Question:
I have a serious summer-time condensation problem in my crawlspace. I have taken care of surface drainage to insure no standing water in the crawlspace, and I have opened all the foundation vents and even installed some fans. I also covered about 80% of the dirt floor with plastic. But I still have major condensation: a blanket of water droplets on the underside of the fiberglass insulation batts between the floor joists and on the HVAC duct-work. I am convinced the problem is due to hot humid air coming in the foundation vents and condensing in the relative coolness of the crawlspace. The coolness of the floor is probably increased by the fact I have brick floors. Anyhow, after thinking and reading, I think the solution is to SEAL my crawlpace. Close and seal the foundation vents, cover the ENTIRE dirt floor with platsic, and maybe even seal the foundation walls and install a de-humidifier down there. My question is, how best to do this sealing. I cannot seal the foundation walls from the outside, because they are stucco-ed and backfilled. Can I just paint the interior of the block walls with that tarry black paint ? Do I need to paint the entire height of the wall, or just up to where the exterior gradeline is ? Or does rigid insulation glued to the block walls make a decent vapor barrier ? I’d appreciate wisdom on these questions or any other comments on this prolbem and my proposed solution.
Response:
Unless you have a french drain system installed around the exterior and in the interior or your crawlspace, cover the entire floor with plastic and concrete the crawlspace (100%), anything else you do is pointless. It is against CODE to seal a crawlspace with a dirt or concrete floor because of radon concerns.
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Response:
> I have a serious summer-time condensation problem in my crawlspace. > I have taken care of surface drainage to insure no standing water > in the crawlspace, and I have opened all the foundation vents and > even installed some fans. I also covered about 80% of the dirt > floor with plastic.
My home inspector recommended that the vents be closed in the hot summer in order to reduce moisture in the crawlspace. Works for me. — Lee King
Response:
That’s fascinating. The inspector who inspected our old house for the buyers told them to leave *all* the crawlspace vents open *all* the time (even in winter). He said that would reduce the moisture in the crawlspace. Let’s see…warm air can hold more moisture (without condensation) than cool air. If you close the vents, the crawlspace will be cooler than the humid summer air, and the crawlspace air will be unable to hold all that moisture–so the moisture will condense. If you leave the vents open, the crawlspace air and outside air should equilibrate. Does that make sense? (I’m making this up.) This is in Alabama–I wonder if there are regional differences? C. Brunner (Address contains a spamblock: "NOT") – Hide quoted text — Show quoted text -> I have a serious summer-time condensation problem in my crawlspace. > I have taken care of surface drainage to insure no standing water > in the crawlspace, and I have opened all the foundation vents and > even installed some fans. I also covered about 80% of the dirt > floor with plastic. >My home inspector recommended that the vents be closed in the hot summer >in order to reduce moisture in the crawlspace. Works for me. >Lee King
Response:
>If you leave the vents open, the crawlspace air and outside air should >equilibrate. Does that make sense?
No, I don’t think it does. Problem is, there are some very cool SURFACES on the inside of your crawlspace, like the cool earth and the cool underside of your floors (especially brick ones like mine); the problem is greatly exarcerbated if your are running the AC, because the ductwork becomes a very cool surface. Now, hot humid air comes in through the foundation vents, touches all those cool surfaces, and condensation occurs. I think closing the vents will help, but I think a little tighter sealing is required in addition (plastic on the ground, waterproofing the block walls) and some folks around here claim you need to install a de-humdifier as well.
Response:
> That’s fascinating. The inspector who inspected our old house for the > buyers told them to leave *all* the crawlspace vents open *all* the > time (even in winter). He said that would reduce the moisture in the > crawlspace. Let’s see…warm air can hold more moisture (without > condensation) than cool air. If you close the vents, the crawlspace > will be cooler than the humid summer air, and the crawlspace air will > be unable to hold all that moisture–so the moisture will condense. > If you leave the vents open, the crawlspace air and outside air should > equilibrate. Does that make sense? (I’m making this up.) This is in > Alabama–I wonder if there are regional differences? > C. Brunner > (Address contains a spamblock: "NOT")
I would agree with that… until heating season. Around here, one needs to heat the crawl space to prevent pipes from freezing. Heating also dries the air, so moisture content decreases. I keep ours at 13 – 14 C (about 56 – 58 F), which is only slightly higher than the core earth temperature (9 C – 48 F) and still warm enough to keep floors at an acceptable temperature. In Summer, however, opening vents is generally the best solution. You have to make sure vents are well located to create a draught, and that they are wide enough to allow a good air circulation. A rule of thumb is that the cross section of openings needs to be larger than 1 ft2 for each 500 ft2 of floor space. If the vents don’t allow adequate air circulation, one needs to add a fan or other vents. Also, a problem with deep crawlspaces is that the deepest parts are not vented. Extracting air from the lowest point will help greatly to lower humidity levels. — Montr
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