I am curious/concerned about advising a tight house (addition) with a cathedral ceiling. I understand that normally, one should keep a 1-2" gap in the rafter bay above the insulation as an air gap. But, if the construction is tight, to achieve a low ACH50 or 1-2, what is the point?How is air (moisture) going to escape if the enclosure, including the roof sheeting, is sealed tight? Also interestingly, the plans did not include any mandatory ventilation for this 780 sqft addition in So. Cal. by the ocean.
It depends, are they going with a hot roof system? If so no air gap is required
The reason for the gap is because even with super low leakage numbers, you still have leakage & the reason the bays get ventilated is to eliminate condensation issues with the sheathing
Most places will not require "ventilation" for additions as they assume the house has it already & it is not a mandatory no matter what if you do X you must put Y in like smoke detectors / carbon monoxide
I agree with Sean, that the main concern is condensation, and in northern climates frost accumulation in the attic. In southern CA, I am sure those are not a big considerations. You should still have some low attic venting, but it is not as critical.
With a vented cathedral ceiling, getting a tight envelope is difficult, but not impossible. Various products are available to maintain the vent gap, some of which are easier than others to air seal.
It's easier, and I believe better, to go with an unvented assembly, in which case the insulation must be fully in contact with the roof, and depending on climate zone, some portion must be air impermeable (less in milder climates, more in colder climates). The purpose is to prevent house moisture from coming in contact a cold roof.
House moisture is removed by spot exhaust in kitchen & baths. Whole house ventilation has a different purpose.
I'm not familiar with California Title 24 requirements for unvented roofs but here's a link to the IRC Chapter that covers that detail (scroll down to Section R806.5): IRC Chapter 8
It is all about the dewpoint. Think of the roof system as a big box. Air can circulate in, out and around the inside of that box. If you could possibly air seal the entire inside plane, and install a durable, continuous and effective vapor barrier over the entire inside plane, you still have the other 5 sides where air and/or moisture can enter (or leave) the box. When the outside temperature falls by a significant amount (as when it is cold enough for snow in the South), you are most likely to hit the dewpoint on those 5 sides. When that condensation pools up in the air permeable insulation, it will be difficult for that water to get out of there unless there is a reasonable connection for air to move in and out. By the time the water eventually gets out, it could promote the growth of fungi, such as wood rot. That is why we ventilate roofs. If there is enough air and vapor impermeable insulation on the roof deck, then the temperature inside the box should stay above the dewpoint and ventilation is not needed (or desired).
I would refer you to this document by Building Science Corp., which tackles the issue of ventilation in cathedral or flat roofs. You don't need to ventilate, but you need to carefully design the assembly.
"Why such a difference between walls and roofs? Walls in most older houses can dry out to the exterior because they rarely have exterior vapor barriers. Roofs have either roof membranes or asphalt shingles that are exterior vapor barriers, and they can’t dry out to the exterior unless the roof assembly is vented to the exterior. We have developed ways of dealing with unvented roof assemblies and insulation, but dense packing them is not one of them. I’ll talk more about the alternative ways later. Walls have “wiggle” room. We don’t have to be perfect because some drying can occur to the outside. We do not typically have the same “wiggle” room with unvented roofs."