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Musings of an Energy Nerd

Are HRVs Cost-Effective?

Compared to a simple exhaust fan, a heat-recovery ventilator saves energy — but it probably won’t save enough to justify the high cost of the equipment

A high-quality ventilation system isn't cheap. A residential ventilation system that includes a Zehnder heat-recovery ventilator (HRV) like the one in this photo costs more than $5,000.
Image Credit: Photo: Martin Holladay — Bar graph and table: John Semmelhack

From 1977 (when the Saskatchewan Conservation house was built) until 2004 (when the first U.S. Passivhaus was built), North American builders completed hundreds of superinsulated homes. In those days, anyone interested in rating the performance of these homes was probably interested in just one metric: annual energy use.

Over the last eight years, however, with the increasing attention paid to the Passivhaus standard, some builders of superinsulated homes are walking along a narrower path. Any builder interested in achieving the Passivhaus standard soon learns that a low energy bill is no longer sufficient to gain accolades.

Unexamined postulates

This narrow Passivhaus path has several restrictions; I call them “unexamined Passivhaus postulates.” Like postulates in geometry, Passivhaus postulates need not be proven; they just are. Here are four of the postulates:

  • It makes sense to deliver space heat through ventilation ductwork.
  • It’s more important to achieve 15 kWh/m2•year and 0.6 ach50 that to calculate whether these goals are cost-effective.
  • The output of PV system should not be considered in one’s annual energy calculations.
  • Every house needs an HRV.

These Passivhaus postulates are not equally binding; for example, North American designers have chosen to ignore the postulate that affirms that space heat should be delivered through ventilation ductwork. (Although the principle is widely ignored, it is still prominently featured on the Passipedia page that establishes the definition of a “passive house.”) When I interviewed Dr. Feist in December 2007, he used the same definition for a Passivhaus that is enshrined on Passipedia: “As long as you build a house in a way that you can use the heat-recovery ventilation system — a system that you need anyway for indoor air requirements — to provide the heat and cooling, it can be considered a Passivhaus.”

Each of the four postulates listed above deserves to be…

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  1. user-723121 | | #1

    Exhaust Only
    HRV/ERV is a balanced system with both supply and exhaust hoods. Exhaust only as I understand it, short of fresh air ports, pulls air through the building envelope leaks to balance the pressure. This air is not filtered and could also cause moisture accumulation in building assemblies.

    When one is spending several hundred k on a new home, an extra couple thousand or so for a controlled system may be money well spent.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Doug McEvers
    You disparage exhaust-only systems because they pull air through building envelope leaks "short of fresh air ports." But if you read my description of John Semmelhack's modeling exercise, you'll note that he assumed that any exhaust-only ventilation system includes passive air vents.

    You are incorrect when you assert that exhaust-only ventilation systems "could cause moisture accumulation in building assemblies." As Bruce Harley has written, “The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effect. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes."

    Semmelhack assumed that the air flow of the exhaust-only ventilation system under consideration was only 64 cfm -- even less than the systems Harley was discussing.

  3. user-626934 | | #3

    Response to Doug McEvers
    I don't disagree with your last sentence - for high-performance homes in the "several hundred thousand dollar" range, a fancier ventilation system may be appropriate for reasons other than energy savings (better control, better filtration, etc.). For more affordable housing, the $2,000-3,000+ savings for a simpler ventilation system can be a big deal.

    Yes, exhaust ventilation systems will pull air through building assemblies in all houses. In cold climates, this is usually considered OK, since the outside air generally contains less moisture than the indoor air. In very tight houses that use passive air inlets, the majority of the incoming air will come through the inlets under most conditions.

    I'm certainly not advocating exhaust ventilation systems for all climates and all houses. But I do think they make a lot of sense in many situations.

  4. user-626934 | | #4

    Passive House certification clarification
    Regarding Passive House certification (at least through PHIUS) - there's certainly no requirement for using an ERV/HRV. However, even in mild climates, using an exhaust ventilation system, for example, would require large increases in assembly insulation (over and above typical PH insulation levels!) in order to make up for increased ventilation heat losses and meet the annual heating demand portion of the standard.

  5. user-626934 | | #5

    It's worth noting that Katrin
    It's worth noting that Katrin Klingenberg at the Passive House Institute U.S. (PHIUS) encouraged me to dig into the details on this topic, after I discussed with her some initial findings. I expect the results will influence (to some degree) the discussions over whether/how to modify the Passive House standard for North American climate zones.

    [Editor's note: See Katrin Klingenberg's posted comment on this page -- Comment #50, below.]

  6. BrianHlud | | #6

    HRV much better IAQ, comfort. Passive inlets ?
    Great info from John. I'm glad Martin finally mentioned the HRV benefits as I don't see a comparison between an HRV vs exhaust only when talking comfort and IAQ. An Hrv/ERV install (at least ours) have fresh air into all living spaces and exhaust from baths, kitchen , laundry, usually 6 on each side and more for a conditioned crawlspace (2500 SF home). That's a robust system providing great IAQ. I can't see a few continuous bath fans with a few passive inlets comparing to that kind of system. Some rooms without pressuring (bedrooms) will not see as much air exchange. In these homes the HVAC is not coming on very much to move air if that is the strategy. Additionally, as a lot of these homes are going towards point source these even less air movement. None the less, great information.

    As far as passive inlets, I'm curious where you would install ? In my cold climate on a winter day, there will be comfort issues.

  7. subahn | | #7

    Exhaust only problems with bedroom ventilation and combustion
    While I strongly agree that strict rules in building often lead to irrational results, I also think that too often computer simulations ignore (or the people interpreting the simulations) ignore the practical variables of real life.

    For example, my wife and I have a well insulated and air sealed 550sqft home that didn't have ventilation. After our first year we ripped out our furnace and installed underfloor hydronic and wood stove with the combustion air vented to the outside. An unexpected consequence was that our bedroom became more "stale" than usual in the heating months because we didn't have the air circulation from our forced air system.

    To improve air quality and manage bathroom moisture issues we installed an 80 CFM bath fan controlled to run 50% of the time every hour. We never had a steamed up mirror again, but we had two problems:
    1) Our bedroom air quality only seemed to improve if we kept our bedroom door open for good diffusion. I think it is very important to have fresh air in bedrooms (since that's where we spend most of our time) and I question how well exhaust only solutions typically deliver fresh air to bedrooms in real-life, especially in bigger homes than ours. Maybe someone here has a study to back me up on this idea?
    2) We had to remember to turn off the fan whenever we opened our wood stove or we would have a room full of smoke. This was very annoying (and I suspect dangerous for some folks that could experience backdrafting issues).

    I believe a balanced ERV system could fix both the wood stove problem and provide warm fresh air where we need it most. A builder's bottom line shouldn't just be about energy usage/cost (the ERV vs. PV question), but it also needs to think about how the house can adapt to the needs of all the future occupants. Also, I think ERVs could be installed for less than $2,000 in existing homes using the existing forced air ducting.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Brian Hludzinski
    For those who want the best ventilation system that money can buy, and who can afford to pay for it, you'll get no argument from me if you choose to install an HRV. The systems work great.

    For those concerned about getting enough fresh air in bedrooms (sometimes a problem with exhaust-only systems), it should be pointed out that supply-only systems don't have that problem. The most common type of supply-only system is a central-fan-integrated supply ventilation system equipped with a FanCycler. Such systems are commonly installed on homes with forced-air heating systems (a type of heating system that Dr. Wolfgang Feist does not like, but one that is very affordable in the U.S.).

    Like exhaust-only ventilation systems, supply-only ventilation systems are considerably cheaper than HRV-based systems with dedicated ventilation ductwork.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Kristopher Steege-Reimann
    Clearly, your choice of an exhaust-only ventilation system in a tiny house with a wood stove was a poor choice. Your backdrafting problems were predictable.

    There is no substitute for whole-house thinking when it comes to designing a ventilation system.

  10. user-626934 | | #10

    Response to Brian Hludzinski

    Relatively poor distribution of outside air in most homes with exhaust ventilation is fairly well known, based on tracer gas experiments in "leaky" homes without passive air inlets. See for example -


    However, in tight homes with passive air inlets, one study (from Sweden) showed satisfactory distribution of outside air. See - I'll be doing my own (less sophisticated) testing on an upcoming house and will try to report back the results.

    Regarding comfort issues - I would argue that register location for any outside air distribution system (whether HRV or passive inlet) is more important than the delivery temperature. 65F air blowing over an occupant's skin is not comfortable. 10F air is, of course, even worse...but both can be mitigated with good register placement. High on a wall with a diffuser that throws air toward the ceiling and/or to the sides should allow good mixing before the outside air reaches the occupancy zone. Also remember - the airflows we're talking about for ventilation are really low...typically no more than 20cfm even for a master bedroom. For an example - see John Straube's presentation (esp. page 11) on the Dorset Street multi-family building in Waterloo - I wish I could have been at "Winter Camp" to ask him if there have been any comfort complaints.

  11. GBA Editor
    Martin Holladay | | #11

    Fresh air distribution in homes with exhaust-only systems
    When it comes to fresh air distribution, at least one ventilation study (Robb Aldrich, Steven Winter Associates, Chicago, 2005) found that an exhaust-only ventilation system performed just as well as an ERV system.

    A report on the study can be found online: Whole House Mechanical Ventilation. In that report, Robb Aldrich wrote, "According to the [CO2] histograms, it is the Air Cycler [central-fan-integrated supply system] and Exhaust systems that have the tightest distributions with the lowest CO2 concentration peaks. Table 2 confirms this, showing the Air Cycler system has the least variability in CO2 range followed by the Exhaust system; the ERV system has the highest variability. Because of the variability in occupant habits, however, SWA cannot definitively label a system as “better” or “best.” SWA can say qualitatively, however, that no system performed poorly in these homes in distributing fresh air."

  12. Tedkidd | | #12

    Having trouble seeing the math on ERV vs vent.
    I like the climate charts! That's really helpful. But I'm having trouble parsing out the incremental cost/incremental benefit analysis. It does not appear clearly stated.

    So, if I go to the total energy cost and see roughly 1000 KWH savings and call that $100 a year, then go to the $2000 incremental cost, I'll call that 5% annualized after tax. To me that looks like the increment pays, but to some it may not.

    (Readers please note - having recently been faced with servicing a bunch of bath fans, the idea of one mechanical device has unfair bias right now.)

    If we went South I'm thinking pretty quickly the increment won't pay, at least until you hit hot latent land. BUT once in that climate, cost benefit be damned I'm going ERV. I don't want a continuous depressurization situation. I'd be very afraid of rotting building components.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Ted Kidd
    The tiny air flows required for a mechanical ventilation system -- 56 cfm to 64 cfm in Semmelhack's study -- aren't going to rot anyone's envelope.

    But if you prefer to pressurize your house, you could install a supply-only system for a lot less than an ERV.

  14. Expert Member
    CARL SEVILLE | | #14

    HRV ERVs and energy models
    Thanks for the great analysis. Although I agree with the issue that HRVs and ERVs in many cases don't provide the roi to justify the cost, here in the south we try to avoid exhaust only ventilation to keep hot humid air out of the house and building cavities. One typical alternate is providing air through outside intakes into return plenum of hvac system with a mechanical damper and timer. In my experience these are costly, inefficient due to using hvac blower motor, and are often poorly installed. They also extract a penalty on the HERS index while ERV and HRV can reduce the index by several points. Whether or not this accurately reflects energy consumption is another discussion. Given the fact that most certification programs incent low hers ratings and problems with alternatives, I am, at least for the moment recommending ERVs to my clients.

  15. user-626934 | | #15

    Exhaust ventilation in humid climates
    I came across an interesting tidbit of information on the topic of exhaust ventilation in humid climates during my research. Up until last year, the ASHRAE 62.2-2010 ventilation standard specifically restricted exhaust ventilation systems to no more than 7.5cfm/100ft2 floor area in the hot, humid climates (1, 2 and the lower portion of 3, I think), with the same restriction on supply-only systems in very cold climates. However, the 62.2 committee fairly recently (late 2011?) approved an addendum that removes these restrictions and included the following rationale: “The committee reviewed Section 4.6, “Restrictions on System Type” and decided the restrictions were not justified by recent field experience. There was general agreement that the problems in both hot/humid and cold climates were caused by specific and easily avoidable errors in envelope design that could not be solved by the system restrictions in Section 4.6.” While this quote is far from a ringing endorsement of exhaust ventilation systems in humid climates, it would seem to imply that as long as you "don't do stupid stuff" with your building envelope, a house with exhaust ventilation in a humid climate will probably not experience assembly failure caused by ventilation depressurization of the house.

  16. user-1140531 | | #16

    How are the passive inlets
    How are the passive inlets detailed? It seems like if you had enough of them, you would get some heat exchange where the inlet comes through the wall. But that may be a hard needle to thread.

