Doors and Windows - New double pane - condensation!?

I'm upgrading my SOCAL home windows from the 25yr old alum frame single panes. I just installed a Certainteed vinyl XOX (double slider) window on the north facing side of my home, replacing an aluminum double slider. About 9pm I noticed a thick condensation coat on the exterior of the picture window, but not on either slider. I wiped it off (to mostly make sure it was on the exterior and not in between panes), it was back 2hrs later and still evident at 9am this morning.

In 20yrs there was NEVER condensation on any of the older alum single panes. That there was no condensation on either slider portion or on a Certainteed single, east facing OX vinyl slider - does this suggest an issue with the picture window of the XOX slider?

The reason you never got condensation on your old windows was because single pane glass is so inefficient at keeping energy in, that your single pane glass was losing enough heat from the house to keep the glass temperature above the dew point. Energy flows from Heat -> Cold.

Now you have double pane glass that is more energy efficient. The outer pane is separated from the inner pane. This is also known as an insulating glass unit (IGU). That means that compared to single pane glass, you will not experience as much heat loss out an IGU. The result is that in cold weather, the inner pane will be warmer, while the outer pane will be colder.

Your question is, why does the center pane have condensation, while the side windows do not? Here is where I start guessing. A hypothesis, you could say... an educated guess. Since you did not mention it, I am just guessing that the center IGU in your XOX window is larger than the IGU's that are on the sides. A large IGU that is square will be warmer inside / colder outside than a small IGU that is narrow. This is simply because the border of the IGU transfers heat from one side to the other. When the IGU is large, the center of the glass is farther away from the edge of the spacer. This is often referred to as "center of glass" and is often where temperature measurements and calculations such as u-value are calculated from, because the center of glass is the optimum point at which the temperature differential is the greatest when comparing the inside temperature to the outside temperature.

So in short, I am guessing your center window has a slightly better u-value than your side windows have, due to its size. This translates into warmer temperatures inside the home, but colder temperatures (closer to the ambient air temperature) outside. When the conditions are just right, dew will form.

If its size is the same as your side windows, then rather than the u-value being the issue, it's proximity to the inside is the deciding factor. You would not think it to be the case, but when a window is set back 2" farther than the side windows, it will be colder than the side windows will be due to the reduced air flow over the surface of the glass, which causes the edge to be colder, which in turn results in a colder center of glass temperature. I've seen this exact thing happen with interior glass edge condensation on XOX sliders in the wintertime, where only the center glass gets condensation even though it is the exact same size as the others. It's depth in the window frame is the only difference.

So in short, the dewpoint must have been just right so that you got condensation on the center window but not the sides, because the center of glass temperature on the center window was colder than the center of glass temperature on the side windows. It could even have something to do with the blinds or curtains. If you have something covering the center glass, but not the sides, it would allow that center glass to get colder, since the blinds would block heat from warming the glass, causing it to be colder than a window that was totally open to the conditioned heated air in your home. Also, windows in the sunshine would not be the same temperature on the exterior as windows in the shade. It may seem unusual, but that's the way it is. It all has to do with the outside temperature of the glass and the dew point. When IGU's are different sizes, they will also have *slightly* different u-values. This might be just enough to cause dew on some but not on others. So be assured that there is nothing wrong with the glass. Additionally, you can expect that there will be some days when all three will have dew on them.

We are on different coasts so I don't really understand your weather. But those old windows were so energy inefficient that nothing could condense anywhere, air flowed too fast. It is not unusual to get complaints about condensation on the inside with new windows, at least until you go thru one heating/cooling cycle(but you only have one season). Those windows are so efficient compared to the old units, that there is no comparison. Where you get condensation may depend on which side has the coolest and most moist air. If one side is cool and the other is warm, you will likely get condensation, even with double glass high efficient glass. Maybe we can get a comment from someone on the left coast.

But I can say that you will definitely save money on energy costs over the old windows, as well as increased comfort(reduced drafts), a quieter house, less fading on the furniture, etc.

The other responses are correct in that condensation on the exterior surfaces of a dual paned window is normal and not a defect. If the condesation is on the interior of the IG (you cannot touch it) then that is a seal failure and it needs to be fixed.
The reason you are getting it on the center IG and not the operable units is because you have screens on the operable sash. That little bit of extra insulation prevented the moisture from forming on the glass. That is not say it can't happen but that the conditions were not right in your case.

Again condensation on the exterior of a IG unit is normal-either on the inside or outside of a building. If it is on the inside surface (you can touch the moisture while standing in your house) you have a potentially serious problem with too high relative humidity. What you see forming on the glass may also be forming inside your walls. Get an HVAC expert to evalute the fresh air exchange and make the changes he recommends. Otherwise your insulation will fail and mold will be growing...and it is not the windows fault!

Appreciate the inputs, thx. I do understand that external condensation can be a normal occurrence. Having said that, the sliders do not condensate at all even without the screens.

Since I posted my orig note, I installed a similar sized window flanked by single hungs (vs sliders) on the same side (north) of the house. This window does not experience exterior condensation while the center window of the XOX slider continues to wet up nearly every night (but not the sliders).

A Certainteed tech came out and inspected the XOX. He noticed a scratch on the LowE coating. Of course this doesn't explain the condensation but does provide justification for replacing the center window. More to come...

In addition to the excellent comments above, here is a brief explanation on condensation on outside of windows in our website's Home Info section: http://www.doityourself.com/stry/condensatedoublepane

moparguy

Did you just replace the slider, or all of the other windows too?

If you replaced them all, the largest pane of the slider MAY condensate. In a house that old, there are literally THOUSANDS of possible air leaks. How about the doors??

Air leaks are what are causing the condensation problems that you have. Keep finding them and tightening things up.

Lefty,

How do air leaks cause condensation on the outside of a window?

Condensation is caused by air being cooled to its' dew point temperature. At the dew point the air can no longer hold the moisture as a gas and it precipitates out as a liquid on the cold surface-in this case the glass but it can be anything that is cold enough i.e. a glass with ice water in it, your lawn, etc.

Plugging up air leaks is a good idea but won't solve his problem unless the window is right next to a leak that is a source of high humidty air (i.e. laundry, kitchen and bath vents.)

If with the screens off the center lite is still the only problem then it is probably a result of air flow across the unit. The sliders are slightly inset and stay just warm enough to not get the condensation.

Because every window/door is in a different location, elevation or room in the building you will get different performances. And it will vary with the time of year for each opening.
Nothing is simple!!