[esp-r] Re: Perplexing U-values with low-e glass

[Jon Hand]

The low-e of the glass must be represented by altering the air gap
resistance. The surface properties of the glass are not taken into 
account in the U value report.

Have a look at other transparent constructions which include 
low-e glass and you will see the pattern.

-Jon

-----Original Message-----
From: esp-r-bounces at lists.strath.ac.uk on behalf of Evan Sterling
Sent: Fri 7/11/2008 5:01 PM
To: esp-r at lists.strath.ac.uk
Subject: [esp-r]  Perplexing U-values with low-e glass
 
I am modelling some triple-glazed windows, by copying the default
double-glazed units provided in the database and adding an extra air gap and
pane (see below for details). But when I substitute low-e glass for plate
glass, the U-value calculated automatically actually rises somewhat, when I
would expect it to drop. Is there something I'm missing here?


*Normal glass*

 Layer|Prim|Thick |Conduc-|Density|Specif|IR  |Solr|Diffu| R    |Descr  |db
|(mm)  |tivity |       |heat  |emis|abs |resis|m^2K/W

 Ext   242    6.0     0.760  2710.   837. 0.83 0.05 19200.  0.01 plate glass
: Plate glass with placeholder s

    2    0     12.0       0.000     0.     0. 0.99 0.99     1.       0.17
air  0.17 0.17 0.17

    3  242    6.0     0.760  2710.   837. 0.83 0.05 19200.  0.01 plate glass
: Plate glass with placeholder s

    4    0     12.0     0.000     0.     0. 0.99 0.99     1.        0.17 air
0.17 0.17 0.17

 Int   242    6.0     0.760  2710.   837. 0.83 0.05 19200.  0.01 plate glass
: Plate glass with placeholder s

 ISO 6946 U values (horiz/upward/downward heat flow)=  *1.874  1.985
1.743*(partition)
1.603


*Low-E glass*

 Layer|Prim|Thick |Conduc-|Density|Specif|IR  |Solr|Diffu| R    |Descr
|db  |(mm)  |tivity |       |heat  |emis|abs |resis|m^2K/W

 Ext   246    6.0     1.050  2500.   750. 0.05 0.06 19200.  0.01 low_e glass
: 6mm CF Low_e glass with placeh

    2    0     12.0     0.000     0.     0. 0.99 0.99     1.          0.17
air  0.17 0.17 0.17

    3  246    6.0     1.050  2500.   750. 0.05 0.06 19200.  0.01 low_e glass
: 6mm CF Low_e glass with placeh

    4    0     12.0     0.000     0.     0. 0.99 0.99     1.          0.17
air  0.17 0.17 0.17

 Int   246    6.0     1.050  2500.   750. 0.05 0.06 19200.  0.01 low_e glass
: 6mm CF Low_e glass with placeh

 ISO 6946 U values (horiz/upward/downward heat flow)=  *1.897  2.011
1.763*(partition)
1.620

Thanks,
Evan Sterling
Mathematics & Engineering
Queen's University