Building Science-Wintertime Interior Humidity

As I write this post at the end of November, our outdoor temperature is 28°F with an outdoor humidity of 75%.  Inside my home, the temperature is 70°F with a humidity level of 21%.  Slightly uncomfortable humidity levels for my family.  During last year’s polar vortex, when the temperatures reached nearly -40°F, my indoor humidity dropped to 9%, much too dry.  Knowing what I know about building science, I will not operate a humidifier.  This post will explain why.

Relative humidity is the amount of moisture content, or water vapor, in the air at a certain temperature.  The warmer the air, the more water vapor that can be present, cooler air contains less water vapor.  Dew point is the temperature at which the air is saturated.

If we take cold air with, let’s say 50% relative humidity, and warm that air, the humidity level will decrease.  The amount of the decrease will depend on the increase in the temperature.  This is why the humidity level inside my home during the last years polar vortex was so low.  My home has a high natural air exchange rate, as the cold, outdoor air enters my home and is warmed, the humidity level substantially decreases.   In my case, humidity levels in the single digits to the high teens are normally seen most of the winter.

Why not just add humidity using a humidifier?  Again, I have a high natural air leakage rate with most of the leakage in the ceilings of my story and a half Cape Code home.  Any moisture added to my indoor environment will end up leaving with the natural air change rates, as it leaves, it may encounter a cold, condensing surface.

Central humidification system tied to a forced air furnace.

Chances are, my roof sheeting will be below the dew point temperature, or the temperature at which the moisture I add to the air condenses into a liquid, or worse, a solid or frost.  This liquid or frost will build-up over the course of the heating season, any frost or ice will eventually melt as the temperatures warm in spring, damaging building materials, ceiling and walls.  Depending on how quickly the moisture dries, there could be rot or mold growth.  If you live in a more moderate climate, the moisture may remain a liquid and simply make the condensing surface wet.  When building materials become too wet, bad things tend to happen.

I took this photo several years ago, a frosty attic.  The areas that are “whiter” are the nail tips from the roofing nails holding the shingles in place.  These nails are colder than the wood boards and are the first place frost forms.  As the other surfaces become cooler, the rest of the roof becomes the condensing surface and frost forms across the attic.  The cure is to lower the moisture content of the air or warm the condensing surface.

How can a home be designed or renovated to handle a higher level of humidity?  This is easier accomplished when designing a new home.  Trying to change an existing home to handle higher humidity levels will most likely require a deep energy retrofit.  The best option in both cases is to warm any condensing surface so the temperature stays above the expected dew point temperature.  This is accomplished by adding exterior insulation to the walls and possibly the roof.  Comprehensive air sealing and using materials that will allow assemblies to dry if they become damp is also a good strategy.  In my climate, high performance windows are a must.  During cold temperatures, the normal window becomes a dehumidifier condensing moisture out of the air.

This window was installed at the code minimum house.  They are a builder grade with slightly higher than minimum performance, U-.28.  Humidity levels were high due to construction moisture, outdoor temps were around 20°F when this photo was taken.

What humidity level can the normal home in a very cold climate handle?  An average home might be able to handle around 30% interior relative humidity, depending on the age and type of construction.  (My home will not handle those humidity levels.)  Even at 30% RH, some frost will form on the average window during very cold temperatures.  A newly built, high-performance home with lots of exterior insulation, very good air sealing details and excellent windows might get as high as 40%.  This will all depend on the outdoor temperature.

Is there a way to increase humidity levels without using a humidifier?  Yes, but…the home should be built with above code minimums in insulation levels, preferably exterior insulation, construction needs to be tight, less than 1.5 ACH50, and there needs to be an upgrade in windows, triple pane, U-.20 minimum.  Once all these improvements are implemented, a balanced mechanical ventilation system can be changed from a Heat Recovery Ventilator (HRV) to an Energy Recovery Ventilator (ERV).  The main difference between the two is the HRV works by removing interior humidity, kind of like a dehumidifier would work.  The humidity accumulates inside the unit as frost or ice.  The HRV has a defrost cycle which changes the solid water back to liquid where is drains out of the HRV.  ERV’s take the water in vapor form and move some of the moisture back into the incoming cold air stream.  You mainly see ERV’s in more moderate climates, not cold climates.  I have only seen a small handful of ERV’s in my market.  The newest ERV models have been built with cold and very cold climates in mind.

What do medical expert say about indoor humidity levels?  The EPA recommends humidity levels between 30% and 60%.  I’ve read a few articles that suggest 40% is the minimum humidity level to reduce the spread of some airborne pathogens, including Covid.  There needs to be discussions and further testing to find a good balance between human health and a healthy, durable home.  Human health should always come before the health and durability of a structure, but cold climates have different considerations, higher humidity may lead to other issues, such as mold growth as temperatures increase at the end of winter.

The main point of this article is humidity is good for us as humans but too much can be bad for our homes.  Finding a balance is much easier in a home that has been designed to be “high performance”.  The owners of older or code minimum-built homes will need to make a decision, increased indoor humidity levels and the risk of damaging the structure with the benefits of higher humidity and human health.

2 Replies to “Building Science-Wintertime Interior Humidity”

  1. Good stuff, Randy

    I will do some research on the cold climate ERV’s. When I first built superinsulated homes we used HRV’s. The units performed well but the lack of timed controls over ventilated the homes and dried them out. The best I could do in those days was a 24 hour dial timer to run the HRV for a period of minutes per hour. My own house currently is at 27% relative humidity. The volume is 28,800 cf and the ACH50 is 3.14 with no mechanical ventilation. In mid winter the humidity reading drops below my weather station minimum of 25%. It takes a long time in the spring before I start getting a reading again. I like the ventilation air standard of 15 cfm per bedroom plus 15. So a 3 bedroom house would need 60cfm continuous.

    Would be interested to know about perceived comfort and indoor humidity. We dislike high humidity in the summer but seek a higher humidity level in the winter for increased comfort. I use AC very sparingly in the summer and this maintains about a 55% RH.

    I have seen attics with a frost buildup much more extreme than your photo. Attic bypasses are top of the list for building performance upgrades.

    1. Thanks Doug. It would be interesting to track building tightness and indoor humidity levels in a very cold climate in homes with no mechanical ventilation. Just over 3 ACH50 and you’re able to hold 25%, me at 10 ACH50 and in the teens. I’m watching to see at what outdoor temp mine drops, today I’m at 25% in my basement with outdoor temps in the 20’s. We are forecast to drop to -10° Monday night, we will see where the indoor humidity level goes then.

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