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

Arpan Bakshi arpanbakshi at gmail.com
Mon Jul 28 15:57:48 PDT 2014


When architects say things like, "our energy loads and modeling software
cannot calculate radiant heat losses in Winter", that is code for "why is
reducing WWR showing better results than any other enclosure strategy".

On Monday, July 28, 2014, Randy Wilkinson <randallcwilkinson at gmail.com>

>  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?
> Randy
> 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 94556yjhuang at whiteboxtechnologies.com <javascript:_e(%7B%7D,'cvml','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
> <javascript:_e(%7B%7D,'cvml','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


Arpan Bakshi

646.704.2880 mobile
212.298.9352 office
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