[Bldg-sim] How can Low-e glass have dramatically lower U-Value?
Philip Haves
phaves at lbl.gov
Mon Jul 28 12:50:45 PDT 2014
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
> _______________________________________________
> Bldg-sim mailing list
> http://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org
> To unsubscribe from this mailing list send a blank message to
> BLDG-SIM-UNSUBSCRIBE at ONEBUILDING.ORG
--
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)
More information about the Bldg-sim
mailing list