We live in Minnesota where the temperature can dip well below zero. The issue we have been seeing is that we are getting frost build up on the bottom of these high performance windows. I am quite disappointed as our older double glazed did the same thing. We were hoping that an upgrade to windows would eliminate this problem. We have been going round and round with the manufacture and they come back stating there is too much humidity in the home. That is where I have trouble.

We have an air exchanger that we monitor and try to adjust with the temperature changes – although not every day… We also have an exhaust fan in the master bath that is most often used by the family for showers and such. The thinking behind going the route of triple glazed was that the glass temp would be higher, thus allowing us to keep the humidity a bit higher in the house yet not get the condensation. We did not ever expect to see frost on the inner pane. To me windows do not seem to be performing as I would expect. Am I off base?

Here are some readings I took in the bedroom where we have the most trouble with frost.

2-15-09
Outdoor temp = 7°
Outdoor Humidity = 60%
M.Bedroom temp = 65°
M. Bedroom Humidity = 29%
Slight amount of moisture only present in corners - only looked at the north window

2-18-09
Outdoor temp = 26°
Outdoor Humidity = 80%
M.Bedroom temp = 66°
M. Bedroom Humidity = 28%
Window was clear and dry

2/21/09
Outdoor temp = 4°
Outdoor Humidity = 41%
M.Bedroom temp = 62°
M. Bedroom Humidity = 25%
Slight amount of moisture present in corners and up about .125" across the bottom - only looked at the north window

2-22-09
Outdoor temp = 1°
Outdoor Humidity = 57%
M.Bedroom temp = 64°
M. Bedroom Humidity = 25%
Slight amount of moisture present in corners - only looked at the north window

Due to onset and hope of continued warm weather the thermometer was moved back to the kitchen.

Readings from 3-11-09
Outside Temp = -1°
Outside Humidity = 83%
Kitchen temp= 61°
Kitchen Humidity = 28%
Frost on all windows in the Master bedroom – corners and up about .25" along the bottom. Frost on rear bedroom and front Bedroom windows. Picture window in the great room had frost in the corners only. Dining window had frost in corners and slightly along the bottom of the window.

Were the windows professionally installed? What type of insulation was used when they were installed ie. fiberglass, low expansion foam, or nothing at all?

A couple of other thoughts - when the windows frosted up were there closed blinds or shades covering them?

I am going to guess that these are casement or fixed casement units?

With LowE on surface 2 and 5 (typical) and a 1/2" argon filled space there is no way that you should be seeing condensation on those units, much less frost, if everything is working as it should be. Even an aluminum spacer should be sufficient to eliminate surface condensation in a typical application.

Looking at your numbers, your indoor RH is pretty darn low. The temps you gave indicate that you keep your house fairly cool in the winter which means at higher temps your RH is going to go down even more. With dual-LowE triples, the glass should not get cold enough to condensate at the moisture level that you have in your air.

Outdoor RH is a non-issue with indoor condensation, but even if it wasn't - for a quick comparison - at 1°F outside temp and an outside RH of 57%, you have about .1614 grams of water per cubic meter of air...raise the temp to 50°F and keep the moisture level the same, and your RH is about 2% - which is pretty darn dry!

Have you talked with the window company rep? I would be really curious to know what they told you.