Bad idea to have foundation vents under a deck?
I’m in climate zone 4-coastal (Seattle area in the PNW — apparently one of the few locations vented crawlspaces can actually work), and have a requirement for 1 sqft ventilation per 1500 sqft. Fairly low (and I’m unable to encapsulate and condition on my budget / at this point in the project). I am shooting for setting up 3 vents to produce cross-ventilation (1 vent on one of the far ends of the house, and 2 on the other). I made a diagram showing potential locations in the same wall at different stages of constructions so you can see what I’m working with, but basically they need to go into the openings of a currently-shear-paneled cripple wall under my deck (in order to avoid rim joist penetrations).
I can either put 2 of these vents under my deck or along the walkway just beyond the stairs of the deck. Under deck is nice (out of sight), but I worry it may result in less effective ventilation of the crawlspace or bringing gross-er air into the crawl from under the deck.
Is there a rule of thumb on this, by chance?
Thanks!
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Alternatively, I could pop out a few vents that we accidentally poured into the foundation and covered, but that would prevent a cross-breeze (See attached diagram/pic). Would that meaningfully reduce the effectiveness of my crawlspace ventilation? The East and West walls are about 30' apart (ie the short length of the rectangular crawl in question is 30').
I am assuming you have a class 1 vapor retarder down or else your vent area would be much higher. With that in place vent placement is much less of a concern. I would not be too concerned about placing a vent under a deck. With that vapor barrier retarder down you will not be drying the area so much as making sure it can dry with incidental moisture intrusion, and things like cross ventilation and corner ventilation aren't as much of a concern.
Very helpful. Thanks!
Are you aware of whether the relative humidity under a vented deck is notably higher than the typical ambient? I suppose I also don't want to bring overly damp air into my crawlspace.
no data but unless your deck is super enclosed I don't think I would worry about it. Especially if you are using a trex product and you protect you girders and joist tops with something like vycor, you would have essentially no moist wood and trex heats up in the sun. But again, no actual data one way or another, maybe I will put a temp/humidity sensor under my porch for a year....
Hah! Good idea, re: sensor. And yeah what you're saying seems reasonable. Thanks!
In Seattle's coastal climate zone 4, your cross-ventilation plan sounds reasonable. While the requirement is minimal, proper vent placement is crucial for effective airflow.
Placing vents under the deck might be neater, but it could compromise ventilation quality. Vents should ideally capture clean, fresh air, so the walkway beyond the stairs might be a better option to avoid drawing in damp or dirty air from under the deck.
Make sure to check with a local expert familiar with the area's specific conditions. They can provide the best advice to ensure your crawlspace stays well-ventilated while considering the design of your home. Visit for more: https://www.roofersofarlington.com/