Positive pressure in vented crawl space
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I was measuring a home yesterday in preparation for a small kitchen addition and noticed that conditioned air was leaking out around the crawl space door. It’s always been our practice to encourage homeowners to roll basic home energy upgrades into elective improvements like this and I’d already given the ductwork a quick check. There were noticeable leaks around the homeowner-installed reflectix/duct tape wrap and I recommended a thorough ductwork overhaul along with insulating and sealing the crawl. I was surprised though by the net positive pressure in the crawl, expecting supply and return to be more or less balancing through leakage, and I would normally expect a crawl space to be under overall negative pressure through the stack effect. My first instinct was to suspect an undersized return, also the homeowner mentioned that supply registers had been closed in a couple of rooms because they were not in use, this would somewhat explain an unbalanced system with excess pressure in the supply side, but I still can’t wrap my head around why this would result in the crawl space blowing air to the exterior. Where’s that air coming from?
BTW, the house – 2-story, 1800 sf, located in central NC, was built to minimum code standard in 1978 and the only mechanical upgrade (other than the ductwork wrap) has been replacement of the heat pump condenser unit a few years ago. Air handler in the crawl space, 2nd floor supply ducts via a chase to the attic. It was over 90° here yesterday when I inspected (though with mercifully low humidity) and the A/C was blasting like nobody’s business.
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Replies
James,
As you correctly guessed, if the crawl space is pressurized, the air escaping the crawl space has to be balanced by air entering the crawl space. The most likely cause of the pressurization is leaks in the supply ductwork.
If leaks in the supply ductwork pressurize the crawlspace, then air leaks out of the crawl space wherever it can; you noticed one of these leaks around the crawl space door.
If I'm right that the pressurization is caused by supply duct leaks, then the source of the air is the forced-air system. Leaks in the supply ductwork unbalance the forced air system, and the house above is getting less supply air than intended by the designer. The return air system is delivering the air to the furnace, depressurizing the conditioned area of the house. Cracks in the home's thermal envelope allow outdoor air to enter the conditioned space, supplying the necessary return air to keep pressurizing the crawl space.
And, if the HVAC system is pressurizing the crawlspace outside of the thermal boundary, then it's also depressurizing the conditioned space. And, during the AC season, this means creating an infiltration drive and a high likelihood of condensation behind interior surfaces.