Q&A Spotlight: Why Is This Airtight House So Humid?

A homeowner recently sealed up the second and third floors of their house during a renovation, adding closed-cell spray foam insulation and new windows. The upgrade made the space airtight, which is usually good news for comfort and energy bills. But it created an unexpected problem: indoor humidity has been sitting in the mid-to-high 80% range all summer, even though the ductless mini-split air conditioners (a type of heat pump system with no ducts) keep the temperature comfortable. The units cool the air quickly, then shut off before they've run long enough to pull much moisture out. Even switching the mini-splits to "dry mode" hasn't solved it. Oddly, the same house gets too dry in winter, when it uses radiant floor heat.

People discussing the case online lean toward one likely cause: once a house is sealed this tightly, it no longer gets rid of moisture through small gaps and leaks the way older, drafty homes do. That means it needs some form of mechanical ventilation, a fan-based system designed to bring in fresh air and control moisture and indoor air quality on purpose, rather than by accident.

The takeaway for anyone tightening up their own home with spray foam, new windows, or other air-sealing work is that a very airtight house changes how moisture behaves indoors, in both summer and winter. Mini-split air conditioners are good at cooling but aren't always good at dehumidifying on their own, especially in a well-sealed house that used to rely on air leaks for ventilation.