Crawl space vapor barrier and insulation

[opiesche]

So, I did a bit more reading and it seems like if you encapsulate your crawlspace you also need a dehumidifier to make sure moisture doesn't accumulate. They seem relatively expensive (on the order of $1200)...

[Daox]

Where is this moisture coming from if you've encapsulated the space?

[Exeric]

I think what he may be refering to is any moisture that accumulated down there before it was encapsulated. I agree that buying or renting a dehumidifier doesn't seem justified if its just temporary. There's other ways of going about it. Here's a link:

Building an Unvented Crawl Space | GreenBuildingAdvisor.com

Like I mentioned before, a permanent solution is venting from the conditioned space above is one option. In this case you would put a fan in the crawlspace that has no more than 30 or 40 cfm and continuously but slowly take the crawl space air directly outside. Then you would put in a couple registers in the floor so the fan would pull warm air into the crawl space. The air exchange would be slow enough that it would also serve as an expulsive ventilator for the house. So you would be achieving two goals with one slow moving fan.

This seems to be a much better option that what I was thinking about before, which was using a fan to directly push warm air into the crawlspace and then have the cold crawlspace air return though registers in the floor. Both ways use open registers between the crawl space and house interior. Its better to have the fan pushing crawl space air outside. Radon issues is also a good reason to do it that way.

Both ways of using a ventilation fan are writen into the building code as acceptable. BTW, for this to work you really have to close up the crawl space reasonably well for outside air movement, except though the fan pushing it outside. Otherwise you would get return air coming from outside instead of from inside the home.

[Exeric]

Actually, maybe you don't even need to go the encapsulated route. According to the article I referenced crawl spaces in areas like ours that aren't humid in summer don't have the same mold problem humid summer areas have.

Again, I admit to personal bias and (perhaps) unexamined assumptions because my own crawl space has water infiltration problems in winter. If you don't have that problem, or if you eliminate the water infiltration problem, then maybe you can get away with just venting the darn thing. Ideas can be overly trendy and not always apply to individual conditions. I'm as guilty as the next person on that, but at least I try to keep thinking about it. You can arrive at the right answer through a very circuitous route sometimes.

[opiesche]

I'm not an expert by a long shot, so all of this is based on what I've read and what common sense tells me (which may be completely off base .

What I was thinking was that unless you create an air- and vapor tight seal between the living space and the encapsulated crawl space as well, vapor pressure should always tend to equalize humidity between the two spaces. If it is very humid then, say in summer, the exchange between living and crawl space could lead to condensation, because the crawl space temperature may be below the dew point.

Like you said though, our summers are dry, so there's not much of a threat there. Winters are humid, but in Winter the crawl space is unlikely to be cooler than the exterior air (and the interior isn't likely to have higher humidity than exterior), so again, condensation should not be a problem.

The only thing I see occasionally is the dirt ground in the crawl space getting wet, to the point of water accumulating a tiny bit in some spots. I think the vapor barrier should avoid most problems in the crawl space with that. I may still go the encapsulated route, simply because it makes sense to me, and would make it easier to avoid things like the rot in my rim joist where the vent openings are.

[opiesche]

I've got a remote sensor, so I'm going to measure humidity and temperatures in the crawl for a few days, see what it looks like at this time of year

[S-F]

Just keep the ground and outside air moisture out of your house. Consider the crawl Space as part of the house. No vents or anything like that required. If you practice good air sealing and moisture management there should be no concerns at all.

[Exeric]

Originally Posted by opiesche

What I was thinking was that unless you create an air- and vapor tight seal between the living space and the encapsulated crawl space as well, vapor pressure should always tend to equalize humidity between the two spaces. If it is very humid then, say in summer, the exchange between living and crawl space could lead to condensation, because the crawl space temperature may be below the dew point.

Well, you are right that warm air holds more moisture. There are two routes you can go with an encapsulated crawl space. Completely encapsulate for both air and moisture like you are leaning towards. It should work.

The other encapsulated route is to provide air and moisture exchange between the living space and the crawlspace. That should work also because along with that exchange you are also creating a temperature exchange that will keep the crawlspace warm enough to keep the moisture from condensing. That will also work.

One big "but" though is that in the second encapsulation method you really have to insulate the perimeter foundation. If you don't do that then the crawlspace will get cold enough to condense the moisture coming from the living space. While it sounds like more work this method allows you to avoid insulating the floor of the house. And you also don't have to insulate the ground.

One thing that you need to know is that it is very hard to completely seal everything around the rim joists. One fairly easy way is to use 2 part closed cell foam. A benefit of using it besides air and water sealing is that it insulates very well. Soooo, since you have to use it anyway then just add thermax polyiso for the rest of the foundation wall and completely insulate the thing. Like I said, its not really more work since you avoid the work of insulating and sealing the floor itself. It's a good tradeoff.

[S-F]

Originally Posted by Exeric

Well, you are right that warm air holds more moisture. There are two routes you can go with an encapsulated crawl space. ......................... Soooo, since you have to use it anyway then just add thermax polyiso for the rest of the foundation wall and completely insulate the thing. Like I said, its not really more work since you avoid the work of insulating and sealing the floor itself. It's a good tradeoff.

Well, Basically

+1