Why Is This Airtight House So Humid in Summer? – GreenBuildingAdvisor

Q&A Spotlight

A newly renovated home stays cool, but summer humidity is much too high. What’s going wrong?

Summer brings high outdoor moisture levels to much of eastern North America. In some places—particularly along the Gulf Coast—high dew points can make stepping outdoors feel like having a wet towel draped over your head. Elsewhere, conditions may be only slightly better: more open-air steam bath than wet towel.

Ideally, that moisture stays outdoors. A well-designed enclosure combined with properly sized HVAC equipment should keep indoor temperatures and humidity at comfortable levels. But what happens when a newly renovated, much tighter house stays cool while the relative humidity climbs into the 80s?

That’s what one GBA member had in mind when they recently posted a question on the Q&A forum. “Madison4159” (whom we’ll just call Madison) describes a recent renovation to the second and third floors of their house, insulating with closed-cell spray foam and replacing all the windows. “The space is now extremely airtight,” says Madison. “I have radiant heat for the winter and Mitsubishi ductless mini splits for air conditioning.”

But Madison has moisture issues, writing that over the past month the humidity inside the house has hovered in the mid to high 80% range. Madison is perplexed: “The mini-splits quickly satisfy the temperature setting, but they do not run long enough to remove enough moisture. Dry mode has also failed to bring the humidity under control. Oddly, I have the opposite problem during the winter, when the indoor air becomes too dry.”

The topic for this Spotlight: What’s causing the humidity problem in Madison’s new remodel, and how can it be fixed?

Forum responses

The audience seems divided. Expert member Malcolm Taylor thinks it’s an issue of ventilation. He writes, “Once you tighten up a house, you need mechanical ventilation to control humidity, but also for IAQ. Depending on where…