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Q&A Spotlight

The Air in My House Is Too Dry

In Ontario, a homeowner wonders why relative humidity inside his house is so low and what he can do to raise it

The owner of this Ontario home is satisfied the house is both well built and effectively air sealed, but low indoor humidity in the winter has him concerned. (Photo: Mike Green)

Mike Green’s Ontario home is well built, and his low energy bills convince him that the house also has been well air sealed. But there is a problem he’d like to solve:  low indoor humidity during the winter — low enough to cause floors to crack.

The 6,000-square-foot house is heated with a forced-air system and is equipped with a heat-recovery ventilator. Green has set the HRV to run only when indoor humidity exceeds 50%.

“In the winter the humidity in the house is around 25-30%,” he writes in a Q&A post. “We are noticing that the floors are cracking a bit at this lower humidity. Why would the humidity be so low in the house?”

Green asks whether running the HRV around the clock would help, or whether he should buy a small humidifier to put more moisture into the air. Ideally, he’d like to see indoor relative humidity at about 40%.

What’s the best way of accomplishing that? That’s the topic for this Q&A Spotlight.

Make sure the HVAC system is balanced

Outdoor air is typically dry during the winter because cold air is capable of holding less moisture than warm air. Even so, says Dana Dorsett, the air inside an occupied house in winter should have a relative humidity of at least 30%. That, he says, is at the low range of what people would find comfortable (50% is at the upper end of this range).

He suggests that a forced-air heating system that is not perfectly balanced, or one that has a makeup air port (an outdoor air duct) connected to the return-air plenum, will increase air infiltration and lower the indoor relative humidity.

The Energy Star program requires that ducted heating and cooling systems be designed and installed to ensure that the air pressure difference between rooms must never be more than 3 pascals (0.012 inches of water column) under all conditions, no matter whether a room’s door is open or closed, Dorsett says.

“Above that the air-handler-driven infiltration rates (even in pretty tight homes) can measurably add to the energy use numbers, but also comfort/humidity issues such as yours,” he says.

However, Green replies, there is no makeup air port on the return path and the system appears to be well balanced.

‘Don’t mess with a good thing’

Peter Engle isn’t so sure that Green’s system is out of whack.

“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,” Engle writes. “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.”

The humidity levels that Green describes are not particularly low, Engle says. He adds that it would be better if Green ran the HRV on a timed schedule, rather than basing operation on a humidity setting, in order to get the required number of air changes.

“With 6,000 square feet and only three people, you’ve got a lot of space for dilution,” Engle says. “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.”

Learning from experience

T. Barker, writing from northern Ontario, has some words of advice for Green. His 20-year-old house was well built at the time, and Barker says he had the same experience that Green did. The bottom line? Green is right: his house is too dry.

“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%),” Barker says. “I don’t care what anyone says, you will need supplemental humidification for comfortable living conditions in the winter.”

This heat recovery ventilator has been set to run only when the relative humidity indoors is greater than 50%.

Barker, in fact, disagrees with typical recommendations on HRVs and their cousins, energy-recovery ventilators (ERVs), which  exchange humidity in indoor and outdoor air streams as well as heat.

“The next house I build in this northern climate will have an ERV to help control the dryness in the winter,” Barker says. “Even for a new superinsulated, ACH0.5 house, I still need to be convinced otherwise.”

In order to avoid cracked skin and nose bleeds, Barker aims for an indoor relative humidity of about 40%.

What should indoor humidity be?

Engle isn’t so sure that Dorsett is correct in his assertion that indoor relative humidity should ideally be between 30% and 50% during the winter. ASHRAE, he points out, used to recommend a range of 20% to 60% but has since dropped the minimum from its 62.1 Standard on Ventilation and Indoor Air Quality because of concerns over condensation and microbial growth.

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

The 30%-to-50% bracket comes from the medical community, but it’s not universal, Dorsett says. It might range from 30% to 60%, or from 40% to 60%. “But none [of the published recommendations] puts the low limit below 30%, even though most people are still reasonably comfortable at 20%-25%,” he says.

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

He cites studies from the Mayo Clinic and the Energy Star program to back up his claims.

“Keeping it [indoor relative humidity] at 50% 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,” Dorsett says.

“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,” Dorsett adds.

Further, there is some disagreement among building scientists over what the best ventilation rates are for a house. The Building Science Corporation recommends a different formula than does ASHRAE, Dorsett says, that would result in a lower rate for Green’s home.

