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

[Jones, Christopher]

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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