On 2007-06-05, Mike E Fullerton wrote:

How can I do framing with 1.5 inches of clearance? In fact I forgot
about the ridges on the ducts. It actually more like 1.25 inches. When
I say 4 feet I mean across and about 15 feet long.

So to paraphrase, you're looking for a material that can be used as a
ceiling that is at most 1.25" thick and will span 4'. Is that right?
If so, have you considered a drop ceiling or ceiling tile system?

Cheers, Wayne