What role is the HRV playing?

Dorsett believes that Green’s HRV will inevitably lower indoor relative humidity — “there’s no getting around it.” But, he adds, if the RH falls below 30%, that’s a sign of that outdoor air is infiltrating at an undesirable rate.

“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,” he says. “You can always turn on the HRV for a while if smells are lingering in some rooms.”

Walter Ahlgrim looks at HRVs a little differently, basing a decision on whether to have one in the first place on how tight the house is. “Some will argue with me, but I see no reason to have or run an HRV unless I have a blower door test result of less than 1 ach50,” he says. “OK, you will need a HRV if you have 12 people living in a 1,000 square foot house with 3 ACH50.”

In the absence of a blower-door test result, Ahlgrim says, there’s no way of knowing just how tight the house might be. He says the upper limit for winter humidity depends on three things: the lowest outdoor temperature Green is likely to see, the U-factor of the windows, and the window-to-wall percentage.

Jon R contends that running an HRV is not about humidity at all, but about diluting pollutants.

“Even with no unintended infiltration,” Jon R says, “it can take a lot of water (e.g., 7 gallons/day) to maintain indoor humidity with HRV use — no surprise that your indoor RH is 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 (e.g., indoor swimming pools).”

Finding and sealing the air leaks

Green isn’t ready to pay for a blower door test to find out how leaky his house might be, but Norman Bunn has an alternative method for him: use a Wizard Stick that generates vapor and a box fan to depressurize the house.

“Get you a Wizard Stick on Amazon for around $25 and a box fan,” Bunn writes. “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.”

The vapor will pinpoint leaks around doors and windows, outlets and baseboards. Green can then caulk and foam the leaks.

If Green invests in a FLIR Pro camera, he will be able to see temperature differences that will guide insulation upgrades and better air sealing.

Calum Wilde suggests Green invest $100 and get two energy audits, one before any upgrades and one after. The work should include a blower door test.

Our expert weighs in

GBA technical director Peter Yost added these thoughts:

I have these recommendations to add to the substantive exchange above:

  1. Know the accuracy of your devices used to measure moisture content. Because there is quite a bit of variation in accuracy from device to device, make sure that you are comparing apples to apples as you assess and adjust.
  2. Add humidification with great care. I checked in with my two local high-performance HVAC contractors (ARC Mechanical and Jeffers & Sons). Both of them said that if the client insists on system humidification, it should only be a steam system, such as the AprilAire Model 800, with outdoor reset. Tim Jeffers added that he normally uses an air prover pressure switch, “a little pricey but it keeps it from raining in your ductwork and over-humidifying.”
  3. The indoor relative humidity you maintain in the winter for safe building assembly performance is a function of the class of interior vapor retarder your assemblies have. In other words, the higher the relative humidity indoors, the more effective the vapor retarder must be.
  4. What’s the main issue: the movement of finished wood surfaces or comfort of occupants? We let our interior relative humidity vary  extensively season-to-season in southern Vermont. All wood trim, doors, and floors expand and contract without harm; they were designed to do this. If there is no harm, there is no foul.
  5. Since the two main causes of dry indoor air in the winter in cold climates are air infiltration and run-time on sensible-load-driven HRVs, I don’t see how you can decide which to focus on without a run-time sensor on your HRV and a blower door test.
  6. For several years before I got my first blower door, I used a powerful window fan combined with turning all house exhaust fans on and a digital pressure gauge to depressurize most homes to about -25 Pa for qualitative assessment of air leaks. It worked pretty well, as long as I didn’t need quantitative airtightness information.

 

 

27 Comments

  1. Alan B | | #1

    25-30% in Ontario suggests high air leakage. And its not a comfortable humidity level. So my advice is get a blower door test done. Yes its a few bucks but if your saving money on bills then spend some of those savings.

    All that said it is a large house with only a few people so the indoor humidity sources are smaller then a typical house with smaller size and likely more occupants.

    Finally if the floors are cracking consider low humidity tolerant floors in the future.

    I'm not averse to supplemental humidification if you know what your doing and don't over do it (something to carefully figure out) but that does add cost and in this case may be a band aid instead of the proper solution. If you have a high air exchange rate then your putting a band aid on it with a humidifier. If you have low air infiltration and its still happening figure out why before you add a humidifier. I like to know why things are the way they are before tinkering with them because you are then either fixing the real problem or you are properly understanding why the solution works.

