# Glass center-of-glass to assembly U-Value?

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Does anyone have any rules of thumb on a quick/easy approximation for
taking a Center-of-Glass U-value and converting it to an Assembly
U-value based on industry standard assembly constructions? I know this
depends on a lot of variables, like frame material and conductance, and
the size of the overall glass assembly, etc., but I'm just looking for a
quick estimate on performance reduction for comparisons to other window
types for a particular building.

Also, is there an ASHRAE 90.1-2004 Baseline glass Visible Light
Transmittance (VLT)?

Josh Greenfield, PE, LEED AP

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According to the California Energy code (Title 24) Non-Residental ACM manual
Section NI.1:

NI.1 Solar Heat Gain Coefficient
This section describes the alternative calculation method for determining
compliance for eligible site-built
products. The following equation may be used to calculate the fenestration
product's SHGC used to
determine compliance. Convert the center of glass SHGC, SHGCc, from the
manufacturer's documentation
to a value for the fenestration product with framing, SHGCfen,
SHGCfen = 0.08 + 0.86 X SHGCc
Where:
SHGCfen is the SHGC for the fenestration including glass and frame.
SHGCc is the SHGC for the center of glass alone

This is how California defines the overall SHGC for code compliance. I'm
not sure where they got the equation from, but I have used it successfully
in the past.

Daniel Grammier, LEED AP

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Josh,

You could probably use ASHRAE fundamentals Handbook to get the assembly
U-values. Go to Fenestration (or 29th Chapter) and look for Table 5
(U-factors for Various Fenestration products) this is from an older version.

Regards

Srikar R Kaligotla

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Actually - when you install eQUEST it should put an XL file in your
\eQUEST\Window folder titled eQUEST-DOE2 Glass Library.xls

This has the DOE2 glass types sorted by SHGC and U-Value. Under U-value
there are numbers for COG, as well as assembly U-Values.

Of course ultimately it depends on the size of the glass lite. For
example if you have a 10' x 10' opening, and you break it into 3 rows
and 3 columns, your U value will be higher than if you break it into 2
rows and one column.

Vikram Sami, LEED AP

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ASHRAE 90.1-2004 Section 5.8.2.5 exception (b) allows the SHGC/SC
center-of-glass to be an acceptable alternative for determining
compliance with the SHGC/SC requirements for the overall fenestration
area.

Josh Greenfield, PE, LEED AP

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Attached is a handy spreadsheet from the Oregon Energy Code User's
Manual. Hopefully this is what you are looking for.

Regards,

Andrew Craig, EIT,LEED AP

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Hi,
Chapter 31 (Fenestration) of Ashrae handbook fundamentals has average
overall U-values for different frame types (weighted average for a frane of
2.5" I believe, which also includes the glassedge effect). Accurate U-values
need to be calculated using NFRC standards, using approved software (in the
US this is LBNL Windows and Therm).

I am not aware of a visible light transmittance requirement, but the lower
the transmittance, the less daylighting benefits you will get, as you know,
so there are no energy benefits for having a low Vt. Glass manufacturers
work hard to keep light transmittances high and SHGC low.

Hope this helps
Kind regards,
Karen

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Table C3.5 from ASHRAE 90.1, 2004 gives baseline Visible Light
Transmission factors which are multiplied by the Solar Heat Gain
Coefficient to determine VLT for the base case when using the envelope

Michael Rosenberg

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Actually - when you install eQUEST it should put an XL file in your
\eQUEST\Window folder titled eQUEST-DOE2 Glass Library.xls

This has the DOE2 glass types sorted by SHGC and U-Value. Under U-value
there are numbers for COG, as well as assembly U-Values.

Of course ultimately it depends on the size of the glass lite. For
example if you have a 10' x 10' opening, and you break it into 3 rows
and 3 columns, your U value will be higher than if you break it into 2
rows and one column.

Vikram Sami, LEED AP

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Joined: 2011-09-30
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Can we use glass center of glass shading coefficient for the assembly ? If not, is there any similar table for assembly shading coefficient calculation?
Thanks.

Ming Zeng, PE, LEED(r)

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ASHRAE 90.1-2004 Section 5.8.2.5 exception (b) allows the SHGC/SC
center-of-glass to be an acceptable alternative for determining
compliance with the SHGC/SC requirements for the overall fenestration
area.

Josh Greenfield, PE, LEED AP

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Joined: 2011-02-17
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Josh,

I have had some luck using the pre-defined values in the tables people
mention. The problem is that the ratio of frame area to total window area
is so variable that I don't think there is a good rule of thumb to use.

Here is some data based on some recent modeling projects.

large curtain wall and store front systems have between 5-25% frame area
punch-out windows have between 18-40% frame area (lower percentage for
fixed, higher for operable)

You can see that there is a wide range which could significantly impact the
accuracy of an energy model. In my opinion, not accounting for window frame
area properly can lead to dubious energy modeling results, especially if
you're doing comparisons between different glazing choices.

Mike

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