[Bldg-sim] How can Low-e glass have dramatically lower U-Value?

Randy Wilkinson randallcwilkinson at gmail.com
Mon Jul 28 13:45:48 PDT 2014


This is exactly what I mean by asking if our energy modeling software is 
inadequate.  Maybe a Senior Analyst or Building Scientist can do this, I 
don't think I can, or should.  If it takes specialty software to model 
long wave radiation coming in AND going out, then it seems like the 
functionality of Window/Therm/Optics should be built into our energy 
modeling software.

Thanks,

Randy

On 07/28/2014 12:03 PM, Jeremiah Crossett wrote:
> Dear Randy,
> What software are you using?
>
> To properly model window coatings you could first use a 2D FEA package 
> such as Window, then for framing Therm, and for optical you could use 
> Optics.
> Then you can use the 2D model results as inputs to 1D software such as 
> Energy Plus.
> http://windows.lbl.gov/software/default.htm
>
> Also a nice, quick way to do analysis is to use COMFIN, (in same link) 
> a graphical UI to E+ that is setup to model windows that have been 
> calculated with Window/Therm/Optics.
>
>
> /
> /
> **
> **
> **
> ​ ​
> Jeremiah D. Crossett***| Senior Analyst**| **LEED Green Associate *
> *
> *
> ​ ​
> 120 E. Pritchard St.  | Asheboro, NC 27203
> ​ ​
>  | Mobile 503-688-8951*
> **www.phasechange.com <http://www.phasechange.com/> **
> *
>
> **
> **
> **
> **
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>
>
> On Mon, Jul 28, 2014 at 11:55 AM, Randy Wilkinson 
> <randallcwilkinson at gmail.com <mailto:randallcwilkinson at gmail.com>> wrote:
>
>     Bldg-Simers,
>
>     I wanted to see if Low-e glass saves energy in the far North (60
>     deg. N latitude or more).  My thought was to use the same U-value
>     for the glass, but change the SHGC to account for the difference
>     in solar heat gain due to the Low-e coatings.  To my surprise,
>     manufacturers data for Low-e glass lists much lower U-values for
>     the same double glazed units except with a Low-e coating on
>     surface #3.
>
>     I'm having a hard time understanding how a coating a few molecules
>     thick, improves the U-value so much.  The Architects in my firm
>     say that the manufacturers are calculating an improved U-value to
>     account for energy saved by blocking radiant heat lost (going from
>     inside, out) in Winter.  They surmize this is done because our
>     energy loads and modeling software cannot calculate radiant heat
>     loses in Winter.  I'm not sure the weather data we use has hourly
>     long wave radiation data that can be used to determine the
>     available IR heat that can be blocked by the Low-e coating.  I
>     don't think our energy modeling software can account for radiant
>     heat leaving the building in Winter.
>
>     For example,
>
>     Pilkington 1" double pane clear glass using air, has a Winter
>     U-value of 0.47 Btu/hr.sq ft F and an SHGC of 0.71
>
>     The same Pilkington unit with their Energy Advantage Low-e coating
>     has a Winter U-value of 0.33  and an SHGC of 0.67
>
>     PPG lists similar improvement for their Low-e coating
>
>
>     Is our energy modeling software inadequate to accurately model the
>     effects of Low-e coating on glass? Both Summer and Winter?
>
>     Can we trust that the glass manufactures are giving us improved
>     U-Values due to Low-e coatings that are valid?
>
>     Randy Wilkinson
>     Spokane, WA
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