Three weeks ago, I wrote about an attic with spray foam insulation and the interesting humidity pattern we observed in it. We placed data loggers near the ridge and floor of the attic as well as in the living space and outdoors. In that last article, I showed the dew point data for the four locations and said I’d come back with more explanation. So here’s the followup article I promised.
First, here’s the graph I showed last time. The really interesting part is the big difference in dew point between the highest and lowest points in the attic, shown by the red and green curves below.
Let’s look at the data for the ridge and floor in separate graphs now. The first one here shows the attic floor, and this time I’ve included the relative humidity. As you can see, the humidity, as measured both by dew point and relative humidity (RH), goes up during the day as it gets hotter outdoors and down at night when it cools off. But the humidity near the attic floor never gets out of control. The dew point and RH stay below 70° F and 70% at their peak (the peak of humidity, not the peak height in the attic).
In contrast, the humidity near the ridge in the attic gets very high. The dew point and RH get up close to 90° F and 90%. That’s way too high. On the bad days, when it’s sunny outdoors, the RH stays above 70% for about 8 hours per day and above 80% for more than 4 hours. Not good!
Why is this happening?
This has been a topic of conversation in Joe Lstiburek’s backyard at Building Science Summer Camp for the past three years. In 2014, Foster Lyons, Joe, and I talked a lot about this. Foster, in fact, kept pressing for better explanations and is the one responsible for Joe’s latest article, Ping Pong Water and The Chemical Engineer. There were also a lot of good comments on the first article in this series. Terry Brennan, a really smart guy with a physics background, nailed it with his explanation
Foster is the chemical engineer alluded to in the title of Lstiburek’s article. He loves to talk about chemical potential, which is part of his explanation for what’s going on. If you want the full, detailed explanation, including discussion of sorption isotherms and pressure swing adsorption, go read Joe’s article. I’ll give a more simplified version here.
What happens is that moisture makes its way into the attic. This could be from cooking, bathing, and breathing in the living space below. It could be from the very humid crawl space, down near the basement and coupled to the rest of the house. It could be from some mystery source. In the living space of this house, the air conditioner and dehumidifier handle the moisture load and keep the humidity under control. In the attic, however, there’s nothing to remove the water vapor that finds its way up there.
At night, the roof cools off. The cool OSB roof deck then offers refuge to the water vapor. Wood is hygroscopic, meaning it’s a porous material that can pull water vapor out of the air and store it on the surfaces and in the pores. The lower its temperature, the more it can store. The amount of moisture a material can store actually increases with relative humidity, and that increases as temperature drops.
So all night long, the roof deck is cooling off and storing moisture. When the sun comes out the next day and starts warming the outdoor air and radiating down onto the roof, the roof deck warms up and drives the moisture out.
When the water vapor exits the spray foam, it increases the relative humidity of the layer of air near the foam. Because water vapor is lighter than dry air (mostly nitrogen and oxygen), that layer of air can move upward in the attic. The result is higher humidity at the ridge than at the floor. Lstiburek calls this the ping-pong effect. The water vapor pings and pongs its way to the ridge through this daily cycling.
Another thing might be driving the upward migration as well. A couple of years ago, Bill Rose spoke about this at the Passive House conference, and I think it makes sense. The ridge of the roof has more exposure to the night sky than lower parts of the roof. That can cause it to cool down more than lower parts of the roof. The lower temperatures can help it pull more water vapor in.
The role of open cell spray foam
Open cell spray foam insulation doesn’t store the moisture. It’s just a pathway. It has a relatively high vapor permeance (~3 perms at a thickness of 6″), allowing water vapor to migrate to the roof decking at night and back into the attic air in the daytime. It’s the wood decking that stores the moisture, though.
The photo below shows a rusty OSB clip in the same house. They had the foam removed and replaced here (because of an off-ratio installation when it was first done) and I got a chance to get up there and inspect the roof deck before they sprayed new foam. I saw a lot of rusty nails and OSB clips. I also saw places where the nails and clips weren’t rusty.
The corrosion here indicates that moisture was indeed migrating through the foam and finding the roof deck. Well, at least most evidence points in that direction. In a 2014 article on spray foam and roof rot, I wrote about Joe Lstiburek’s research on different underlayments on top of the roof and how he showed that the moisture in the roof deck was not coming from above. The humidity data above show that there was plenty of moisture below. The conclusion is pretty simple. The moisture comes from below.
What’s the solution?
So, how do you fix this? Or better yet, how do you prevent it? Let’s look at prevention first. If the problem is moisture migrating through relatively permeable foam, you can make it less permeable. Put a vapor retarder on open cell spray foam. That’s actually required in climates 5 and higher. Or you can use closed cell spray foam. Those options would slow down the moisture migration and prevent the problem with the sheathing. It wouldn’t solve the problem of high humidity in the attic, though.
If you don’t do that, you’ve got to reduce the amount of water vapor in the attic. You can do that by directly conditioning the attic with supply air, by blowing conditioned air from the house into the attic, by exhausting air from the attic to outdoors, or by using a dehumidifier.
In his article, Lstiburek mentioned that he had proposed this for the 2018 International Residential Code (IRC). I just spoke with him and he told me his proposal passed, so this will be acceptable in the 2018 IRC. Note that it applies only to climate zones 1, 2, and 3, though.
The upshot of all this is that we need to start thinking of open cell spray foam installations differently. The days are over when you can spray open cell foam in an attic, close it up, and walk away. We know now that this can cause problems.
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