    BTW is the HRV still an open path with outside is it not even when its not running (albeit with a thermal exchange core) or does it have baffles on the inside?

  2. Trevor Lambert | | #2

    I keep seeing references to <30% RH being low, uncomfortable, or destructive. I find this kind bizarre. I came from a place that was genuinely dry (Calgary, AB). In the winter it was very common to have indoor humidity of 10% for weeks or months continuous. I experienced this both at home and in the workplace. We had a lab we struggled to keep the humidity of one room above 20% with 24/7 humidification. I have a hard time believing that 25-30% RH contributed to cracked floors. If that was the case, I would have expected to see nothing but cracked floors where I lived, yet I don't actually recall seeing even one. Aside from using a lot of lip balm, I also don't recall being particularly uncomfortable at 10-20% RH.

    1. User avater
      Dana Dorsett | | #5

      An RH of 30% is the low end of optimal human health, not comfort. ASHRAE draws the comfort lines at 25% & 65%. Below 25% split fingernails & chapped lips rise in prevalence, as well as more significant health issues.

      My office is normally running 15% in winter, and noticeably less comfortable than at home, but nothing much to complain about. My house (in a local climate comparable in nearly all respects to Toronto) stays in the low to mid 30s most of the winter until/unless there is a sustained cold snap with daily mean temps cooler -15C for longer than just a couple of days, at which time it can drop as low as 25%, usually not lower. This isn't the tightest house on the block but perhaps tighter than most, and there is ~20cfm of HRV running 24/365, and consistent appropriate use of bath & kitchen exhaust venting.

      The gaps in the 95 year old hardwood flooring installed parallel to the plank subfloor (why did they do that) moves a bit seasonally but not a whole lot. The gaps in the 20 year old flooring with plywood subfloor in an addition doesn't move much at all. We usually hold the line at 60% RH in summer, but occasionally it'll stray to 65% when outdoor dew points are north of 20C unless we're running the AC. There's very little sensible load here (a favorable PM shading factor from trees & hill, and a modest 1% outside design temp), so it tends to creep up a bit during the summer, but can stay high during the fall shoulder season.

  3. Lance Peters | | #3

    I grew up near Toronto and am now in Ottawa, both CZ6A though Ottawa is quite a bit colder. Dry winter air is definitely a problem in average houses here. I currently live in a 2004 built townhouse and I installed a humidifier to keep our RH above 30%. Without it RH would drop to 20% or less in Jan-Feb, and problems like nosebleeds, dry skin, static electricity were a reality. I installed shrink film to our average 2004 2-pane windows and can get by without excessive condensation at 30-35% RH even with night time lows well below -20C.

    People on this forum have constantly told me that humidifiers are evil, unnecessary etc. While I completely understand their point of view regarding exfiltration moisture risks, I have yet to be given a reasonable solution that does not involve supplementary humidification. In a house that's already built, there's only so much you can practically do to increase air tightness.

    ERV vs. HRV? There's absolutely no question in my mind; cold climates require an ERV if they are to be operated with any reasonable duty cycle. An HRV brings outdoor air in that's unbelievably dry; at -20C and 50% RH the dewpoint of that air is -28C. Bringing that air up to 22C drops its RH to 2%! Ventilating with an HRV has the same effect on indoor humidity as infiltration, while an ERV conserves indoor humidity by passing some of the outgoing moisture to the incoming air. An ERV will still dry out a house in the winter months, but it will do so at a much lower rate than an HRV.

    The opposite is true in the summer as well. When it's 32C and 65% RH outside (dewpoint 25C), an HRV brings that humid air indoors to an environment where it's at 100% RH, driving indoor humidity UP. An ERV will pass some of that incoming moisture to the outgoing air, increasing indoor humidity much less than an HRV will.

    Tip: it takes a lot of energy to evaporate water, so a humidifier will cost you energy-wise. In the winter, avoid running exhaust fans when appropriate. Shower without the fan, just leave the bathroom door open. Boil water on the stove without running the range hood fan, only use it when cooking greasy/smelly foods or things that otherwise pollute indoor air (unless you use a gas stove, then use your hood all the time).

    Tip: shrink film on windows works at decreasing condensation, put it on bathroom and kitchen windows or anywhere you plan to generate moisture. Putting it on all windows will allow higher indoor humidity levels without windows sweating and dripping. It's tricky to install cleanly, patience is a virtue. Leaving bug screens off windows in the winter also helps, with or without the film, as screens slow airflow and increase condensation.

