Adding second pane to wood window

I have a fixed wood frame window with the glass secured by flat 3/4" strips of wood on the inside. To prevent condensate forming between the two panes, do I need to air seal the two panes or just secure with a second set of wood strips?
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On 1/7/2005 3:05 PM US(ET), Martik took fingers to keys, and typed the following:

The less air leakage between the two panes, the better.
--
Bill

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that's why they are sealed in commercial windows. sometimes they have a silicate material in them, or are filled with dry argon.
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It's not that easy a problem.
Condensation forms in proportion to the humidity of the air and the temperature of the surface.
If warm, humid air leaks through the new inner pane and gets to the cold outer pane, condensation will form on the inside of the outer pane.
If cold air from outside leaks through the outer pane and cools the inner pane, condensation will form on the inside of the new inner pane.
Most window makers make the seals airtight and fill between the panes with an inert gas. That's a great solution, as long as it works. Once the seals deteriorate and air leaks in, the window clouds and the entire glazing system needs to be replaced.
Pella windows let a small amount of outside air between the panes. That seems to prevent any condensation between the panes, but doesn't allow the inner pane to cool to the point where indoor humidity condenses inside.
So, to put it simply, you're trying to replicate the results of the best window manufacturers and their labs in a jerry-rigged system. The answer, I think, is that you won't be able to do so very satisfactorily, and you'd be better off getting a real 2-pane window and installing it.
--
Doug Boulter

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If you could place some descecant (sp?) between the panes and seal the space, you would reduce the amount of moisture available to condense.
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I asked a glass man about this once, and what he showed me was an aluminum strip with desiccant inside about 3/8 thick. this would be placed between the two panes and them a sealer was used all around. This whole unit was then set into the frame.
This set up might be suitable with your situation.
--

Roger Shoaf

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that's the only real solution. take off the moldings and take a good measurment of the glass inthe window opening. then go see your local glass shop and order at minimim a 5/16" insulated unit. do not measure the glass tight from side to side, you want at least 1/8" less on your width and height measurements. he'll tell you about installation tips, but it's easy and do not re-use the old moldings they will need to be trimmed down to allow for the new thickness of the unit. you can use flat stock material, but make sure your bottom and top are installed first. you can use latex caulk on the outside to make a good seal on glass to wood moldings for painting purposes, but set the new unit in a clear silicone on the backside or interior side of the opening *or* if the window is set from the inside do the opposite.
good luck...and make sure the opening is square before ordering anything.
mike...............
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Be sure to do some cost calculations first. I considered replacing the single glass windows with double paned insulated glass in my Minnesota house a few years ago. I was going to use the existing sash and replace only the glass. I gave up on the idea when I calculated the payback to be something like 15 years (if I remember right). Going from one pane to two only increases the R value from about 1 to about 2 which is still very low. Such a small change in thermal transfer for such a small area does very little to improve heat loss and insulated glass is expensive.
Going to insulated glass may offer the benefit of more comfort if you're sitting near the window, but don't count on it to save you any money.
Spending money on air infiltration by weather stripping is much more cost effective.
Tom

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I agree but, I'm only spending $2000 to replace 10 large windows averaging 20sqft each, with low-e argon which increases the R-value from 2 to 4 for only 20% more. I expect to receive approx $1500 in rebates which makes my net cost about $500. My main concern is comfort and resale value not payback but I just could not resist such an excellent deal. Of course my labour is 'free' :)
Reducing air infiltration, increasing attic and basement insulation are all part of the project.
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If your in a mild climate, you will probably be ok with fabbing your own dbl panes. My dad (Atlanta, GA) slaps a piece of plexiglass over the windows (interior side) in the heating season to create a temp dbl pane. Doesn't have any problems with condensation. Here in central TX I have taken apart dbl pane windows to clean out the smoke (damn my renters that smoke)that leaked in and then reassembled/installed. No problems, didn't even use any sealant. The problem with sngle panes around here is when a cold front blows thru and the temp drops 20dg in 1hr, the humid air inside the house condenses on the now cold glass. Some day I will try making my own dbl panes. Awhile back I checked on buying some of the spacer strips, they contain dessicant and have the adhesive preinstalled. About $1 per ft and that was a few years ago. That adds up fast, I will first try the wood strips. Also, the bigger the space the better the insulation factor.
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Acually the bigger the space the greater the more interpane convection and therefore condensation . About 1/10" ispace is ideal....mjh
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glass screen door about 6" apart and there is never any condensate.
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I will have to dig out my HVAC manuals to check but seems that the table showing R values for dbl pane windows indicated that the bigger the air space the better the R. seems like the triple pane were very impressive. My previous house (central TX) had storm windows installed. They created not much more than a primitive dble pane, maybe a couple inches of air gap. Never had any condensation problems but I am sure they are not very airtight. Wasn't insulation value the marketing hype back when these things were sold? Guess they are obsolete now with dbl pane windows so prevalent.
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Martik wrote:

You are not going to be able to seal those panes will enough to keep the moisture out of them. Even the manufactured windows can loose their seal and when that happens, they cloud up quickly. Yours will not be that tight.
Your only home made solution is to vent the windows to the COLD side "generally outside" That air is drier than the air inside. The other way the warmer moister air will reach the cold outside window and condense on it. Make sure you have a way of disassembling this window from time to time as some dust etc. will get between the glass and need to be cleaned off.
You may want to see if you can mount a thermo-sealed window in that space. You can buy the sealed glass units in any size you need at prices that may be a lot less than you think. Most larger cities have local shops that will make them up for you.
--
Joseph Meehan

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Thanks for all the replies!
I'm replacing 10 single pane windows with double low-e and will have a lot of glass left over.
This window is quite small 31/2' x4' and set in stonework so I think I'll just add another pane (removable) and see what happens. I can always replace it with a sealed unit later. Temperature here rarely goes below freezing.
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