Thursday, July 17, 2008

What’s wrong with this picture?

a) The rim joist is too heavily notched.
b) Using framing cavities as duct runs frowned upon by codes and professional associations.
c) This passed the framing inspection.
d) All of the above.

The answer is d, all of the above.

While it appears that this rim joist isn’t structural (there are studs below it), The Engineered Wood Association says that when cutting multiple holes in a rim joist, the spacing between the holes should be at least twice the length of the longest side of the longest rectangular hole. And according to Joe Lstiburek, the photographer, this also passed the framing inspection.

This leaves option b, “Using framing cavities as duct runs is a bad idea”. This photo shows both stud and joist cavities being used as return air ducts. In their duct design guide Manual D, the Air Conditioning Contractors of America (ACCA) recommend not using panned joists for return ducts, and the 2006 IRC and IECC codes prohibit joist cavities for supply ducts. It’s too bad that neither does both.

Framing cavities make bad duct runs because the leak and they’re connected to every part of the house via framing gaps and holes for wire and pipe. These leaks suck in air from unintended sources (outdoors, attics, garages, and crawlspaces). Along with this unintended air comes humidity, mold, dust, and whatever else is in these spaces (radon, carbon monoxide, gasoline, paint thinner, pesticides,). Not only does this poison the indoor air supply, but it increases the heating and cooling loads while decreasing the efficiency of the mechanical equipment. A bad idea all the way around.

— Bruce Harley is technical director of Conservation Services Group and author of Insulate and Weatherize (Taunton 2002). Photo by Joseph Lstiburek

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1 comment:

Pete said...

It was in association with this photo that I heard the comment (not sure from whom, but probably Joe Lstiburek) that one HVAC contractor defined non-
structural as "any framing that is in my way..."