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lbaccari 08-21-2007 03:26 PM

Crawl Space Air
I have a 1 story home on a typical crawl space, except that instead of a dirt flood it is concrete. All my HVAC ducts run through the crawl. In the dead of Summer, when the vents are carring cold air, they sweat.

Should the vents which are in the premiter block wall of the crawl space be open or closed (seems to be 2 schools of thought here) and what do we know about installing power vents in place of the normally used venting.

yudamann 08-21-2007 08:28 PM

I am a mechanical engineer and know about the 2 schools of thought [one being from the guys who sell auto louvers]. I have always had a relatively dry crawlspace floor [sand or dirt] and I have always added poly film vapor barrier over it. As long as it is relatively dry, do not open the crawlspace vents; this only adds ambient [hot or cold] air to this space to increase your floor heat loss/gain. If it is wet, you need to open them to help keep it as dry as possible. Sweating ducts are caused by insufficient duct insulation and lack of [or poorly installed] duct wrap vapor barrier. Overwrap with duct wrap with an integral aluminum jacket. Seal ALL joints and seams w/ pressure sensitive aluminum foil tape made for this purpose [NOT duct tape] and use a plastic flexible "spreader] similar to a thick credit card to press down hard to insure a good tape joint. If your duct is leaking air, by all means remove the existing insulation and seal ALL leaks w/ a duct seal mastic coating or butyl backed aluminum tape made for this. Duct leakage is the primary reason for inefficiency in HVAC systems.

shawn T 08-21-2007 09:08 PM

Even with your cement floor in your crawl, radon may still be an issue and I would err on the side of saftey and leave your vents open. Just insulate your ducts with ductwrap. I would assume the problem would either be high humidity in your crawl, or a restriction in airflow by undersized ducts or blocked vents which would drop the temperature and dewpoint of the duct surface. I would say the primary source of inefficency problems by far would be poor installations.

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