  17. Mike Eliason | | #17

    a few thoughts...
    1. if we're

    a few thoughts...

    1. if we're going to talk about cost effectiveness of PV v. HRVs, then we need to be honest about full cost (subsidies, credits, maintenance, etc). there was a session in hannover on centralized HRV units in a PH having significantly lower maintenance costs over decentralized units, noting that there are other associated costs of mechanical systems other than initial outlay. also, john's cost numbers will change (significantly for some regions) if/when a carbon tax is initiated.

    2. this HRV discussion doesn't hold as much accuracy when comparing to larger buildings (esp. MFH/institutionall/commercial) - where mechanical systems are routinely oversized, even without utilization of HRV/ERVs. i believe adam cohen discussed this topic at the PHNE event last month. the PH argument really needs to be spread beyond detached single family housing if its to really have any impact, because it's less costly and easier to achieve on larger buildings - which, by the way, tend to have higher EUIs (thus, PH = significantly realized savings).

    3. i need to look back at my notes/conference proceedings, however there were several presentations on PH in warm/humid climates - and there were several locations identified as being appropriate for extract air systems over HRVs. 'happy climates' if i recall correctly (san francisco?)

    4. utilization of passive vents in some areas can lead to health issues where there's poor air quality (so again, associated costs of system).

  18. GBA Editor
    Martin Holladay | | #18

    Response to Kristopher Steege-Reimann
    You wrote, "You claim that when it comes to fresh air distribution exhaust only ventilation will perform just as well as an ERV system."

    Actually, I made no such claim. In the article, I wrote, "Defenders of HRVs will probably ... point out that an HRV provides better comfort and delivers fresh air more evenly than does an exhaust-only system. This is undeniable."

    In Comment #9, I wrote, "Getting enough fresh air in bedrooms [is] sometimes a problem with exhaust-only systems."

  19. GBA Editor
    Martin Holladay | | #19

    Response to Mike Eliason
    You wrote, "If we're going to talk about cost effectiveness of PV v. HRVs, then we need to be honest about full cost (subsidies, credits, maintenance, etc.)"

    I agree. The cost figures used in Semmelhack's analysis were unsubsidized costs, for both HRVs and PVs. His conclusion that PV systems are a better investment than HRVs holds true for unsubsidized PV systems.

    From my experience, HRVs definitely have maintenance costs over 20 years (filter changes and repairs). PV modules don't, but interters may. (My first inverter lasted 19 years.)

  20. subahn | | #20

    Response to Martin Holladay
    "When it comes to fresh air distribution, at least one ventilation study (Robb Aldrich, Steven Winter Associates, Chicago, 2005) found that an exhaust-only ventilation system performed just as well as an ERV system." That post was what I was responding to. I didn't mean to mis-represent you.

  21. user-874928 | | #21

    General Response
    I was in Denver, and attended John's presentation. There are several flaws in the metrics of the presentation, and I am preparing a detailed response. There are also studies being conducted to address this entire discussion with DATA. It is data that is missing here. Modeling, modeling, modeling. What we think will happen. I also attended Robb Aldrich's presentation, which he made at the EEBA conference in September. That study also lacks complete data.

    Martin, I would encourage you to attend a presentation coming up at the NESEA conference in Boston in March. Eberhard Paul from, you guessed it, Germany will be presenting on H/ERVs, and about IAQ in homes. He has done more study and data collection on the subject than anyone I know, and I think that you will find that his data and studies will have you re-think things. One study that he did concludes that opening windows is a better combined solution for IAQ and energy efficiency than using bath fans, even in cold climates.

    More to come.

    Barry Stephens

  22. curtkinder | | #22

    A couple $200 Panasonic
    A couple $200 Panasonic WhisperGreen 80 CFM bath fans set to exhaust 30 CFM constantly at 4 Watts each is looking better and better!

  23. user-1119494 | | #23

    Watching out for perverse incentives
    Amory Lovins talks about spherically perverse incentives & I think that you are doing us a great favor by looking at things that may make sense from one vantage, but be unproductive from another: not spherically perverse, but not spherically sensible either.

  24. user-1087436 | | #24

    What about a Panasonic ERV?
    Talk about cheap and simple. But is it relevant? I'm looking specifically at the "Panasonic WhisperComfort - (ERV) 40/20, 20/10 CFM, 0.8 sone APPA04VE1," which is available at Wal-Mart for $353. Supposedly it can be a Spot ERV or a whole-house ERV, though I'm not sure how the latter would work. This is not an exhaust-only fan, obviously, and it supplies fresh air without an expensive ducting system. Used in a master bath, could it solve the problem of stale bedroom air (in a master bedroom, at least), while a separate exhaust-only fan could be mounted in another bathroom? I'd love to see some comments on this.

  25. GBA Editor
    Martin Holladay | | #25

    Response to Gordon Taylor
    The advantage of the WhisperComfort is that it provides some heat recovery. The disadvantage is the fresh air is delivered to the same room that the exhaust air is pulled from.

    Many people with small homes or apartments are perfectly satisfied with the WhisperComfort. I have also heard of larger two-story homes with one WhisperComfort on each floor.

    While a traditional ERV has four ducts to the unit, the Whisper Comfort has just two -- an exhaust duct and a fresh air duct. That simplifies ducting.

  26. GBA Editor
    Martin Holladay | | #26

    Response to Barry Stephens
    Like you, I welcome more data, and look forward to hearing more from any disinterested third-party researcher who presents good, solid data. I plan to attend the NESEA conference in a Boston.

    GBA readers may be interested in the fact the Barry Stephens is the national sales director for Zehnder, a manufacturer of HRVs, while Eberhard Paul is the owner of a company that manufacturers or distributes HRVs. When we consider data, we need to look at it with a critical eye, and consider not only the numbers, but also the source of the numbers.

    I mean no disrespect by pointing out these facts. But most of us pay more attention to a Lawrence Berkeley National Lab study than a study sponsored by Venmar or Zehnder.

  27. user-723121 | | #27

    Response to John Semelhack

    My intention was not to diminish the work you have done on this, my speed reading missed some key points as usual. I agree with you, an HRV/ERV is not cost effective in moderate climates. When exhaust only systems are specified, I believe the passive inlets should be used in conjuction. Fresh air in bedrooms is a concern for me and I have doubts about a single bath fan without passive inlets supplying this. I believe the MN code for ventilation air for a house is 15 CFM per bedroom plus 15 CFM. A 3 bedroom house would require 60 CFM continuous.

    As you stated, cold climates have special considerations and heat recovery ventilation pencils out. I had heard a few years back about some slab on grade townhomes in the Twin Cities with exhaust only systems being very uncomfortable and expensive to heat. The comfort isuue may have been due to poor air sealing and and an improper slab insulation detail.

    60 CFM is a lot of fresh air and in a 7,500 hdd climate would take 116.64 therms of energy annually to bring up to room temperature. A 75% efficient HRV would save 87.5 therms on a yearly basis less the extra energy to run the HRV. With natural gas as the heating fuel the ROI is borderline in this climate, a better payback when heating with electricity, propane or heating oil. I do like having fresh air delivered to every room in the house via the HRV, we use remote timers in the bathrooms and eliminate the individual bath fans.

  28. user-626934 | | #28

    Response to Mike Eliason
    Regarding PV cost - my estimate was very conservative - $4,500/kW with no subsidy. Current prices in my area for medium-sized systems (5-6kW) are coming in at $3,800-3,900/kW. The Germans are apparently installing residential systems for $2,240/kW! (

    Yes, my study was only geared toward single-family houses, and I modeled a relatively small (for us) house. For larger houses with higher ventilation rates, the economics will change. More ventilation flow = more energy savings from heat recovery. The unfortunate situation for designers of small, low energy homes on a budget is that the available high performance HRV/ERV equipment is oversized (max. flows of 200+cfm) and comes with a price tag to match. A hypothetical product with a peak flow of 100-120cfm (and a fan energy “sweet spot” of 50-60cfm), along with a reduction in price could make the difference in terms of economics for small projects. I mentioned this in my presentation at the conference.

    In terms of bigger buildings – yes, the economics are different…in some cases drastically different. In commercial buildings with high ventilation rates, ERV/HRV’s can displace large amounts of cooling/heating equipment cost….enough to almost make them pay for themselves in upfront equipment savings. In these cases, it’s a no-brainer! In multi-family buildings, I think it’s a mixed bag. It depends a lot on occupancy density/airflow rates, and whether or not a central system with little/no occupant control is feasible. In condominiums, I expect owners would not be willing to give up control over their ventilation rates. In tenant housing – maybe it’s OK. There can be metering issues as well. For instance, in the multi-family housing in my area most units have their own electricity meter, which makes it harder to justify a central system.

    Regarding outdoor air quality – this is an issue for any ventilation system, balanced, exhaust or supply. Yes, currently marketed passive air inlets have low performance filters, but I don’t see any reason why this issue couldn’t be addressed. Most ERV/HRVs also come standard with low performance filters. The only unit that I’m aware of that comes with a standard high performance filter is from Ultimate Air (MERV 12).

  29. subahn | | #29

    Response to Martin Holladay
    You claim that when it comes to fresh air distribution exhaust only ventilation will perform just as well as an ERV system, but I still am skeptical because I believe that closed doors prevent fresh air getting to where it needs to be (and away from where it shouldn't be, like an unoccupied basement) and ERVs will tend to be ducted for better fresh air distribution.

    For example, I've noticed that if I leave my bedroom door closed for the night there is a tendency for room to have noticeable body odor and more condensation on the windows than if the door is open (room doesn't get colder with the door shut with in floor heat).

    Also, when doing a blower door test or testing the bath fan, you know that opening or closing doors can significantly effect air movement. In my life, I see this when I take a shower. If I leave the door closed I end up with a steamy mirror and if I leave the door cracked I get much better ventilation.

    The bathroom humidity problem wouldn't be solved with a ERV, (I probably need to cut a larger gap at the bottom of the door), but I think it demonstrates that things like partition walls and doors can significantly affect how air is distributed through a house. Also, here's a link to a study about the return pathway of air with exhaust fans:

    And a serious (maybe stupid) question: If an exhaust fan provides fresh air distribution through the entire home, then wouldn't you be wasting significant energy by providing fresh air to an unoccupied basement, especially in a single story home where a basement can be 50% of the home's volume? Or do basements need to have ventilation anyways?

    EDIT: This is the post I was responding to. Didn't mean to mis-represent Martin: "When it comes to fresh air distribution, at least one ventilation study (Robb Aldrich, Steven Winter Associates, Chicago, 2005) found that an exhaust-only ventilation system performed just as well as an ERV system."

  30. GBA Editor
    Martin Holladay | | #30

    Comments from Max Sherman
    Max Sherman, senior scientist at Lawrence Berkeley National Laboratory and former chairman of the ASHRAE 62.2 committee, sent me an e-mail with comments, along with permission to post his comments here.

    Max Sherman wrote:

    “Interesting discussion and I generally agree with you.

    “In using the system that our passive friends want, you do not exhaust contaminants from bathrooms and more especially from range hoods. Cooking is probably the single most polluting activity done in homes and we do not want to recirculate the exhaust stream, we want to, well, exhaust them. Putting that air through an HRV (or far worse and ERV) means that a substantial portion of those nasties do not get exhausted.

    “An HRV pulling from a bathroom is OK as long as it does not leak too much. An ERV pulling from a bathroom is going to recycle the moisture one is trying to exhaust and therefore most redistributes rather than exhaust moisture (and odors, etc.). The situation is much worse for the kitchen as the capture efficiency of the cooking contaminants is terrible without a range hood.

    “ASHRAE 62.2 removed the prohibition against exhaust ventilation in hot, humid climates. The reason is because the small amount of mechanical ventilation (e.g., 40-90 cfm) is not going to make a wall rot unless the wall is made very poorly. Even with a balanced system half the wall is going to have some infiltration because of stack and wind effects.

    “Joe Lstiburek was the leader of the removal of that provision from ASHRAE 62.2 because he says it’s about the wall design, not about a piddly amount of exhaust flow.

    “Now, if you have a 1,200 cfm kitchen exhaust fan, as some high-end homes have, that is potentially another story, but nevertheless it [potential damage] is still about the wall.

    “Passive houses can be so tight that they cannot manage even a reasonable amount of exhaust flow. This is a separate issue -- that their spec is foolish.”

