Humidity Issue

A homeowner who recently renovated the upper floors of their house, adding closed-cell spray foam insulation and new windows, is dealing with a summer humidity problem. Indoor humidity has stayed in the mid to high 80 percent range for the past month. The home uses radiant heat in winter and Mitsubishi ductless mini-split units for air conditioning, but the mini-splits satisfy the thermostat's temperature setting so quickly that they don't run long enough to pull moisture out of the air. Even switching the mini-splits to dry mode hasn't helped. Oddly, the same home turns too dry in winter.

A building science expert responding to the question pointed to the likely root cause: the renovation made the space very airtight, and tight homes need mechanical ventilation, both to manage humidity and for indoor air quality. Depending on location, the right ventilation setup might control moisture well enough on its own, without needing a separate dehumidifier.

For anyone doing a similar deep energy retrofit, this is a reminder that sealing up a house tightly and switching to right-sized heating and cooling equipment can change how moisture behaves indoors year-round. A mini-split that cools fast may not run long enough to dehumidify, and an airtight envelope can trap moisture in summer while also drying out air in winter. If you've recently tightened up your own home with spray foam or new windows, it may be worth looking at whether your ventilation system is doing enough on its own, or whether a dedicated dehumidifier is needed alongside it.