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Old 11-06-2013, 03:48 PM  
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Jb, welcome to the forums!

As your crawl is connected to the basement, the walls should be insulated (as existing) AND a house-air supply should be installed. This will move warm air currents around in the area to remove moisture so it won't condense on the cold plastic/earth. OR--- close it off from the basement air, and ventilate it per minimum code requirements. Once moisture condenses on plastic- you have a moisture problem there, that will only get worse as the temps get colder. Expect condensation on the wood joist framing/wood deck sheathing boards/ply/OSB and mildew (mold). Crawlspace air RH tracks outside RH- this is where your water is from- not a leak;

Condition the basement /crawl with house air supply and numerous choices of exhaust;

If the basement is unfinished, you could insulate the crawl/basement floor/water supply lines;,d.cGE&cad=rja

Though for a premium install, add foil-faced foam board (PIC) - air sealed and leave a gap at fibrous insulation/floor sheathing; Check with local AHJ per code left uncovered.

Fibrous insulation is rated for R-value in a six-sided chamber, leaving a side off (attic/crawlspace application) takes a hit in the R-value, exposed to air movement; pp.12;

In a heating climate, the pressure drive (air/moisture) is usually upwards, any crawlspace air leaks find their way to the attic (stack effect) due to pressure/temperature differences (natural/mechanical);

First; insulate/air seal the rim joists, a major contributor of air infiltration/exfiltration due to the seasonal shrinkage/expansion of older solid wood rim joists (R-1.25 per inch);


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Old 11-08-2013, 05:52 AM  
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There is no point in insulating crawl space walls, or in adding heat.

The crawl space is not heated and it is outside the home comfort zone, therefore, whatever happens there will not effect the home temperature or humidity once the house floor is sealed and insulated.

Bringing warm air into the crawl space is pointless, if the idea is to raise the crawl space air temperature,to enable it to hold more water vapor and to stop condensation. All this does is to raise the cost of heating a home, at a time when we are trying to save money, the Earth and carbon emissions.

It is perfectly normal for condensation to form on a cold surface that is below "dew point" it happens every day all over the world. Water vapor always moves towards a cold area and/or a cold surface to condense, Condensation forming on a plastic sheet does no harm.

Keep in mind that as the air gets colder as we move into winter, the airs ability to hold moisture decreases, there is less water in the air, once the temperature drops to 40f the air is almost dry, once it reaches freezing the air is dry.

Water vapor always moves towards the coldest area/surface, because of the heat leakage from the home, the wood surfaces that are part of the home are subject to heating by radiation and conduction, as long as the home is heated moisture/water vapor will not enter the structure of the wood.

As mentioned previously the structure of a crawl space is not water vapor proof, therefore as warm wet air arrives outside, and where and if, the temperature in the crawl space is lower, then the water vapor will pass through the various holes in the crawl space walls, floor, ceiling enter the crawl space and condense on the coldest surface, most likely the plastic sheet.
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