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FixitDragon 04-21-2011 12:38 PM

Anyone ever seen two windows not double pane?
Since purchasing my 1981 track home 5 years ago I have run across sever different interesting characteristics done by the previous owner. One of the most interesting is my windows.

I have double windows, a single horizontal sliding pane on the outside like normal, and then a second one in the inside of the wall. Kind of like a early attempt at double pane maybe? So there is 4" of air space between the panes when closed, and to open I slide the inside one, then the outside one.

I had thought about replacing with a nice double pane, but my stepdad said I actually get very good insulating value from the 4" of trapped air, and would gain nothing by replacing with a double pane that has 1/2" of gas.

So what do you all think? Ever see this before? Would I gain anything by replacing them? It doesn't bother the way they are, just trying to improve my cooling load during summer (most of summer is 95-110).

gotogregg 04-21-2011 12:46 PM

Hey Fixitdragon,

Your dad is right that you get a good insulation value from the gap between the two panes of glass, but it's not better than new insulated low-e double pane glass. It wouldn't be difficult to replace them entirely with wood or vinyl replacement windows. I would leave it until you can replace it with a new window. It's not that uncommon, but I wouldn't recommend it. -Gregg

oberon 04-21-2011 04:55 PM


Totally dead air (absolutely no movement at all) has a R value of about 6. If the air moves then the R value drops.

It seems to make sense that the 4" space that you have would then substantially outperform the 1/2" or so in a typical dual pane, but the wider the air space between two panes of glass the greater the air movement in that space due to the formation of convection currents. Also the greater the difference between the interior and exterior temperatures, the more the air moves in the space.

What all that is saying is that the 4" airspace is not an advantage from an energy performance stand point. It's going to be just about the same as the performance in the thinner (clear glass) dual pane. So no real advantage there either. Note I said "clear glass" dual pane.

The primary advantage of dual pane windows over a set up like yours is that the dual pane allows the manufacturer to include LowE coatings and gas fill between the two lites. Given the addition of those two upgrades, the performance of the dual pane would exceed what you have described in your home.

But is it worth it to change out the windows? In my opinion, nope, not at all.

You live in a very mild climate, so assuming that the windows that you have work well and aren't rotting and falling out of their frames, and you really don't hate the sight of them, then the energy advantage of newer windows just isn't going to be a cost effective upgrade.

I would suggest that you perform whatever maintenance might be required to maximize the performance of the current windows and let it go at that. I might also look into adding a solar reflecting exterior shade to help with the cooling load. That would be a good bit less expensive than replacement.

HomeSealed 04-21-2011 04:58 PM

Fix it, dead air space is a good insulator, however, leaky windows allow that air to move around between the two panels. You WILL be able to realize some significant gains by replacing those windows, but not even so much for that reason, but in sunny southern cal, low-e glass will do wonders for you. Look for a low SHGC, like .2 or lower.

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