside the ERV, not on the larger scale of the enclosure.

@Roy, my original comment was indeed wrong. When you took issue with it, I didn't go back to read what I wrote, otherwise I wouldn't have continued to defend what I wrote. Mea culpa. What I meant to write is 'on the side of the enclosure with the highest dew point.' Thanks for persisting!

Allison, that is a typical response from the FAKE MEDIA! Just kidding. I will admit that I am a nitpicker, but somebody has to do it, otherwise these myths are perpetuated, like people calling forced-air furnaces "dry heat" and hydronic heating systems "wet heat" which is then interpreted to mean that force-air furnaces lower the humidity in a house and hydronic systems raise it.

David, I figured that is what you meant and that you just wrote it a bit wrong. I can be a nitpicker when it comes language, which is a necessary condition for being on the Std. 62.2 committee.

I also attend the ASHRAE 62.2 meetings and know most of the members and attendees. I think that there is a broad range of expertise, including a lot of practical field experience. I do get frustrated when we argue about whether to add or subtract another 10 cfm to or from the requirements, especially when we have little in the standard to verify that the equipment is delivering what it should, especially after a year or two of operation.

Dave, please keep in mind that Std. 62.2 has no requirements for humidity control. It only talks about ventilation with outdoor air and a little big about filtration. So it does tell you that you have to bring that nasty hot humid stuff inside in Houston, but it doesn't tell you how to control the humidity. So please explain what you did to get fresh air with humidity control. Are still providing Std. 62.2 levels of ventilation?

I too experienced a humidity control problem in a brand new tight home in Houston. I traced the problem to a 150CFM outside fresh air intake that was programmed to operate 40% of the time. I realize that this std does not address humidity control but maybe it should. Humidity levels have a much more profound effect on my comfort and health than ventilation. Although I can not perceive any difference between 10 and 40% ventilation times. It is blantanly obvious the difference between 50% and 60 RH I saw. I did two things to improve the situation.

1. Reduce the vent time to 10%, lock it out at night when humidity is high.

2. Reduce the AC airflow to give a 50 degree evaporator.

I plan on installing an outside temp/ humidity sensor so the thermostat can sense that and gate the ventilation off during periods when the dew point is 72 or higher or outside temp is above 95.

I also plan on adding a whole house dehumidifier to process the incoming wet air stream so the air added to the house is much drier.
It will also function during the periods of time that there is not enough sensible load to cause the AC to adequately control humidity by itself (about 4 months per year here in Houston)

@Charles, you referred to outside dew point... As I'm sure you know, when comparing outside air to inside air, it's important to use dew point, not RH. For example, when it's cooler outside, the RH naturally goes up, but that doesn't mean it necessarily has more moisture than indoor air. It could be raining outside and if it's cool enough, the dew point may be lower than indoor air, in which case ventilation would act to reduce indoor RH.

If your home is relatively tight, operating your ERV at an effective rate of 60 CFM shouldn't spike your RH. If you need mechanical dehumidification, there's likely something else wrong. See my comment to Dave Yelovich above in case it applies to you.

Allison
Bailes

Charles, to add to what David Butler said, it's important to look at dew point (or another measure of water vapor concentration, like humidity ratio) when talking about moving air between indoors and outdoors.  Yes, the relative humidity is higher at night but that doesn't mean the dew point is higher.  Also, if you're not going to bring in any outdoor air in Houston when the dew point is 72° F or higher, you're going to go for long stretches for several months in the summer without getting any ventilation air into your house.  You might want to reconsider that and just do more humidity control once the outdoor air comes in.

Charles, I agree with your Comment 2, that reducing indoor airflow is an effective way to increase latent capacity, but I have to disagree with Comment 1. In most climates, the absolute humidity (e.g., dew point) is pretty constant throughout the day and night unless a weather change passes through. If anything, the dew point tends to rise a little during the day as more water evaporates due to the higher dry bulb temperature. I believe that you are thinking of relative humidity which is higher at night due to the lower dry bulb, but it is the dew point that determines how much humidity enters the house with the ventilation air. Bringing in ventilation air at night instead of during the day does help reduce sensible loads.

