[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?
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
> 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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