  31. Mike Eliason | | #31

    'Passive houses can be so
    'Passive houses can be so tight that they cannot manage even a reasonable amount of exhaust flow. '

    granted, i'm not an LBNL scientist, but this is contradicted by all the literature and data i've ever reviewed, as well as personal experience visiting and staying in passivhaus buildings. quite the opposite, in fact. i've always been impressed at how clean and fresh the air quality is in a passivhaus compared to other buildings (especially commercial). RH appears to be more stable in a PH than in code built and existing houses as well.

    in every comparison i've seen between a PH, code and existing building, the CO2 concentrations of the PH have been far better than others. here's one recently published from scotland:

    monitored community center retrofit showed CO2 concentration at 750ppm or less 80% of the time, very little if any above 1350ppm.

    i wish the PHI would translate their papers on monitoring rehabilitated projects because those are a minefield of data as well (and again, i've yet to see one project where CO2 concentration post PH Rehab is worse).

    there is a developer of dormitories for foreign students in vienna that was so enthralled at how the moisture/mold and food odor issues that had plagued his previous buildings weren't an issue once he built a passivhaus. only building PHs now.

    there have been a number of retrofits where the improvement from air quality in moving to passivhaus was so great, folks were able to stop taking medications they had been on for years - a number of different sources on this, most recent example i can think of are a few clients of bere architects (UK):

    this is where guenter lang's 'sex in a Passivhaus is safe sex' riff comes from.

    also, is sherman thinking that range hoods aren't used at all?!? it's actually the inverse in much of europe, where ange hoods aren't the norm (and even in the US, recirc hoods are something like 80% of installations).

  32. GBA Editor
    Martin Holladay | | #32

    Response to Mike Eliason
    For the record, I agree with you on all the points you made.

    I agree that Passivhaus buildings tend to have excellent interior air quality -- because of their tight envelopes and high quality ventilation systems.

    I'm not sure, however, whether the publications of PHI are a minefield of data or a goldmine of data... perhaps both... Wade in at your own risk!

  33. user-626934 | | #33

    Follow-up with Mike Eliason regarding multi-family buildings...
    In climates where clients are willing to forgo cooling systems AND the winter design temperatures are relatively mild (I'm thinking Portland and Seattle, for instance) the peak heat load in multi-family buildings with HRV/ERV systems can be low enough that ALL of the space heating energy can be delivered via the ventilation air (without increasing the ventilation rate!). This, of course, is the classic PH space heating strategy - high performance HRV/ERV, high performance domestic water heater tank, a small pump and a water to air heat exchanger to move heat as needed from the tank to the ventilation airstream. Maybe $8,000-10,000 installed for everything (ventilation, space heat and DHW)? These situations are where the economics start to swing back in favor of HRV/ERV, in my opinion.

    On the other hand, this concept is much more difficult to achieve for buildings/clients that require space cooling.

  34. user941025 | | #34

    exhaust-only at cold temps
    Martin Holladay wrote:

    "The advantage of the WhisperComfort is that it provides some heat recovery. The disadvantage is the fresh air is delivered to the same room that the exhaust air is pulled from."

    Another disadvantage, if we're discussing the ERV's advantage in cold climates, is the WhisperComfort's function as exhaust-only below 20F. "Frost-prevention mode." See attached image.

  35. user-874928 | | #35

    Response to Martin
    There will be a lot more data coming out on this subject shortly. The NorthWest Energy Efficiency Alliance has commissioned a residential ventilation study, and Washington State is doing the study. So your third party data and study is under way. I expect that 2013 will be a very interesting year for ventilation!

  36. GBA Editor
    Martin Holladay | | #36

    Response to Barry Stephens
    That's excellent news. GBA looks forward to publishing new data on this issue, so you should feel free to e-mail me with any developments: martin [at] greenbuildingadvisor [dot] com.

  37. greenophilic | | #37

    A few thoughts on HRV/ERV, IAQ and ventilation efficiency
    First of all, HRV/ERV provide the least efficient spot ventilation possible. For example, when a shower occurs and a humidistat ramps up the unit to high airflow, the ventilation rate of the entire home is increased, rather than the ventilation rate of the room with the pollutant. This provides exceptionally low ventilation efficiency (mass pollutant removed/cfm). Similarly, kitchen ventilation using an HRV exhaust in the ceiling is not sufficient; once again, ventilation efficiencies are horrible. And even worse, essentially none of these systems ramp to high in response to cooking in the way they do to humidity events. My main problem with these systems is the belief that a kitchen range hood exhausted to outside can be eliminated if a fully ducted HRV is installed. The kitchen is one of the primary sources of pollutants in a home (especially one with low emitting materials), the bedroom is not. Even distribution of ventilation air is not necessarily desirable, rather you want ventilation when and where it is needed...and then some low rate for general purpose.

  38. Bronwyn Barry | | #38

    Local Superinsulation Club meeting
    I'm just catching up on this post now after a local Superinsulation Club meeting at the SF Elks Lodge. It took me a while because of the ball and chain I drag behind me. (Mine says 'made in the Netherlands' by the way Martin, because the Zehnder HRV's I've spec'd are actually Dutch.)

    Great post by the way. You've knocked this one out of the ballpark. All that data included in the hypothetical study (by a Passive House postulate given at a Passive House conference) has scored huge points for the Superinsulation team. (And everyone should get exactly how Passivhaus and Superinsulation are so different by now.) It has me really thinking hard about spending $2K more on HRV's. All that fresh air I've been spoiling my clients with may be total overkill. I see now I could easily just spec a few alternates (made in China) that just suck. Wow. So much easier and cheaper.

    It's so great to have you really nailing all the unexamined postulates of the Passive House movement too. It's crazy how those folks are all working so hard to build low energy buildings. I mean, high quality products that cost a bit more are so stupid. Nobody should want those. Not even folks who live in the richest country in the world who use the most energy per person, globally. It will be so good when everyone simply goes back to the way it used to be, in the 70's, with you SuperInsulated guys who created such good software for everyone to use. I'm sure it's still available everywhere. Cheaply. Isn't it?

  39. GBA Editor
    Martin Holladay | | #39

    Response to Bronwyn Barry
    Sorry to hear that the ball and chain are slowing you down. (The device on your ankle also appears to be making you a little bit grumpy, unfortunately. But at least you still have your sense of humor.)

    Let's see where we agree. We both like superinsulation; that's good. We both believe that it's a good thing to work hard to help others build low-energy buildings; that's good. We're both a little worried about the fact that our country uses more energy per person than almost any country in the world.

    You raise a new question: one of software. My blog doesn't really discuss the issue of software, but I'm glad you brought the question up. Back in the 1970s, the available energy-modeling software wasn't very good at all. PHPP is much better than anything available back then. That's why John Semmelhack chose to use PHPP to model the buildings in the study you dismissively refer to as "a hypothetical study by a Passive House postulate given at a Passive House conference." Actually, any time that anyone uses PHPP -- even you -- the results are "hypothetical." That's the whole idea behind modeling.

    So, where is the disagreement? It appears to be the issue raised in this blog: whether it makes sense in all climates to pay an upcharge ranging from $2,023 (for an American ERV) to $3,273 (for the Dutch equipment you prefer) for a balanced ventilation system with heat recovery.

    Here's my take: such as system shouldn't be mandatory. It should be up to the client to determine whether to spend that much for the upgrade. You appear to disagree with me on that point, evidently defending the Passivhaus Institut stand that these ventilation systems should be non-negotiable. Fair enough, I suppose. But I remain unconvinced.

    Now I have to head down to the local Elks Lodge to have another beer with my superinsulation buddies.

  40. Ted Clifton | | #40

    Balanced ventilation without HRV
    Great article, Martin, you have finally reiterated what I have been proving for years! I put together a spreadsheet a while back that does the calculations performed by Semmelhack, I have attached a copy hereto. There are a few calculations he missed: 1. The cost of money over time, and 2. The expected inflation of energy costs over time.
    My spreadsheet does not include the inflation on energy costs, which would probably skew the results more in favor of the HRV, but when the comparison with more PV is made, the PV would clearly win every time. The cost of money is included, because regardless of how you spend it (HRV or PV), the cost will be the same.
    What all of you seem to be missing is that there is more than one way to achieve balanced ventilation. We have been balancing our range hood with a powered HEPA filter for years, solving the air quality problems that an HRV does not. The HEPA filter puts fresh air directly into each bedroom, and the exhaust comes from the kitchen/great room. We use a slightly smaller range hood fan than the HEPA fan, to give a slight positive pressure in the house. This partially makes up for the occasional bath fan use. When the bath fans are running, the 8" inlet for the HEPA filter provides the make-up air, not leaks in the envelope.
    Finally, Martin's most important point is being lost in the weeds. The most important point is that the dogma of the Passivhaus system (or any other system not open to innovation) is clearly a detriment to the development of the next generation of homes. We are producing net-zero-energy homes at a cost of less than half of what "passive homes" are costing in our area. In the process, we are also meeting the EPA's IndoorAir Plus standards. Our homes are using far less energy (before counting the PV) than the passive house system requires, without "super insulating" them.

    1. barkingeater | | #110

      What is this HEPA powered fan product?

  41. GBA Editor
    Martin Holladay | | #41

    Response to Ted Clifton
    Thanks for your vote of confidence. As Mao said, let a hundred flowers bloom.

    My intent is not to endorse the Carter Scott approach to building, nor the Ted Clifton approach, but simply to affirm that we need to choose our specifications according to rational principles rather than blind obedience to a system that resembles a cult or some type of religion.

    Especially in light of the many different climates found in North America, rigid rules are unlikely to yield consistently good results in all climates.

  42. user-729621 | | #42

    In Canada...
    In Canada, where HRVs are required by the building code for every new house, you can buy decent ones for 800 bucks. I can't imagine doing a new home where you didn't have such a system, balancing the exhaust and the input.

  43. Lizzieplants | | #43

    Ted's System

    We are building a near passive house and I am interested in your HVAC system with the powered HEPA filter. What is your heating source and do you have AC? What is the brand of HEPA filter system. We are located in Upstate NY so I am wondering if we should or should not install the Ultimate Air??

  44. lutro | | #44

    Does cost per sq ft matter?
    I'm wondering if a house's cost per square foot has any importance in the HRV vs exhaust only cost/benefit analysis. Running some naive numbers, an HRV might have a footprint of 10 sq ft, and construction costs on a passive house might be over $250/sq ft. So that could add $2500-$3500 to the calculated cost of HRV. But is an HRV system really adding 10 sq ft to the size of the mechanical room and to the whole house? And since a sq ft of a mechanical room costs far less than the average for the whole house, what price should we put on the space required for an HRV system?

    Do you think the space calculation has any significance in this discussion?

  45. Bronwyn Barry | | #45

    Smack talk at the Elks Lodge
    Glad you saw the humor in my post Martin. There really is so much misinformation out there about how to build high performance buildings. My main point is that too few of us are doing it, so you may want to ease up on the Jerry Springer comments towards the folks on your own team.

    No doubt there is plenty of B.S. out there, but it is not limited to the Passivhaus community. You've done a pretty spectacular job yourself here of building a whole argument based on incorrect info about Passivhaus: an HRV is NOT REQUIRED to build a Certified Passive House. However, it is recommended for most climates (yours being one of them.) Should you install one, you do need to provide a third party commissioning and balancing report to receive certification.

    Now that we've cleared that up, the informative sections of your post are very interesting, but not substantiated by enough real data for my own taste. I contacted my friend at LBNL, Dr. Iain Walker, to see if he had any solid info on this topic. Unfortunately the only study that includes any real data related to this topic is limited to 10 deep energy retrofits all here in California. Here's what Iain wrote:

    "The homes in our study tended to have ERVs - that have a major drawback because they recycle moisture into the home from the kitchen and bathroom exhausts. An HRV would have been better. The answer is complicated by the ERVs not being operated in an optimum way. For example, we found that they were not set up to deliver 62.2 air flow rates (usually operating at higher total flows than 62.2 required - so ventilating more and using more energy), or to exhaust enough air from kitchens to meet 62.2 (too little exhaust - really needed exhausting kitchen range hoods). Generally they used too much fan power that offset any possible energy savings.
    My opinion is that they are really marginal in our climate and you need to make sure that they are installed well and the controls are set up correctly and you should provide additional source control - particularly in kitchens."

    So, looks like the data so far backs up John’s premise that an HRV/ERV may not be necessary for the California Bay Area. It does not address other climates with higher temperature deltas. It also does not look at the efficiencies of these specific units, duct leakage, actual wattage draw and actual air exchange rates supplies. Ergo: The Jury is still out on this topic.

    Iain did go on to say that LBNL would love to do a more extensive study but is limited (much like the Passive House Institute) by resources of both time and money. I hope that the study Barry Stephens mentioned earlier will shed more light on the subject. I also hope that we see a few new developments from the manufacturer community in the interim, and that the cost and complexity of these units improves.