Ventilation in humid climates is a gamble. Especially since most in this business think too highly about, and depend too heavily on, ERVs. An ERV is not a device that helps indoor summer moisture control. In fact, it is a common contributor to moisture problems.
At best, when perfectly installed, an ERV will only remove about one part of the three parts of moisture coming in. Your home gets the other two parts. And this ratio is very dependent on the manufacture's ideal test environment with balanced intake and exhaust. How many of our real world ERV installations have this perfect balance. Stray from this balance and the moisture removal goes down.
Using an ERV in humid conditions can be a downward spiral in home comfort. The indoor moisture levels continually increase. Why does this happen?
Think about it. What is the "tool" that allows the ERV to remove moisture from outside air? Assuming it is a conventional ERV, there is no compressor. No refrigeration or condensation occurring. The "tool" is your perfectly conditioned, DRY, indoor air. If you have some dry air to spare, you can push this through your ERV and it will help draw some moisture out of the incoming air. But as you build up the moisture in your home to 70% RH like Dave's home, you just lost your tool. You do not have a very good drying tool for your ERV. The more moisture in your home, the less you remove in ventilation, and the less you remove, the more moisture in your home. It's a toilet flush.
As I write this, from Indiana, June 29, 2018, it is 91 degrees and our dew point is 75. That is some nasty stuff and I want none of it in my home. My Geo HP is running and my whole house dehumidifier is taking a break for now. I am about 40% RH inside and a very comfortable 76 degrees. Later tonight when it is 75 degrees outside with a 75d DP, the dehumidifier will take over for complete 24/7 comfort.

There seems to be a common misconception that ventilation through an ERV is supposed to reduce humidity in the home compared to having no mechanical ventilation at all. An ERV does add humidity to the indoors. However, it should add less humidity to the indoor space than other mechanical ventilation strategies (like supply only or exhaust only systems). If you don't want to bring in outside air to dilute contaminants other than water vapor, then don't use any ventilation at all.

@Dan, to your point, the larger the difference between indoor and outdoor absolute humidity (dew point is one way to gauge this), the more effective the ERV is in mitigating the ventilation load. Ratings data indicate 50% total efficiency at 95F, but I haven't been able to get any manufacturer to provide separate numbers for latent, or at any temperature other than 95F (for cooling season). So I agree that an ERV's actual effectiveness at moisture removal is unknowable. On the other hand, as Roy just said, non-recovery ventilation (or using an HRV) would be even worse, so there's no question that an ERV is the most appropriate means of ventilation in humid climates. Just don't use them for primary bath exhaust, and don't exceed the lowest ventilation rate that is supported by code (in my opinion).

You guys are right. To clarify I am not using an ERV or HRV just an outside vent with a damper. The dew point referred to outside air. The setpoint for allowing fresh air in would have to be adjusted experimentally. The Honeywell sensor does not have good accuracy specs but does appear to be stable so if I was to empirically arrive at a setting that will alow ventilation only during the driest times of the day that could work. With the historic data I looked at there was about a 4-6 degree dew point variation each day and maybe 74 or even higher would be the right setting to use. See the follow graph for the next week.
https://www.wunderground.com/forecast/us/tx/hooks-memorial?cm_ven=localw...

I am able to control humidity pretty well without a dehumidifier as long as I keep tight control over the amount of fresh air brought in and its moisture content. But as soon as sensible load goes down humidity rises. What I propose is to bring the fresh air thru a ventilating whole house dehumidifier and run it when necessary to dry the incoming stream. I would then mix this with the supply side of the ac. The dehumidifier would of course also have a dedicated return near the top of the stairs where I have measured the highest temp and humidity in the house. This should allow good humidity equalization throughout the house. The secret to getting all this to work well is a couple of notes of tweaking to get the settings just right.
The ac is adjusted to have a low SHR during periods of humidity removal. I recognize this is rather Rube Goldberg and would love to a tightly integrated electronic control but I am not aware of proper device to do the job.

