I have a problem I am having trouble with and I was hoping that you might help me. I have secured a pice of double glazed window glass with a low e surface between the two sides. I have two questions I hope you can help me with: How do I determine which of the two panes has the coating? Which way does that low E pane face - towards the outside or towards the interior? Thank you in advance for your assistance. Will

Willallen-

Take a laser pointer and shine it into the window at about a 45 degree angle, and look at the reflections on the same side of the window as the laser. Notice that there will be four reflections spaced out in a line. The reflection closest to the laser is from the first surface, and so on. Either the second or third spot will be brighter than the others, and that will indicate that surface 2 or 3, as numbered from the side that the laser is on, is the coated surface.

I think that low-e windows are usually oriented such that the inside surface of the outer pane is the coated surface (surface #2 in the parlance of windows manufacturers that number from outside to inside). In that orientation, the long-wavelength infrared energy radiated from the inner pane is reflected back toward that pane when trying to radiate warm energy outward in the winter. If the coated surface were placed as surface #3, it would also work by reducing the radiated energy outward at the source in the first place. There is a small advantage when considering multiple reflections, and I think that favors using surface #2 over #3 to reduce heat losses in the winter. Some say that these coatings reflect long-wavelength energy back into the room, but that is inaccurate. The window panes are completely opaque to long-wavelength infrared energy.

Lee Dodge
www.ResidentialEnergyLaboratory.com
in a net-zero source energy modified production house

Will-

Following up on your question, according to Pilkiington that makes a low E glass:

"A room at 70 F radiates energy in the far IR (maximum radiation at around 10 micrometers). Uncoated glass readily absorbs this energy, heats up and radiates heat to the cold exterior in winter. Low-E coatings, because they are electrically conductive, reduce the transfer of this long wave IR energy. (Pilkington Energy Advantage™ Low-E glass has a Fluorine doped tin oxide coating making it conductive)."

"If the coating is on the 3rd surface (the exterior side of the room-side light) of a two light insulating glass (IG) unit, it cuts the radiant heat loss to the cold exterior by reducing the radiation or emission, towards the exterior, of the energy absorbed within the room."

"If the coating is on the 2nd surface of an IG unit, it controls winter heat loss by reflecting the long wave IR, coming from surface #3, back towards the room. A coating such as Pilkington Energy Advantage Low-E with an emittance of 0.15, has an IR reflection efficiency of 85%, as compared to uncoated glass with only 16%.
The thermal insulation effects of the coating on either #2 or #3 surfaces are identical for winter nighttime heat loss. The two equal window U-Factors (thermal conductivity) show this fact."

Thus, according to this information, it does not matter which way you mount it the window. That result will save you having to identify which side is coated!

Lee Dodge
www.ResidentialEnergyLaboratory.com
in a net-zero source energy modified production house