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

Jeremiah Crossett jcrossett at phasechange.com
Mon Jul 28 12:03:48 PDT 2014

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.

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 503-688-8951www.phasechange.com <http://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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