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

[Randy Wilkinson]

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