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.”
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.”
“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:
- 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.
- 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.”
- 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.
- 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.
- 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.
- 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.