Ventiliation and Dehumidification in Temperate Climate
I have a 100+ year old home in Seattle that uses mini splits for heating and cooling. In Seattle, it rains 3/4 of the year where temperatures range from 40-65F, and the summers are dry at 70-80 (a few weeks of 90 or 100+F these days) . The goal is freshier, drier air indoors during the winter months for my bedrooms.
My first thought was using and ERV/HRV. My understanding is ERVs try to equalize indoor/outdoor humidity while HRVs just push the outdoor humidity inside. According to online calculators, when it is 40-55F and 90% relative humidity the total humidity linearly increases from 5-8.38 g H20/kg air outside. When inside it is 68-75F and a desired 42% RH yields 6.14-7.82 g H20/kg air inside.
Playing with the numbers a bit, it seems like any rainy days at 55F or above would push humidity into the house. For example, a 60F, 90% RH outside would cause the 72F house to be at about 65% humidity. So, ERV/HRV would work for the winter but in the fall and spring would cause indoor humidity to be too high.
I also looked at ventilating dehumidifiers. Unfortunately since there is only supply side ventilation I am concerned the resulting positive air pressure would cause wall cavity condensation during the winter when the inside 70F, 45% RH air hits the uninsulated 20-30F siding on cold days.
It seems like my best option is an ERV with a backup dehumidifier as needed, or using my mini-splits’ dehumidifying mode. I am wondering what GBA thinks of my calculations though, or if there are any other existing options that I’ve missed that might be useful for this climate.
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Replies
James,
This is what I find on Southern Vancouver Island, which is probably quite similar. As you say, in the winter I can keep inside RH below 50% with ventilation, but have problems doing that in the shoulder seasons (like right now).
The summers surprise me. We have no appreciable rain for months, but with the temperatures about the same inside and out, and the windows open, the RH sits in the low 60s.
I guess the question is, if you can control interior RH in the wet winters, what are the downsides to letting it get a bit high in the shoulder seasons, or when the house is open to the outside in the summers?
My wife is getting serious about fixing her allergy issues and has learned lower humidity decreases airborne mold spores and prevents dust mite breeding.
I'm thinking of using a ductless ERV and a backup dehumidifier, perhaps an in wall unit. Pricey but seems like it's my only option
James,
That's probably a good strategy.
Why not an HRV if low humidity is ideal? Curious because I’m in a similar situation.
If the humidity is coming from the outdoor air you want an ERV to reduce that humidity before it gets in the house. If the humidity is coming from indoor sources you might want an HRV unless you also have a dehumidifier. If you have a dehumidifier that can keep up with indoor humidity sources then you still want an ERV.
Gotcha. Thanks.
Malcolm, quick question. I always see people say to try to keep the indoor humidity below 60% to avoid mold. I'm also in the PNW, and like you, I have no problem at all keeping humidity below 50 from about November through about May. But in the summer, and especially here in Sept-Oct, my home's humidity has crept up into the low 60s.
The last few weeks, because it's been a bit cooler outside, the temperature inside my home is around 68-70, humidity is average maybe 62% or so, and dewpoint is often around 55-56, which is a perfectly comfortable and objective good dewpoint as far as I know.
In your understanding, is low 60%-s humidity a mold issue for these few months per year? Or would you view it to be totally fine?
I actually bought a portable dehumidifier about 2 weeks ago but to keep the humidity below 60 uses 2ish kWh per day, and the thing is a bit louder than I like. If 62% RH for a month or two isn't a meaningful mold risk I'd probably return the dehumidifer and save the energy use!
pnw_guy,
Mine is 62% this morning, and has been in that range all summer. We have been in this house for about 25 years and this is what I have found.
I've had both walls and the roof exposed for various projects and have not seen any problems with the structure. I did have some issues with a hipped portion of my roof, but that was unrelated to indoor RH.
We do get some mold on piles of things stored tightly against exterior walls and left undisturbed for several years - luggage in closets and the like.
For some reason we get some blue mold on the underside of our antique dining room table in late summer, but not any other pieces of furniture.
I'm not willing to close my house up in the summer to lower RH. That's the whole point of summer. I have toyed with trying to do something about the shoulder seasons, but in the absence of anything more than the problems I've posted I'm not sure it's worth it.
Maybe wait until you see something worth addressing before going back to dehumidifying?
Thanks so much for sharing your experience. It does give me a bit more confidence to "wait and see" whether I need any dehumidification.