FAIRBANKS — Everyone in Fairbanks knows summer is when the ground thaws and your bones warm. But it’s also the season when your walls dry by shedding moisture deposited in the wall cavity during the winter. When walls warm in the sun, built-up water vapor wants to go back outside where there’s less moisture.

“The way we build walls controls how much that happens,” said Colin Craven, product testing director at the Cold Climate Housing Research Center. “You want your walls to be able to breathe one way or the other.”

Today, many homeowners add outer foam-board insulation to their houses to save energy, but the extra layer can trap moisture inside the walls, causing mold and decay.

“We always thought, ‘Should we really do this?’ It seems to be working, but there’s no real data besides ‘We haven’t seen houses fall down yet,’” said Terry Duszynski, a Fairbanks energy rater who helped jump start the project.

This summer, the research center is completing a yearlong test of how various wall systems handle moisture in Alaska climates to avoid this type of problem.

Craven manages the Mobile Test Lab, a 100-square-foot trailer containing a patchwork of nine walls, each with different combinations of studs, insulation and vapor barriers. Researchers tested the walls under extreme temperatures and humidity levels to learn which ones succumb to moisture problems and how to avoid them.

Data gathered from dozens of sensors embedded in walls will be compiled in a report geared at homeowners who are considering retrofitting their house.

During the winter, the lab simulated harsh conditions for walls by setting a 70-degree indoor temperature with 40 percent humidity (average is 25 percent), puncturing the vapor barrier with picture frames and windows and pressurizing the interior to push air into the walls. The researchers wanted to know whether adding exterior foam would help walls by keeping them warmer or hurt walls by encouraging condensation. The answer, they discovered, depends on how much insulation you use.

A balancing act

Water vapor, generated by cooking, showering and breathing, permeates your walls from the inside through little leaks and seams in your vapor barrier, a polyethylene sheet directly behind your drywall (if you have one). Walls can handle a certain amount of vapor without growing mold, but problems develop gradually once they become moist enough.

Every summer, the water vapor tries to escape, typically through more-permeable outside walls but also through the same channels it entered.

But adding exterior foam blocks this escape route and creates a vapor barrier on each side. A major question behind this study was whether this double vapor barrier would cause moisture problems.

Testing showed that adding enough insulation to the outside kept the walls warm enough to deter moisture problems, because the wall surfaces couldn’t get cold enough for water vapor to condense into liquid.

“It allows the water vapor to stay a vapor instead of letting it condense to a liquid that can then accumulate,” Craven said. “But (not everyone) does that. They’re putting a couple inches of foam on the outside, and that reverses the ratio.”

A good ratio is two-thirds of thermal insulation on the outside and one-third on the inside, but the research center is testing what works best for various houses.

While this research highlights potential risks of insulating outside, it doesn’t discourage it, said Craven and other local building scientists.

“As an energy rater, I don’t hesitate to recommend an insulation to the outside for every house that hasn’t yet,” said borough assemblyman Mike Musick, who helped create the trailer.

The trick is maintaining the thermal balance and vapor control of your walls.

“It’s a little bit of a tough message because insulation costs money. Because of the costs, people prefer to put a thinner layer on the outside. What we’ve been advising for a while is how much you can get by with and be safe.”