Completely insulate / cover a window in an open cell foamed attic?
You guys answered a lot of questions for me last year during my retrofit of open cell foam in the cathedral attic of a 20 year old home I bought. For the most part, things are great, but the dehumidifier that I have in the attic still seems to have to work way too often to keep the humidity around 50% even though I also have a couple of registers cut inline on the supply trunks up there as well. The temp stays below around 82 when its 95 or so outside and I have the upstairs at around 74-76, so that part is also OK, but I was wondering if the single window that is in the attic may be allowing humidity to infiltrate and constantly have to be removed. Is there a good method or product, ideally removable, that I could use to stop and infiltration and insulate / cover / seal that window? In the long run it is *possible* that I turn this large space in the attic into a real heated/cooled room of the house, so I dont want to irreversibly cover the window. Thanks for all suggestions!
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Replies
John,
1. Paint the glass black (on the interior of the glass) so that it doesn't look too weird from outdoors.
2. Use a high-quality tape to seal the seams between the sash and the frame.
3. Cover everything with an oversized piece of foil-faced polyiso, installed with attention to airtightness.
4. Install drywall on the interior side of the polyiso for fire safety. (You'll probably also want a minimal frame around the polyiso, made of sticks that you make on your table saw.)
Ok Great thanks! Do you agree that the window is likely to be a source (or maybe *the* source) of excess humidity up there?
John,
Q. "Do you agree that the window is likely to be a source (or maybe *the* source) of excess humidity up there?"
A. That's hard to say. It depends on whether the window is leaky. Take an incense stick up there and use the incense to search for leaks.
There are lots of potential air leaks in an attic -- even one insulated with spray foam. You might want to investigate possible air leaks near the eaves and ridge.
> I also have a couple of registers cut inline on the supply trunks up there as well.
Consider a) closing these supply registers and b) changing the setting to 60% humidity.
With supply air and no or limited return air, infiltration is unlikely to be your problem,
Oh, so the supply air could be causing an issue? I only put it up there because I thought I read it would HELP the area to dry... ugh!
John,
I disagree with Jon R.
The supply air is helpful (assuming, of course, that you house is air conditioned in summer, and the supply duct is delivering cool, dry air).
John and Jon,
The long-standing recommendation from Building Science Corporation for this type of attic is to include supply air from the forced air system, and to include a grille (call it a floor grille or a ceiling grille) that allows relief air to move from the conditioned attic to the floor below.
The only problem with the recommendation is that it violates some building codes. Joe Lstiburek is trying to change the code.
The best thing to do (too late for John) is to use closed-cell spray foam, not open-cell spray foam, to create a conditioned attic.
More information on all of these controversies here: High Humidity in Unvented Conditioned Attics.
It's causing the issue in that your dehumidifier is dehumidifying the whole house. Pressurization (typical with a supply and no return) causes wasteful exfiltration. Cooling a space where you don't care about the temperature is also wasteful.
Follow that link and search for "Alternatively one of the following shall occur".
...
3) mechanical dehumidification shall be provided to the unvented attic air space.”
I see no recommendation to combine methods (eg, supply air and a dehumidifier).
Jon,
Fair enough. One or the other makes sense. But if John follows your advice and seals the supply-air register, it's essential to keep running the dehumidifier.