Two part question:

1. From 2007 Ca. Building code: "1203.2 Attic spaces. Enclosed attics and 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..."

This current language REQUIRES ventilation and drops the following language which was in the 2001 Ca. Building Code:"1505.3 Ventilation. Where determined by the building official...due to atmospheric or climatic conditions,enclosed attics and and enclosed rafter..."

Q.: What is the background leading to the explicit requirement for ventilation and the deletion of the contingent requirement based on a Building Official determination?

2."Rafters" generally are defined as sloped members.
Q.: In the same assembly for a flat roof
(low-slope)using roof "joists" wouldn't the same requirement apply? In other words since the word "joists" is not in the code language does that somehow imply that ventilation would not be required in a flat or low slope if everything else was the same?

Thanks for your help,
Original Post
I would like to know about this too, as we build alot of mini-marts with truss roof systems that have 1/4" per foot slope ....and we have had to go to a foam roofing system on top.
Spell "foam" as e-x-p-e-n-s-i-v-e.
Building collectors or having to vent each rafter space is a pain and freaks out engineers when they can't tie their roof diaphram down to the walls.

I will attempt to answer your questions now as follows....

A1) The 2001 CBC was based on the 1997 UBC. The attic ventilation requirements were in Chapter 15 (Roofs).

The 2007 CBC is based on the 2006 IBC, an entirely different building code that does not have exact requirements and differs in many ways from the UBC. In addition, the IBC was a culmination of three model building codes that were used throughout the USA into one (The UBC was one of the three). So by the time the IBC was developed, the wording for attic ventilation was changed to accommodate the three model building codes coming together and to account for all the states in the USA (with varied climates) and not simply the Western States. The wording “determined by Building Official and due to atmospheric or climatic conditions” was dropped.

The attic ventilation requirements were also moved from Chapter 15 in the 2001 CBC to Chapter 12 in the 2007 CBC.

A2) See the attached commentary from the 2006 IBC Handbook regarding Section 1203 for additional clarifications.


Ventilation of roof rafter spaces is usually an issue for a couple of reasons which I'll try to explain:
1. First, in cold areas, the intent is make certain that any moisture that condenses due to hitting the "dew point" does NOT do it in the insulation [where the change in temperature occurs]. This means that venting and vapor retarders should be used together. In cold areas the vapor retarder goes on the warmer inside of the insulation and the venting is on the outside.

2. In direct sun [hot] conditions, venting of the joist spaces underneath the sheathing where asphalt shingles [or even building paper] is located does a lot to lower the temperature of the shingles. This extends the life of shingles, sometimes dramatically [especially if the shingles themselves are not the very best grade].

Often we acheive the vent above the insulation using an insulation baffle such as "Proper Vent" which just leaves a little air space from the ridge to the eave. Naturally you'd use a vent of some sort at those locations to allow for air movement.

From what I see, there is a lot of misunderstanding about vent locations: I;ve seen some designs where the vent penetrates the insulated envelope of the building: Naturally we should never vent a conditioned space [even if it is an attic or concealed space] or we're just trying to heat or cool the exterior. It depends on where the line of the insulated envelope is in the building.

Finally, in many commercial buildings [where you are definitely not building a "Swiss watch"], the ceiling space should be part of the insulated envelope, thus putting the insulation on the deck [polyisocyanurate or other rigid foam] is the easiest way to get the job done and actually make the building pretty energy efficient [along with making roofing conditions easier to deal with]. For me and my clients, that's always been a pretty good tradeoff.

Clear as mud?

Mike E
The concept of attaching the insulation directly to the underside of the roof sheathing, thereby eliminating the attic, is in the IRC R806.3, which California did not adopt.

Even so, most jurisdictions tweak the standard there to account for the local climate conditions...mostly so that there remains a way out of the insulation for any condensation that might form. With the space between the insulation and the roof sheathing removed and the corresponding ventilation space non-existent, either an air-impermeable barrier needs to be applied to the underside of the roof deck and/or the insulation is of the closed-cell variety, and not blown-in or firberglass-batt type.
Thanks for all the discussion-mostly to my first question but please allow me to seek add'l clarification.

Referring to Q1, the elimination of the building official determination. Was this just a 'paper' issue to coordinate language or was there consideration of historic performance of non-vented spaces that elevated ventilation to a requirement? Was there a discussion of historic failures that led to elimination of BO determination and made venting an explicit requirement?

I didn't see response to my 2nd question.
Would '07 Ca. BC also require ventilating "flat" roof joist spaces in addition to sloped rafter spaces? Other than movement of warmer air to peak don't the same physics and condensation risks apply.
If somebody said they didn't ventilate "joist" spaces because the code only requires this for "rafter" spaces would anybody really buy that?

Thanks for your input!

I didn't see response to my 2nd question.
Would '07 Ca. BC also require ventilating "flat" roof joist spaces in addition to sloped rafter spaces?

I must be confused because I don't know why this is so hard: If there ain't no "joist space" [insulation is tight to deck, top or bottom] then there's nothing to ventilate.

A designer should NEVER "vent"* the building envelope, or the HVAC design won't work, you won't meet the energy code, etc.

*Please don't include overpressure or gravity vents which are part of the HVAC design here.

Ceilings are applied directly to underside of rafters. Insulation is on ceiling with space between top of insulation and bottom of sheathing.
1203.2 REQUIRES cross ventilation for "rafter spaces" which are sloped.
Same assembly for flat roof "joists" presumably has same requirement for cross ventilation even though the word "joist" is not mentioned? Y/N?
If there's a space between the insulation and roofing, you do have to vent it.

Forget the code: If you don't, that space will be too hot in summer and will probably develop and trap condensation in the winter. There's lot of ways to deal with it, up to and including roof vents on a flat roof.

I don't know how big the joists are, but if you are gyp boarding the bottom of them, the easiest fix is to fill the void with fiberglass. If you are talking about 22-inch deep wood I-joists at 16-inches on center, I don't know what to tell you except that you could use wires and hold the fiberglass up at the deck...

Sorry, I don't have any better ideas...
I have run into similar conditions regarding unvented areas above the top floor ceiling and roof. The area is comprised of Joists conected to a heavy ridge beam for sloped roofs and just spaned joists for flat roofs. The ceiling is sheetrock followed by r-30 insulation then the roof sheating and finally the roof membrane. There is no airspace to allow for venting. Several years later the roof is replaced and rot is discovered because the warm water vapor travels up through the sheetrock, insulation, and sheathing. When the vapor hits the cold roof membrane it condenses and stays in the area because it is unvented. After repairs I have required spray foam insulation. Are there any other acceptable ways to address this? How about a vapor barrier after sheetrock befor the insulation? I have attached a picture of what happens to unvented spaces.



Images (1)
Along this line. Would the application of 1" of closed cell foam insulation suffice as an adequate moisture/vapor barrier under a flat foam roof to then put R19 or R30 bat?

Thanks Chris
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