FYI, an good article by Joseph Lstiburek on the subject of venting a roof, and not venting a roof.
Located on the Building Science website (http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting)
and another link on the same subject, with a video so we can hear our gurus voice.
On the Green Building Advisor website (http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs)
Thanks for posting this excellent article. Currently having some difficult exchanges with my building inspector over what he will accept for ventilation in the 20x30 1-1/2 story and this article helps a lot...thanks
It would be interesting to hear his concern and how this worked out. Glad you could make use of the post.
I really wanted to have a natural draft from the crawl space to the attic to exchange both air plenums but the county wouldn't allow it--- they thought it created a fire hazard in the structure
The chimney created by old unblocked balloon frame construction allowed many unseen fast fires, they are rightfully cautious. It's always a tradeoff between venting and keeping the fire blocked.
Fire blocking is a wise safety issue learned from quickly spreading fires in unblocked balloon framed walls. This unblocked open wall design was standard practice in 19th century wood frame buildings.
Modern balloon framed walls will have blocking at each floor level to solved this issue. Platform framed walls have the platform itself acting as a fire block.
In the late 1970's there was an interesting "double envelope house (https://en.wikipedia.org/wiki/Double_envelope_house)" That used a north wall with an open chamber between two framed and insulated walls. Warmed air from a south facing solarium was assumed to circulate around the inner insulated house that saw this moderated air chamber as the "outside environment". These ideas got a bit more speculative as the design took on more and more magical properties.
(http://ekoseahomes.com/images/mode2.jpg)
These houses did perform fairly well as a well insulated structures, but most of the earth tempered solar warmed air chamber magic proved to be little different from the work (and expense) of the additional insulated wall.
Many building departments required that the double north wall cavity be protected by fire dampers with thermal triggers that would close down the air circulation in the case of fire.
Here's an article from Mother Earth News (http://www.motherearthnews.com/green-homes/the-double-envelope-house.aspx#axzz2dV8bDk8j) circa 1982.
I'm a little surprised this article makes no mention of radiant barrier use.
Because fire science still out on roof top or ridge top venting in the roof systems they are pushing here. (That according to two architects - one very trusted engineer and a pretty up to-date contractor we talked to.) We opted out of doing such. It may have been a real mistake or ??? maybe are real mistake the other direction.