[Bldg-sim] aluminum window frame U-values

Alex Krickx akrickx at seriousmaterials.com
Tue Mar 1 12:00:46 PST 2011


Hi James,

As you said, Window5 does offer a default value for Thermally Broken AL (U-1.0 BTU/h∙ft²∙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<http://www.seriouswindows.com/commercial/products/heavy-commercial.html>. 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<http://windows.lbl.gov/software/therm/demo/Sample/index.html>) 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


Alex Krickx, LEED AP
Building Energy Specialist
[cid:image001.jpg at 01CBD808.4C57C6B0]
1250 Elko Dr, Sunnyvale, CA 94089
(t) 408.541.8124

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From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Tuesday, March 01, 2011 9:46 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] aluminum window frame U-values

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!

GHT Limited
James Hansen, PE, LEED AP
Senior Associate
1010 N. Glebe Rd, Suite 200
Arlington, VA  22201-4749
703-338-5754 (Cell)
703-243-1200 (Office)
703-276-1376 (Fax)
www.ghtltd.com<http://www.ghtltd.com/>


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