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

James Hansen JHANSEN at ghtltd.com
Mon Jul 28 12:10:20 PDT 2014

By decreasing the emissivity of glass, you improve the glass' ability to
reflect long-wave energy.  If the glass doesn't have to absorb the heat,
then it's not going to convect and/or radiate that heat to the indoor
space (which is the method by which it improves the U-value).  That's
how a very thin coating can have such a dramatic impact on U-value.

I'm not sure if that answers your question!

GHT Limited
James Hansen, PE, LEED AP, BEMP
Senior Associate
1110 N. Glebe Road, Suite 300
Arlington, VA 22201
703.243.1200 (main)
703.338.5754 (direct/cell)

-----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?


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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