What do building science wonks do in the vague days between Christmas and New Year's? They eat and rest and eat and rest, and then get bored and start to wonder about things like relative humidity, indoor air quality, air cleaners and filtration, and how they relate to their own house. Well, that’s what I did this year.
I live in a house that was built in the mid 1980s (for BS nerds: 2x6, R20 nominal batt in walls, R40 nominal attic/flat ceiling w/standard heel trusses, poured concrete walkout basement with R12 standoff interior wall, no subslab insulation, 2x pane windows, most low-e w/air fill, wood frame, some new w/low-e+argon, insulating spacers and vinyl frames. ATT = 4.5ACH@50Pa. 65% efficient HRV). It has a slowly imploding radiant ceiling panel heating ‘system’ that we augment with radiant space heaters and a decent airtight woodstove.
The house is a work in progress (shoemaker’s kids, etc.).
I have plans for a phased deep energy retrofit, including a mid-velocity ducted cold climate heat pump. I’ll talk about that, and other plans, in future articles.
Low Humidity in Houses Makes Your Nose Itchy
In the meantime, we struggle with humidity levels all year round: RH too high in the summer, too low in the winter. Too low in the winter is what I want to write about today. What’s too low? How about 20-25%. Ideal RH is 30-50%. In past years, we’ve put a pot full of water on the woodstove to keep the RH at least in the high 30s. In past years, I’ve relied on a humidistat randomly to show where we are, and resigned myself to a permanent, slightly dry nose and scratchy throat as the tax for my yummy woodstove and cozy main floor as we save up to replace the stupid heating non-system.
Early in the spring of 2021, I bought an Awair monitor, because I was curious about any ongoing correlation between running the woodstove and PM 2.5 (particulate matter)/VOC (chemical) levels. Turns out, there wasn’t any.
The woodstove causes very slight bumps in particulate matter in the main living area when the door is opened too quickly after pulling the damper open, very occasionally when there’s not enough temperature difference to establish a good draft up the chimney, or, typically, when cleaning the ash dump. Opening the door too quickly can push the PM 2.5 from 5 to 20 µg/m3, like it did this morning.
Awair tells me 15 is the threshold for ‘not excellent’ (where the line in the chart changes from green to yellow). And it clears within 5 minutes in both those situations. Stoked and cheerily keeping us warm and happy, there’s nothing but low RH associated with the stove.
Getting Particular About Particulates
What I did find affecting the air quality in the whole house, not just the main floor area is this: we do a lot of cooking. A lot. Always have, and likely always will until I can’t. When the house is closed up during the cold season, the Awair shows that using the cooktop or the oven consistently causes a rise in PM 2.5 and/or chemicals that doesn’t clear, even though we religiously use the exhaust hood when the cooktop is in use, and operate the HRV in continuous high speed mode.
For instance, coffee making causes a peak in chemicals (volatile organic compounds or VOCs), but no jump in PM 2.5. When I cooked 2 meals on New Year’s Day (brunch and dinner: French toast and bacon, veggie stir fry, if you’re wondering), the PM 2.5 was elevated for ±90 minutes for brunch and over 120 minutes for the stir fry. Each cooking event itself was less than 15 minutes, with the range hood on high.
So that’s interesting. As we know, range hoods are supposed to suck. As in suck air out, not suck at doing their job, but most of them are horribly ineffective. For more on that subject, here’s a whole rant.
Back to the low humidity problem.
I bought a humidifier. A fancy-pants one with about a million settings and a big water tank because there was a $75 savings/incentive on them. I filled that tank up from the tap (well water, no chlorine) and pressed go, target 40% RH, at 2:00 on December 27th, and…PM 2.5 went off into the red zone and stayed there with some bobbling up and down as the humidifier turned on and off when RH hit 40%. Peaked at 352 µg/m3 on the 27th, 722 µg/m3 on the 28th (low 308), back down to 2 µg/m3 when the tank ran dry and then back up again after I refilled the tank.
What happened next? I know you’re all on the edge of your collective seat to find out!
I plugged in a HEPA filter and built the Corsi-Rosenthal box I’d been meaning to get around to for our household while there was nothing else to do. How much would that reduce the PM 2.5 being caused by the humidifier?
I recorded the Awair readings for the morning and early afternoon of the 31st, because that’s what a nerd would do on New Year's Eve. Here are some readings (outdoor conditions were 2°C, 94% RH, Dewpoint 1°C).
While the HEPA filter dropped the PM2.5 somewhat, the CR Box kicked butt in cleaning the air. However, changing to reverse osmosis water from the tap water was the game changer. The tap water was causing the jumps in the PM2.5, and there was so much particulate matter launched into the air that neither the HEPA unit or the CRBox could sufficiently clear it out of the main floor living area.
We have a softener system to treat our hard well water. ‘Softened’ water is supposed to be okay for humidifiers as it has lower levels of soluble minerals in it. After 2 and a half days of running on tap water, the humidifier had splotches of fine white dust on it. The dust is the result of soluble minerals being pushed into the air, and it can ironically cause more health problems than the humidifier solves. I thought that it might have been mitigated by the softener system, but nope. Good thing we have that reverse osmosis system and spigot installed for our drinking water, because distilled water is $$$$$!
And now, we’re back to reasonable RH levels with lovely, low PM 2.5, except when we cook.
Hardcore nerds: this is the main living area, with position of woodstove, Awair monitor, humidifier, HEPA/CRBox, and cooktop noted.