Fred:

I imagine the key question, like all the others, is whether the garage is enclosed and conditioned (or semi-heated).

If it's unconditioned and open to ambient, then its walls do not count as part of the exterior envelope.

If it's enclosed and conditioned, then its walls do count as part of the exterior envelope.

I could also imagine a situation where a garage was enclosed and had no heating / cooling system, but it was adjacent to conditioned space, in a way that made it an indirectly conditioned space.

Probably the most helpful resource I've found when trying to make these decisions is the ASHRAE User's Manual discussion of spaces and envelopes (pp 5-2 and 5-3 etc from the 2004 User Manual). Making the determination about indirect conditioning would require a careful parsing of where the insulation is and the overall relative u-a heat transfer factors between the enclosed spaces and the potentially indirectly conditioned space. This is more fully described in the definition of conditioned space in the standard.

Basically, the key to this is figuring out what spaces in your building are conditioned and semi-heated, then drawing the line of the "envelope" around them, as per the User Manual.

What's in the envelope then becomes the basis for the window to wall calculation.

Aaron Dahlstrom , PE, LEED(r) AP