Window and Door Performance

The best performance is expensive, but is it more expensive than it is worth? I hope not! Here are the tested performance values for the 525-S Alpen window and doors that I chose. All the 525-S window panels are glazed with 2 panes of glass and 1 of film with argon fill. The depth of the panel is 1 3/8″.

Style U-Factor R-Value SHGC VT Air Infiltration
Picture Window LP 0.17 6.3 0.3 0.49 0.01
Casement Window 0.19 5.3 0.24 0.38 0.01&.02
Picture Window LP 0.17 5.9 0.48 0.62 0.01
Casement Window 0.2 5 0.38 0.48 0.01&.02
Awning Window 0.19 5.3 0.24 0.38 0.01
Patio Door Glass 0.19 5.3 0.41 0.53
Patio Door Frame .27/.18 3.7-5.5 .02 & .04

The air infiltration figures are particularly high performing. I read that the NFRC cannot test lower than .01. I found this chart comparing the difference between air infiltration rates for a 12 x 12 room in hours to change the air completely.

Window Air Infiltration Rates

Window Air Infiltration Rates

Although the u factor and r value are higher for the sliding patio units, the uncontrolled air change rate according to this chart would be 9.1 hours, while the patio doors would leak a full room of air in 16 hours. The casement windows will take 64 hours to allow enough air in to replace all in a 12 x 12 room. Lots of difference in the air infiltration rates.

The solar heat gain coefficient numbers are not high for the south facing glass but they are the highest I could find with low u factors. Visible transmittance or the “clearness” of the glass is about half that of untreated glass except for the low solar heat gain windows on the north, east, and west.

These are current day high performance windows and doors, but they are sure to improve in the next 30 years so that one day, another owner will replace them with truly energy efficient units!

UPDATE: At the last minute I decided to upgrade the door inserts to the 725 glass. This unit has two layers of film inside and for the doors, the upgrade was very reasonable.

Style U-Factor R-Value SHGC VT Air Infiltration
Patio Door Glass 0.18 5.6 0.41 0.53
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