  4. Jon R | | #4

    > "indoor relative humidity you maintain in the winter for safe building assembly performance is a function of the class of interior vapor retarder "

    I'd say it has more to do with the total moisture robustness of the partition. For example, some well vented roof/attic designs with Class III interior vapor retarders have been shown to perform fine at 50% RH and other designs with little or no outward drying and Class I interior vapor retarders have been shown to not work well even with much lower humidity.

  5. Peter L | | #6

    The solution is simple. Start cooking Italian and boil pots of pasta. The indoor humidity will surely rise.

    1. sethw | | #12

      Right on! Dinner parties over wine help too...bodies giving off moisture and sweating from all that dancing!

  6. SteveTheInalienable | | #7

    Funny, I have the other problem.

    Just moved into a new build, and I'm struggling to get the humidity down under 50%.

  7. User avater GBA Editor
    Martin Holladay | | #8

    Steve,
    You're right, of course. Some homes (the leaky ones) often have low indoor humidity during the winter. Other homes (usually, the tight ones) have high indoor humidity.

    In your case, you're probably dealing with ordinary construction moisture. In the summer, you can deal with this moisture by operating an air conditioner (and in some cases, a dehumidifier). In the winter, you can address construction moisture by operating your ventilation system.

    (I hope that your house has a whole-house mechanical ventilation system.)

  8. SteveTheInalienable | | #9

    She's definitely not leaky!

    We did the blower door test on Friday; 0.6 ACH50! A new record for my builder.

    I'm running my ERV, but we're kind of shoulder season still do is not doing much.

    1. Lance Peters | | #10

      Steve, congrats on the tight build! Where are you located and what climate zone?

      1. SteveTheInalienable | | #16

        Southern Ontario, climate zone 6.

  9. sethw | | #11

    Great article & detailed info. My comments:

    1) If floor cracking means seams opening up, there may be a lack of Client education by the design and construction professionals going on here. I find that many Clients nowadays don't know that wood floors (non-engineered types, especially in wider board widths) will have joints which open up during dry seasons. That's the deal. It's wood. It's ok. Some expected movement and change is part of wood's natural beauty. Some design and engineering professionals don't talk to Clients about this - and so some Clients think wood flooring seams opening up are a problem.

    2) If the heating is forced air heated by flame burner (rather than hot water coil), that may be contributing to uncomfortably dry air circulation - regardless of what the overall relative humidity is in any one given spot in the house. Consider changing to hot water coil for the forced air heating system.

    3) Though the Aprilaire system mentioned above has been good in my experience, and adding spot humidifiers could help, I'd do the above #1 and #2 items - because adding more gizmo's might only complicate the issue. Start simple, with the simplest solutions using the least number of variables, then only proceed is those don't work.

    4) Or do like my grandparents and parents did, and put a few pots of water around the house...let them evaporate into the air. Cheap and simple. BAM!

    Cheers.

    1. User avater
      Dana Dorsett | | #13

      sethw: Regarding #2, please 'splain me how a condensing gas furnace dries more than a heat pump or hydro-air?

      It just DOESN'T happen! There is no moisture being added or removed from the air with any of those heating systems. A bad DUCT design/implementation that isn't well balanced &/or leaky can drive excessive outdoor air infiltration while operating though ( whereas ductless solutions don't). Set up properly the exit air at the registers would normally be between 100-125F (38-52C) with any of them (possibly lower with some types of heat pumps.)

      The higher register output temperatures of "scorched air" non-condensing fossil burners has a very localized stream of drier RH air directly in the much warmer air stream (a lower RH due to the higher temperature) but with proper mixing/diffusion at the register grilles that's not really an issue either.

    2. Lance Peters | | #15

      Sethw:

      1. True, however, controlling temperature and humidity keeps wood from moving around. Keeping indoor RH between 30-50% will take care of the majority of wood product expansion/contraction issues. This should also be communicated to new home owners, especially those with hardwood flooring.

      2. A gas furnace does not humidify or dehumidify a living space, it simply adds heat. Unless the occupants are sitting directly over a register, the overall humidity in the house is all they should be worried about. For a given air temperature rise in the forced air system, the heat source (gas burner or water coil) makes no difference in the RH of the air moving through the system.