    Anecdotally, we (One Sky Homes) just installed a 'hybrid' version of the equipment options studied by John Semmelhack in our current Sunnyvale EnerPHit project. We eliminated most of the (expensive) supply ducting for the HRV and are dumping all the fresh air supply into the main living area. This large area is serving as a plenum for fresh air, heating and cooling. The 'plenum' space includes a Fujitsu mini-split unit to meet the home's heating and cooling requirements. We've then installed a Panasonic WhisperGreen fan to push fresh air (plus heating or cooling) to the bedrooms. Both bathrooms are direct extract vented to the HRV. Kitchen hood extracts directly to the exterior. The project is being monitored extensively, so I’ll let you know what the data says on how this system is working when we have enough to share.

    Interestingly, when Davis Energy tested the Panasonic fan, it was drawing well over the 7 Watts claimed on the box. We adjusted the ducting to reduce the flow resistance and it then reduced the power draw on the fan. Logical, but only caught because it was tested.

    All this is to say that a lot more work has to be done. Real data needs to be collected and those of us trying to figure this stuff out have to make sure that all our systems are tested and are working correctly. Getting all frothy at the mouth and telling people that they're wearing a 'ball and chain' is not helpful. Or polite. This is still a free country and nobody HAS to build Passivhaus (yet.) We, and our clients, want to. It is interesting and the PHPP is still the only modeling tool that has been calibrated (that I know of.)

    So now that I know you and your buddies are still having fun at the Elks Lodge, I look forward to an update on this topic when the real data is available. Always good to hear (most of) the smack you and the 'oldtimers' (your description) come up with...

    1. barkingeater | | #111

      how does the wispergreen push the conditioned air to the bedrooms?

      1. GBA Editor
        Martin Holladay | | #112

        I'm pretty sure that the system that Bronwyn is talking about has a WhisperGreen exhaust fan mounted in the living room. It delivers air to the bedroom(s) through a duct.

  46. GBA Editor
    Martin Holladay | | #46

    Another response to Bronwyn Barry
    If the Passivhaus Institut is willing to certify a building that lacks an HRV, that's good news. Someone should tell them that they need to correct the errors on the website that indicate that HRVs are required. If the main websites of a well-known standard-setting institution include incorrect information, I can hardly be blamed for taking the websites at their word.

    Like you, I'm always interested in more data. But the basics of the energy use of exhaust fans and HRVs is not as complicated or unsettled as you pretend. You wrote, "All this is to say that a lot more work has to be done. Real data needs to be collected and those of us trying to figure this stuff out have to make sure that all our systems are tested and are working correctly." But here in my corner of Vermont, we've been installing HRVs since the mid-1980s, so Vermont contractors have a 27 year track record to go on. There are no huge mysteries yet to be solved.

    It's possible to screw up the installation and commissioning of any HVAC appliance, and you are correct that many exhaust fans as well as HRVs are installed very poorly and don't perform as they should. All the more reason to prefer simple equipment over complicated gadgets!

    By the way, the calibration of energy modeling software is a routine part of energy software development. Some software developers do a good job of calibration, while others do a not-so-good job -- but it is simply untrue that "PHPP is still the only modeling tool that has been calibrated." But I nevertheless take off my hat to PHPP -- it's good software.

    The big guns at LBNL when it comes to ventilation are Max Sherman and Iain Walker. Max Sherman posted a comment on this page, “Interesting discussion and I generally agree with you," while you quote Iain Walker as saying, "My opinion is that they [HRVs and ERVs] are really marginal in our climate and you need to make sure that they are installed well and the controls are set up correctly and you should provide additional source control - particularly in kitchens."

    So I don't think your arguments in favor of HRVs and ERVs are getting a lot of support from researchers at LBNL.

  47. Bronwyn Barry | | #47

    There are HRV's, ERV's and HAHR's
    Now you know this is a nit-picky community and not all HRV's or ERV's are built alike. Most of what is currently available on the market should more accurately be classified as HAHR's (Hardly Any Heat Recovery.) These units absolutely don't make sense to install because their recovery rate (some as low as 40%) does not offset the energy expended by their fan motors. Using a broad brush to disqualify all heat/energy recovery units is therefore simply foolish.

  48. GBA Editor
    Martin Holladay | | #48

    Response to Bronwyn Barry
    You're incorrect when you accuse John Semmelhack of using a broad brush. He evaluated the Ultimate Air ERV and the Zehnder HRV -- the two ventilation units that are most often specified by Passivhaus builders -- and he evaluated them using PHPP.

    There was no "broad brush" inclusion of 40% efficient units in his analysis.

  49. Katrin Klingenberg | | #49

    Terrific article and
    Terrific article and discussion, Martin! Thank you!

  50. GBA Editor
    Martin Holladay | | #50

    Response to Katrin Klingenberg
    Thanks very much. I look forward to any developments by PHIUS that might lead to more flexibility in the specification of ventilation equipment.

  51. Bronwyn Barry | | #51

    Not talking about John's study
    I was referring to your comment about the units that Vermont contractors have been installing for the past 27 years. Is there a study on these projects and a spec for the HRV's they installed? I'm not sure LBNL has a study that is that nuanced either, so I hope one will be forthcoming.

  52. GBA Editor
    Martin Holladay | | #52

    Response to Bronwyn Barry
    The first published study of HRV performance that I am aware of is "Heat Recovery Ventilation for Housing," written in February 1984 by the National Center for Appropriate Technology in Butte, Montana for the U.S. Dept. of Energy. Since that time, many researchers have published papers on heat-recovery ventilation. As a former editor of Energy Design Update, I am familiar with many of them, because EDU reported on these findings as they occurred over the last 28 years or so.

    A full literature search and summary of EDU articles on the topic would take a long time to prepare.

    I hope that your reference to "the units that Vermont contractors have been installing for the past 27 years" was not intended to disparage the knowledge or expertise of Vermont contractors. The premier Vermont contractor installing HRVs in the mid 1980s was David Hansen, who is still active in the field today. He is an HRV pioneer, as well as a published author and acknowledged expert on heat-recovery ventilation.

  53. chramiec | | #53

    In The Real World/ Farts Of Yore...
    In 2002 I built a small medical clinic that included an apartment for the doctor to reside in. The clinic had to be divided from the residence by a firewall and the plans examiner wouldn't allow the HRV on the clinic side to penetrate the fire wall so, to save costs, we ventilated the apartment with a single panasonic 80 cfm bath fan on a timer (and a range hood) with a fresh air intake. I think we left the intake closed though and relied on the Milgard windows to leak (for which they are probably quite reliable). The walls were insulated with soy-based spray foam so the shell could be assumed to be pretty tight (although I shudder to think how patients allergic to soy may react upon entering).

    Now, I'm no scientist, but to my senses, the air quality on the clinic side compared to the residential side is dramatically better. When you walk in the clinic the air feels fresh and invigorating. When you enter the residential side you are immediately struck by a staleness that is obvious DESPITE KNOWING THAT BY CODE YOU HAVE SUPPLIED THE REQUIRED AMOUNT OF CFMs. The constant mixing of air from the HRV provides more fresh air at a more even temperature to more locations throughout the building. The bath fan/inlet system short circuits rooms that aren't between the fan and the intake leaving large areas of stale, un-excellent air. There are probably still farts from my painter haunting corners of that apartment. You could put more inlets in but everyone that's not crazy closes them in the winter (and most likely forgets to open them back up in the spring).

    I've almost always built with HRV's since, and the few times I haven't I've confirmed that the air quality provided by the bath fan-only system is way less awesome. The small Panasonic ERV is a decent substitute (air quality-wise) when not trying to achieve 'high performance' in small spaces.

    Cost-wise, I've always been pleasantly surprised with the price of an HRV compared with the result. My own house is another HRV success story. Before I retrofitted it with two small HRV's (with charcoal filters), my timers would kick on in the winter and fill the house with my neighbor's wood smoke (he is a 'green' in that he only burns 'green' logs). Now, I love my neighbor at least 10% more.

    Call me crazy, but I want to keep installing HRV's that are powered by solar panels. Someone please add a link showing the cost benefits of living in a home with great IAQ, un-haunted by farts or yore.

  54. Bronwyn Barry | | #54

    Disparagement Free Zone
    Asking for data does not equal disparagement, at least not on my side of the equation. I have great respect and admiration for Vermont builders, especially one's who installed HRV's before the rest of the world knew what they were and had built really great one's. They were probably also installing incandescent light bulbs in those homes. Amazing how far we have all come, and how far we still need to go...

  55. GBA Editor
    Martin Holladay | | #55

    Response to Bronwyn Barry
    You wrote, "They were probably also installing incandescent light bulbs in those homes."

    I was a Vermont builder during the 1980s, and I can assure you that I was installing CFLs in the homes I built, at least during the second half of the 1980s. I clearly remember ordering two cases of Osram CFLs for a house I completed in 1988. They were reflector CFLs, and the cost $23 each -- a lot of money back then.

    One reason that I remember the details so clearly is that I am still using several of the CFLs from that order. They are now 24 years old, and I still use them every day. Here's a photo of one of the old bulbs, still on my living room ceiling.

  56. Bronwyn Barry | | #56

    That's a great picture Martin. Thanks for sharing it. I'll bet you're now very happy you spent the extra bit of cash for something you knew would last, despite everyone at the time poo-poo'ing them?

    Quality/durability is always the most sustainable choice and I hope that variable can somehow be included in the cost calculations on H/ERV's vs multiple exhaust fans. Coincidently, I've just ordered two new extract only bath fans for my loft condo here in SF. The existing one's don't have easily removable filters. The motors are now so clogged and so damn noisy that they need total replacement. The building is 11 years old. I'm looking at just under $700 for cost and install for these two fans. They've already been replaced once, just before we moved in six years ago.

    Does anyone have any anecdotal info on the life expectancy of their existing H/ERV's? How many of those installed by your Vermont builder friends are still working? That will be interesting to know.

  57. user-1061844 | | #57

    HRV requirement for Passive Houses
    PHI does not require an HRV. IF you read the certification requirements that can be found here: - and this hasn't changed recently. My PHPP 2007 manual doesn't list it as a certification criteria either. Why it is generally recommended is that it will be very hard to get your heating (or cooling) demand below 15kWh/m2*yr without an HRV in most climates.

    The tricky part is that if you would like to get certified (voluntarily) you need to conform to all the PH criteria, which includes comfort - which is defined by ISO 7730 (typo on cert. requirements lists ISO 7720). A good explanation can be found here:: - which means if your supply air is more than 3.5C (6.3F) lower than the room temperature, your building in my understanding would no longer be comfortable and certifiable by PHI as a Certified Passive House. This is why you need an HRV with a high recovery rate in most climates - otherwise those pesky Virginia nights (average low's in January below 0C, 32F) at 75% efficiency, would supply air at dT20*0.75=15C (59F.) You would actually need a post-heater to remain comfortable. With an 85% efficient HRV you would be ok, w/o post-heater. Trickle vents or other passive supply methods would bring in 0C-32F air - quite uncomfortable.

  58. GBA Editor
    Martin Holladay | | #58

    Response to Floris Keverling Buisman
    Thanks for the further information on the requirements for Passivhaus certification. I have edited my article to reflect the information you provided. The information is very helpful.

    I can assure you that I am familiar with the web page that you linked to. On that page, there is a further link, labeled "Criteria for certification of Passive Houses for residential use."

    Clicking that link, one arrives here: "Criteria for certification of Passive Houses for residential use". That is the document that I quoted from in the article -- the document that lists “HRV commissioning report" as one of the “Documents necessary for Passive House certification.”

    So, as I say, I don't doubt the information you provided, but the information provided on the Passivhaus Institut web site is misleading at best.

    However, I'll take the information your provided at face value. According to what you write, builders in most climates find it difficult to use an exhaust-only ventilation system because of the 15kWh/m2*yr limit. Pardon my metaphor, but that sounds like one of the "ball-and-chain" limitations that is the subject of my blog.

    You have shown me that it is very difficult to achieve Passivhaus certification with an exhaust-only ventilation system in most climates. But that doesn't mean that the requirements constraining builders -- pushing them in the direction of an HRV -- make any sense.

  59. hughsdb | | #59

    Thanks for a great post and discussion. It seems that we have almost enough personality and not nearly enough data. I appreciate both. I am still trying to wrap my head around the variables and their variable weight. I'm a hot humid climate, which seems to negate a lot of what is known. A house is a dynamic pressure zone, yet, so often, it is treated as if it were static. Every time a door opens or closes, especially if it is an exterior door, a clothes dryer goes on or even when the wind changes direction or the outside temperature goes up the system changes. To what degree is any ventilation system "smart" enough to take all of these variable into account. It is unclear, in reading many of these scenarios, what the exact set up is and wouldn't this make a huge difference? Is the range hood tied into a ventilation system or does it stand alone? Same with the bathroom. Are there room jumpers that allow air to move easily between rooms with closed doors? Aren't some of these variables almost as impacting as the type of system itself? In fact, shouldn't they be thought of as part of the ventilation package? It seems that that there are three primary things to accomplish with a ventilation system: energy conservation, comfort and IAQ. To what degree is the right system going to be dependent on how you weigh these goals?

