Yesterday I attended a workshop put on by NESEA, the Northeast Sustainable Energy Association entitled "Residential Retrofits for Energy Efficiency and Sustainability" by Larry Harmon. Usually I have to travel to Boston or Burlington to attend these which can be costly and time consuming so it was nice to have one a mile down the road from my offices. There was a lot of good information and Larry is an engaging speaker. The big things that I came away with were the reinforcement of what I have been learning about air sealing, venting, and insulation. 1-Seal completely before adding insulation. - Most contractors or architects probably don't grok the importance or level of thoroughness that is required here or realize the ramifications of doing a less than perfect job. It is not just about energy loss and heating bills. It is very much about how to make a house that will last 100 years or more. There was much discussion and many slides of imperfect air sealing jobs and how they acellerated rot and mold problems.
2-Don't ventilate your roof! That was so 80's and 90's. