Yes, it's certainly nice if you can keep things simple.

I've done some research and gathered a few contenders for a good dual pane window for cold climates, especially for passive solar purposes.

There's a new glass made by Guardian: called ClimaGuard 75/68 HER that has 75% light transmittance and 0.68 SHGC, and the U-factor is 0.28 with argon. The soft-coat low-E coating is on surface 3 (best for the heating climates of Canada). You could put this on all sides of the house.

Another interesting glass is the Pilkington Energy Advantage (noted in another post above) that has a SHGC of 0.70, and U-factor of 0.29 with argon fill. I couldn't find the data for light transmittance... This is a hardcoat prodcut.

Cardinal has the low-E 178 glass that is a soft coat high solar gain product, but is still put on surface 3. It has 0.78 visible light transmittance, 0.63 SHGC, and U-factor of 0.27 with argon.

ACG makes the Comfort E2 glass that is hard coat on surface 3. SHGC 0.73, U-factor 0.30 with argon, visible light transmittance 76%.

Getting a vinyl window casing filled with foam is also good. I'm not sure if going triple pane is a good idea--I guess you would have to compare the numbers with the ones above. Now I've got to phone around and see if the local window suppliers can make windows with one of these glass products.

Jimmery,

Good information.

One quick follow-up, Cardinal has replaced LoE-178 with LoE-179.

LoE-179 has VT of 79%, SHGC of .70, and U factor of .28 with argon fill.

Do you know of window makers who use any of these high SHGC and Low U glass?  I am in San Jose and want some high SHGC for my south windows?

Thanks,

John

Many window makers can use nearly any type of glazing you specify for the windows. Marvin and Pella are two quality manufacturers that can put in high SHGC glass for passive solar purposes. Because of the cooling requirements, high SHGC glass has been less popular in the United Sates, but many Canadian and European manufacturers have even more experience with it.

Posted By Oberon on 24 Apr 2008 09:04 AM

Addressing a couple of possible misconceptions - <!--?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /-->

... And as an aside, technically LowE coatings do not reflect IR (or any other sort of radiation).  They are "low emissivity" which is not the same thing as being reflective.  I am using reflect or reflective in this posting because it is easy to understand (and picture) the coating as being reflective and it works for a basic understanding of different coating properties.

I realize that I'm resurrecting an old thread but I've some questions. I'm not sure that Oberon is still active here. If he is I'd appreciate his comments, if someone else has something to add, I'd appreciate that as well.

My understanding of emission is that a body receives radiation, becomes warmed and then emits that energy as radiation, usually IR. So if these coatings are not reflecting the heat, is the glass absorbing the heat and then emitting it toward the exterior more than it emits it toward the interior?

TIA

Bill

Bill-

You have picked out the most confusing paragraph in oberon's discussion of coatings. Let me ignore his paragraph, and suggest the following.

R + T + A = 1 at a given wavelength (or frequency) (Eq. 1)
where
R = fraction of electromagnetic energy reflected
T = fraction of electromagnetic energy transmitted
A = fraction of electromagnetic energy absorbed

E = A at a given wavelength (or frequency) (Eq. 2)
E = emission energy normalized by maximum possible emission energy for perfect radiator

It sounds like you want to know how a low-e window works to reduce heat loss from a room in the winter. So the room is hotter than outdoors, and tries to get heat through the window by conduction, convection, and radiation. The room is at about 300 K (27 C) and the radiation peaks in the infrared (IR) at about 10 micrometers. Glass is opaque to this wavelength, so the inner glass pane absorbs that radiation, as well as absorbing heat from the room by convection and a little by conduction. The inner glass pane tries to get heat to the outer glass pane by the three heat transfer methods, conduction, convection, and radiation. Conduction is limited by the spacers used between the panes. Convection transfers some heat, but the gap between the panes is chosen to minimize convective heat transfer. The inner pane is a little cooler than 300 K. With no coatings, the inner pane radiates some IR energy like a blackbody that peaks at roughly 11 micrometers or so, and the outer pane is a good absorber at that wavelength, so a fair amount of energy is lost due to radiative heat transfer.

With a low-e coating on surface 3, the surface of the inner pane that faces outside, then the radiative heat transfer is reduced. So we need a surface that has a low value of E (emissivity) from above, which, from Eq. 2, means a low value of A at 11 micrometers (and wavelengths in that region). To have a low value of A, it must, from Eq. 1, have a high value of T or R. It would not be good for it to have a high value of T which is transmission, or we would be back to the case for uncoated glass. Therefore, the coating must have a high value of R (reflectivity) at that wavelength. A reflective surface at that wavelength would make it a poor emitter (a low value of E), and would reduce radiative heat transfer.

Instead of coating surface 3, you could instead coat surface 2, the side of the outer pane that faces indoors. Then the inner pane would radiate heat well since it is just uncoated glass, but the coating, if it were reflective at around 11 micrometers, would reflect the radiation back toward the inner glass pane.

You can check out the Cardinal glass catalog (older version) at http://www.cardinalcorp.com/wp-content/uploads/pdf/residential-brochure.pdf to see some numerical values for coating properties and relative amounts of heat energy transferred for low-e versus standard windows.

Thanks Lee!

I'll have to sit with what you posted for a while.

I'm glad to see that you believe reflection is a large part of the technology. I'm employed in the fenestration industry and can testify to the greater intensity of reflected heat from a Low-E unit compared to uncoated glass.

Bill