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General energy efficiency housing question

MikeGreen | Posted in General Questions on


I am hoping someone can help me out with a general question.    

I am in a newly built home (3 years old).  It was well built and air sealed. Our energy bills are quite low so that helps support my theory of a well sealed house. I can give more details upon request. We have an HRV installed to be used for fall/winter/spring when the house is generally closed up and requires ventilation. We have found in the winter that the house tends to get very dry. I have our HRV set to not continuously run but set at a level of 50%. The house doesn’t get to that humid and the HRV doesn’t kick on. In the winter the humidity in the house is around 25-30%. We are noticing that the floors are cracking a bit at this lower humidity. Why would the humidity be so low in the house? Should I be running the HRV on low continuously during the winter months? The house is large (approximately 6000 sqft) with only three occupants. We use forced air natural gas to heat. Are we just not generating enough moisture for a house that size to keep the humidity high enough? Should I be looking at getting a small humidifier to help generate more moisture.  

I live in Ontario Canada so we do get cooler winters but nothing that would be that extreme most winters. 

Any insight or advice you may have would be greatly appreciated. I have asked countless HVAC contractors but they are not experts in efficiency.

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  1. Expert Member
    Dana Dorsett | | #1

    The outdoor air is pretty dry in winter, but if the house is occupied there is usually more than enough moisture emitted to keep the house at 30% or above (the low end of the human comfort & health range, 50% being the high end of that range.)

    Forced air heating with imperfectly balanced ducts or that has a makeup air port on the return path will drive air infiltration while operating, which would ventilation air at some unknown/uncontrolled rate, which would lower the indoor humidity. Under the US Energy Star homes program, with ducted heating & cooling system the room to room pressure difference must be under 3 pascals (0.012" water column) under all operating conditions, room doors open or closed. Above that the air-hander driven infiltration rates even in pretty tight homes can measurably add to the energy use numbers, but also comfort/humidity issues such as yours.

    1. MikeGreen | | #2

      Thanks for the response. I know theer is no makeup air port on the return path. And i beleve the system to be well balanced.
      If I wanted to keep the humidity at say 40% in the winter what would be the safest way to achieve this? Install a humidifier right on the ducting or just use a portable one? Would you even recommend using a humidifier or is my issue elsewhere.

  2. Expert Member
    Peter Engle | | #3

    I would not recommend humidifying the air, given your conditions. 25%-35% is still well within the range of human comfort.

    Aside to Dana: Where do you get 30%-50%? ASHRAE used to use 20%-65%, and they still use 65% as a maximum for occupied buildings. ASHRAE has now removed any minimum humidity from 62.1 for residential ventilation because of condensation and microbial growth issues.

    The more humidity you add to the air, the more chance you've got for condensation causing damage to the building and for microbial growth, both in the HVAC equipment and in building cavities. In my area (Zone 4), 35% is just about the limit for safe interior humidity in the winter before I start seeing condensation and damage in standard, code-minimum building shells. Ontario is zone 6 (I think), and lower humidity would be a good idea.

    Risk of condensation is managed by the quality of insulation, air sealing, and vapor management. All of that depends on the local codes, enforcement and construction quality. Still, adding humidity has its risks, and the humidity levels you describe are really not particularly low.

    Also, operating an HRV based on humidity is probably not the best control method. It is better to do it based on timing and required air exchange rates. However, with the humidity you report, it is likely that you are already getting enough accidental air exchange and the HRV is probably unnecessary. With 6000 square feet and only three people, you've got a lot of space for dilution. It sounds to me that you've got a pretty balanced house that's performing well and you seem satisfied with the energy costs. Don't mess with a good thing.

    1. Expert Member
      Dana Dorsett | | #19

      >Where do you get 30%-50%? ASHRAE used to use 20%-65%, and they still use 65% as a maximum for occupied buildings.

      ASHRAE levels have little to do with human health, nor are they concerned about the health of the building. They're more interested in what is perceived as comfortable.

      The medical world concerned with indoor air quality brackets 30-50% RH, but that's not universal. Some put the optimal human health range brackets at 30-60% others 40%-60". None put the low limit below 30%, even though most people are still reasonably comfortable at 20-25%.