  10. Don Fugler | | #14

    Even if that 6000 square foot house is tight (as I suspect it is), three people will have a hard time creating enough ambient moisture to sustain moderate to high indoor humidities in winter. Do the math. More recently built houses have better basements too, so that the soil is not adding moisture to the house over the winter as happens with older basements. This sounds like a case where a humidifier would be useful. I would still limit its use to providing 30-35% in the depths of winter to avoid moisture problems in the envelope.
    For Steve in the new house at 50% RH, it takes a year or two for construction moisture to leave and the house come to equilibrium. It pays to over-ventilate for the first winter in a new house so that the moisture leaving the framing, drywall, and concrete does not create a high RH.

    1. SteveTheInalienable | | #17

      We're trying! Even with the ERV set to Max all day, we're struggling to hold 50% humidity...

      1. Lance Peters | | #18

        Dewpoints are dropping fast; it’s -1C and 70% RH in Ottawa right now, dp-5C. Continuous use of your ERV will start dropping your RH soon.

        Are your ERV exhausts located in bathrooms and kitchen? How big is your house and how many occupants?

      2. Jon R | | #19

        I haven't run the numbers, but I expect that over ventilating with an ERV is sometimes less efficient (and certainly less effective) than running a dehumidifier.

        1. User avater
          Peter Engle | | #20

          The advantage of over ventilating with the ERV is that you also wash out the VOCs that are still outgassing from all of the new stuff in the house - cabinetry, finishes, carpets, furniture, etc.

          Steve does mention that he has an ERV. That's not going to dehumidify as effectively as an HRV, because it's capturing the outgoing humidity and transferring it to the incoming airstream. During the swing seasons when the outside DP is low and the temperatures not too low, opening the windows is going to dehumidify much more effectively than the ERV. With outdoor temperatures in the 10C-20C range, open windows are probably the most efficient and effective ventilation and dehumidification. Adjust the windows to be open just enough that you don't have to add much heat and let the indoor RH float for a few weeks. Of course in the great white north its probably already too late for that. I'm still enjoying the no heat - no cooling season in zone 4.

          1. Jon R | | #25

            On the other hand, when it's 10-20C and raining outside, opening the windows (or any amount of ERV or HRV) won't provide adequate dehumidification.

        2. Lance Peters | | #21

          Jon, you bring up a good point. Dehumidifiers add heat to the living space, but that's not an issue in the heating season. Perhaps a dehumidifier is a great way to step-up dehumidification in winter.

          Of course, Peter brings up a good point about new construction air quality. I certainly wouldn't want to under-ventilate in a new home simply because a dehumidifier was in use. I would think the proper ERV duty cycle should be priority #1 with the dehumidifier used only as a backup to keep RH in check.

          1. Jon R | | #22

            They even add heat efficiently - I calculate a COP of 2.8 when using a portable dehumidifier for heating. On the other hand, at low outdoor temperatures, you are going to want a lower %RH than a typical dehumidifier can produce.

            Maybe the moisture recovery of an ERV can be disabled.

  11. Trevor Lambert | | #23

    Jon,

    I'm pretty sure the moisture recovery is integral to the material of the enthalpy exchanger core. The only way to disable it would be to swap it out with an HRV core. Zehnder is the only one I know of that you can field swap cores.

    I'm curious how you calculated the COP for the dehumidifier. Seems suspiciously like a "free lunch" scenario, aside from the fact that the heating season is rarely the time you'd want to lower house humidity.

  12. Jon R | | #24

    I added the heat of condensation to the waste heat. 0.63 kWh/L * 2.89 L/kWh (a common dehumidifier efficiency) + 1 kWh/kWh

    Steve's case is a good example of wanting to lower house humidity during the heating season (and an ERV not doing enough).

    1. SteveTheInalienable | | #27

      Wow! Seems like I missed a bunch of the discussion while I was off working on my basement.

      We definitely have a lot of VOCs that we're trying to vent, from things like carpets, and new mattresses and the like.

      I likely would use my dehumidifier, but we're still in a half moved in state, and it's in storage!

      Luckily, my Mitsubishi heat pump has a dry mode. We've had a couple of warm sunny days, where the sunlight just pours in my big South facing backx windows, and we've been operating in dry mode. Just 2-3 hours is enough to drop the humidity down to around 40%, which is great.

      Operating the heat pump in this fashion seems to be very efficient, per unit water removed, and the house would be uncomfortable anyways without it, so it's a win-win!

  13. Trevor Lambert | | #26

    Yes, it's a very good example, but probably a very short lived example.

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