I reject the idea that a dehumidifier is a baseline necessity in new construction. There are lots of other factors that can cause or lead to high indoor humidity. Let me count the ways: ineffective use of spot exhaust, leaky shell, foundation or site drainage issues, oversized AC, return side leakage, over-ventilation, lack of commissioning, and as I previously described, using ERV as primary exhaust for a high moisture area.

Bottom line: I don't think the the moisture load from a properly designed and installed ERV operating at IRC mandated flow rate is large enough to cause RH to exceed 60% (upper end of ASHRAE comfort zone) as long as the mechanical system is also properly designed and installed.

As an aside, when I moved from the muggy southeast to the arid southwest back in '06, I assumed I could increase my t'stat set point due to low indoor RH. I was surprised (actually, shocked) to discover that bumping my stat by more than a degree was equally unacceptable in AZ at 40% or lower as it was in NC at, say, 55%. People tend to exaggerate the impact of humidity on comfort, especially referring to the 50% to 60% range. In humid climates, I typically design for 55% and tell the client to expect up to 60%, especially during cool rainy weather when there's no sensible load. I see no need to expend energy to maintain RH at 50% or lower in the typical home. It's amazing what our bodies can adapt to.

Quality DH units are expensive buy and all DH units are expensive to operate. DH operating costs loom especially large in high performance homes with much lower than average heating and cooling costs. I see dehumidifiers as a band aid rather than a legitimate part of mechanical design for new homes. Unless money is no object.

One other point of clarity, For some reason you have to click on options and select dew point to add it to the chart. You can also clearly see the highest dewpoint is when the air has cooled. So using the Honeywell 8321 setting of "lock out ventilation during sleep periods" is appropriate for this climate.

The external thermohygrometer is to just lock out ventilation when the temp or dewpoint is very high.

The whole house dehumidifier / control strategy worked very well in our last house and resulted in not only a more comfortable house but also lower utility bills. That house did not have an external air intake. It was built in 1978 and was plenty leaky. Our new house in comparison is quite well sealed for Houston and has been blower door and duct blaster tested. The lesser of .25 cfm or less per square foot of envelope area at 50 pascals or ? 2012 IECC ACH50 target in jurisdictions that have adopted the 2012 IECC code.

Air quality is so much influenced by the individuals in the building that what they do should feed back into required ventilation rates. Two elderly people living in a 25000 SF house, who do not smoke or have smoking guests, tend to maintain a cleaner air supply that generates less need for air exchange.

Good an timely debate! We are working on a passive house project in the hot and humid (relatively) climate of Austin. Our HVAC as designed has a VRF system with dedicated dehu and a second duct line with an ERV that exhausts from the bathrooms continuously and returns to the bedrooms. In order to certify under the local green building program, we are required to put in exhaust fans in all bathrooms that are on timers of humidistats, but there it seems that there are conflicting schools of thought as to whether or not that would be a good thing. Aside from additional costs of installing those vent fans and penetrations, we would be either depressurizing the house when they are on and sucking in hot wet air anyway, or adding the cost of installing make up air systems to go along with them and sucking in hot wet air.

I'm reminded of a post on here from this winter that really changed my thinking about bathroom exhaust and humidity and it seems to me that having a high performance system with a dehu would prevent any really issues from occurring even if we have occasional high RH situations from a long, hot shower.

@Trey, bath fan timers are good. Humidistats are good in theory to prevent over-ventilation but I don't specify them due to accuracy and reliability issues.

you wrote: "Aside from additional costs (for) vent fans and penetrations, we would be either depressurizing the house when they are on and sucking in hot wet air anyway"

The risk associated with using ERV as primary bath exhaust is much higher in cold weather (and in colder climate zones) where tight construction + ERV already yields elevated wintertime RH. But keep in mind that even in hot humid climates, shower exhaust will have a higher dew point than the outside air, so using ERV to exhaust during a shower adds more moisture back than spot exhaust.

