Attic ventilation- 2007 CBC, Section 1203.2

We have a bit of a different situation for which we would like your interpretation and input.

A new wood-framed school buildings with R-30 rigid insulation on top of the roof sheathing and R-19 batt insulation in the walls (see sketches below).

We have an enclosed soffit (3'-11" overhang) and rake with exterior cement plaster applied to the bottom of the shaped 2x12 roof framing.

We are unclear as to the code-required ventilation provisions in this situation (for the enclosed overhang space).

Your insight and opinions are appreciated.


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Original Post

So the first thing I see is how are the enclosed attic spaces (enclosed overhangs) cross-ventilated?

I see the continuous 2-inch soffit vents at the bottom, but that's it! Typically, you would have intercommunication with the main attic (though holes in the blocking above the wall top plates, etc) and that would provide the cross ventilation. But as drawn, the main attic is part of the conditioned space since the rigid insulation is located above the roof sheathing.

As drawn, only ventilating taking place is at the bottom (in the soffit). It does not comply with Section 1203.2. Im stumped to what you can do given that you have rigid insulation above and batt insulation vertically sealing off this space!

Anyway to provide vents through the deck/rigid insulation above the enclosed space- good luck Big Grin

Now I'm wondering how many plan examiners and inspectors would have caught this or commented on it.
where you have the "one hour" conditions you are required to ventilate the joist bay.

1203.2 Attic spaces.
Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing members shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain and snow. Blocking and bridging shall be arranged so as not to interfere with the movement of air. A minimum of 1 inch of airspace shall be provided between the insulation and the roof sheathing. The net free ventilating area shall not be less than 1/150 of the area of the space ventilated, with 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents.
Exception: The minimum required net free ventilating area shall be 1/300 of the area of the space ventilated, provided a vapor retarder having a transmission rate not exceeding 1 perm in accordance with ASTM E 96 is installed on the warm side of the attic insulation and provided 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet above eave or cornice vents, with the balance of the required ventilation provided by eave or cornice vents.
You don't ventilate the building envelope, only the concealed spaces which are not conditioned. The intent is to relieve heat build up and eliminate condensation, right?

In the example, I'd say that a couple of continuous vents in the exterior soffit [one at the front, one at the back] will take care of the issue.
mikeelia: How do the continuous vents at the bottom only (in the soffit) provide cross-ventilation for the enclsoed spaces? They don't!

You are only getting ventilation from the bottom and nothing from top! Section 1203.2 specifically requires "cross" ventilation for enclosed attic spaces. I don't see how that is accomplished in the sketches.
Section 1203.2 is titled "Attic Spaces". the enclosed rafter spaces formed by sheathing the underside of the rafters, in my opinion refer the spaces directly above air conditioned spaces. where you will most likely have the most heat build-up and potential for condensation.
Question: Why would you insulate beyond the exterior wall / envelope?
The eave vents do and are adequate for the rafter tails.
In the relentless search to deal with this issue, I was thinking I might find some help in the metal building industry, and their on-going dealings with the best way to insulate those buildings. I found, NOTHING,,

But, I did some additional research, and found one manufacturers solution. The product on the following web page seems to satisfy the question, as "How" to insulate and ventilate a roof with rigid insulation.
As a point of clarification - The Building envelope consists of the R-19 batt wall insulation & Rigid roof insultation, above the shear ply. I don't see penetrating the envelope as a viable option, as the entire space below is conditioned.

The rigid insulation on the roof at the soffits is to continue to roof profile. The soffits are unconditioned spaces (not attic spaces) It would seem that the soffit vents supply adequate ventilation.
As a point of clarification

Regarding the rigid insulation over the overhangs/eaves why would you extend the insulation to the roof edge? there is no value in it. the OSB would span block to block
there will probably be a block at the OSB edges (joints) for there will be a void between the plywood and OSB around the eave/overhang perimeter..
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