      Below 30% susceptibility viral & bacterial infections increase (pneumonia, anyone? OK, how about a plain old cold?) Above 50% dust mites will survive and procreate (arguably only a problem for those allergic to dust mites). Above 60% susceptibility to fungal & yeast infections increase, even though 65% isn't a comfort problem for most people.

      There are many versions of this sort of diagram being promoted by different organizations to be found on the web:

      The optimum zone and the points on the wedges will vary a bit from diagram to diagram (depending on what they're selling? 😉), but there is general agreement in the health care industry about the type of health risks that occur below 30% or above 50-60%.

      FWIW: Energy Star also cites the 30-50% numbers. Last sentence of the first paragraph on their mold discussion page:

      " Indoor humidity levels between 30% and 50% are ideal."

      So does the Mayo Clinic:

      "Ideally, humidity in your home should be between 30 and 50 percent. Humidity that's too low or too high can cause problems."

      ( From: )

      "If your indoor humidity is higher than 50 percent, you may have increased exposure to mold in your home."

      (From: )

      Keeping it 50% RH indoors all winter is pretty comfortable and healthy during the winter, but in the spring the much higher stored moisture in the building materials induced by humidity levels that high can lead to high mold spore counts, which is can be a human health problem and potentially a long term problem for the building. Building scientists tend to look at even 40% RH indoors as excessive and an increased risk to the building. The IRC prescriptives for exterior insulation presume indoor dew points under 45F, which would correlate to 7oF/40% RH air. If keeping it more humid than 40% indoors it takes more exterior foam or a Class-II vapor retarder to keep the sheathing from accumulating potentially damaging levels of moisture over a winter.

  3. Expert Member
    Dana Dorsett | | #4

    An indoor RH of 40% is on the high side for wintertime in a cold climate, since that increases the likelihood of excessive moisture accumulation along exfiltration paths.

    Believing a system is well balanced isn't the same as measuring it. A dual-port hand held manometer designed for measuring the static pressure drops in duct systems with a resolution of 0.01" water column isn't very expensive (under CDN$200) and can be useful in finding the worst offenders on room-to-room imbalances. Very few houses have ever had room to room pressures measured as part of commissioning the house. Indeed many duct systems have never been commissioned/verified with static pressure measurements. When measuring room to room pressure differences with that instrument anything over 0.01" would be suspect. Since it's at the very bottom end of the measurement range it's worth measuring it in both directions to rule out potential error.

    Houseplants can raise the indoor RH and improve the indoor air quality by other measures at the same time. Table top water features/fountains or heated fish tanks can add moisture to the air too, but without the air cleaning/oxygenation benefits of plants. Active humidification with humidifiers is a last-ditch band-aid.

  4. MikeGreen | | #5

    All, thankyou for the responses. You are getting me answers that I was unable to find through the local HVAC people and to have expert advice is great.
    I am just surprised to hear that humidity levels down to 30% would be possible on a newly built house(even in winter). I figured the HRV would need to be run continuously to keep the level in check with a well air sealed house. I did do a bit of an experiement last winter where I boiled water and placed water by the intakes to the forced air system to increase the humidity in the house. I was able to get the humidity (measured in various places through the house) up to a level of 45% with no condesation on any of the windows. I am not sure what that means...
    We do have several large indoor plants in the house but perhaps we should get some more. :)
    Would it hurt the humidity to run the HRV on a very low setting continuously? I like the idea of getting fresh air into the house during the winter but don't want to make my lack of humidity worse. Any thoughts?

    1. Expert Member
      Dana Dorsett | | #8

      Running the HRV in winter always removes moisture- there's no getting around it.

      If you're dropping below 30% RH @ 20-22C you're almost certainly getting some "natural ventilation" (aka "infiltration") from somewhere, possibly quite a bit. The path that incoming air is taking and where it's delivered may or may not be all that great from an air quality point of view. You can always turn on the HRV for awhile if smells are lingering in some rooms.

      Bumping the RH up to 45% in winter is a potential moisture accumulation risk along the path the air is taking out, and not recommended.