The reason I brought this up was in response to Dave Y, who is experiencing high moisture loads when his ERV operates. It's just another moisture source that must be considered when diagnosing excess moisture issues. Appropriately controlled spot exhaust for showers puts a lower moisture load on the building than an ERV.

To the extent that we want to reduce a home's energy footprint, it makes no sense to use a dehumidifier to remove additional moisture that could have easily been avoided in the first place.

Thanks David. In that case, would you recommend a bathfan with a timer hooked up to a make-up air system, or do you think that the temporary depressurization is not an issue?

@Trey, since you mentioned yours is a 'Passive House' project (e.g., super tight), you'll almost certainly need a makeup air source, mostly for the range hood. A single makeup vent will serve kitchen, baths and laundry. Ideally it should be barometric, meaning that it won't open until the house reaches a negative 4 or 5 Pascals, thus avoiding becoming a source of infiltration. (If someone knows who makes one of these, please post here. The one I used to specify is no longer produced.)

In general, there are two arguments for makeup air: a) a makeup air vent allows for filtration rather, otherwise it's pulled in though building cavities (or worse, the garage); and (b) it's important to avoid negative pressure in hot-humid climates, as that could cause condensation on the back side of drywall, especially where supply diffusers are directed toward the wall (a bad idea in any case).

It's easy to test whether makeup air is needed once the house is fully sealed. I'm not sure what Passive House requires along those lines but I recommend zonal pressure diagnostics for all homes, especially super tight homes. If a enclosure is leaky enough to not require makeup air (range hood + clothes dryer + bath fan don't pull a negative 5 Pascals), then a makeup air vent won't fix what's already broken!

@David, we will have a make up air duct with a motorized damper and a current sensing relay that is connected to the kitchen vent hood. Im not sure if we could connect that same damper to the bathroom vent hoods as well or exactly how that would work but will have to look into it. I agree that we would want some sort of make up air if we do have bath exhaust fans.

I'm still not convinced that we need the bath fans/additional make-up air since we will be exhausting continuously from the bathrooms anyway with the ERV. Without them we might have occasional positive enthalpy exchange (which would be taken care of by the dehu) but with them we would have added loads from the make-up air.

Thanks for your feedback on this, BTW, much appreciated :)

Here is another issue with exhaust fan depressurization in tight homes. Drain traps. In a previous job, we built some research houses that were quite tight. We put in quiet, high-performance bathroom exhaust fans. They were so quiet that they often got left on. Whenever that happened, I noticed sewer gas smells. The house was unoccupied, so I had to keep adding water to the drains to keep the traps full, but I would still get that sewer smell when the bath fan was left running. I finally figured out that it was coming from a floor drain in the mechanical room. Pouring water in it did not help, so I contacted the plumber and he admitted that he did not install a trap on that drain, since these types of drains are rarely used so would likely be dry. He was intending to put a rubber "dry trap" (one-way check valve) in that drain, but he forgot it when he trimmed out the house. He came back and installed it and the problem went away. The moral of the story is that drains can be your source of make-up air if you are not careful. The second moral of the story is that there may be an advantage to having noisy bathroom exhaust fans.

@Trey, you may not need MUA for bath fans (you'll need to test for that), but conventional clothes dryers typically pull > 200 CFM, so you'll need to tie that to the damper control (or use a barometric damper).

I'm building a new home in the lower desert in Arizona. No gas products. All electric. Is this ventilation even something I have to worry about? Seems to me to not make sense to make the house supertight, then bring in outside air when I will have zero gas and zero LP appliances.

Debbie, the whole-house ventilation requirements are not based on cooking or gas combustion processes. They are primarily based on occupant and building material contaminant sources. There are separate exhaust ventilation requirements for cooking, regardless of whether it is gas or electric. I commend you for building your "super tight" house without any combustion appliances, but research shows that cooking with electric also generates significant contaminant levels, especially with electric ranges with high temperature heating elements. If you want to really minimize your exposure from cooking, you might consider the newer induction cooktops, but Std. 62.2 still requires exhaust ventilation due to the other food-generated contaminants. I guess the bottom line is that it is OK to eat that stuff, just don't inhale it.

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