In Winter During COVID-19, You Want the Goldilocks Humidity

Not too high, not too low, but just right.

condensation on window
Condensation from too much humidity!.

Burak Karademir/Getty Images

It's below freezing outside as I write this post, a temperature where people in cold climates should be thinking about the relative humidity. That's the percentage of the amount of water in the air RELATIVE to the maximum the air can hold at a given temperature. Warm air can hold more water than cold air, so outside cold air at, say 90% humidity, might not have much moisture in it at all. Bring that same air inside to room temperature and the relative humidity might drop to almost nothing. Doctor (and architect) Stephanie H. Taylor wrote in her paper "We need hydrated air to fight infections":

"Research continues to reveal that dry indoor air is connected to MORE infections in people. This helps explain why the flu season is in the winter, when cold outdoor air – already low in moisture, is brought inside and further dried out when warmed. The obvious solution is to provide indoor humidification to achieve a beneficial relative humidity (RH) level between 40 to 60%." 

But that's not so easy. And unfortunately during this pandemic, it's even more important. Joseph Allen, Linsey Marr, and Akiko Iwasaki explained in an op-ed in the Washington Post that humidity affects the transmission of the virus in three ways:

  • The mucous membranes in the respiratory tract do a better job of capturing virus particles, but there is less mucous at low humidity.
  • The coronavirus decays faster in higher humidities; "a new study shows that the coronavirus decays faster at close to 60 percent relative humidity than at other levels."
  • In drier air, the droplets carrying the virus dry out faster, leaving the smaller particles to travel farther and stay up longer.

Here is where we have a problem with most buildings. In this new video Tomás O'Leary, Co-Founder and CEO of Passive House Academy, explains how relative humidity works in buildings, noting that "If you over-ventilate a building in winter, you can end up with uncomfortably low indoor relative humidity. If you have cold surfaces in your home or holes in your vapour control layer, you can end up with mould and condensation." If it gets too high, you can get condensation that rots your windows, you can get mold forming on your walls, and if the moisture condenses in your walls, can cause severe problems.

In many colder climates, it can be really hard to get to between 40% and 60% without running into trouble. In its latest advice on COVID-19, the Canadian government didn't even try. Its recommendations:

"While humidifiers do not remove SARS-CoV-2 virus from the indoor air environment, they could impact the duration that particles that contain virus are suspended in the air. It is therefore important to maintain an optimal humidity level, between 30% and 50% in indoor settings. Lower humidity levels can cause droplets to shrink, and smaller droplets can stay suspended in the air for longer. However, increasing humidity too much can lead to condensation on surfaces, as well as inside walls and building areas where it cannot be seen. This can lead to  mould growth and the proliferation of mites."

I suspected the reason that they didn't specify 40% to 60% was because of the lousy building envelopes that are in Canadian houses. I asked engineer David Elfstrom, who told Treehugger:

"Notice that PHAC [Public Health Canada] used 30-50% instead of 40-60% that is the widely accepted range for minimal virus transmission. In Canada's cold climate, cold interior surface temperatures of typical building envelope components place an upper limit on the humidity level, such as on thermally unbroken window frames, and simpler window glass. It depends on the situation and region but around 50% RH in the winter is the upper limit for most buildings. To achieve that requires active humidification and if someone sets a humidistat for 55% or worse, 60%, it will never be satisfied and will greatly increase the water deposition leading to the issues noted."
Lloyd Alter

In my 100-plus-year-old home, I have almost no control of humidity, and in the past week it has bounced between 30% and 40% in the evening when we're cooking; I suspect that if we get a cold front it will drop further. It's not where you want to be.

This is one of the reasons I am so fond of the Passive House concept, where the exterior walls have so much insulation that their inside surface is as warm as the inside air, and where the windows are triple-glazed; you probably never get condensation and have no problem keeping the relative humidity between 40% and 60%.

That's the Goldilocks humidity; not too high where you get condensation problems, and not too low where the virus stays airborne and your throat dries out. It's just right.

In an earlier post, Winter is Coming; Time to Start Worrying About Indoor Air Quality there is a long list of things you can do to control indoor humidity.