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

[Philip Haves]

Randy,

Most higher performance windows have a coating that is both low 
emissivity and spectrally selective.  It's the spectral selectivity - 
blocking the parts of the solar spectrum beyond the visible - that 
reduces the SHGC. The low emissivity reduces the U-value, as others have 
noted.

Phil

On 7/28/2014 11:55 AM, Randy Wilkinson wrote:
> 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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-- 
Philip Haves
Leader, Simulation Research Group
Lawrence Berkeley National Laboratory
One Cyclotron Road, MS 90R3145
Berkeley, CA 94720-8134
(510) 486 6512 (office)
(510) 486 4089 (fax)