Preventing mold in an under-construction crawl space
We are constructing a home with an unvented crawl space. We’ve been setting the i-joists for the floor and are nearly ready to start laying subfloor, but a storm has dumped more than eight inches of snow in the crawl space. With the wind blowing, some additional snow has accumulated there as well.
We have not yet encapsulated the crawl space, so as the snow melts it can be absorbed into the soil in the crawl space floor. However, if we lay the subfloor we’re concerned we may create an environment suitable for mold growth on the i-joists and subfloor. It could be several months (we’re in Montana) until the snow completely melts and/or it stops snowing, so waiting isn’t desirable. We’re now unsure how to proceed.
In researching mold prevention, it appears that air circulation can be helpful. Maybe we could lay the subfloor and then run fans in the crawl space? Any thoughts or advice would be appreciated.
Thanks in advance, Doug
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Doug,
I'm afraid I don't have any useful advice and hopefully someone else can chime in, but for others building similar un-vented crawlspaces: The surface of the area (preferably a concrete scratch coat) should be sloped to a floor drain. The vapour sealing of the floor, as well as any insulation that is going to be added, should be done before the framing above commences.
Thanks Malcolm. We live in a relatively dry climate (less than 13" of rain per year) and are building on a site that is not susceptible to high ground water, etc. For these and other reasons, we made the decision not to use a scratch coat or include a floor drain. Maybe that was a mistake. We also didn't complete the sealing, in particular installing the vapor barrier membrane, because we didn't want to walk on it during installation of the floor joists. We could complete it now before we put the subfloor on, except for the snow issue.
Doug,
I would consider adding a floor drain while you still can. In most areas of the US, and in all of Canada it is required by building codes.
"The surface of the crawl space floor must slope to a drain or sump pump. Any drain located in the crawl space must be allowed to run off and terminate outdoors or to an interior crawl space drain or sump pump. Crawl space drains may not run off to gutters or foundation perimeter drains, and dryer vents must be terminated outdoors. As a more general observation, any building grade should allow water to drain away from the foundation of the crawl space (Source: 2015 IRC – Sections R405 and P2719)."
Hi Douglas -
Bit of a bummer that you did not move that snow before you laid the floor sheathing.
Until your unvented crawlspace gets warm, you can't grow mold. When your crawlspace gets warm, you will need to dehumidify it.
Once the snow melts, I would get a moisture barrier down immediately to keep that load from making its way back up into your building/materials.
How tall is the crawl? Could be a real pain but no chance to shovel the snow out?
And also, since the floor is now capped, that means no additional snow/water will make it into your crawl?
Best - Peter
Thanks for your response Peter. We haven't laid the floor sheathing yet, so snow removal is a possibility, although it will likely be a challenge. Joists are 16 oc, which makes access cumbersome. We have about 3.5 feet of space under the joists. Maybe we should try and melt it somehow (propane heaters?) before installing the subfloor.
It sounds like you’ve framed the floor, but not laid the subfloor yet? Maybe you could get a snowblower into the crawl and blow the snow up and out between the joists? A little snow on the joists won’t really hurt anything, and mold won’t grow when temperatures are below freezing.
Best to get as much of the snow out as possible before laying that subfloor.
Bill
Thanks Bill. Appreciate your thoughts. You're correct that the subfloor is not installed yet. I'll have to think about the snow blower option and do some measuring to see if it will fit in there.....
I'd remove two or three joists to make pulling out the snow easier. And then I'd get the snow out of there, before sheathing the floor. If the ground is frozen, seems likely, you'll have standing water until the ground thaws, greatly complicating the work there and risking the mold problem you're trying to avoid.
All I can offer is my diy experience, but maybe there's a lesson in it. I have a building with a vented crawlspace we built a couple years ago. We ran out of time before winter to get the roof on. To add to our stupidity, we closed up the crawlspace vents. I diligently travelled to the build site after each storm to shovel everything I could off of the subfloor and framing through the winter. After 2 months, we re-entered the crawl to work down there and had quite the mold factory waiting. Not fun.
Before installing your subfloor, it might be easier to shovel out than expected if you push the snow with a plow style shovel parallel to the joist cavities towards the edges, then shovel it up and out with a square shovel. Obviously, if the snow fell while it was cold, if it has stayed cold, and if you do this before it melts too much, you will have a much easier time of it. Even if you only get half of it out, you'll still be improving the situation. Good luck!
I think it's helpful to realize that an unvented crawlspace is useful for many reasons and in many locations, maybe including where you live. However my understanding is that in locations where it is dry in summer there is one less reason to keep a crawlspace unvented, which may apply to where you live in Montana. I'm not sure.
At any rate it seems reasonable that you DO want to vent the crawlspace while you have bulk water down there. It is very important to put down a moisture barrier as soon as possible and anchor it to the foundation footings. It seems like you could do it even with some snow or ice down there as long as you level it and get somewhat close to grade. However, you shouldn't bring that moisture barrier so high up the foundation footing that you can't put in at least temporary vents. As that moisture beneath the barrier melts it will simply be absorbed in the ground. You won't be able to have a completely dry exposed surface of the moisture barrier due to the conditions you'll be working in. It doesn't matter though if you have temporary vents. That water on top of the barrier will then evaporate in summer and no further water can get down there. Then if you want you can seal the vents after all the moisture on top has evaporated.