Crawl space

Published on June 8th, 2013 | by Henri

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Insulating a wet crawl space

Q.    The old part of my ranch house has a four-foot crawl space. I have had installed automatic louvered vents that purportedly close when it is cold and open when it is hot. There is no insulation in the “ceiling” of the crawlspace.

I put an addition on, which also has a four-foot craw space with non-automatic vents. The contractor insulated the “ceiling” of the crawlspace. The house is heated and cooled with forced hot air by way of ductwork in the crawlspaces. However, during the hot summer months with the A/C on, the insulation gets very wet with condensation.

I’ve heard conflicting theories. Some say that if a crawl space is vented, you should not have insulation. Others say insulation is good. I do not think that the condensation is a good thing. Should the insulation stay or go?

A.    Recommendations for the treatment of crawl spaces have dramatically changed in the last 20 years or so. Most building codes still specify ventilation although it has proved to be the wrong thing to do. The best strategy for crawl spaces is to thoroughly cover the bare ground with 6-mil plastic.

If the crawl space leaks, fix any grade problems outside to keep water away from the foundation, but bring the plastic up the walls to a point a few inches higher than the outside grade and tape it to the walls. Make sure that the plastic applied to the walls is under any plastic you will need to add on the soil (if the wall plastic is on top, any water will flow right over the floor plastic.)

Close the vents permanently: venting a crawl space in winter makes the floor above cold and in summer, it introduces huge amounts of moisture in a cool space. This is probably responsible for the condensation you have experienced if you live in a moderate to warm climate.

It is best to insulate the walls of the crawl space with extruded polystyrene rigid insulation. Don’t forget to insulate the band joists as well. This will condition the crawl space, which now robs a lot of heat from the ducts in winter and increases the loads on the air-conditioning in summer.

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