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

Randy Wilkinson randallcwilkinson at gmail.com
Mon Jul 28 14:50:00 PDT 2014

Thanks Joe,

So you are saying that use of an improved U-value is a valid work-around 
in energy modeling software that can't or doesn't model long wave 
radiation in and out.  What if I put that improved U-value in there and 
the program then calculates the effect of long wave gains and 
losses...would we then have double accounted for the radiation effects?

Since I recognize you as also an expert at simulation weather data, do 
our typical hourly simulation weather data files contain sufficient 
information to model energy losses and gains from long-wave radiation?  
Especially gains from the Sun?


On 07/28/2014 02:34 PM, Joe Huang wrote:
> 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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