      As a point of reference, my antique house isn't the tightest house in town, but tighter than when I moved in, and a bit less than half the size of yours, also with an occupancy of three. The local winter climate & outdoor humidity is comparable to Toronto. There's a continuous 22cfm HRV running in the basement, but no active background ventilation on the first floor. I try to hold the line at 35% max when it's below 0C outside, which sometimes requires running the bath or kitchen fan for several hours if it's been creeping up on 40%. While there is a larger first floor heating zone covered by a hydro-air air handler & ducts, most of the time that zone is heated with a wood stove. When heating that zone solely with the hydro-air for long periods the air-handler driven air infiltration on that 90+ year old VERY unbalanced duct system keeps the humidity in check, but doesn't drive it to the point where active humidification is required. The experience at my house suggests your ventilation/infiltration rates are probably higher than you think.

  5. walta100 | | #6

    In my opinion how much you need to run a HRV depends on how tight your house is.

    Short of having a blower door test anyone saying how tight a house is flat out guessing and or hopping.

    Some will argue with me but I see no reason to have or run an HRV unless have a blower door test less than 1 ACH50. Ok you will need a HRV if you have 12 people living 1000 sf house with 3 ACH50.

    The upper limit for winter indoor humidity depends on a few factors.
    1 the lowest outdoor temp you will see.
    2 The U value of your windows.
    3 Your window to wall percentage.

    It will be impossible to make your indoor air dew point above temp of the inside of your window glass. As any moisture you add by boiling water will be condensed on the window glass and run down onto the sills fast as you boil it.

    You need to be aware you are playing a dangerous game with mold and rot when you push the upper limits of winter humidly. Because if that moisture finds a cool spot anywhere in the house and may start mold or rot. How sure you do not have one under insulated corner? If you set back your thermostat far enough at night some spot will fall below the dew point and collect water and may mold or rot.


  6. Jon_R | | #7

    Running your HRV is about diluting pollutants, not controlling humidity - without data (say > 5-14 [email protected]) indicating otherwise, run it continuously at ASHRAE 62.2 rates.

    Even with no unintended infiltration, it can take a lot of water (eg, 7 gallons/day) to maintain indoor humidity with HRV use - no surprise that your %RH is/will be low.

    What interior %RH is safe for the house is highly dependent on the design. With some designs (often involving pressure control), high can be safe (eg, indoor swimming pools).

  7. MikeGreen | | #9

    Everyone. Thanks for the information. This is really useful.
    I am guessing that everyone would recommend a blower door test to confirm leakage. I am also interested in this result. However, I am also a bit hesitant to shell out the money for one at this point (perhaps my cheapnes). I guess i assumed that a newly built home above code would be fairly "tight". I am assuming i guessed wrong. Is there any way to do a basic DIY check to see if I have any really large leaks?
    Also, from the things I have read is it reasonable that I use my HRV continuously on low if only to help with indoor air quality.
    I am actually located north of Toronto. Looking at statistics. January had an average low of -12C and an extreme of -26C. February had an average low of -9C and extreme of -20C. We didn;t have many warming cycles so it was quite dry all winter. Is it expected my indoor humidity should be in the 25-35% range with those temps, even with a super well sealed home?

  8. Expert Member
    Dana Dorsett | | #10

    When it's below -5C out every day the amount of moisture in a volume of air is quite low compared to the amount of moisture in 30% RH @ 20C air. Dropping the outdoor temps to -25C doesn't really change how much moisture is lost in the air exchange by very much.

    At 30% RH @ 20C every kilogram of air has about 4.5 grams of water.

    Air at -10C/90%RH it's about 1.5 grams of water for every kilogram of air, so you're giving up 3 grams per kg with a straight exchange.

    That's about 67% of the moisture gone with each exchange.

    At -25C/90%RH there's about 0.35 grams per kg, so you'd be giving up 4.15 grams /kg or about 90% of the moisture.

    At a more humid 20C/40% RH the indoor air has about 5.8g/kg, so even with the more humid -10C/90%RH each exchange is giving up ~75% of the moisture.

    Whether it's "only" 67% of the moisture disappearing vs. 90% or some other number above 50% doesn't make a heluva lot of difference- it's still ALL about reducing the numbers of air exchanges to a manageable level.

  9. NormanWB | | #11

    Get you a Wizard Stick on Amazon for around $25 and a box fan. Close all the doors and windows and shut off the HVAC and any fans. Place the box fan in a doorway venting out and seal the rest of the doorway and around the fan with 6 mil plastic. Turn the fan on high and fire up the Wizard Stick. Using the "smoke" for the WS, go around to the common leak areas (around doors and windows, outlets, baseboards, any wall protrusions, etc.) and let the smoke tell you where you might have a problem. Caulk and foam as appropriate.

