In this short podcast episode, Bryan explains how leaky ducts can lead to sweaty, uncomfortable houses.

Moisture issues are common all across humid and “green-grass” climates in the United States. Hot, unconditioned attics and crawl spaces also often have high dew points, which can cause moisture problems in those spaces. Plants also release moisture, so agricultural communities also have those special considerations. There are several HVAC and building design factors that can prevent or worsen growth from moisture problems. Duct leakage is one of those factors.

Return and supply leaks can cause moisture to enter the ductwork. Return leaks suck in air from around the leak. Supply leaks create a pressure differential that can cause mechanically-driven infiltration (MAD-AIR); when a supply duct leak puts the attic under positive pressure, somewhere else has to go under negative pressure. There are lots of gaps and cracks around boots and can lights, which provide pathways for hot, moist air to seep into the envelope. Longer runtimes and keeping the fan on will worsen these conditions.

We need to control the source of moisture and then seal the ducts and boots before we focus on the equipment. Visual inspections at the connections and boots will go a long way, but there are also plenty of tools for testing duct leakage, especially by Retrotec and TEC. You can also measure house pressure to the outdoors with the air handler running and all the doors closed, but that's a qualitative measurement that has a lot of other variables to consider.

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