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

[Timothy Moore]

Randy,

I don't know what energy modeling software you're using, but you certainly don't need a finite-element analysis to do what you've described. Indeed, most of the tools used by regular contributors to this forum take into account all of the following glazing properties:

- U-value (as a function of low-e coating, gas fills, edge spacers, and so forth);
- SHGC as determined by spectral selectivity of coatings, absorption, re-radiation, and convection;
- Visible transmittance as a function of the glass composition

Most of the detailed data for many hundreds of actual glazing products and assemblies of these---needed if you want to use a particular set of inputs to represent this information accurately---can be found in the International Glazing Database from Lawrence Berkeley National Laboratory (LBNL) as viewed within their free Window 6 tool.

Once you have that data, you then need a software tool capable of modeling solar radiation striking your windows (including what passes directly through, what fraction is absorbed, and to which side of the glass this is re-radiated or convectively transferred), heat transfer from the air in the building to the interior of the glass at a given delta-T, and long-wave radiation trying to escape the building through the windows. This capability is common in all the major whole-building simulation packages.

If the software you're using does not account for these fundamental properties of glazing, perhaps you should consider upgrading your software, rather than abandoning your project.

Timothy Moore 
Senior Product Manager 

T: +1 (415) 983-0603
T: +44 (0) 141 945 8500
timothy.moore at iesve.com 
www.iesve.com 

Integrated Environmental Solutions Limited. Scotland registration SC151456
Helix Building, West Of Scotland Science Park, Glasgow G20 0SP

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-----Original Message-----
From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Randy Wilkinson
Sent: Monday, July 28, 2014 1:35 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] How can Low-e glass have dramatically lower U-Value?

I went into this thinking that the Low-e coating would simply change the SHGC and not change the U-Values much.  I suspected that Low-e coatings would NOT save energy in the far North since they would block the wanted solar heat gain.  Since the manufacturers are claiming a U-value improvement from the Low-e coating, It looks like I can't calculate the savings using our normal energy modeling software. If the U-value improvement they give is in lieu of a more direct calculation method, and is accepted by the community (you all), then I will accept and move on to the next problem.

Thanks to all for your comments.

Randy

On 07/28/2014 12:56 PM, Jones, Christopher wrote:
> Keep in mind that you want a higher SHGC in the north to maximize passive heat gain.
> The framing and glazing spacer selection is important.  Argon fill is a must have.
>
> Christopher Jones, P.Eng.
> Tel: 416.644.4226 * Toll Free: 1.888.425.7255 x 527
>
>
> -----Original Message-----
> From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On 
> Behalf Of Randy Wilkinson
> Sent: Monday, July 28, 2014 2:55 PM
> To: bldg-sim at lists.onebuilding.org
> Subject: [Bldg-sim] How can Low-e glass have dramatically lower U-Value?
>
> 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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