Attic Ventilation and Ceiling/Roof Assemblies

If you would like to offer an opinion about attic ventilation and ventilation of ceiling/roof assemblies, read on. The code passage referred to is the California building Standards Code, but similar language is in the IBC, Section 1203.

A plan checker for a State of California agency recently sent me the following email:

Going back about six or seven months you sent us a question on asphalt paper and weather resistive barrier(WRB). Although we were not able to really address the issue, we appreciated that you sent us your paper that you presented at the ASTM symposium.

Since you are the chair of the ASTM Task Group on WRB and flexible flashings, I am wondering if I can bring up a slightly similar topic. DSA is in the process of reviewing our procedure of ventilation, and an opinion from subject matter expert like yourself would be a tremendous help.

Do you think building officials like DSA should enforce Code Section 1505.3 which states in part that " ,,, due to atmospheric or climatic conditions, enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for ....."?

Recently there have been alternative systems for unvented system, or sealed attic construction by excluding vents to the exterior. Then there are other concerns that ventilation creates penetration into shearwall, notching of rafters, and other problems with encapsulated roof and other hard ceiling construction. This issue can become more and more complicated.

As a long time design professional and expert in WRB, your opinion of this ventilation will most be appreciated in this office.

I responded as follows:

Thank you for asking.

This is not really a WRB issue, but it is an issue involving the hygro-thermal performance of building envelopes, including roof/ceiling and roof/ceiling/attic assemblies that may include WRBs or vapor retarders as components.

The short answer is that, in my opinion, this is a complex and complicated issue. There is no one size fits all answer. There are, however, increasingly sophisticated computer modeling tools available to address it. One example is WUFI (See

I know from personal experience that, under certain circumstances, attics can function well without ventilation. My own home, built in 1985 in Richmond, CA, has an unventilated attic with insulation incorporating a vapor retarder just under the roof deck. There have been no adverse consequences over the years.

On the other hand, as a forensic architect, I have seen a number of roof/ceiling assemblies over the years without ventilation experience dramatic failures from decay of the structural framing and sheathing.

As a rule of thumb, 1505.3 makes lot of sense as the default requirement. You can't go wrong following it. If I were a building official, I would enforce 1505.3 unless a qualified design professional could make and take responsibility for a compelling case, backed up with calculations, computer modeling or other reliable written information, that ventilation is not required. I had to do that in 1985, and the local building official accepted it.

The subject of ventilated eaves or soffits that conflict with fire-resistive requirements is a continuing problem that the ICC and the CBSC needs to address. There are no readily available solutions.

I will shop this to several committees to which I belong and try to get you some more opinions.
Original Post
in trying to comply with the latest CA energy code, our consultant informed me of the following;
"rigid foam board used for cavity insulation must fill the entire cavity between the rafters and sealed properly to prevent air gaps, and must be secured properly to prevent any future discrpancies in the construction assembly." and, "The ventilation space requirement would have to be waived by the building official for the case of cellulose insulation or foamed plastic, since the entire cavity would be filled."

from my understanding, this is from the manual of the computer program that determines compliance.

wow, is this true? i know i'm old school when it comes to venting attics (thou shall ventilate all attics!), and with mold being such an important issue, is the state really dictating a code revision without the usual code review and approval?

and does my local building official know this, or do i have to carry this manual in and show him the new rules?

but more importantly, is this the right thing to do. i've just spent an entire day online researching this issue to see if perhaps there has been verified testing to show this works. i've found tests for unvented cathedral ceilings, but they are limited in scope and do not address the broadscope requirement of california's new energy code. can someone show me where to look? i would really appreciate the help.
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