Sloped window sills – Fad or a new best practice?
I’ve seen folks on Youtube promote the benefit of framing their rough openings with window sills that slope outwards. When they install the window, they then use a number of shims (with the opposite slope) so that there are a handful of points of level contact for the window.
I believe the objective is that if there is a failure of the window frame, the water will hit the sloped window sill and then run down the flashed sloped sill and out of the building.
My questions:
1) Doesn’t this create a significant weak spot in the thermal envelope? Folks are going to great lengths to minimize thermal bridges with continuous insulation and buying very expensive windows. But this just introduces a full-on air gap from the outside of the building all the way to the inner side of the window frame. What’s even the point of a thermally broken window frame if you are going to put an air gap underneath it?
2) Regarding the above, does anyone ever try to jam something into that gap that will be an insulator but also allow water to flow out? Seems like that is not very practical.
3) Are you increasing the likelihood that you will get a leak by not setting the window on a solid piece of wood across the entire bottom edge?
4) What’s the general verdict on whether this is better than frame a flat and level sill and then setting the window in a few healthy lines of caulking?
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Replies
canada_deck,
We should probably start the discussion by noting that all the Canadian building codes I'm familiar with require sloped sills.
Leaks invariably affect the sill. That's where you always find the damage around windows. I think the best preventative measure for that is a sill pan with a back dam. I'm not as convinced that sloping the sill is that important as long as there is a drainage path for any water that accumulates on the sill pan to move to the exterior - but many others disagree, and I don't see any downside to the practice.
1. The current best advice around insulating around the window frame is to only insulate the inside 1/3rd of the gap, and leave the area beyond that open as both a drainage path and to equalize pressure. You can still do that at the sill too.
2. See #1.
3. No. I don't see how this affects the flashing and cladding which keep out moisture.
4. Even if the sill is level, bedding the window in caulking is a bad idea., which will trap any moisture that accumulates.
What I do is bevel the portion of the sill framing that is under the window, not the whole sill. I also hog out two narrow kerfs on the outside face of the sill about 1/4" deep, so that when I put on the membrane sill pan it leaves two small drainage paths under the bottom flange into the rain-screen cavity.
Thanks! Re #4 - That advice is different if it is a sliding glass door, right? Or am I wrong about that.
These two videos from Architect Steve Baczek may help.
Part 1
https://www.youtube.com/watch?v=yqjA-lYAwd0
Part 2
https://www.youtube.com/watch?v=JwgIn5UopEc
Thank you!
"I believe the objective is that if there is a failure of the window frame, the water will hit the sloped window sill and then run down the flashed sloped sill and out of the building."
Yes, but not just a failure of the window frame, leaks come from missing flashing, siding installation errors, fasteners, modifications after construction, missing sealants, caulk wearing out, weather damage, accumulation of debris, building movement, and a million other causes. At some point there will likely be something that will introduce a bit of water so having a path for it to exit is always a good idea.
That exit path doesn't have to introduce any of the compromises you mention. A little low expanding foam fills the bulk of the side and top gap, flexible caulk and backer rod stop any water or air leaks from coming in, a properly taped or WRB wrapped frame keep the water from getting into the sheathing, and the window itself is attached by the flange (or clips) so it's not resting on the sill anyway.
Setting windows has become a little bit more involved than it used to be way back when, but once you get the concepts and materials it's really not that hard to execute well.
Thanks!