Rose red COVID virus particles on a pale aqua background, one large in-focus particle in the foreground on the left, several smaller soft focus ones in the middle and back ground

3 Building Science Rules You Can Rely on When Covid Hits Your Household

Shawna HendersonMarch 01, 2022

Following on the heels of my own COVID adventure, I’ve got some things to say. First: the actions we took are completely in line with building science actions (eliminate, ventilate, filter/clean) to maintain a healthy indoor environment, regardless of COVID. Second: The likely vector was high school as my partner and I work from home (WFH) and have been able to control contacts and situations, where the kids have not had as much control and we know several of their friends have had to isolate over the past several weeks. Both high schoolers tested positive the same morning I did (12 Feb) but were asymptomatic. I had significant symptoms, which is why we all tested. I was positive, my partner wasn’t. We immediately swung into action: 

  1. Everyone masked up with KN95s (Layfields, super easy to wear for long periods of time, inexpensive, made in Canada). 
  2. Then we got the CR box and the HEPA air cleaner running, to protect my uninfected partner. (We’ve been running our HRV at high speed all winter, to keep viral load down regardless.)
  3. I moved my electronics/biz work stuff, some clothing, another duvet, some books and some craft stuff into the spare bedroom, along with lots of Tylenol, Advil, saline nasal spray, cough medicine and my puffer. 
  4. I then closed the door and cranked open the window (I had access to a space heater, if needed). 
  5. We isolated the asymptomatic teens here so the other set of parents, whose work is outside the house (we both WFH), would be at less risk of infection.
  6. I was bedridden, so partner brought food to me and I only moved, masked, from bedroom to bathroom shared with teens. 
  7. Teens fended themselves in a separate shift from partner, with masks on while prepping food, eating in isolation, and we slotted time in between each shift for the CR box to do it’s thing. 
  8. Teens tested negative (18 Feb) and bugged out to the other parents’ home as fast as possible to be back in the world. 
  9. I finally tested negative on 22 Feb. 
  10. Partner remained uninfected.

So what did we do right?

We followed the 3 key rules in building science for dealing with indoor pollutant levels to create and maintain good indoor air quality and a healthy indoor environment:

  1. Elimination
  2. Ventilation
  3. Filtration 
Bubble diagram showing the many factors involved in creating and maintaining a healthy indoor environment and indoor air quality

Eliminate (Isolate the Source)

In terms of building science, the first thing to do to provide and maintain a healthy indoor environment is to eliminate as many sources of pollutants as possible. For instance, if you were trying to establish a better living space for someone with severe asthma, that could look like stripping out carpets, eliminating possible sources of moisture/mold, rehoming pets if it was really desperate. For COVID, we can swap out isolation and masking for elimination.

Ventilate (Dilute the Load)

Closed spaces concentrate pollutants, so bringing in fresh air helps to dilute pollutant loads. Ventilation dilutes pollutant loads but does NOT solve the problem. However, sometimes, you cannot eliminate the source, or guarantee that it will stay eliminated, and so the second defense is to dilute the pollutant load inside the closed-in living space. Spot ventilation such as bathroom fans and kitchen ranges are obvious for diluting and clearing smoke, smells, and humidity. A balanced whole-house ventilation system (HRV or ERV) works on a constant basis to provide fresh air.

Filter (Clean the Air)

Sometimes it’s not possible to eliminate or ventilate a space very well. That’s where filtration comes in. This is simple cleaning, not treating the air. HEPA filters and high efficiency furnace filters (MERV 13) are the go-to products here, they provide large amounts of surface area and capture pollutants like layers of swiss cheese. The more surface area the air has to move through, and the finer the mesh of the filter, the better. That’s why Corsi-Rosenthal boxes work. That’s why KN/N/95s work.

Infographic showing how to stop spread of COVID in three columns. First, eliminate or isolate the source of pollutants by masking and isolating. Second, ventilate or dilute the load by opening windows and turning on fans, using space heaters as needed. Third, filter or clean the air with air cleaners and masks

What did we do right?

We eliminated pollutant sources. Me and the teens. Well, we really didn’t eliminate people that would be nice. But we were isolated. And yes, it’s boring but we all went through months of lockdowns, so we can cope with 5 to 10 days to keep our loved ones safe.

We ventilated the house. HRV on at high speed, windows open in the bedrooms where we isolated, bath fans on periodically.

We filtered the air. 2 filtration units on full speed, full time, masks on for everyone as much as possible, and definitely when the infected ones were moving around the house. 

Inverted pyramid showing the importance and impact of industrial hygiene measures, starting with elimination and moving through mechanical controls to personal protective gear




Look at how nicely those 3 key rules for houses line up with industrial hygiene. Like there’s science behind  this approach or something, eh?



Is Your House Gasping for Air?

A caveat on ventilation. Houses with oil or gas fired space and/or water heaters (or a wood burning appliance) can be prone to backdrafting, or combustion spillage when exhaust fans are running. How this happens: all fuel-fired appliances need air for combustion. That’s how fire works. At the same time, combustion appliances rely on chimneys or vents to exhaust waste products. 

Depending on the condition of the building envelope, it becomes harder or impossible for the vent or chimney to work properly when exhaust fans also start pulling air out of the house. Opening a window to dilute the viral load conveniently allows more air into the house, compensating for the air pulled out of the house by the exhaust fans, so combustion spillage is not an issue. 

If you are concerned about possible backdrafting or combustion spillage in your house in general, you can have an energy advisor do a depressurization test with a blower door. If the test shows that your house is at risk of combustion spillage, there’s a straightforward fix: have a make up air duct installed in the vicinity of the appliances.

To learn more about indoor air quality, healthy materials choices, and ventilation, check out our 3 module mini course Healthy Indoor Environment.

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