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

[Joe Huang]

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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