Like many building-science mavens, Allison Bailes has been an advocate for the practice of running heating and cooling ducts inside the building envelope. Little by little, this practice is catching on, as builders and design professionals come up with smart ways, beyond the utility closet, to house air handlers in dropped ceiling and run supply and return ducts through interior soffits. But the exact same principles that make these smart solutions apply to mechanical ventilation ducts, too.

How often do you see (or have installed yourself) a duct for a bath fan through the attic? And how often do you see these get blocked with ice, or drip with condensation, during the coldest and hottest times of the year? The simple solution to cure these ills is to run the ductwork inside conditioned space.

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