I had my attic insulated with blown cellulose and air sealed. Since that time my home has gone from around 45 to 48 percent RH to 52 to 56 percent RH, sitting at 73 to 74 degrees. I live in Nashville, so humid is nothing new to me. What caught me off guard: we had a much more unpleasant summer the year before, with higher heat and humidity, yet the interior humidity back then was lower.

Fiberglass batt insulation being installed by hand
Photo: Erik Mclean / Unsplash

Insulation, AC Runtime, and Latent Load

Reduced AC Runtime and Latent Load

With more insulation, a house has much less heat gain, so the AC runs less. Since the AC runs less, it removes less moisture from the air, and that's the high humidity. Air tightness lowered the sensible cooling load, so the AC now runs at a lower duty cycle, but the latent load didn't drop in the same proportion. Latent load handling is a common problem in high performing homes.

The Dehumidifier Fix

The fix people keep landing on is a room dehumidifier near the main AC return register. Anything under 60% RH is healthy and comfortable for most people. Under 50% is recommended for dust mite allergies. That's my target now, alongside operating the bathroom exhaust fans and range hood fan more regularly.

Spray Foam’s Health and Structural Risks

Isocyanates and Offgassing

Spray polyurethane foam contains isocyanates, widely documented respiratory sensitizers linked to asthma. Improper mixing or incomplete curing can lead to prolonged offgassing, and indoor VOC concentrations in these homes can run two to five times outdoor levels, with some instances surpassing that by more than 100 times. In Vermont, homeowners have reported severe air quality issues and forced displacement, with remediation costs exceeding $50,000.

Trapped Moisture and Lender Pullback

Closed cell foam acts as a vapor retarder. Moisture that gets into a wall assembly has no way out, and wood assemblies need to dry out fast to stay intact. Insulation that blocks vapor has been documented causing hidden decay and mold growth not caught until failure. Nearly 25% of UK mortgage lenders now deny financing for homes with spray foam insulation, affecting an estimated 250,000 properties.

Choosing Vapor Open Materials

Every product has pitfalls, and insulating too little has its own. Vapor open materials like cellulose, wood fiberboard, wool, and cork are hygroscopic: they buffer and redistribute moisture instead of blocking it, supporting passive drying and reducing microbial growth. They're also low emission and not toxic. True energy efficiency isn't just reducing heat loss. It's keeping a house durable, safe, and responsive to moisture and the people breathing it. The dehumidifier and exhaust fans are holding the line for now.

Here's roughly how mine went, in order: