[Greenbuilding] Designy metal frame windows with good U-value?

[Stephen Thwaites]

A few days ago, Rob Tom made a reference in this thread to a conversation 
Rob and i had some time ago.

What i said at the time was that a fibreglass(Canuckistan spelling) window 
frame doesn't necesarily guarantee good energy performance.

The example i gave was of a west coast company that moved into the Denver 
area some years ago.
They found some of their customers were getting condensation on the bottoms 
of  their window jambs.

The manufacturer's technical wiz was wondering if they got a bad batch of 
fiberglass!

Being based in a mild climate they didn't put insulation in their frames, so 
their window's frames had large hollow cavities.

Insulating glass is also a large hollow cavity. Hollow cavities experience 
convection within the hollow. This natural convection is what cause 
condensation to form across the bottom of insulating glass before it forms 
elsewhere.

So any guesses as to the cause of the condensation at the bottom of their 
fiberglass jambs?????

The best answers are not simple. In the case of windows, good thermal 
performance is not just a matter of picking a fibreglass frame and being 
happy.

The glory, as always, is in the detail.

Steohen Thwaites
Thermotech Fiberglass Fenestration
still frozen Ottawa, ON  Canada

P.S.
 For the latest on ice conditions on the longest skating rink in the world:
http://www.canadascapital.gc.ca/bins/ncc_web_content_page.asp?cid=16297-16299-10080-10081&lang=1&bhcp=1