Exterior Continuous Insulation window drainage beyond foam
wood0619
| Posted in Energy Efficiency and Durability on
I am in the middle of adding continuous exterior insulation. The window pan extends beyond the foam and into the 1×4 furring rainscreen. The gap there from the underside of the window to the foam due to the pan slope is 1-1/8”. I plan to pack the interior frame with wool insulation (sheep wool). I am tempted to fill the bottom gap of the exterior drain pan with wool as well. Since wool is excellent at dealing with moisture, do you think this is a bad idea?
(picture attached)
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Replies
> The gap there from the underside of the window to the foam due to the pan slope is 1-1/8”
Seems like a big gap unless the wall is incredibly thick.
> I am tempted to fill the bottom gap of the exterior drain pan with wool as well.
Why?
> Since wool is excellent at dealing with moisture, do you think this is a bad idea?
Yes. If you have to choose one of "let water drain" or "add some insulation", the former is better. It's hard enough to protect our assemblies. I wouldn't add a further impediment for the sake of a tiny bit of thermal envelope improvement.
Regardless of what you do, the interior of the window frame should be air sealed to your air barrier.
woo0619,
How did you end up with such a big gap? The suggested slope at the sill is 6 degrees.
It's 2” of foam. The slope on the foam is too much, and I will dial it back to 6 degrees for the remaining windows. It just feels a shame with a high-performance assembly to have this gap.
wood0619,
The sheep's wool will be fine, but I share Patrick's reservations about impeding the drainage path. I would make a wedge of foam to fill the gap, but cut channels in the bottom and back for drainage. Or, since it's just one window, why not pull it and add the wedge of foam under the sill-plate?
> It just feels a shame with a high-performance assembly to have this gap.
I understand this intuition, but you have to consider it on the balance. Every time you decide to cut a hole in a high performance wall, you're making a choice. The window will never be as good as the wall, so an insulation gap here is comparable to if the window was 1 1/8" taller.
Patrick,
Good point.
I think it is true that, from a thermal standpoint, I have enlarged the window. I will probably leave this alone and reduce the slope for future windows. One thing to note is all the windows on this side of the house have a 24" soffit overhang within 30" of the top of the window. All of the 1970s windows I removed had no drip cap, no sloped sill, no sill flashings, and zero signs of water anywhere. But with exterior foam the risks are higher, so I will leave this one as is.