I was wondering if anyone would be willing to share thermally broken
aluminum frame conductance values that they've been using in recent
Windows5 center-of-glass calculations. I don't have information on the
particular aluminum frames for my current project, so I don't really
want to try and learn THERM quite yet.
Most of the older documentation online that I've found shows that
thermally broken aluminum frames typically have a U-value of about 1.0.
This jives with one of DOE-2s window entries for a PPG glass with a
frame conductance of 0.94.
Almost every commercial building constructed these days is made with
aluminum framed windows...I just have a hard time believing that
manufacturers haven't figured out better ways to insulate the frames.
With vinyl and wood having U-values < 0.4, I have to think that aluminum
frames should be able to at least approach 0.5, if not lower.
Unfortunately not a lot of manufacturers of aluminum frames post their
thermal characteristics.
What are people using?
Thanks in advance!
James Hansen, PE, LEED AP
Hi James,
As you said, Window5 does offer a default value for Thermally Broken AL (U-1.0 BTU/h?ft??F). I've seen some thermally broken frames with much worse performance (U-1.5). I'm currently looking at modeling a YKK frame right now with our glass, and the frame performance is around U-0.9. I haven't seen anything near U-0.5.
I believe that the U-value of the frame will vary based on the glass package that it is modeled with, but I'm not sure by how much.
I know SeriousMaterials now has a fiberglass framed window that are Heavy Commercial (CW) rated - link. Fiberglass has comparable thermal properties to wood and vinyl - I believe these windows have as high as U-0.19 (frame+glass performance). The frame conductance is around U-0.5.
Although I think using THERM is a good approach (I just learned how using this 17 minute video by LBNL - link) you can create a new generic frame material in WINDOW5 and assign the frame conductivity. I would say U-0.8 or 0.9 is the best you'll do with thermally broken AL and U-0.5 is where fiberglass is.
Regards,
Alex Krickx