I figure I'd post this here first before another forum. I'm framing my basement and creating soffits aound the hvac ductwork. Well I thought I measured the lowest spot and I was being very tight because I wanted in minimilize losing height undertneath. Well a few spots the 2x3 are just slightly pushing up on the underside of the sheetmetal. I don't suspect any big deal here but was wondering about expansion/contraction causing poping noises. Anyone ??
I would leave around 1/8-1/4" gap between your framing and the duct. Not so much for expansion and contraction, but because of movement in the floor system could cause squeaking between the wood and the metal.
If headroom is an issue, how did you frame it? There are often ways to fur down ductwork without having lumber running underneath the duct itself. I've done jobs where the ducts weren't too wide or the trunks had gaps between them, which allowed framing on each side of the duct to act just like a ceiling joist to attach the sheetrock very close to the duct.
When I'm inspecting a finished basement at rough-in, I take the way that the builder framed the furdowns around beams and ductwork into account when applying the headroom code requirements. If they've made every effort to get it as high as possible, I'm easier to get along with.