Engineer:

A key line item request was that the duct system be of the rigid pipe variety.  Since our residence is single story with upwards of a 35 foot roof, this was a doable option, just more expensive than flex duct.

This was an important consideration for us, not an optional item.  It was one of the key go/no-go consideration points for us when selecting a contractor.

Indeed, the contractor we agreed to proceed with stipulated a newly designed and installed rigid pipe system, with the exception of the last 6 feet being flex duct.  We agreed to this (there are noise benefits to flex duct terminations).

What we got was a considerable flex duct system, unfortunately.

I don't want to debate the merits of rigid vs. flex - this is done continually over at hvac-talk.com.  We simply wanted rigid pipe ductwork, were willing to pay for it, had it in the contract, but didn't get it.

Best regards,

Bill