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Choosing Between HRV and ERV

Joe_Adams | Posted in General Questions on

Choosing between HRV and ERV

Hello everyone,

I am currently retrofitting my grandparents’ house. However, due to the excessive indoor moisture (occurring mold and condensation), I am considering applying some decentralized HRV or ERV extract fans. Or even centralized equipment.

I am on marine climate. During the winter, the external RH could be very high during the night and morning periods (80-100% and 40-60 F). During the afternoons, as the external temperature rise, the RH also decreases.

My question is, is there a general guideline to choose an ERV over an HRV? Like % of the time of external RH above some threshold, etc.

Kind Regards

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  1. Expert Member
    Malcolm Taylor | | #1


    You may find this discussion useful:

    I'm on Vancouver Island (Marine zone 5) and find that despite the very high outdoor RH in the winter it's fairly easy to bring down the interior humidity with ventilation. In the summer, when the indoor and outdoor temperatures are the same and the windows are open, it's hard to get it below the mid-60s. I guess an ERV or dehumidifier might help then, but that would mean the house would have to be closed up, which to me sort of defeats much of the joy of summertime living.

  2. GBA Editor
    Kiley Jacques | | #2

    Be sure to read this article HRV or ERV?, as well as Scott Gibosn’s recent Product Guide post, Balanced Whole-House Ventilation—especially the section on choosing between an HRV and ERV.

  3. Charlie Sullivan | | #3

    If your goal is to lower indoor humidity in the conditions you describe, HRV will accomplish more of that for a given air flow rate. The possible reasons to consider ERV would be

    1. In colder weather, if the humidity inside gets too low, an ERV would be nice because it wouldn't lower humidity as much.

    2. In hot weather, if you are running air conditioning inside and that is cooling and dehumidifying the inside air, the ERV will help you keep that dryness inside.

    If you want to understand the fundamentals of this, an HRV will bring in outside air keeping the dew point constant, not the relative humidity. So in considering what it will do in a particular climate, dew point is the number to look at, not the humidity. If the dew point is lower outside than in, the HRV will help lower the humidity. If the dew point is higher outside, an ERV is preferred because it will help maintain the low humidity inside.

  4. PBP1 | | #4

    While the OP is about humidity, I was looking again at ppm CO2. Is RH the main factor? When is CO2 a factor? RH is definitely a truly proven issue, little debate about that.

    However, there seems to be some debate about health and CO2 levels, I've seen some that say 1000 ppm can make a difference but I also found this article ( that says: "Impairment of cognitive performance is not expected from exposure to 20 000 ppm CO2".

    Physiological responses, self-reported health effects, and cognitive performance during exposure to carbon dioxide at 20 000 ppm

    "24 subjects (20-58 years) were exposed to carbon dioxide (CO2 ) at 770 ppm and 20 000 ppm in an exposure laboratory for 4-h, including 2 × 15 min of cycling to investigate the effects on acid-base balance, physiological responses, cognitive performance and acute health."

    "In sum, the findings suggest that the observed changes in acid-base balance and ventilation can be classified as physiological adaptation responses. Impairment of cognitive performance is not expected from exposure to 20 000 ppm CO2 , neither as direct effect on central nervous system function nor as a distraction related to perception of health effects."

    Also this one from 2021: Human responses to high levels of carbon dioxide and air temperature (Indoor Air
    . 2021 May;31(3):872-886. doi: 10.1111/ina.12769. Epub 2020 Dec 7.) (

    "30 subjects were exposed to different combinations of air temperature (Ta : 24, 27, and 30°C) and CO2 level (8000, 10 000, and 12 000 ppm) in a high-humidity (RH: 85%) underground climate chamber"

    "Increasing CO2 from 8000 to 12 000 ppm at all Ta caused subjects to report higher rates of headache, fatigue, agitation, and feeling depressed, although the results were statistically significant only at 24 and 27°C. The text typing performance and systolic blood pressure (SBP) decreased significantly at this exposure, whereas diastolic blood pressure (DBP) and thermal discomfort increased significantly. These effects suggest higher arousal/stress. No significant interaction effect of Ta and CO2 concentration on human responses was identified."

    1. Expert Member
      Malcolm Taylor | | #5


      Neither can be said to be "the main factor", and both need dealing with, but for different reasons. While both are affected by ventilation, RH is usually only a problem for the building, while CO2 and other indoor pollutants typically affect it's occupants and the building couldn't care less.

    2. Trevor Lambert | | #6

      An ERV and HRV of equal flow rates will reduce CO2 equally. They will affect RH somewhat differently, in a way which is difficult if not impossible to generalize. Reducing air pollutants, of which CO2 is the primary one, is the intended purpose of both ERVs and HRVs, with a secondary purpose being to evacuate sources of internal humidity. This gets complicated or even impossible if the outdoor humidity (absolute, not relative) is significantly higher than the desired indoor humidity. If high humidity is a problem and any part of it is due to external sources, then choosing between an ERV or HRV won't solve it. You'll need a dehumidifier.

    3. Eric Habegger | | #7

      I would take the cited study with large quantity of grains of salt. I, and millions of others, have noted strong subjective feelings of sluggishness and decreased ability to concentrate when co2 goes over 1000 ppm, much less 20000 ppm. Millions of scientific studies come out every year and don’t represent a lasting contribution to science. It takes a lot to move the scientific consensus and that study does not accomplish that.

