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

[Robin Mitchell]

The LBL WINDOW program can calculate a U-factor and Solar Heat Gain Coefficient for a particular IGU, i.e., you create a glazing system with the different layers from the Glass Library, make sure the low-e coatings are on the right surface, and the program will do that calculation for the glazing system without a frame. If you want whole window properties, you can put the glazing system into either a generic frame or a frame modeled with THERM, and WINDOW will calculate the U-factor and SHGC for the whole window.

 

Energy Plus can correctly model a window with low-e glass, and uses algorithms from WINDOW that have been incorporated into the program. You can use a file generated by WINDOW to define the glazing system and frame and insert it into the Energy Plus IDF, or you can built it up natively in whatever interface (or no interface) that you use to run Energy Plus. The DOE2 simulation engine can also correctly model low-e glass.

 

Robin

***

Robin Mitchell

Building Technology and Energy Systems

Lawrence Berkeley National Laboratory

 

From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Randy Wilkinson
Sent: Monday, July 28, 2014 1:46 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] How can Low-e glass have dramatically lower U-Value?

 

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. 

 

Image removed by sender.




 

 

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Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate 

​ ​

120 E. Pritchard St.  | Asheboro, NC 27203  

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 | Mobile 503-688-8951

 <http://www.phasechange.com/> www.phasechange.com 

  

 

 

 

 

On Mon, Jul 28, 2014 at 11:55 AM, Randy Wilkinson <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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