  60. GBA Editor
    Martin Holladay | | #60

    Response to Hugh Stearns
    Good questions.

    Q. "Every time a door opens or closes, especially if it is an exterior door, a clothes dryer goes on or even when the wind changes direction or the outside temperature goes up the system changes. To what degree is any ventilation system 'smart' enough to take all of these variable into account?"

    A. This question is easy to answer. I have never seen any residential ventilation system that responds to doors opening, directional changes in the wind, or clothes dryer operation.

    Q. "Is the range hood tied into a ventilation system or does it stand alone?"

    A. Since most range hoods are ducted to the exterior, the exhaust fan in a range hood is part of a home's ventilation system. In almost all homes that include an HRV, the operation of a range hood is independent of the HRV. HRV manufacturers specifically forbid range-hood exhaust ducts from being connected to the HRV.

    Q. "Are there room jumpers that allow air to move easily between rooms with closed doors?"

    A. The answer varies from house to house. To read more about this issue, see Return-Air Problems. In most cases, this is treated as a space-heating problem, not a ventilation system problem -- although of course some homes use space-heating ducts to supply ventilation air. Most Passivhaus buildings don't have a furnace or forced-air ductwork, however.

  61. hughsdb | | #61

    Think about ventilation as a system
    Thanks Martin, for the quick and thorough response. So why isn't the dryer included in the ventilation calculations? After all the hood and bathroom vents can be a consideration depending on how you do the calculations, and the dryer, I think, is pulling between 100 and 150 CFM. And, in most homes, the dryer is running a lot more than bathroom or hood vents. It seems that even if you are not using a balanced system, you want to create as much balance as possible. So, for example, if you are using an exhaust only system, you will improve IAQ if you allow free flow of air between rooms, allowing for more mixing. If you deal with the ease of moving air between rooms in a return air system for space conditioning, why wouldn't you do it for ventilation? Again, I have to ask what we are trying to accomplish by providing fresh air? If the air exchange is only happening in a limited amount of space, are we accomplishing the aim?

  62. GBA Editor
    Martin Holladay | | #62

    Response to Hugh Stearns
    Q. "So why isn't the dryer included in the ventilation calculations?"

    A. Nothing is preventing you from estimating your daily use of the clothes dryer, and using that estimate in adjusting your ventilation rate. Note that many Passivhaus builders prefer to install condensing clothes dryers, which are unvented. More information here: Alternatives to Clothes Dryers.

    Q. "If you are using an exhaust only system, you will improve IAQ if you allow free flow of air between rooms, allowing for more mixing. If you deal with the ease of moving air between rooms in a return air system for space conditioning, why wouldn't you do it for ventilation? Again, I have to ask what we are trying to accomplish by providing fresh air? If the air exchange is only happening in a limited amount of space, are we accomplishing the aim?"

    A. You are raising questions about fresh air distribution. I can assure you that builders have been considering the issue for years, and have developed various approaches to address your concerns. To learn more, see A New Way to Duct HRVs.

  63. GBA Editor
    Martin Holladay | | #63

    Response to Mark Siddall
    Thanks for your comments.

    First of all, a note to GBA readers about Mark's abbreviations: in the U.K., the type of ventilation system that we refer to as an exhaust-only ventilation system is called a "mechanical extract ventilation" (MEV) system.

    Frankly, Mark, I'm not sure that I understand your point. You wrote, "I would contend that for the cost per kWh to be determined for MEV and HRV/ERV that you need to compare these options to natural ventilation." I really have no idea what you mean by "natural ventilation." Perhaps you mean one of the following options:

    1. A system whereby occupants are urged to open the window when a room is stuffy.

    2. A system whereby the builder is deliberately sloppy, so that the building shell is leaky, with enough infiltration and exfiltration so that the leakage (somehow -- I'm not sure how) provides fresh air to the occupants.

    3. A system whereby the builder includes a deliberate, specified hole to allow air to enter the building as well as a deliberate, specified hole to allow air to exit the building, with the hope that the stack effect and wind will provide enough of a driving force to allow fresh air to enter and stale air to leave.

    Frankly, none of these options is used by knowledgeable builders in North America, and all three options have significant disadvantages. In the context of anyone interested in meeting the Passivhaus standard, these options are clearly off the table.

  64. user-971945 | | #64

    Datums and ventilation options
    Greetings from the UK. Thanks all for an interesting debate, I've been following it for a while and have formulated a couple of observations and queries that relate in some form to this discussion. 

    The study in question seeks to compare HRV/ERV with MEV, however, I am concerned that this is not the full story and that as a consequence accidentally results in a slight distortion and may or may not change the results. In reality ventilation of various types may be regarded as something of a continum – spanning from natural ventilation to forced ventilation of one kind or another (MEV and HRV/ERV).

    I would contend that for the cost per kWh to be determined for MEV and HRV/ERV that you need to compare these options to natural ventilation, only then will a fair and balanaced view be achieved. In this respect the study does not, from what I can see, contextualise any relationship with the national/state average airtightness of existing homes nor does it compare with existing building codes in these regions.

    In order that the cost analysis may be undertaken in a fair manner HRV/ERV and MEV I would suggest that the study should also be compare against this datum. If this is not done then the cost per kWh saved will be distorted (this is because the cost per kWh saved is calculated relative to a given datum.)

    If this analysis were undertaken then I suspect wht will happen is that the cost/kWh saved for the HRV/ERV will fall significantly, that ERV will remain the cheaper option in some climates (through a reduced number), and that the relationship between the cost of PV and HRV/ERV will be changed to favour HRV/ERV in a greater number of climates. 

    In a nutshell, it would be interesting to see how this study may be evolved to reflect cost effective construction acriss the range of options that are available, thereby avoiding, or addressing, possible distortions in the analysis presented to date. 

    Keep up the good work!

  65. user-971945 | | #65

    A contextualised response?
    I can appreciate that knowledgable builders would not rely upon the vagaries of natural ventilation. I wouldn't either (for winter conditions in the UK.) To help try to explain the logic I will offer a English context in the hope that parallels may be drawn by you and others, but before I do I would note that for a study is to be truly rational then it will examine all options on a fair and even footing before coming to a conclusion. The current study, by excluding national/regional averages and building codes fails to draw a complete picture that would ultimately allow a completely rational decision to be based upon the cost analysis.

    1) Natural ventilation
    The average existing UK dwelling has an airtightness of about 16 ach/hr@50pa; about 0.8 ach/hr at normal background pressure (call this Business as Usual, BAU).

    Building Regulations in England require that homes achieve an airtightness standard of 10m3/m2@50pa. This translates, (very) roughly, to 10 ach/hr@50pa for a dwelling. Limited intermittent mechanical extract is required in kitchens and bathrooms (often interlinked to lights/cooker hood for control). In addition to opening windows manually controlled trickle vents are required so as to enable natural (non-mechanical) ventilation. Theory is that 10 m3/m2@50pa (10 ach/hr@50pa) equates to about 0.5 ach/hr in a domestic building. For both of these scenarios (BAU and B.Regs) building occupants are required to operate the building in order to achieve adequate indoor air quality.

    I, like you no doubt, do not consider the above Building Regulations to be "good practice". Nor do I consider the average existing building stock to be particularly praiseworthy. They are however a part of the norm/business-as-usual and consequently provide the fair benchmarks/datums that should be included in any further cost analysis.

    2) Why should BAU be kind of datum be used? Let us consider insulation.
    If I have an uninsulated wall and install 200mm (10") of insulation then the cost per kWh saved is X. If I then at a later date sought to install an extra 100mm (5") insulation then the cost per kWh saved for this new retrofit would be prohibitive, however, if I had installed the 300mm (12") insulation in the first place - instead of 200mm - then the cost per kWh saved of this greater quantity of insulation would have been affordable.

    The reason for these observations is that the cost per kWh saved is derived from the difference between the estimated existing condition and the proposed condition over time. If we assume fixed costs and fixed time then smaller the difference between the two conditions,I the greater the cost per kWh saved. In this basis, if someone is to analysise the generic cost per kWh saved accurately then you should really base the cost analysis upon a datum and this should, I argue, reflect the most common national/regional condition (business as usual, BAU, if you will.)

    Building codes should, in theory, result in improvements relative to BAU and should consequently be compared to that datum, as should any other efficiency improvement (Passivhaus, super insulated house, Okay house ...whatever.) If this is not done then you risk distorting the model and getting results such as those in the 100mm (5") retrofit, things look expensive, but that is because you are starting from the analysis from the wrong point.

    When considering alternative (and hopefully better) ventilation options such as MEV, HRV and ERV (as compared to English BAU) it should be noted that within this context airtightness must also be considered within the cost analysis for any ventilation calculations as should the ventilation efficiency (the reliability of the ventilation strategy to deliver appropriate ventilation - mech vent is more reliable than natural vent and therefore permits a lower air change rate). Only by undertaking whole systems analysis will you begin to really establish a more realistic appreciation of the cost per kWh saved.

  66. GBA Editor
    Martin Holladay | | #66

    Another response to Mark Siddall
    Your discussion applies to retrofit work rather than new construction. The whole issue of recommendations for mechanical ventilation systems during weatherization work (or energy retrofit work) is complicated and, frankly, controversial. It is a discussion worth having, but irrelevant to the point of John Semmelhack's paper.

    John was looking at the issue of mechanical ventilation for new buildings. He was approaching the issue from the perspective of a builder interested in energy-efficient construction or perhaps the Passivhaus standard. Most GBA readers fall into that category.

    Ever since ASHRAE 62.2 was published 9 years ago, that ASHRAE standard has established a best-practices benchmark for U.S. builders. While ASHRAE 62.2 has not yet been referenced by most residential building codes, it's fair to say that builders in the U.S. who are paying attention to airtightness are usually also paying attention to ASHRAE 62.2.

    Builders interested in energy efficiency in the U.S. have been preaching the mantra, "Build tight and ventilate right" for at least 25 years. Hoping that natural infiltration will provide enough fresh air for occupants is, frankly, not an option.

    The situation you describe in Britain -- one in which most homes have an exhaust fan in the bathroom and an exhaust fan in the kitchen -- isn't much different from the situation in the U.S.

    ASHRAE 62.2 recommends that builders who depend on these exhaust fans for ventilation should control them with a timer, and should regulate the timer to meet certain minimum ventilation rates. That's not very complicated, and that is the minimum acceptable system considered by John Semmelhack in his study.

    Your comments highlight an ongoing problem: the extent to which marketers of various devices and programs depend on the existence of inefficient buildings, because these buildings provide a false benchmark that allows the marketers to exaggerate the virtues of the devices or programs they are promoting.

    The Passivhaus Institut in Germany is not immune to this problem, since they have long boasted that Passivhaus buildings use 90% less energy for space heating than "typical" older buildings in Germany. This is a false benchmark, because it would be illegal to build such a building today -- either in Germany or the U.S.

    The worse your benchmark, the better the touted advantages of a program appears.

  67. user-971945 | | #67

    It's all in the lifestyle?
    I find the concept of cost /kWh saved quite intriguing as their is no accepted norm form such calculations. Energy efficiency, being what it is (the mysterious negawatt) is somewhat difficult to quantify. It is true that the worse the benchmark the better the results may appear. But what is the appropriate benchmark? What I am trying to do is highlighting the fact that the results of these calcs have a sensitive dependence upon initial conditions and therefore need to be treated with a degree of caution.

    ....Conceptually the debate that I am opening up does not, I believe, only apply to retrofits rather it may also be dependent upon a clients current standard of living. Where does the client live? If they live in a leaky old home and seek to move to an energy efficient new one then this is the datum that could justifiably be used to provide the basis for any cost evaluation. They are in effect "retrofitting" their lifestyle with a new house. You on the otherhand (living in a super insulated home) may not find it economic, over a given lifecyle of say 20 years, to move to another super insulated home that has lower energy demand - the cost per kWh hour saved compared to your current lifestyle could be prohibitive.

    Cost per kWh hour saved calculations are perlexing, what is the appropriate benchmark for undertaking such studies?

  68. GBA Editor
    Martin Holladay | | #68

    Response to Mark Siddall
    Q. "What is the appropriate benchmark for undertaking such studies?"

    A. If we are discussing a new home, the appropriate benchmark would (at the very least) be a home that meets the current building code in the country where it is being built. However, I think that John Semmelhack's approach is more responsible, since he assumes that any new home that aims for energy efficiency will need a mechanical ventilation system that complies with ASHRAE 62.2. I agree with his benchmark.

    John raised the question at a Passive House conference. Many of his listeners were shocked to hear that he would consider using an exhaust-only ventilation system. I think your proposed benchmark of "natural ventilation" -- whatever that means -- would be received with even more shock.

