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

Joe Huang yjhuang at whiteboxtechnologies.com
Mon Jul 28 14:34:43 PDT 2014


Randy,

I think you're being misled in a way. ALL building simulation programs model long-wave 
radiation between the building surfaces and the environment, because otherwise you would 
get erroneous results, a case in point being  night-sky radiation that causes roofs to be 
significantly colder than the outdoor air at sunrise.  How different programs handle 
long-wave radiation varies, but that's more an issue of modeling methodology, whether to 
combine the radiative with the convective or calculating them separately, what temperature 
to assume for the
environment (ground, sky, air, etc.), etc.

As for the LBNL Suite of window simulation software (Window/Therm/Optics), I don't know of 
anyone except the NFRC Simulation Laboratories that use all three, and only for the 
purpose of getting an NFRC rating  of a specific product.  Outside of that context, the 
most I've seen people
do in building energy simulations is to obtain or create a "Window-4" file using Window 
(but not Therm or Optics) and then import that into their building energy software.  Even 
there, the main advantage is to get better representation of the angular-dependent 
properties of the window.
As far as capturing the long-wave radiation, inputting the U-value from an NFRC Rating or 
a Window-4 file should work fine.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"


On 7/28/2014 1:45 PM, Randy Wilkinson wrote:
> 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/> **
>> *
>>
>> **
>> **
>> **
>> **
>>
>>
>>
>> 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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>
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