Humid Crawl Space Question in SE
I live in the southeast, so I have the common problem of a humid crawlspace in the summer. I’ve seen and heard various opinions on what best to do with a crawlspace in this part of the country and have found the articles on this site thoughtful and informative. I recently had an approach suggested to me that I had not heard before, and seems to be a hybrid approach between a vented crawlspace and conditioned crawlspace. The suggestion was to (i) moisture barrier the floor, (ii) seal all of the vents, except for one and (iii) add a dehumidifier. The one vent would be left open to meet code (I guess can’t seal all vents unless conditioned), provide air for gas furnace, and provide some fresh air for the crawlspace to avoid build up of soil gases, etc.
While this would obviously not be the most energy efficient setup since the dehumidifier would be running with one vent open, it seems to me as a layman it might be a good suggestion when you take into account the numerous considerations for dealing with a crawlspace. Not too high of an install cost (compared to conditioning it), allows some fresh air in the crawlspace (which to me seems to be the downside of a sealed/conditioned system), and should control humidity (albeit with a slight energy penalty from having a dehumidifer running with one vent open). Although the monthly energy costs would be slightly higher, the initial install cost should be much less than creating a completely sealed/conditioned space. Also, I struggle to get comfortable with a conditioned space because then all of the crawl space air would enter the home.
Although energy costs are an important consideration, I am more concerned with air quality in the home, which is why this approach seems on the surface to have appeal. I’m happy to be corrected on any of this, so any thoughts or critiques are welcome. This is a fairly new home, but being in the SE, i’ve got to do something or else the crawl space will quickly become a mess of dripping insulation and mold/mildew.
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With a vent open, running a dehumidifer isn't a slight energy hit, it's a HUGE energy hit.
Even if you don't insulate the foundation making it into conditioned space, a 10-mil poly or EPDM vapor barrier can be layed down covering the entire crawlspace floor and walls, up to and including the foundation sill. Seal it to the sill with foam or fiber-reinforced duct mastic, , then caulk or foam seal the band joist. If code won't allow permanently sealing the vents, install tight-fitting weatherstripped removable hatches.
THEN running a dehumidifier to keep the RH in the crawlspace at ~60%, would have zero mold issues, and would run at a very low duty cycle, whether the floor above is insulated or not, and whether the foundation walls are insulated or not.
Mind you, the thermal benefits of insulating the foundation walls with a couple or three inches of EPS (and coating it with an intumescent paint or half-inch wallboard) is significant in a SE US climate, even if you have joist insulation, since it couples the house to the thermal mass of the soil, which is at a fairly temperate temperature compared to the seasonal outdoor highs/ lows. With joist insulation the crawlspace would stay pretty near your deep subsoil temp year-round:
http://mb-soft.com/solar/soilmap.gif
Without joist insulation but with R8-12 on the foundation walls, the temp & relative humidity will track that of the conditioned space above, with little heating energy use penalty, and a modest amount of cooling energy SAVINGS.
Brian,
Like Dana, I can't support your suggested approach. You shouldn't consider running a dehumidifier until you have first sealed all of your crawl space vents.
The steps necessary to convert a vented crawl space to an unvented crawl space are well established. I suggest that you follow the advice in this article: Building an Unvented Crawl Space.