    In rural Mongolia, of course, your benchmark might be a yurt (or ger).

  69. user-971945 | | #69

    ....forgive me for using
    ....forgive me for using insulation as the reference the concepts of cost/kWh saved are easier to convey when discussing insulation.

  70. GBA Editor
    Martin Holladay | | #70

    Still another response to Mark Siddall
    It sounds like you are a proponent of Passivhaus-levels of insulation. You realize that these very high R-values are impossible to justify, so you are grasping at straws to make the math work out the way you want it to. That's why you want to compare the very high R-values of a Passivhaus with a leaky building that doesn't even meet code.

    You wrote, "Here's the thing with the code approach, due to the sensitive dependence upon initial conditions, the cost/kWh saved buffer (break even) is hit far sooner than it need be or in fact would be for another scenario."

    Right. So if you compare a Passivhaus to an illegal building, your math works. However, I'm unconvinced of the logic of this approach.

    Here in New England, building codes require R-49 insulation on an attic floor. That's about 14 inches (35 cm.) of cellulose. But some Passivhaus builders are installing R-100 insulation on the attic floor. That's 28 inches of cellulose (70 cm.)

    If a Passivhaus advocate wants to justify the additional 14 inches of cellulose that it takes to get from R-49 to R-100, you can't compare the performance of the Passivhaus building to a tent. You have to compare it to the worst building that you can legally build -- one with R-49 insulation in the attic.

    The payback period for the last 14 inches is much, much, much longer than for the first 14 inches. You know that, of course. That's why it's useful for people with an agenda -- people who already know the answer to the question -- to be able to compare the Passivhaus to a tent.

    As I said before, hucksters and fraudulent marketers are delighted by the existence of illegal buildings, leaky buildings, uninsulated buildings, and even tents. If these terrible buildings didn't exist, they'd have to create a few, to help with their marketing campaigns. When you see an advertisement that reads, "Save 50% on your energy bill," look closely for the asterisk. A classic marketing campaign based on comparisons between a building with sub-par insulation and a tent has been waged in the U.S. by Icynene; I wrote about it here: It’s OK to Skimp On Insulation, Icynene Says.

  71. Mike Eliason | | #71

    the problem with that


    the problem with that logic is that passivhaus was never intended just for new construction - in fact it's highly applicable and even more cost effective on existing buildings.

  72. Mike Eliason | | #72

    ack that sent and deleted
    ack that sent and deleted half of what i wrote...

    the plethora of uninsulated buildings that dot the global landscape can be retrofitted to passivhaus, resulting in greater than a 90% reduction in energy consumption.

    this 32k s.f. multi family project in frankfurt dropped energy usage from 63.5kBTU/ft2a to 5.5kBTU/ft2a, not an insignificant drop.

    so i don't think there's any merit in insinuating that passivhaus is somehow promoted by fraudsters and hucksters.

  73. GBA Editor
    Martin Holladay | | #73

    Response to Mike Eliason
    No, I wasn't thinking of Passivhaus when I was railing against fraudsters and hucksters -- I was just warning Passivhaus advocates not to join the crowd of marketers who sell radiant barriers, P2000 insulation, and insulating paint.

    John Semmelhack was analyzing ventilation options for new buildings in his paper. It isn't that complicated! New buildings need ventilation systems, not "natural" ventilation.

    Benchmarks matter. If we're talking about new construction specifications, no one will be convinced by exaggerations. Passivhaus specs often make sense, but let's compare those specs fairly to code-minimum buildings, so owners can decide whether the upcharge is worth it.

    If Passivhaus advocates avoid exaggeration, they are more likely to be taken seriously.

  74. GBA Editor
    Martin Holladay | | #74

    More on exaggerations
    As long as we are on the topic of exaggerations...

    As you know, the Passivhaus Institut boast is that new Passivhaus buildings use 90% less energy for space heating than "typical" older buildings in Germany.

    However, this boast isn't enough for many Passivhaus advocates. They up the ante -- from 90% less energy for space heating to "90% less energy" -- which is an entirely different statement, and which is totally untrue. Almost always, the Passivhaus advocates who make this false "90% less energy" claim also forget to add the important qualifier "than older German buildings."

    Examples abound:

    Passive House DC: "Passive Houses save 90% of household energy."

    Passive House Alliance: "Passive House: no boiler, no furnace, highest comfort and up to 90% less energy."

    Passive House and Home: "A passive house is 90% more efficient than a standard house."

    TE Studio: "Passive House aims to reduce energy in buildings by up to 90% while providing superior comfort and indoor environmental quality."

    Clarum Homes: "A passive home is an extremely comfortable, healthy, economical, and sustainable home, designed and constructed to use up to 90% less energy than a traditional home."

    Ecology Ottawa: “Passive House in Ottawa uses 90% less energy.”

    It’s the Environment, Stupid: “No Solar, No Wind, but 90% Less Energy Use for this English House”

    Brett Sichello Design: "Passive House: Up to 90% Energy Savings"

  75. albertrooks | | #75

    Meat on the bones
    It sure is good to see such a lively discussion on a new topic. Good comments numbering into the 70s don't happen enough.

    Martin, I'd bet the false energy statements are unintentional. You don't promote passivhouse to get rich. From subslab foam to ventilation... it's a tough row to hoe.

    Doesn't mean they don't need to be noted and corrected. They do need to be. Intentional or not, they are incorrect and hence false statements.

  76. Mike Eliason | | #76

    i realize this is a


    i realize this is a side discussion (and yes, the misinformation bit isn't good for anyone...), but two things...

    one, it is entirely plausible for a passivhaus retrofit to reduce total energy consumption by 90%. the numbers from the tevesstraße multifamily project aren't heating usage, that's total usage. the retrofit reduced consumption from the existing condition by 91%. i can pull up a number of retrofits where that's been true.

    second, even on new construction, it's possible that a passivhaus could achieve a 90% reduction in energy usage (not just heating). i've seen a handful of MFHs in the EU w/ a site demand 90% lower than the typical site demand of new MFHs here in the NW. furthermore, worst case-all electric PH commercial buildings going to have a site EUI of ~12kBTU/ft2a - which represents an 88% drop of the CBECS average (91kBTU/ft2a) from the new building institute's study on LEED buildings (new construction) from 2008 (the one henry gifford references). it's worth noting the study tossed the 21 highest energy hogs because they skewed the data the wrong way. in terms of energy usage/sf, single family housing is on the low end (outside of storage facilities). the lowest numbers i've seen for detached housing, PH represents about a 75% reduction over 'code minimum'. the salem PH represented a ~2/3 reduction over code minimum (per the mechanical engineer, who designs & crits NW energy codes)

    the reality for most buildings, however, is that PH represents between 50-80% over code-built buildings outside of hotels, dorms, uni labs and hospitals (where worst-case PH can represent greater than 90% savings).

  77. Ted Clifton | | #77

    Response to Elizabeth Kormos
    Elizabeth, sorry I was not able to respond more promptly to your query about which powered HEPA filter system I use. I use a lot of different systems, depending on the application, but I have had the best results using the FanTech CM3200 HEPA filter (240 cfm), coupled to a FanTech RV6 range hood (206 cfm). They can both be run off the same variable speed control, which is wired first through a simple dial timer. The power feeds first through the timer, then through the speed control, then to the HEPA Filter and Range Fan. The range fan is a remote-mounted unit, installed on the outside wall of the house where it is completely silent from inside. The combination of the two fans running simultaneously has the same effect as a much more powerful single range fan, because it is never "gulping for air" in the tight house.
    The timer allows you to air-condition the house for less than two cents per day, by running the fans in the early morning for an hour or so. This will fully exchange the hot air inside the house for cool air from outside. The speed control, in addition to giving you the level of control you want while cooking, will let you set the fan speeds low enough to run continuously, or intermittently, as the conditions would warrant.
    One added benefit of this system is that it allows you to buy a relatively cheap range hood. As long as you like it's looks, it is good enough. Take the fan out of it, and connect your exhaust duct directly to the housing, using the existing filter and light. The installed cost can be significantly less that that of a custom range hood with the same amount of useful power.
    There is a great on-line source for these products, and many more, right in your neighborhood; Energy Federation Inc.

  78. user-971945 | | #78

    Building Code Benchmark?
    I'm not proposing natural ventilation (non-mechanically assisted ventilation) it is purely one of the options that should be considered when discussing costs. .....There are many sound reasons NOT to use natural ventilation of that I am certain but these are not driven purely by energy costs (health and well being etc.) In a macroeconomic analysis then avoided healthcare costs would feature in cost analysis of ventilation options also.....

    Here's the thing with the code approach, due to the sensitive dependence upon initial conditions, the cost/kWh saved buffer (break even) is hit far sooner than it need be or in fact would be for another scenario. For instance, if I went to a cool/cold (UK-ish) country that had no energy efficiency standards and proposed a Passivhaus it would (for arguments sake) be economic, however, if I went to an almost identical country that had modest energy efficiency standards then Passivhaus then the same calculations could suggest that it is uneconomic simply because of a prior decision to choose a random less ambitious standard at some random point in the past.

    I must admit that this rationale makes no sense to me. It is like saying that you can afford to retrofit to Passivhaus due to the poor datum but not build new homes to the same standard simply because there are already Building Codes for new build (but not retrofit.)

    .....The initial shift, that conceptual install of <50mm (

  79. user-971945 | | #79

    Here's a quote from Dana Meadows, it struck me the quote was pertinent to this discussion:

    "Ideally we would have the mental flexibility to find the appropriate boundary for thinking about each problem. We are rarely that flexible. We get attached to the boundaries our minds happen to be accustomed to.....
    ......It is a great art to remember that boundaries are of our own making, and that they can and should be reconsidered for each new discussion, problem, or purpose. It's a challenge to stay creative enough to drop the boundaries that worked for the last problem and to find the most appropriate set of boundaries for the next question. It's also a necessity, if problems are to be solved well."

    I do not consider myself to be grasping at straws. In terms of pure economics legally mandated regulations are not an issue. Sometimes we need to step back, look at the whole picture and re-assess the situation with a clear mind - and that's what I try to do.

    The intellectual game of undertaking cost/kWh saved calcs for "retrofitting" a legally madated house that has not been built strikes me as odd. In England we do not have Building Regulations that require energy efficient retrofit but we do have regs for new build, conversions/extensions etc. Why because of some random legal mandate would super insulation become uneconomic for new build but not for existing build? Also, as energy efficiency regulations get incrementally improved (they are revised every 3 years in England), the cost/kWh saved for super insulation standards - using the method that you propose - get more and more expensive (less and less obtainable.)

    Rather than basing the cost/kWh saved on current standards there needs to be an datum that underpins all decisions. How were the economics of ASHRAE62.2, in terms of cost/kWh saved, derived? (If such calcs were not undertaken then it is like comparing apples and oranges as they have be evolved on the basis of completely different modes of decision making i.e. cost/kWh saved and some other rationale.

  80. GBA Editor
    Martin Holladay | | #80

    Response to Mark Siddall
    I feel like we are going in circles here, so I'll be brief.

    Q. "The intellectual game of undertaking cost/kWh saved calcs for retrofitting a legally madated house that has not been built strikes me as odd."

    A. John Semmelhack's paper refers to new construction, not retrofits. Whenever a new house is built, the designer creates specifications. These specifications must, at a minimum, meet the local building code. Of course, a designer can choose equipment that is more expensive to install than equipment that barely meets the local code (or that barely meets some other minimum standard) -- but it's always helpful to know whether there are energy savings associated with the expensive equipment that help justify the higher costs. John Semmelhack's paper provides useful calculations that can to help designers specify ventilation equipment for new homes. In a sense, you are correct that John Semmelhack's calculations represent an "intellectual game"; but the game is a useful one.

    Q. "How were the economics of ASHRAE 62.2, in terms of cost/kWh saved, derived?"

    A. ASHRAE 62.2 is a residential ventilation standard. I will quote from the standard: "Purpose: This standard defines the roles of and minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality in low-rise residential buildings." The standard was developed with this purpose in mind. There was no economic analysis -- in the same way that there is no economic analysis behind a code requirement that calls for a roof that can handle a snow load of 30 pounds per square foot.

  81. GBA Editor
    Martin Holladay | | #81

    Response to Albert Rooks
    Thanks for the feedback.

    You wrote, "You don't promote Passivhaus to get rich."

    That's true -- which makes it difficult to understand the motives of the people who exaggerate. The best explanation I can come up with is that these people are in the grips of a quasi-religious fervor that prevents them from seeing the data clearly.

  82. GBA Editor
    Martin Holladay | | #82

    Response to Mike Eliason
    You wrote, "The reality for most buildings, however, is that PH represents between 50-80% over code-built buildings." That's a big range. My gut tells me that, once we get more data on U.S. Passivhaus buildings, the savings will be much closer to 50% than 80%.

