2 Fairbanks Daily News-Miner, Saturday, September 11, 2010
Study looks at insulation practices for climates similar to Fairbanks Molly Rettig
There are many good rea-sons to retrofit your house, including saving energy, sav-ing money and adding com-fort. And a new building study tells you how much insulation you should add and other factors you should consider to increase energy efficiency and avoid molding walls.
The Cold Climate Hous-ing Research Center recently completed a report from a yearlong test of how various wall systems handle moisture in Fairbanks. The study test-ed whether the conventional ratio for insulation applied to the local climate.
The rule of thumb is that two-thirds of the R-value (thermal insulation) should be outside the structure and one-third inside, said Colin Cra-ven, product testing director at the research center. What we found for the worst-case scenario is that this is defi-nitely true.
Craven manages the Mobile Test Lab, a 100-square-foot
trailer containing a patchwork of nine walls, each with dif-ferent combinations of studs, insulation and vapor barriers. Last winter, Cravens team simulated a worst-case sce-nario by setting an extreme indoor temperature 70-degree indoor temperature with 40 percent humidity (average is 25 percent).
Humidity is important because your walls need to dry in the summer by shed-ding 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 theres less moisture.
You want your walls to be able to breathe one way or the other, Craven said.
Today, many homeowners add foam board outside their houses to save energy. But the side effects of this type of ret-rofit in Fairbanks have never been researched.
We always thought, Should we really do this? It seems to be working, but theres no real data besides We havent seen houses
fall down yet, said Terry Duszynski, a Fairbanks ener-gy rater who helped jumpstart the project.
During the winter, the lab not only exaggerated the heat and humidity but also punc-tured the vapor barrier with picture frames and windows and pressurized 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 them by encouraging condensation. It all 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, typi-cally 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. But would this double vapor barrier cause moisture problems?
Testing showed that add-ing enough insulation to the outside kept the walls warm enough to deter moisture problems, because the wall surfaces couldnt get cold enough for water vapor to condense into liquid.
It allows the water vapor to stay a vapor instead of let-ting it condense to a liquid that can then accumulate, Craven said. But (not every-one) does that. Theyre put-ting a couple inches of foam on the outside, and that reverses the ratio.
The recommended ratio usually translates into three or more inches of outer insu-lation, Craven said, but varies depending on your wall com-position.
Insulation that was prob-
lematic was only a couple inches of foam on the outside, three inches or under, he said.
While this research high-lights potential risks of insu-lating outside, retrofitters shouldnt be deterred, said Craven and other local build-ing scientists.
As an energy rater, I dont hesitate to recommend an insulation to the outside for every house that hasnt yet, said energy rater and borough assemblyman Mike Musick, who helped create the trailer.
The trick is maintain-ing the thermal balance and vapor control of your walls.
Its 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, Craven said. If youre com-mitting to do this, we want to make sure you do it right.
Beginning this April, the research center will study the same walls under more realis-tic conditions.
We anticipate that will be more forgiving, Craven said.
3Fairbanks Daily News-Miner, Saturday, September 11, 2010
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