It is very necessary so that the roof deck can be ventilated, removing warm moist air in the winter and hot air in the summer. The roof shingle warranty requires adequate ventilation.

This may help.

Michael

Here is a link that might be useful: Principles of Attic Ventilation

Michael- thank you so much for your post. I looked up the information on the link that you provided. My problem is that there is no soffit in the addition (and no soffit anywhere else in the house). I am very confused as to how to ventilate the room with no soffit and a cathedral ceiling.
Thanks again.

"Does anyone know whether the standoff to keep the insulation off the plywood is written up anywhere?"

New York State Residential Building Code requires ventilation for every rafter space for cathedral ceilings.

Section R806.1 states:

"...enclosed rafter spaces formed where the ceilings are applied directly to the underside of roof rafters SHALL have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow."

Section R806.3 further requires:

"Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1-inch (25.4mm)space shall be provided between the insulation and the roof sheathing at the location of the vent."

You can access New York State residential code on-line at the link below. Click the left column for Chapter 8 and look at the sections cited above.

Here is a link that might be useful: New YorK State Residential Code

Manhattan42
Thank you - that is exactly what I need!
My only other question...in a cathedral ceiling roof that has neither soffits nor eave vents, how would ventilation be established?
Thanks so much once again..

If your project were permited, the ventilation as well as the insulation should have been addressed in plan check and there should also be a praticular detail addressing the ceiling.

Make a point of being present at the inspectors next visit and ask questions.

"My only other question...in a cathedral ceiling roof that has neither soffits nor eave vents, how would ventilation be established?

The rafter spaces need to be "cross ventilated" according to the building code.

Vents need to be installed for each rafter space at the eave end and at the roof peak.

Where no soffits exist, it is common to vent the rafter spaces using drip-edge vents.

Here is a link that might be useful: Drip-Edge Vent

You place a ridge vent at the top.

You leave a space between roofing plywood and insulation.

You can bore holes at the bottom ends of your roof and place in vent plugs. The holes should be placed so air can travel just under the plywood all the way up to the peak. I had to do that with a home I had once. As I remember I had to make the holes in the tops of the exterior walls.

Make sure air can move all the way from the vent plugs (intake) and freely flow up through the ridge vent just above the insulation.

All of this is done to keep interior heat from melting snow that refreezes into ice dams (been there!). Also, moisture can quickly build up in insulation that has been stuffed right up to the roof boards leading to rotting (been there too!).

I WOULD LIKE TO THANK EVERYONE WHO HAS HELPED ME WITH THIS THREAD. THE INFORMATION HAS BEEN EXTREMELY VALUABLE. I AM TRULY MOST GRATEFUL!!!
I have begun a separate thread regarding the wall insulation is this is now becoming an issue, as well.
Thanks again.