On Fri, 14 Apr 2006 16:56:28 -0400, clare at snyder.on.ca wrote:
I hadn't. After about an hour of Googling, I still don't know exactly
what that term means. Some people seem to be using it to identify
"soft coat" or sputtered Low-E as opposed to "hard coat" (pyrolytic
CVD) Low-E. Several pages seemed to imply that the "Low-E^2" name is
proprietary to Cardinal Glass, while others use "Low-E2" and "Low-E^2"
As I've heard some places before,
The U-factor does not change whether the coating is on surface 2 or 3.
But their page on solar heat gain coefficient
gives numbers showing more summer heat gain with #3 coated., and
repeats the "surface matters" explanation:
Low E glass coatings work by reflecting or absorbing IR light (heat
energy). The thickness of the Low E coating and the position in the
window (#2 or #3 surface) dictate how the window will perform.
When installed on the #3 surface of an insulated glass unit (IG), the
Low E coating will reflect IR heat from inside the room to help reduce
the energy loss during the cold months, thereby reducing heating
When installed on the #2 surface of an IG unit, the Low E coating will
reflect or absorb IR heat from the outside, thereby reducing solar
gain and cooling costs during the warm months.
Back at the Cardinal site,
LoE² coatings on the other hand are solar selective, and will commonly
be referred to a low solar gain product. The multiple layers within
the LoE² coating block solar infrared while allowing the visible
energy to pass through the glass.
Perhaps the confusion arises from companies using the term "Low-E" as
a generic identifier for "solar selective" coatings as well as for
coatings that modify long-wave IR (sensible) emissivity?
I could believe that when a high-emissivity surface and a low
emissivity surface face each other, the resulting long-wave sensible
heat transfer is the same regardless of the heat transfer direction -
limited by the lower of the two values. I believe that is the
I could also believe that if more short-wave IR solar radiation is
reflected away by a coating on surface 2, less of it will hit surface
3 to be absorbed and raise the inner glass temperature. If surface 3
has the Low-E coating, the energy that is not reflected from it has
already contributed heat to the inner glass. The re-radiation from the
inner glass surface adds to the solar heat gain coefficient.
I haven't seen a standard quantity that identifies how much of the
solar heat gain coefficient is due to this re-radiation, and how much
is due to the overall transmission or reflection of solar IR,
regardless of which surface is coated. I suspect the larger component
of SHGC is due to the wavelength of the emissivity cutoff the coating
provides. Cold climate "Low-E" allows shorter (solar) IR wavelengths
to pass but traps longer (sensible) IR wavelengths indoors. Hot
climate solar selective coatings block both sensible and solar IR,
moving the cutoff up to allow only visible light through.
I guess companies feel they need to simplify this for the consumer...
Finding out what is really going on certainly isn't simple! I hope I'm