    While not as cool or measurable as a blower door test, this technique will identify many leaks for around $50.

    You can also get a FLIR Pro camera for your phone. It will identify temperature differences and help target insulation and air sealing issues. When done, sell it on Ebay to get a good bit of your outlay back. BTW, the Pro is worth the extra over the standard one. Much better pictures and better detail.

    1. Expert Member
      Dana Dorsett | | #12

      A large reversible 16" window fan such as the Lasko 2155A (or larger, if it fits) in a window is quite a bit easier to use for leak detection than the box-fan in the door approach. To use that particular model you need a window opening at least 27" wide and 22" tall. They run about $100, sometimes less.

      Even the $200 lo-res version of the FLIR One (the Flir One-Pro is twice the price) are good enough for this purpose. The difference between the One and One Pro are negligible if you're using a phone for the display. If using a tablet computer for the display the difference in resolution of the the thermal imager is more obvious, (160 × 120 vs. 80 x 60 pixels) but still not a big deal for this use. For other applications being able to pin-point a hot/cold spot calls for the higher resolution, but air leaks make substantial sized cold spots.

  10. calum_wilde | | #13

    For $100 you can get two home energy audits, one before any upgrades and one after, that include a blower door test. At least here in Nova Scotia. I'd recommend you look into it for your home. There might also be an incentive program that will pay you back a portion of the money you spend on any upgrades.

    ASHRAE 62.2 lists a calculation to help set your HRV. In recent years the required ventilation rate has been dropping, it's likely you might be over ventilating. If you look up the reference you'll have to make a couple of assumption and you'll need the specs for your HRV, but it should get you in the ball park. When I did the gazintas I realized I could drop mine from running constantly to running 20 minutes per hour and still be a touch on the high side of 62.2.

  11. Expert Member
    Dana Dorsett | | #14

    ASHRAE 62.2 isn't really built on much science, and has had a lot of pushback in the past decade, notably from the folks at Building Science Corporation. You can now take an "infiltration credit" deducted from the ASHRAE 62.2 active ventilation levels, but only if you run a blower door test and run the results through their magic credit formula.

    Building Science Corp's proposed standard is for 7.5cfm per occupant, plus the whole house floor Area x 0.01 cfm. The generic occupancy assumption is the number of bedrooms, plus one, but with a known number of occupants you can adjust that accordingly. The assumed double-occupancy of one of the bedrooms is for sizing the equipment when the actual occupancy is unknown.

    The primary difference between BSC and ASHRAE is area multiplier: ASHRAE 62.2 2013 calls out Area x 0.03 cfm + 7.5 (bedrooms + 1), instead of BSC's proposed Area x 0.01 cfm + 7.5 (bedrooms + 1).

    For a 6000' house that would be 60 cfm for the floor area ventilation component per BSC, compared to 180cfm under ASHRAE. With three people add 22.5 cfm and you'd be at ~ 200 cfm per ASHRAE, but only ~ 80cfm per BSC. That's less than half the ventilation rate.

    In reality most of the time for non-smoking families who don't use aerosol spray products indoors it will be safe to cheat those numbers, as long as you're religious about using kitchen & bath exhaust fans appropriately. With only very modest and well controlled indoor air pollution sources you could still have reasonable indoor air quality even at 50 cfm background ventilation or about 1/4 the ASHRAE recommended rate.

    Of course none of this fine-tuning of active ventilation matters if you're dealing with large amounts of air-handler driven infiltration. It could be akin to adjusting the showerhead flow to conserve water while there's a fire-hose sized leak going in the basement.

    More details here:

    1. calum_wilde | | #15

      Thanks Dana, I didn't realize the air handler would cause that much more extra infiltration than an HRV. Also, it's time to redo my numbers to see if I'm still over ventilating.

      1. Expert Member
        Dana Dorsett | | #16

        A typical 100KBTU/hr hot air furnace's air handler is typically pushing between 1000-1500 cfm, a 150K furnace is pushing 1500-2000 cfm. What size furnaces / air handlers are you running?

        If even 5% of that air flow is taking a path outside the building envelope it'll be a large fraction of the BSC's recommended ventilation for your house. If 10% of the flow is taking an external path it'll be more than the BSC recommended ventilation, more maybe even more than ASHRAE 62.2. One of the arguments BSC has with ASHRAE on ventilation is the excessive wintertime drying at rates that high.

