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

Judson Brown judsonb at krahn.com
Mon Jul 28 12:28:07 PDT 2014


When the building is in heating mode in the far north ( 60 degrees N latitude) the low emissivity coating on the 3d. surface of glass in the glazing unit works in a similar manner to that described below to block long-wave radiant heat transfer from the warm interior of the building to the outside.    The long-wave radiant heat is transmitted through the first layer of glass, then reflects off the low-E coating (on the 3d. surface of glass in the glazing unit) back into the heated interior space of the building.  

In this way, the radiant heat loss component of heat transfer from the building is blocked and the U-value of the glazing unit with a coating is significantly higher than a unit without a coating.  Hope this is helpful.

Judson Brown, PE, P. Eng., LEED AP

-----Original Message-----
From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: July-28-14 12:10 PM
To: Randy Wilkinson; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] How can Low-e glass have dramatically lower U-Value?

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)
www.ghtltd.com

-----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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