Thermal breaks for window bucks for PHIUS certified
Hi Guys,
I am building a PHIUS-certified house in Canada as a self-build project.
I am finalizing the design for the window installation and I am encountering some difficulties.
My wall is about 14 inches thick and its composition is as follows: a Gutex wood fiberboard (~4 3/4″ – 120mm) for the exterior insulation and a 2×6 wooden structure filled with cellulose.
My initial idea was to adapt this drawing: https://2.bp.blogspot.com/-Na-1hWwQ38M/Va_6Ogy7cLI/AAAAAAAAAL0/w5aJb6St2SE/s1600/Post08_WindowDetail-Full.jpg
But I am curious to see if I can find a better way to eliminate the thermal bridge as required by PHIUS, without creating a larger opening than necessary.
Thank you for your help!
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Replies
That is the best way to do it that I have seen or used, and I have put a lot of thought and research into it. If you don't need the PH certification, there are many options that are almost as good. But that drawing is the best way.
The next-best way, depending on your window type, is to eliminate the 2x4 inner buck, but leave one on the bottom. It makes everything physically tighter and with tilt/turn windows it can be really tight, especially at the drywall directly behind the hinges, but it usually works out.
Do you know who did the drawing? I know I've seen it recently but I can't recall who the designer was. Maybe Steve Baczek?
I don't know who did that. I found the drawing through a Google search, and based on my research, I believe it to be a good solution.
Found it: https://bldgtypblog.blogspot.com/2012/08/air-sealing-step-03-poking-holes.html. They have some good supporting images as well.
Michael,
I have a couple of small reservations about the detail as drawn.
There are a lot of pieces of casing and trim that sit on foam with no obvious way to fasten them to anything. You can see that most clearly in the first and second images.
The opening for venting the rain-screen under the sill will admit more moisture than it will expel.
I was thinking of adapting the details in this way. Any feedback is welcome!
killix,
You don't show any sloped sill-pan under the window. It's required here, and good practice everywhere.
If you have both vertical and horizontal rain-screen strapping, you don't need to vent below the windows. Air will move around them. The chance of water intrusion from the outside outweighs any improved drying you may get.
I'd suggest drawing in your finished surfaces - the drywall, trim, interior and exterior window casing, exterior sill. Show how they are fastened, and to what.
Oh, I didn't notice that the sill and drywall lines are very light in color, making them pretty much invisible. I'll make a new version to show that clearly.
I didn't catch the part about the "you don't need to vent below the windows", Can i ask you more details?
killex,
The trade-off with venting the top of the cavities under windows is between the increased air-flow it provides, and the risk of water intrusion from wind blown-rain, pressure washing the siding, etc.
The solution which has emerged is to leave the vertical strapping down from the sill a couple of inches, which allows uninterrupted airflow around the opening. If you also have a layer of horizontal strapping, you don't even have to do that as it provides a path for the air movement.