largest duct/joist combination

[hawaii]

is the interstitial space depth for duct/joist determined by the largest duct size with the same joist direction in the same room roof area where the cut line is?
or one needs to also consider to use the largest duct from the other adjacent room remaining roof area as well?
how is the depth actually determined?

[Coach]

You must consider all joists and ducts within the each area having the same ceiling height. The typical problem will have 3 areas.
Look at all joists, find the largest duct under each joist size. Add the pairs and use the largest result.
The largest joist/duct combination will not necessarily include the largest duct or the largest joist.

[hawaii]

ok thanks…

[remy]

Coach, I'm a bit confused by your response "You must consider all joists and ducts within the each having the same ceiling height."

Hawaii, I think you have to base the interstitial space on the largest duct coming from the vertical riser. So for example in the practice vignette, the ducts in the 2nd floor side room that are cut in section measure 20 x 12. You still have to consider the 32 x 20 duct in the corridor which is connected to the side room. Because of the corridor duct dimensions, the total interstitial space of the 2nd floor side room: 24" (for joists) + 20" (due to adjacent room ducts) + 8" for the lights. That is my understanding. If I'm wrong, maybe someone else can chime in.

I don't think there is anything in the practice vignette program that suggests that if the ceiling joists in the corridor were larger (i.e. 30" in lieu of 24" shown on the plan), that the interstitial spaces of the side rooms would be affected.

[Coach]

Coach, I'm a bit confused by your response "You must consider all joists and ducts within the each having the same ceiling height."

Sorry, it should be within each AREA.

Hawaii, I think you have to base the interstitial space on the largest duct coming from the vertical riser. So for example in the practice vignette, the ducts in the 2nd floor side room that are cut in section measure 20 x 12. You still have to consider the 32 x 20 duct in the corridor which is connected to the side room. Because of the corridor duct dimensions, the total interstitial space of the 2nd floor side room: 24" (for joists) + 20" (due to adjacent room ducts) + 8" for the lights. That is my understanding. If I'm wrong, maybe someone else can chime in.

I don't think there is anything in the practice vignette program that suggests that if the ceiling joists in the corridor were larger (i.e. 30" in lieu of 24" shown on the plan), that the interstitial spaces of the side rooms would be affected.

Incorrect. You must use the largest joist/duct combination.