    Space heating represents a declining percentage of residential energy use; plug loads keep climbing, as we all get more electronic gadgets. These trend lines are not headed in a direction that the Passivhaus standard is prepared to address.

    Clearly, the best way to address the rising plug load problem (besides the obvious advice - "don't buy any new gadgets") is to advise buyers to choose efficient appliances. Beyond that, you can always install a PV array -- but of course the Passivhaus standard doesn't consider the output of a PV array in its calculations.

  83. Bronwyn Barry | | #83

    'These people'
    Discussion policy: The editorial staff at does review user submissions. Please be respectful of others. Inappropriate content will be removed.

    For someone who has himself included a few misstatements about Passive House in this very (data-light) blog post, it’s a bit harsh of you to go pointing fingers. To then go on and accuse ‘these people’ of being ‘in the grips of a quasi-religious fervor that prevents them from seeing the data clearly’… Oh dear. Your office must be getting very warm. Please check your security settings (and read your own discussion policy) - Jerry Springer may be hacking your blog.

    But let’s focus on the meat of this juicy debate: real data. The item that tends to cause confusion (yes, even in the PH community) between heating/cooling energy and whole building energy is that a Certified Passive House is also required to meet a Source/Primary Energy target. That calculation includes energy required for DHW, lighting and appliances, essentially limiting whole building energy, so this may be why many get the two confused. (The source energy calculation does not include plug loads - I’m not sure if any other modeling tools predict plug loads? Additionally, can we as builders and designers ever really control plug loads? A topic for its own blog post perhaps?)

    Source energy differs from Site energy calculation in that it factors in line loss and generation inefficiencies for electrical use (3x) whereas gas use is usually calculated as 1:1. These multipliers are location specific and can be adjusted in the PHPP to reflect local energy supply efficiencies. The PH certification limit is < or = 120kWh/m2.yr.

    I did a quick calculation for you on how PH whole building energy would compare to current usage. It’s a rough ‘back of envelope’ comparison with the following assumptions:
    - Single family residential only as most PH buildings in the US currently fall into this category
    - Data source:
    - Average home of 1,971sf (183.11m2) uses 89.6 mil BTU/yr (26,259.18 kWh/yr)
    Using the above info, this crunches out to 143.40 kWh/m2.yr for the site energy of the average US single-family home, based on the above 2009 data. If we assume these homes use 50% electric energy, we reach a source energy number of 286.2 kWh/m2.yr. That’s roughly 60% higher than what is required for PH Certification, so your guess of 50% overall building energy saving for PH was pretty close. Still, not to be sneezed at for anyone looking to improve building performance and, as Mike indicated earlier in this thread, the number will vary for other building types.

    As a side note, I have heard rumors that the next iteration of the PHPP will include a tab that calculates offsets to source energy by renewables such as PV. I’m betting on it, particularly since the upcoming International PH Conference ( has a special segment devoted to ‘the regional implementation of strategies combining energy efficiency and renewables as part of the new EU funded PassREg (Passive House Regions with Renewable Energy) project. I’m excited to see this next iteration of a constantly evolving standard, along with a few other cool developments that I hear are also in the pipeline… so much to look forward to, n’est pas?

  84. GBA Editor
    Martin Holladay | | #84

    Response to Bronwyn Barry
    You're making the same error that Mark Siddall was making -- comparing a brand new Passivhaus to an "average" American house rather than to a new code-minimum house. If I'm planning to build a new house in the U.S., and you are trying to convince me to build a Passivhaus, you have to compare the energy performance of the Passivhaus with a new house that meets the current building code -- not the average energy used by Americans living in older houses.

    I'm don't have the time now to do the necessary calculations, but a study performed by Jeff Christian in Tennessee has data showing that a 2,643 square-foot (245 sq. meter) house in that climate that meets the Building America benchmark (basically, the 2003 IRC) uses 57.1 kWh/day, or 20,841 kWh/year. That means it uses on 85 kWh per square meter per year, which (if I understand you correctly) is within the PH limit.

    That doesn't seem right, so I need to double-check the figures. Nevertheless I maintain that comparing a brand new Passivhaus building to a collection of older buildings is misleading. Anyone building a new house these days has to meet minimum code requirements, and these code requirements bring the energy use of the occupants of a new building to levels that are lower than the average energy use of Americans living in older buildings.

  85. Bronwyn Barry | | #85

    Thanks for the link to the
    Thanks for the link to the study. Also don't have time to dig into the numbers until Friday, but my initial response is to check whether the consumption numbers cited are site energy or source energy. I'll be interested to see how this plays out as I too am curious as to where this number will fall.

  86. Mike Eliason | | #86

    PH numbersplug loads are
    PH numbers

    plug loads are most definitely accounted for in the source number for a PH, albeit at a very low, EU-centric rate. In the US, PHPP users should be modeling add'l electronics as necessary. the study looked at a net zero house, so all electric for comparison (makes it easier)...

    worst case all-electric Passivhaus will have a PHPP calc'd site number of 14.07kBTU/ft2a (38kBtu/ft2a/2.7 source factor)

    because PHPP's numbers are based on TFA, there's a slight adjustment in the floor area for calcs. we're finding on residential projects that TFA is approximately 92% of net area. GSF = 2,643sf, net area of project listed as 2,313sf x 0.92 = 2,128sf

    2,128sf x 14.07kBTU/ft2a = 29,940kBTU or 8,775 kWh/a divided by 2,643sf = 3.32kWh/ft2a <-- this is our apples to number for comparing the study.

    85kWh/m2a = 7.9kWh/ft2a

    therefore, the worst-case all-electric passivhaus represents approximately 58% reduction over baseline.

    Blake Bilyeu's Salem Passivhaus was measured at 2.87kWh/ft2a, which is a 64% reduction from the study's baseline. As I noted before, compared to new construction - the Salem house (as determined by the folks who determine policy here in the NW) represents a 2/3 reduction in energy consumption. that's new construction, apples to apples. And it could have been better, the users have relatively high plug loads and as occupants figure out their consumption patterns and the house dries out, it also tends to drop (at least on the monitored PH data i've seen).

    in terms of energy consumption per square foot - single family houses are the low end, it only goes up for there for multi family and commercial/institutional. thus, PH represents potential for an greater reduction over baseline than detached housing. this is a big deal because as gifford and others have shown, LEED and other programs are notoriously awful for achieving hyper-efficient buildings. Passivhaus hasn't had that issue, and this is one of several reasons it's being folded into the building codes in europe.

    another interesting passivhaus tidbit (at least in WA state) - our energy code is mandated to have incremental tightening to achieve a 70% reduction in 2031 over our 2006 energy code. this means a passivhaus will meet or exceed code minimum housing through ~2028.

  87. GBA Editor
    Martin Holladay | | #87

    We're getting close to a consensus
    So a conserving family like Blake Bilyeu's family living in a Passivhaus uses 64% less energy than a code-minimum house in Tennessee, while a more typical American family (the family of Matt and Laura Beaton, who live in a certified Passivhaus in Shrewsbury, Massachusetts) use 4.34 kwh/sq.ft*year -- roughly 45% less than used by the hypothetical residents of a code-minimum house in Tennessee.

    I think that range sounds about right. Not really "90% less energy" -- but still quite impressive.

  88. eyremountllc | | #88

    Are these HRVs/ERVs too large?
    I think purely from an economic perspective, the HRV/ERV modeled in John's study make less sense than the exhaust only option but I think an ERV/HRV offers so much more than what Exhaust only option offers.

    The exhaust only option though less expensive, creates issues that the green building community in general have been working so hard to move away from, i.e. indoor air quality, distribution, house depressurization just to name a few. John discussed all of these and one of his suggestions was that ERV makers should make smaller, more cost effective ERV/HRVs. I think he is right on the money.

    I wonder what the results would be if modeled with an HRV like the Lunos E2. They are $2195 for two pairs. They require no duct work. Reading the 8 page installation manual, the installation is so simple even I can install them. So let's put the cost of installation at say $150, which would put this HRV option at $2,345, only $200 more than the exhaust only option. Taking this out 20 years, I would pick the HRV option any day.

    I realize these Lunos HRVs have a lower CFM rate but my point is perhaps the problem here is not whether or not to use HRV/ERV but rather, are the ERVs/HRVs currently widely available simply oversized. I have the Ultimateair which has a maximum of 200 CFM, ComfortAir is probably similar. It simply doesn't make sense to use these in small houses that only require a total flow rate of 56
    Don't you think this is similar to us complaining about HVAC contractors oversizing the heating and cooling equipment?

    During my training with PHIUS, we were taught to decouple ventilation from heating and cooling. The standard practice was to use a ducted ERV for the whole house ventilation but then use a point-source minisplit to condition the space, relying on the ERV for distribution. I understand PHIUS is not necessarily preaching that anymore from reading Katrin's blog. As a practical matter, this HVAC strategy does not work well in larger houses. At Arlington Passivhaus, we followed that strategy knowing that the heat pump would provide plenty of BTUs to heat and cool. But we thought completely relying on the ERV to distribute air to 3800sf of house was not possible. We had transfer fans strategically placed to provide more direct distribution and that helped but still not perfect. We could have ducted the minisplit but that would cost way more, killing the savings of right-sizing the equipment.

    It also doesn't make sense to build this great ventilation network throughout a house but only use it for ventilation. So, one compromise I think might work for larger homes is to use the smallest conventional HVAC system you can find. I think Carrier has a 2 ton system, then strap an ERV to it as in some retrofit ERV installs. Sure, the SEER ratings are lower on the conventional systems but if the difference in operating costs is only a few bucks a year, and it does not require any tweaking, I'd do it any day too.

  89. Mike Eliason | | #89

    good to heat


    good to heat consensus is near! i'd venture that the salem house is closer to avg. US consumption and the beaton house is excessive, but will readily admit to being biased from too much exposure to living abroad.

    i don't see the 90% savings being applicable outside of hospitals or retrofits, but there are instances where it can come really close for new construction (e.g. uber efficient libraries/dorms - maybe even larger scale multifamily). almost all would require some sort of MVHR, however.

  90. GBA Editor
    Martin Holladay | | #90

    Response to Roger Lin
    Thanks very much for your comments. I'm in agreement. Like you, I was very excited when I first heard about the Lunos fans. They are simple; they are right-sized; they are elegant; they use very little electricity. (They also operate on 12 volts DC, so they can even be used in an off-grid hippie house.) Bingo!

    The only problem is that they are ridiculously expensive. I hate to say it, but a Chinese factory could easily churn these out for $150 a pair -- instead of the $1,200 a pair charged by Four Seven Five for these German-manufactured fans. (I know, I know -- Chinese labor practices.... But where do you think your laptop and TV came from?)

    According to Chris Corson, the Lunos fans work great.

  91. eyremountllc | | #91

    Great idea! We don't

    Great idea! We don't need to go all the way to China. I wonder if I might be able to get someone in Taiwan (where I grew up) to manufacture them.


  92. CalBungMelb | | #92

    Best combination of AC/HRV/Exhaust etc in Mediterranean climate
    Great thread.

    So what might be a reasonable combination of technologies to achieve air quality and minimal energy heating/cooling in a Mediterranean climate? HRV might be useful in winter when it is 5C outside. But what about during summer? It's heading for 36C here today and around 40C forecast for tomorrow. How best to maintain air quality and an indoor temperature not above, say, 25C.

  93. CalBungMelb | | #93

    Using a thermostat as well as a timer for range hood fans
    Hi Ted,

    Have you used an interior/exterior thermostat as well as a timer to control those range fans?



  94. GBA Editor
    Martin Holladay | | #94

    Response to David Coote
    According to the committee that established the U.S. residential ventilation standard (ASHRAE 62.2), a good ventilation rate is 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. However, many experts think that this ventilation rate is too high, especially for those who live in a hot climate. You can read about the debate here: Designing a Good Ventilation System.

    Here's the bottom line: it's a good idea to have equipment that can operate at the ventilation rate suggested by ASHRAE 62.2 -- but you don't have to use it all the time if you don't want to. If occupants use smelly cleaning products, or if they do a lot of cooking, they will need a higher ventilation rate than occupants who are careful about the cleaning products they allow into their home, and who cook more rarely.

    Experiment with your ventilation rate. Using a ventilation rate that is less than suggested by ASHRAE 62.2 will save energy, and it may be perfectly reasonable for your family and lifestyle. The more you ventilate, the higher your energy bill.

    When it comes to cooling indoor air, you have several choices:
    1. seal up your house and use an air conditioner,
    2. seal up your house and use a swamp cooler (an evaporative cooler);
    3. open up all the windows and turn on a fan.

    (Actually, there is a fourth choice: if the outdoor air temperature drops to comfortable levels at night, you might want to operate a whole-house fan at night or during the early morning hours, and then seal up your windows in the middle of the day. For more information on whole-house fans, see Fans in the Attic: Do They Help or Do They Hurt?)

