[BLDG-SIM] glass u-vals

[D. Charlie Curcija]

The overall window U-factor is usually not significantly higher than center
of glass (cog) one.  It is only significantly higher if the frame U-factor
is significantly higher then cog U-factor and if spacer is highly
conductive.  This is typically true for Aluminum framed windows and Aluminum
spacers.  If you know frame U-factor and spacer material you can simply
calculate overall window U-factor by doing area weighting using the
following formula
 
U = (Af*Uf + Aeog*Ueog + Acog*Ucog)/A
 
where,
 
Af = total projected frame area
Uf = frame U-factor
Acog = Ag - (W-0.127)*(H-0.127)
Wg = exposed glazing width
Hg = exposed glazing height
Ag = Area of exposed glazing
Ag = W*H
Aeog = Ag-Acog
Ucog = center of glass U-factor
Ueog = edge of glass U-factor
A = Total window area
 
Uf and Ueog is usually determined using 2-D numerical analysis.  There are
number of numerical tools for this but the best one and easiest to use is
THERM program (windows.lbl.gov).  You will also need WINDOW program (same
web site) to prepare glazing system for THERM.  However, this is
time-consuming process and it requires that you know all of the details of
glazing, spacer and frame.  If you don't have those details and you don't
need absolute accuracy, easy way to get Ueog is to look in ASHRAE Handbook
of Fundamentals from one of prior years and to read Ueog from Figure Ueog
vs. Ucog (Figure 6 in 1989 Fundamentals).  If you don't have access to this
book, let me know and I will send you scanned Figure.
 
Note: The above formula is a simplified version of the full window U-factor
calculation formula, and it assumes that all of the Uf and Ueog are the same
regardless of the position in a window (i.e., jambs, sill, head, mullion,
etc.).  In your case, with not having all of the details, this formula is
appropriate.  Otherwise, members in the above formula are under summation
signs, since each component of a window is accounted for separately.
 
Use of ISO 10077 standards is not appropriate here because we use different
boundary conditions, so unless this is for some calculations in Europe I
would strongly advise not to use these standards.
 
Regards,
Charlie Curcija

  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Annalisa
Simonella
Sent: Wednesday, April 18, 2007 4:37 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] glass u-vals



Hi Aaron,

The overall U-value is usually significantly higher than the centre-pane
U-value. In order to estimate the overall U-value of curtain walls or
windows you need to include the frames U-values and the edge effects and the
additional effect of point thermal bridges if present (structural
connections through the facade or window).

 

You may find some guidance in the NFRC standards
http://www.nfrc.org/technicaldocs.aspx  and you may use the software THERM
(downloadable from the LBNL website http://windows.lbl.gov/software/ ) to
calculate the U-value + edge effects of frames. 

You may also find some guidance in the EN ISO 10077-1 and EN ISO 10077-2
which define a different method from the NFRC one, also the first one gives
simplified method and includes a table with pre-calculated linear thermal
transmittances (edge effect).

 

I hope it helps.

Best regards,

Annalisa.

 

Annalisa Simonella 

ArupFacadeEngineering 
13 Fitzroy Street 
London W1T 4BQ 

T +44 (0)207 75 54593 
F +44 (0)207 75 53626 
M +44 (0)77 1781 4793 

mailto:annalisa.simonella at arup.com 
http://www.arup.com/facadeengineering/ 

  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Dahlstrom,
Aaron
Sent: 18 April 2007 04:24
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] glass u-vals

 

ASHRAE 90.1 window U-vals are for the "assembly."

Manufacturer's data for my project is for "center of glass."

 

Is anyone familiar with a way to get from one to the other?

 

I have a frame type and u-value; however, Fund. Chap 31 seems to caution
against doing hand calcs due to edge-of-glass effects.

 

Thanks,

 

Aaron Dahlstrom

AKF Engineers, LLP
1500 Walnut Street, Suite 1400
Philadelphia, PA 19102 
Phone:   (215) 282-2169
Fax:        (215) 735-6706
 <http://www.akf-engineers.com/> www.akf-engineers.com

 

 
 
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