Scroll down to the bottom.

Michael

Here is a link that might be useful: Exterior Condensation

Thx for the link. Yes, I've been reading about exterior condensation and the conditions were right last night for it. However, I still wonder why the slider glass portions of the window did not develop condensation at all. Heat transfer and such should be about the same.

The screens are keeping the condensation off the glass.

Remove the screens tonight and examine them in the morning.

Michael

What Michael said - any disruption of the window to the open night sky - be it clouds, trees, overhanging soffit, screens, whatever will disrupt the formation of exterior condensation.

OK, last night I removed one of the slider screens. The center window developed condensation but the slider without the screen didn't. So this puzzle continues. The window company is sending out a tech to look at it.

I will offer up our experience and a hypothesis that condensation on the outside surface of the outer glass pane, in this case, is most likely not a bad thing.

Condensation forms when a surface is cooler than the warm humid air. Most normal solid surfaces respond to temperature changes much faster than air and it explains why we often feel a surface is colder or hotter than air. The single pane has no condensation most likely because its surface temperature is identical to, or higher (>=) than, the ambient temperature. Condensation cannot form on a warmer surface.

The interior warmth of the house heats the single pane temperature to that higher than the ambiemt temperature outside so condensation cannot form. This is why single pane does not insulate well in winter. The glass is conducting heat right out of the house.

The warmth of the house cannot leak into the tightly sealed double-pane airspace of the new window. The outer pane then is always cooler than the ambient air outside so condensation can form. This is good for keeping out winter chill and keeping in interior warmth.

As for why one has condensation and one does not, it most likely has to do with overhang and airflow. We see this on our French doors as well. Two banks, each 10-ft long separated by a 2-ft center post, both facing the same direction with identical overhang and ground characteristics. One has condensation, one does not. Surprisingly, the side with condensation is the one that has a bit more breezes towards late evening. We believe it is cooler than the other bank.

A little overhang also makes a difference and this often shows up on our cars. Park one car to within 24 inches of a tree's canopy and by morning, the side closest to the canopy is dry while the exposed side is dripping wet. The canopy and car manage to keep that area cool and still so there is no moisture laden air to cause condensation. The exposed side is of course open to condensation. This is also why NWS always forecast freezes for "wind shelter" area where it can remain still and cool.

As long as there is no condensation in the airspace of the window, it is most likely not having a seal failure.

Good luck with your analysis.

Calbay,

Very nice answer. I would agree completely with everything that you said with one minor caveat...

"As long as there is no condensation in the airspace of the window, it is most likely not having a seal failure."

I would change that line to say - "As long as there is no condensation in the airspace of the window, it is not a seal failure".

Exterior or interior condensation does not indicate an IG seal failure. Only condensation between the lites would indicate a problem with the IG seal.

Very minor change to a great answer.

Appreciate everyone's input.

Update: installed a similar sized picture window flanked by single hungs on same side (north) of house. No condensation on this one at all while XOX center still wets up every night. Certainteed tech came out and looked at it. He noticed a scratch on LowE coating. Doesn't explain the condensation of course but did provide a justification to replace center window.