        At least air handler isn't running at a 100% duty cycle, but the duty cycle increases as temperatures grow colder, which is when the air is colder & marginally drier.

        1. calum_wilde | | #17

          I don't have an air handler, just ductless mini splits with baseboard electric for back up.

          I did the math for my place, roughly 2200sqft with four occupants, and it came out to 50cfm. IIRC, That's ~ what I set my HRV. I think when I did it I was reading those articles on BSC vs ASHRAE and forgot about the reference.

          As always, thanks.

          1. Expert Member
            Dana Dorsett | | #18

            I was getting confused, making reference to Mike Green's situation. With a 6000' house and typical (=ridiculous) oversizing factors on the hot air furnace he could be looking pretty substantial air handler driven infiltration while it's running. Fortunately that's never a problem with ductless.

  12. MikeGreen | | #20

    wow... this is a lot of information. I have a lot of reading to do to start understanding everything you guys have discussed.
    Thanks for the input.

  13. T_Barker | | #21


    I went through almost the exact same situation in my custom built 2,400 sq. ft. house last year. I'm in Northern Ontario, zone 7A, forced air, 2 storey + basement, windows R=4+, HRV, etc.. My house is almost 20 years old so even though it was built "very well" at the time using 2000 era energy standards, your house is most likely more air tight.

    You are absolutely correct that with the HRV running even at low speed and/or intermittently, your indoor relative humidity in the winter will drop to uncomfortably low levels (i.e. ~30%). I don't care what anyone says, you will need supplemental humidification for comfortable living conditions in the winter. In fact, I strongly disagree with almost every current recommendation regarding which climate to use an HRV vs. ERV. Contrary to current "wisdom", the next house I build in this northern climate will have an ERV to help control the dryness in the winter. Even for a new super insulated, ACH=0.5 house, I still need to be convinced otherwise.

    Balancing the health chart recommended RH levels with my personal experience and desire for non-cracked skin or nose bleeds, we aim for close to 40% RH inside our house during the winter. Low to mid 30's is just too low. And it works. Consistent RH in the high 30's to 40% makes all the difference for comfort. And no mold on the window sills or other obvious issues - AS LONG AS THE HRV IS RUNNING. If there happen to be minor hidden mold spores behind the walls (which I doubt), frankly I don't give two hoots - when I'm long gone the next guy can deal with that, lol.


    The motor on my LifeBreath HRV wore out and started this whole issue a year or so ago. Because we were so much more comfortable at the resulting increase to ~40% RH when the HRV was broke, I procrastinated and purposely didn't fix the HRV for a few months. Then we started to see mold issues on the window sills. But I told my wife I wasn't going back to low 30's RH all winter (6 solid months here). So at the same time we replaced the LifeBreath motor and turned the HRV back on, we also added improved humidification to the HVAC system. We now maintain 40% RH inside during the winter with the HRV running, but no mold. So it's a combination of the two that matters. Enough circulation of fresh air, combined with humidification up to 40% in my experience.


    1.) You need the HRV running for fresh air. My HRV is set to LOW, on all the time (I could retrofit and add more sophisticated intermittent control, but it works well as is). My furnace fan (Carrier Infinity) is set to CONTINOUS run even when there is no heat being generated. So there is always fresh air being circulated in the house.

    In my case I have a hybrid system, where the HRV fresh air goes through the HVAC ducts, while the HRV exhaust air has it's own set of ducts. It actually works pretty well, although not the way I would do it today. Would I prefer a system where an inline CO2/VOC monitor called for the correct amount of fresh air in each room as needed? Absolutely. But don't get me started on the current lack of such technology in the residential construction industry. Of course, that's a discussion for another day.

    2.) Have your HVAC contractor install/attach a flow-through humidifier on your furnace. Period. End of story. I was a skeptic because we suffered for many years with a drum humidifier attached to the furnace and it was a piece of crap. But the flow-through humidifier is a huge improvement. For my house, 17 gal/day model. Anything smaller won't be enough. For your house, I suspect you'll need significantly more capacity. It keeps our RH at high 30's and 40% - exactly where it's comfortable. Your HVAC guy might try to sell you on an attached steam humidifier, but I had concerns about condensation in the ducts, maintenance, etc., as well as the cost.

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