      1. PBP1 | | #12

        Thanks, I found a 2021 review article:

        "Low Level Carbon Dioxide Indoors—A Pollution Indicator or a Pollutant? A Health-Based Perspective":

        "To summarise the existing CO2 concentration guidelines, CO2 levels 1000 ppm represent good or excellent indoor air quality, 1000–1500 ppm represent acceptable or moderate IAQ and concentrations >1500 ppm represent poor IAQ. These levels appear consistent with the existing literature, which reports effects starting at as low as 1000 ppm CO2 [38,43,44,46]. Two human studies investigating lower concentrations (~400–800 ppm) [66,70] found no significant correlation with health outcomes. Only one of the 51 papers reviewed reported significant effects at <1000 ppm [75]."

        Less than 1000 ppm CO2 seems to be the consensus proxy for good/excellent IAQ. I imagine effects vary from person to person and would depend on numerous factors such as altitude/pressure, other fumes from people and/or things, etc.

        People do crowded 90 minute "hot yoga" in small studios where there can be back to back classes at 105F and 40% humidity. Would be interesting to start measuring CO2 in those environments. A quick search and of course someone has already studied gyms ( One crowded gym hit over 7000 ppm CO2.

        Also a thank you to all the HRV/ERV humidity posters, quite interesting ;-)

        1. Expert Member
          Malcolm Taylor | | #13


          The problem with using CO2 as a proxy for other indoor pollutants is that high levels come from people being present in the space, while high levels of other things like VOCs could build up while the house is uninhabited, and the CO2 levels would remain low.

          1. PBP1 | | #18

            Good point, oil-based stain on wood flooring is still out-gassing. If we're away for a month, when we get back, we open up the house to get some quick air changes (6 sliders on first floor). CO2 in winter, when occupied, cycles about 250 ppm between min and max (max within acceptable range). So, I guess I have proven your point ;-)

  5. Trevor Lambert | | #8

    Absolute humidity is what matters, not RH. In order to understand what you're up against and how the ERV or HRV may help or hinder, you should convert those RH and temperatures to dew point temperatures. Do this for both external and desired internal environment in varying conditions. I suspect that in a marine climate, you'll find that at least some of the time neither an HRV or ERV will be capable of nudging your RH in the right direction. Or at the very least, you'll find you need one at one time, and the other at another time. I suspect a dehumidifier is going to be the solution.

    1. Expert Member
      Malcolm Taylor | | #9


      I suspect you are right for the summertime when the interior and exterior temperatures are very similar, but ventilation should work in the winter.

      I'm 1/4 mile from the ocean here on Vancouver Island. Even in our prolonged winter rains I can take a couple of percentage points off my interior RH by simply leaving the front and back doors open for five minutes. Continuous mechanical ventilation should be able to do that much more effectively.

      1. Trevor Lambert | | #16

        Yes, ventilation can address the humidity issue under certain conditions, but if it can't do it under most conditions you need a plan to solve it another way. Winter is usually easy, tropical climates excepted, because there's no such thing as humid, cold air in the absolute sense. If it's legitimately cold outside, the dew point has to be below desired indoor dew point even if it's 100% RH.

  6. Joe_Adams | | #10

    Thank you all for your help,

    As far as I know, ERVs keep humidity where it is desired, right ?

    Meaning that in cold dry climates it will keep humidity inside, and warm humid climate will keep humidity outside. It works both ways, not only keeping the humidity inside.

    1. Charlie Sullivan | | #14

      ERVs keep humidity where it *is*, not where it's desired. If it's humid inside but you want it dry, the ERV will keep it humid inside. If you are in a climate like Malcolm's, an ERV won't help, but an HRV will help lower the humidity.

      If you want more specific analysis, let us know your specific location.

      1. Trevor Lambert | | #15

        I don't agree with the assessment on Malcolm's climate. If it's humid outside, an HRV will raise the humidity indoors, not lower it. An ERV will raise it less in most cases by transferring some of it back outside.

        I think the way to look at it is that an ERV will do whatever an HRV would do under the same conditions, just less so. If outdoor humidity is higher than indoor, HRV will raise indoor humidity. ERV will redirect some of that incoming moisture back outside, but it won't actually be able to lower the indoor humidity.

        The only exception I can think of to this is if you have a localized area of high humidity indoors (e.g. an active shower). By capturing that moisture at the source and sending it outside, you can actually lower the future humidity of the house even if the outdoor humidity is higher than the average indoor humidity, assuming the humidity in that bathroom is higher than outdoors. The ERV will perform worse than the HRV in this scenario.

        1. Charlie Sullivan | | #17

          You seem to understand that it's absolute humidity that matters, not relative humidty. Now apply that understanding to the climate in question:

          The dew point rarely gets above 55 F and essentially never gets above 60 F. The humidity that the original post is talking about, say 80% humidity at 72 degrees inside, is a dew point of 65. An HRV will help throughout the year in that location, regardless of the outdoor relative humidity.

  7. Expert Member
    Malcolm Taylor | | #11


    I'm not sure that's an accurate description. Unlike an HRV which just transfers heat, an ERV allows the transfer of both moisture and heat. So if you are trying to use it to lower indoor RH by introducing air with a lower moisture content, it will transfer some of the indoor humidity you were trying to expel into the air being introduced.

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