We’re planning to build a new 2,200sf house in Phoenix, climate zone 2B. The single story flat roof will have 1″ spray foam roofing on top and 10″ of open-cell foam underneath. A conventional air-source heat pump will be located in an interior, conditioned mechanical room. Ductwork will all be within the open-web truss cavity between the ceiling and roof-deck insulation.

Since the ductwork will be completely within the thermal envelope, I don’t see a benefit to installing duct insulation. But the architect is insisting on R6 duct insulation w/ vapor barrier facing.

Questions:
1) Is duct insulation needed?
2) Would duct insulation in this circumstance provide a benefit?
3) Is condensation on uninsulated ducts likely to be an issue in this climate?

Since the IEEC minimum ceiling insulation requirements will go up to R49 in the next code cycle, it seems like it would make more sense to spend money on additional roof-deck insulation rather than duct insulation.

One related question:
4) The specs also say that exhaust ducts “shall not be” insulated. Why? Since the exhaust damper will be at the fan, the air inside the exhaust duct will basically be “outside” air. It seems like we would want insulation between that and the conditioned ceiling cavity.