To vent or not to vent a crawl space in the Pacific NW?
I bought a house in the city of Goldbar half way between Seattle and Steven’s Pass.
The roof is metal and has gutters on one side only; so I have rain falling to the ground near the foundation.
There is a crawl space: about 40″ tall. There is no standing water. The ground has a poly vapor barrier that covers entirely and part way up the stem wall. The stem wall is concrete and has not been treated or covered. The 2X12 floor joists are full of bat fiberglass insulation. No visible mold.
When I open the access door to the crawl space I can smell musty moldy smells. Many of the crawl space vents have been blocked. I believe that the previous owner thought that closing the vents would help keep the house warmer in the winter.
Considering that the house is in the NW, do I encapsulate the crawl space and completely block the vents? Do I open the vents to allow air to circulate in and out? Do I add a De-humidifier and pump the condensate water outside? Do I install an exhaust fan tied into a humidity sensor that turns on when the moister reaches a curtain level?
Do I put up gutters and divert the roof water away from the foundation?
Please reply.
Jim
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Replies
I live in San Juan County and inspect several crawl spaces every month. The large majority are vented, and are in good condition, without mold, mildew, or elevated MC in the framing lumber. We have fairly breezy conditions up here, and that helps quite a bit. We also have well-draining soil for the most part, with little clay. My own crawl space is 80 years old, well vented, and in mint condition.
If you have no ductwork in the crawl space, I would keep it as vented. If you have ductwork, there is a possible case for a sealed crawl, but IMO executing it correctly is harder than a vented crawl. If you hire a contractor to seal and insulate your crawl, do your homework on best practices.
The biggest issue I see is poor stormwater management around the outside of the foundation that leads to wet soil or standing water in the crawl space. This is a drainage issue and needs to be dealt with by footing drains, sometimes a curtain drain, and in rare cases a sump pump. You should definitely have gutters and subdrains that lead to daylight well away from the house.
Jim,
If you decide to seal the crawl space, this article will tell you how to do the job: Building an Unvented Crawl Space.
If, on the other hand, you decide (after reading David Meiland's comments) that you prefer a vented crawl space, you should open up the blocked-off vents, and then monitor the crawl space to see if the musty odors get better or worse.
In either case, you should install roof gutters where they are missing and direct the roof water away from the foundation.
Also, keep an eye on those fiberglass batts. Over time, they tend to fall out. If you want an insulated floor, it's a good idea (in the long run) to protect the batts on the underside with foil-faced polyiso, OSB, or both.