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The facts about thermal pane windows
7 May 2005


Q. How about leveling with your readers on the facts about thermal pane windows? About 11 years ago I had my wooden frame single-pane windows replaced with vinyl thermal pane windows purchased from a reliable dealer. These windows had E-film, argon gas, and a ten year warranty. After 8 years moisture entered one of the sas hes. I was lucky the manufacturer was still in business. The manufacturer replaced the window and the dealer charged $60. for installation. Currently, two more windows have gone bad. What is the life expectancy of thermal panes (In the good old days a pane of glass lasted generations)? What happened to the argon gas? How did the moisture get in? Can a retail glass dealer replace a thermal pane in a vinyl sash?

A. Unfortunately, you can't reseal insulated glass that has lost its seal and has formed a condensation fog between the panes. I have seen people try to do it but it's a doomed effort. The whole glass assembly has to be replaced as you have learned. I frequently walk into houses as old as yours and see a number of windows on the sunny side of the house in some state of seal failure or another. It's a common problem.

The space between the glass uses air in between as an insulator. The less expensive insulating glass technology is not really sealed completely tight and that has to do with the thickness and the strength of the glass. Fancy, high priced insulated glass windows-- like the ones you bought-- now are completely sealed using a stronger grade of glass and inert gasses placed between the panes such as argon. They also anneal a micron or so of metal to the inside face of the inside pane as a reflector for th ose heat radiating sunrays and the whole package is quite remarkable from an energy standpoint. Those are called low-E windows, for low-emissivity, and they are the minimal quality that I would install for both room comfort and energy savings during both heating and cooling seasons.

The less tightly sealed spacers have a semi-permeable membrane to allow pressure equalization between the outside air and the air between the panes. It has a dryer in it called a dessicant and when that fails the windows fog. Those windows usually have a five years warranty and last just about that long if they get direct sunlight sometime during the day. Insulated glass windows in the shade tend to last and last. It’s the cyclic temperature changes from the sun that do in the spaces between the glas s. When the dryer fails over time, the air entering the space between the panes of glass will drag whatever moisture is in the outside air along with it. When temperatures change, the moisture in the air now trapped between the panes condenses and you get the "fog" that signals that the seal has failed. Once begun, the fog will worsen over time both from additional moisture getting in even creating drips and a photo-chemical reaction from sunlight sets up that will etch the inside surfaces of the glas s.

Your seals, as air-tight as they were designed to be, are failing and the action of the sun has driven the argon out and allowed moist air in that condenses between the panes. They mostly made it past warranty. That doesn’t do you much good.

The longest seal warranty in the window business of which I am aware is from Andersen and is longer than 20 years.

It's quite confusing when you see all that's available these days in the window market especially comparing quality against price. You can contact the National Fenestration Rating Council (NFRC) in Silver Spring at 1-301-589-1776 or visit their website @ www.nfrc.org/ for some info on how to compare one window to another

Local glass companies can and do replace insulated glass. They do it every day but I can’t vouch for any warranty periods. You’d have to ask.

Generally, you’d be better off replacing the entire window rather than just an upper or lower pane that’s fogged up. It's a pretty safe bet that the other pane will soon lose its seal. After all, it's eleven years old too and experiences the same env ironment. Why fork out a fistful of cash to fix the upper sash only to have to do the same for the lower one sometime in the not too distant future?

In the old days we had single pane glass windows and maybe storm windows over that. The windows were leaky at the edges and the taking the storms on and off was work. Windows have gotten better and better over the years and old windows are like Model Ts cars compared to what we have now. I like looking at them but wouldn’t want to rely on one.

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