  95. CalBungMelb | | #95

    Thanks, Martin.
    It looks like

    Thanks, Martin.

    It looks like we would need a combination of direct ventilation and AC for summer and an HRV during winter. That's a lot of kit, unfortunately. More to go wrong :(

    Re night purging you can tell when a cool change hits our street of 1930's houses (after a day like yesterday that was over 40C) by the sound of sash windows opening up and down the street.

  96. Bronwyn Barry | | #96

    Circling back
    I sent an email to the Passive House Institute a few weeks ago, asking for more info on the ventilation issues raised in this blog post. Today, Wolfgang Hasper, who works on the certification of mechanical units, confirmed that while PHI has yet to certify a project without an H/ERV unit, that it is indeed possible to do so.

    He included a link to a recently completed study that references the 'happy climates' Mike Eliason mentioned. In these locations extract ventilation combined with natural ventilation at night and thermal mass "are of high relevance to minimize the energy demand (and) for achieving the Passive House Standard here." (See section 5.2 on Pg.17.)

    The study, commissioned by the Mexican Government, may also be relevant to David Coote's question for Mediterranean climates. The conclusions on building shape and density, life-cycle cost analysis and impact on CO2 emissions make a $2K upcharge on an H/ERV pretty insignificant. Happy reading:

  97. jinmtvt | | #97

    few comments and questions from a n00b :)
    1- Please explain why not all PH and recent buildings are not using mini-split type of HEAT/COOL system with their super high recent efficiency ?? doesn't it have the highest efficiency for both heating and cooling ?? why settle for something else at the price they are now ?

    2- then, why aren't designer/architect including some kind of very simple recirculating air system
    similar to what is done for central ac, in a simpler way with very light cfm ?? then all your fears of comfort and fresh air distribution, heat spread etc magically vanish ??
    Also "cold" air intake could be mixed up in this recirculation system giving it time to get back to similar temperature with the remaining of the inside air.

    3- the only point in using HRV ( at the prices you are mentionning it costs installed ...seem ridiculous prices to me but you guys down state seem to like paying extra high fees to all "trades" while building houses ) would be that it at least provide with some payback term ..not 50 years
    as Martin is mentionning, on from the numbers i read, mild climates do not justify a 4000$+ system.

    Still don't understand why using some kind of passive HRV with existing bathroom fans
    would work wonders in mild climates.

    4- lastly, i wish to be enlightened on the numbers provided by mr Semmelhack
    from the table picture


    is this number the meaningfull one in calculating the different of efficiency ( $$ ) from using HRV to only exhaust fans ??
    would give ( for VT situation ) ~ 4500KW - ~1500KW = ~ 3K KW/H of difference total per year ?
    So that is the maximum money back gain from using HRV VS same similar CFM direct output ?

    that would give something in the ~300$/year of $$ gain @ 0.1$/ KWh ???
    please correct me if i get this wrong

    because it is, this is a serious economy
    and a very 'fast " payback as far as i am concerned
    ( regular HRV here in Quebec are installed for something like 2K to 2.5K$ ..far from 4K$figures
    and lack of inspection/law makes it legal for a homeowner to simply install it himself ..then u can assume in the 1K$ to 1.5K$ with material of total expenses .. )

    Neway this is what i am looking to understand, is there 3K KW/h of difference between direct and HRV in the study of mr Semmelhack ?? :)

  98. GBA Editor
    Martin Holladay | | #98

    Response to Jin
    You have correctly noted that in cold climates (for example, Vermont or Quebec), an HRV is a good choice. The high incremental cost of an HRV can be justified by the energy savings in a cold climate.

    Obviously, if you can install an HRV for less money than Semmelhack assumed, your payback will be faster.

    According to Semmelhack's modeling exercise, ventilating with an Ultimate Air 200DX ERV in Vermont incurs an energy penalty of 1,329 kWh per year, while ventilating with Panasonic exhaust fans incurs an energy penalty of 4,691 kWh per year.

    Using the ERV saves 3,290 kWh per year. If you pay $0.10 per kWh, that amounts to $329 per year of savings.

  99. jinmtvt | | #99

    Thanks for the confirmation
    Thanks for the confirmation Martin, those numbers are @ ~60cfm continuous right ?
    So the energy difference will be affected by different cfm numbers linearly ??

    Pete Chramiec: could you please elaborate on your personal house air systems ??

  100. GBA Editor
    Martin Holladay | | #100

    Response to Jin Kazama
    The answers to most of your questions are in the article on this page.

    Semmelhack assumed that the ventilation rate was 56 cfm continuous for the HRV and ERV, and 64 cfm for the exhaust-only ventilation system.

    If you reduce the ventilation rate, you will also reduce the energy penalty associated with ventilation in proportion to the ventilation rate reduction.

  101. chadludeman | | #101

    Philly Building Ventilation
    Great article. Wanted to put out our current strategy in Philly that is in the middle of the mild and severe climates. Didn't have a chance to read the 100 comments yet, so forgive me if this has been covered.

    We started with the expensive UltimateAir product 4 years ago and created separate ductwork all over the homes to Passive House specs. It was expensive as this article confirms. We now have a new system.

    We have something similar to the 3rd option in the tests here, but instead of using exhaust only, we hook up a $500 HRV to our dedicated bath and kitchen ductwork. We then bring in fresh air to only one spot per floor of living area (not each bedroom and living space).

    We have not noticed an upcharge from our HVAC contractor since he was already including ducts for bath and kitchen ventilation in his historic price. We take $100 per bath from our bath fan budget now and put it towards the HRV, which means our premium cost ends up at about $300 for a house.

    Yes, I know it's not ideal to use an HRV for bath and kitchen exhaust. Neither is paying $5K for the alternative and in tight rowhomes it's hard to find a good place to bring in that cold passive makeup air with exhaust only. We also typically use recirculating range hoods unless a client specs otherwise. Just our 2 cents. Hope it helps and keep up the great posts!

  102. tgpine | | #102

    Wisconsin Radon problems
    Agreed this is a great article with wonderful discussion. I think my problem relates at times to the discussion. We are considering putting in a ERV system to help with out Radon problem. We have done several Radon Mitigations but still have a winter Radon level in the 5- 8 range.
    While it would be great to put in a whole house ERV system it seems for Radon reduction it is recommended to just do the basement area. Does anyone have experience placing an ERV for Radon reduction? Have they only done it for the basement or whole house? Does anyone know other sources available to assist in this decision?
    For background we built our house on an Esker, which apparently can produce very high radon levels. We started with levels in the 20 pCi /L range. We have had 3 different radon mitigators come work and then eventually give up on our house. We have sub-slab decompression at different sites in the basement and are hoping that ERV will get us to an acceptable radon level. We have no problems with the radon level in the summer, only the winter. We test our radon level with a safety siren.
    I apologize if this is the wrong forum for this question but it certainly would pertain to ERV and in home pollutants (or is a cancer causing material more than a pollutant?)
    Thanks for any help or direction you can give me. We are very frustrated and concerned.

  103. GBA Editor
    Martin Holladay | | #103

    Response to Terrence Gaynor
    I haven't heard of the use of an ERV to lower radon levels, but I have heard about the use of exhaust fans for that purpose. Peter Yost used an exhaust fan in his retrofit work to address radon in his house in Brattleboro, Vermont:

    "Unfortunately, radon readings spiked from 6 pCi/L (already 2 over the EPA action limit of 4 pCi/L) to 12 pCi/L in this now tighter space. In response, we sealed off and insulated the vented crawl space under the front porch (paving the way for the front porch to become a home office), removed a concrete block to have the crawl communicate with the basement, and added a 24/7 high-efficiency exhaust fan. While this didn't lower the radon readings in the basement, the continuous exhaust ensures that radon readings in the first-floor living space are consistently 2.5 pCi/L or less."

    This information comes from Peter's article on the retrofit project; read it here: Deep Energy Makeover.

  104. capnkent | | #104

    opening windows and doors
    One aspect that is rarely mentioned and calculated is the occurrences of opening of windows and doors. I'd postulate that most houses are over-ventilated because of the window & door opening factor?

  105. GBA Editor
    Martin Holladay | | #105

    Response to Kent Mitchell
    Most homeowners don't leave their windows open unless the weather is pleasant and the outdoor temperature is comfortable. Hopefully, they aren't running their furnace or air conditioner while their windows are open. (I know -- some people do -- but there is no easy cure for stupidity.) If the weather is pleasant, there is no down side to overventilation.

  106. srivenkat | | #106

    Panasonic APPA04VE1
    I am a little intrigued that Panasonic has an ERV in APPA04VE1 as opposed to an HRV considering these are likely to be installed in Bathrooms which need to be dried out and hence no moisture transfer back into the incoming stream is desirable.

    Does anyone know if Panasonic has plans for an HRV in the style of APPA04VE1 ?

  107. GBA Editor
    Martin Holladay | | #107

    Response to Venkat Y
    As far as I know, Panasonic is not marketing its ERV for use in bathrooms. You are correct that it doesn't make much sense to install an ERV in a bathroom.

    If you want to ventilate a bathroom, the most common method is with a bathroom exhaust fan. Another approach is to install a ceiling grille connected to the exhaust port of an HRV.

  108. srivenkat | | #108

    COP of heater for exhaust-only ventilation
    It's not clear to me what was assumed to be the efficiency of the heating system in the exhaust-only case and how that efficiency compares to the state of the art heating/cooling equipment. Thanks in advance for any info.

  109. GBA Editor
    Martin Holladay | | #109

    Response to Venkat
    John Semmelhack provides his assumptions in his paper. Semmelhack wrote that he assumed the use of a "Heat pump heating/cooling system with heating-season COP ranging from 2.75 to 3.50 depending on climate (warmer climate = higher COP) and cooling season COP of 5.0."

  110. Alan Abrams | | #113

    errrr, pardon the late comment. I just stumbled over this thread, now more than 6 years old. I also recognize that the initial topic was HRV's.

    All that aside, and on the chance that the participants are still interested, I would like to comment and raise a question.

    For one thing, kudos to John Semmelhack, who was in my PH training classes, back in '09. His analysis makes a lot of sense to me, at least in terms of cost benefits. Maybe even moreso, with the trajectory of energy prices flattening out, due to fracking and other extraction technologies.

    However, in my cursory review of the thread, I did not see any comparison of the moisture content of incoming fresh air between the various systems being discussed.

    The significance comes into play if you are building wall and roof assemblies that are effectively vapor-tight. If, for example, you are using Zip-R type wall sheathing, or a hot roof with membrane roofing.

    In such cases you are vulnerable to the accumulation of moisture within the framing cavities--which can progress over years or even decades, before reaching problematic levels.

    The question is, then, at what point does an ERV with at least some capability to extract moisture from incoming fresh air become a necessity? A full blown WUFI analysis would inform the question of course. MemBrain or Intello on the interior faces of the framing would help reduce the effect. Even so, the ultimate result may be unknowable. Add to the mix that climate trends presage rising RH levels, making the issue more acute.

    When all these factors are considered, doesn't an ERV sound like good insurance?

    1. GBA Editor
      Martin Holladay | | #115

      I'm not sure who told you that "the accumulation of moisture within the framing cavities ... can progress over years or even decades, before reaching problematic levels." That's not an accurate description of moisture risks to framing or sheathing.

      High indoor humidity levels in cold climates can cause sheathing or framing damage, but only if the sheathing or framing is cold during the winter. If the house has a continuous layer of insulation on the exterior side of the sheathing, there will be no cold surfaces -- and therefore no worries.

      In general, the risk increases if the house has air leaks (because air leaks provide channels where moist air can travel in winter) or if the house has sheathing that is cold during the winter. The risk decreases if the house has low levels of air leakage and a continuous layer of exterior insulation.

      Keeping the indoor humidity in a reasonable range is always wise and helps lower the risk of the type of sheathing damage we're talking about. That said, you don't need an ERV to maintain a reasonable range of indoor humidity. Other approaches also work: exhaust-only ventilation, for example, or controlling the sources of your indoor humidity (by covering dirt floors in crawlspaces or basements with polyethylene, for example, or reducing the number of houseplants).

  111. qofmiwok | | #114

    Have costs really gone up this much since 2012? I'm at $27k for a Zehnder system installed. I'm conditioning 2.5x the air, but 1800 sf was only $5400 in 2012?

    1. GBA Editor
      Martin Holladay | | #116

      Q. "Have costs really gone up this much since 2012?"

      A. Yes. Construction costs are crazy right now, due to huge jumps in materials costs and a booming construction economy, which allows contractors to raise their bids without fear that they will run out of work.

      Your $27,000 bid for an installed Zehnder system allows you to claim the prize for the most expensive ventilation system ever mentioned on GBA. It's a dubious prize.

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