4 Comments

1. 
   o2bee | May 07, 2020 05:38pm | #1

   Martin

   Thanks for your solid summary. Some additional information follows.

   Gas in the spaces created by two or more “lites” also circulates in convection currents like you describe inside the room. Outside cold moves inside via conduction to this gas, thence inside the room via conduction through the inner lite. Heat moves the other way. Air, which is 78% nitrogen, circulates faster than heavier gases such as argon, xenon or krypton, which are gases used to better insulate windows by slowing the convection currents and thereby heat transfer. Coatings, mostly called “low e” with various numbers, are placed on various glazing surfaces to either reject heat from outside (cooling climates) reflect infrared radiant heat energy back to the inside (heating climates). Not all transparent glazing is glass. Using more glazing surfaces reduces heat loss. Alpin Windows offers quad glazed high performance windows with R values approaching 10. Window frames rarely offer R values over 5 so have room for improvement.

   Many people incorrectly think that insulated glass (IG) windows are vacuum insulated. Like the vacuum glass thermos bottle in your lunch pail, vacuum insulated glazing (VIG) has been prototyped with various technologies. VIG units can achieve R15, so R30 glazing with two units (4 sheets of glass) is conceivable, albeit expensive. Windows with clear or translucent aerogels sandwiched between two sheets of glass, fiberglass, acrylic or polycarbonate are available. Frost on the exterior of these highly insulated windows is more common since less interior heat reaches the cold exterior glass surface.

2. 
   john_prospect | May 11, 2020 02:00am | #2

   Martin, I love your dry sense of humor.

3. 
   DPiranesi | May 14, 2020 11:05pm | #3

   A good read and also a reference I can use later. I wonder if anyone would care to remark about another aspect. Windows endure weathering. How well do modern windows endure?

1. GBA Editor
   Martin Holladay | May 15, 2020 04:52am | #4

   David,
   Window durability depends on the materials used to make the window, the quality of its assembly, and (especially) its level of exposure. Old windows that endured for centuries were often installed as "innies" and were protected by generous head trim that helped to deflect rain and shield the window from exposure.

   Some architects--especially those enamored of the "midcentury modern" look--have forgotten basic rules of water deflection, and have decided to expose windows to the weather. This shortens their life.

   Windows with aluminum or vinyl cladding on the exterior are very durable. Windows with exposed wooden components on the exterior are sometimes short-lived, especially when exposed to sun and rain, and especially if the manufacturer used finger-jointed lumber.

   If you're interested in information on vinyl window durability, you might want to read my Fine Homebuilding article on the topic, "Will Vinyl Windows Last?"