Modeling Windows: Center of Glass vs Assembly

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Does anyone know definitively if eQuest calculates window assembly U-values
based on user input?

As many have pointed out, the USGBC is now being careful about the
distinction between center-of-glass U values and assembly U values. I?ve
been entering center-of-glass U-values and precise window geometry, counting
on eQuest to do the heavy lifting for me. Report LV-D says ?U-VALUE
INCLUDES OUTSIDE AIR FILM; WINDOW INCLUDES FRAME, IF DEFINED?. However,
combing through several posts to this forum, some say eQuest doesn?t really
calculate an assembly?s U-value, because it ignores the edge-of-glass
effects due to the frame. (Not to be confused with the thermal conductance
of the frame itself, which I?m sure eQuest uses in its calcs.) I?m hoping
that one of the programmers can answer this, or maybe someone who got the
story from a programmer.

I plan to read the ASHRAE Fundamentals Chapter on Fenestration this weekend.
Wish me luck.

Steve Samenski, PE, LEED AP, BEMP

STEVE SAMENSKI's picture
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Happy reading. I'm not sure exactly how eQUEST handles all of this but I
would assume, oooh, dangerous, that if you put in your glass type carefully
and take care to specify the width and type of frame you have, all of which
are in the Wizards, you should be good. You could test the output by
comparing it to a Win5 calculation of your COG U value and frame, or just
import the Win5 data into the model.

Carol

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I've had some looks, nothing too deep, but I don't think I've ever seen a reference to edge-of-glass (EOG) performance, only COG and frame conductance... And I would also guess that when looking at COG values of U-0.3 (typical double pane low-E) and frame value U-1 (TB aluminum) that the edge of glass performance is not that good and would noticeably affect building energy use.

I don't know of any way of taking this into account. One option would be to figure out the area-weighted average conductance of the COG and EOG (1 U-factor that takes both components into account), the problem is that in order to input that into eQUEST, you would need to do it as a "simplified input" which I know people on this list-serv dislike doing.

I can't think of another way to account for EOG performance.

Alex

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Thanks, Carol. I have two problems with Window 5:
* It can?t seem to handle complicated multi-pane geometries (though perhaps
there?s a way to finesse that), and
* It makes as much sense to me as your average Lady Gaga music video

After I work my way through the Fenestration chapter I plan to wade into
Window 5.

Steve Samenski, PE, LEED AP, BEMP

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There are several ways to enter window properties in eQUEST. If you are in wizard mode, you can choose from the library (but this will not work for LEED compliance models in eQUEST 3.64). The other two choices in Wizard mode are "NFRC U-factor/SHGC" and "U-value/Shading Coefficient". NFRC U-factor/SHGC automatically sets your window frame width to 0 because the value you enter (NFRC rated U-factor provided by the manufacturer or installer) should be for the entire assembly. This is the way to do it for LEED compliance models.

For the "U-value/Shading Coefficient" method, you should enter the center-of-glass U-value including air films. Often, the glass manufacturer lists two U-values, summer and winter. You should enter the average value. You must also enter a frame width and choose a frame type. Both "NFRC U-factor/SHGC" and "U-value/Frame Conductance" map to the simplified input in detail mode.

Do not confuse GLASS-CONDUCTANCE with U-value or U-factor. GLASS-CONDUCTANCE is the thermal conductance of the glass without the external air films. eQUEST calculates this automatically if you have entered your window properties in Wizard mode. If you want to enter window properties in detail mode, you will need to look up the appropriate GLASS-CONDUCTANCE value from the table in the DOE2 help file (attached).

Kelsey Van Tassel

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