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

Randy Wilkinson randallcwilkinson at gmail.com
Mon Jul 28 18:31:25 PDT 2014

There is my problem.  For decades I've been trained to understand that 
energy exchange thru glass contains 2 components:  1) Conduction - 
Calculated using a U-Value with units of Btu per hour per SF per degree 
F., and 2) solar - calculated using the sun's energy output, adjusted 
for latitude, shading factor for glass, tinting, drapes, etc.

The U-Value is only for conduction...by definition, and is confirmed by 
looking at it's units (per degree F).  Now a few molecules of metallic 
coatings added to one surface adds to the U-value?  That's the part I 
find hard to believe.  Or at least I did until today.

I can accept that an improved U-value can be determined that would make 
it easy to calculate the overall equivalent heat transfer, averaged over 
a season, including the effect of long wave infrared radiation from the 
Low-e coatings.  That would make less sophisticated simulation programs 
be able to get better results. I guess I just need to verify that the 
improved U-value given is an NFRC value that has been determined using 
LBNL's Window software.

Thanks to all for helping me understand.  As others pointed out, E+ can 
accept a Window glass definition file and can also allow you to define 
glass parameters using layers.  I better figure that out next.


On 07/28/2014 05:05 PM, Joe Huang wrote:
> Randy,
> I'd like to dissuade you of the idea that a U-factor that includes 
> long-wave radiation is some sort of work-around.  In fact, it's the 
> standard definition of U-factor as applied to windows because of their 
> high conductivity.  Actually, the more I think about it, it would be 
> the conduction-only U-factor that would be misleading. I suppose it is 
> possible that a program might use such a U-factor, then also do a 
> external radiative exchange, and thus be double-counting, but that 
> seems unlikely because to do that you would have to solve for the 
> external surface temperature, which would require a detailed heat 
> balance, etc.
> Sure, standard weather files have all the information needed to 
> account for long-wave radiation, because during the day, the long-wave 
> radiation from the sun is included in the reported solar radiation, 
> which is all spectrum, while the long-wave radiation exchange with the 
> ground, air, and sky are all calculated using assumed temperatures for 
> each and generally minimal.  During the night, the main long-wave 
> radiation exchange is with the night sky, for which there are various 
> algorithms to estimate the night sky temperature, depending on the 
> atmospherics, clouds, etc.
> 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 2:50 PM, Randy Wilkinson wrote:
>> 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 94556
>>> yjhuang at whiteboxtechnologies.com  <mailto: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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