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.