Dual pane windows are also known as IG or Insulated Glass windows. Insulated really isn't the correct term to use for multipane windows however, although it is commonly used by pretty much everybody. Correctly, IG really means INSULATING glass since the glass isn't insulated in any way, but it does insulate. See where I am going with this? R-value describes a material's resistance to thermal transmittance. Insulation is generally rated by its R-value. A single lite of 1/8" glass has an R-value approaching 1. Add a second lite and the R-value jumps up to almost 2 (I have always done well in higher mathematics), so by adding a second lite, we have doubled the insulating value of our window. If we have a basic, simple, no-frills dual pane IG unit we have a center-of-glass R-value approaching 2. However, if we happen to have a single pane window and a single pane storm window installed then we find that we have an R-value approaching 2. So, two lites of glass is two lites of glass. The difference between the IG unit and the single lite with storm is negligible - when dealing with the R-value insulating capabilty of the clear glass lites. However (there's that word again), R-value only deals with conductive thermal transfer and doesn't take into account either radiant or convective thermal transfer. And on first glance it appears that your question specifically concerns radiant energy gain. "...the light passes through the storm, bounces off the single-pane or your house, changes wavelengths, but it can't pass back out through the storm window because it changed wavelengths, so the heat just stays in your house." Simply put - no. Consider the three "light" source we deal with. First is UV, then visible light, then IR light. UV stands for Ultraviolet and ultraviolet really doesn't account for any appreciable heat gain in your home (or anywhere else for that matter). UV does account for that burning sensation that you get when you are in the sun for two long (that would be UVB, btw), and UV does account for that gradual fading that you see in your home's furnishings, carpetsa, drapes, etc - and that would be UVA. And as an aside, since I mentioned UVB results in sunburn, UVA results in tan - and it is also the primary contrubutor to skin cancer. Go figure. Ordinary glass is opaque to UVB - in other words, UVB is blocked by ordinary glass. UVA, on the other hand, passes right thru glass. About 95% of the UV that we see down here is UVA and the other 5% is UVB. We don't get natural UVC (remember the ozone layer?), which is a good thing since it would potentially/eventually kill us. Next in the spectrum is visible light. Visible light is a good thing for many obvious reasons. As intended, visible light does pass rather well thru clear glass, about 90% of the visible lite that impacts a clear glass window goes right on thru. The remainder is either absorbed or reflected by the glass. Again, 90% transmittance is a very good thing for many obvious reasons. However, visible light entering our homes does result in an appreciable heat gain within the home. Visible light is a source of radiant heat gain and visible lite in the shorter wavelengths is responsible (along with UVA) for that annoying fading thing. Next on the list is IR or Infrared light. Infrared is the primary source of radiant heat and it is the primary source of heat on our world. No infrared, no us. We break down infrared into different wavelength or frequency bands. Near or short wave infrared is direct solar gain. Far or longwave infrared is not (re: what you were told). Infrared light passes quite readily thru clear glass. About 80% of near infrared passes thru "ordinary" clear window glass and up to about 85% of far infrared passes thru. I will have to finish this later. It is already pretty long and I have run out of time this evening. Hopefully, someone might find it interesting or even a bit informative.

Thanks Oberon, I didn't know one could specify a coating on the "outside" of an IG unit. That's a good idea for storm windows. My idea of using the big box to order replacement glass now won't work----they won't sell it unless I am specifically using it for one of their manufacturer's windows. I'm not---I'm making storm windows. So, now the price of glass has doubled and tripled based on quotes from the mom & pop shops I've talked to (none of which seem to know much about coatings and IG). So, I may be forced to use single-pane 1/4" glass. But, before I give up on IG, I want to poke around a bit more. EfficientWindows.org (if I'm reading it correctly) says clear, single panes get 1.04 U-factor and double-pane clear is 0.50 That seems a big difference and that's without any coating. What they don't say is the thickness of the glass panes in either example. Is that likely to be 1/8"? http://www.efficientwindows.org/glazing_.cfm?id=1 Their stated 0.50 U-factor is based on double-pane, clear with a 1/2" airspace. But again, they don't say the thickness of the glass. http://www.efficientwindows.org/glazing_.cfm?id=3 My winter temps average 20's nighttime & 30's daytime. Since my old drafty home never gets warm in winter or hot in summer, what are the best thickness choices for an IG unit? I was thinking 5/8" overall (two 1/8" lites with 3/8" airspace) but maybe another configuration is much better given performance & prices. Maybe 1/2" overall is good enough and much less expensive. What do you recommend? I am gearing up for winter! I'm new to this area (I'm from Los Angeles where I also had an old, drafty home but never really noticed). Thanks very much for all the great advice Oberon. You are a real asset to this forum. -Chris

Wow, so many ideas. Has the smart tint been used for very many years. Does it have a very good warranty. It sounds almost too good to be true. Why would Patricia Colwell put the solar shades on the inside instead of the outside. It would be hard to hang the shades above the arched windows on the inside. I could put them on the lower windows on the inside, and then put a tint on the arched windows. Or I could put solar shades on the outside of the house above the arched windows. Although I still don't feel confident that these outside solar shades will stand up to the wind, rain, snow and sleet of the seasons when they are left down. They will face the lake and have a fairly good wind come their way in the winter time. The screen shades look great for the lower windows, but I keep coming up queasy on what to do with the upper arched windows?

As long as you don't need them for privacy or light blocking, go for it and remove them. I would think you might need them for the sun but if you don't no reason to keep them. I guess. I happen to like blinds.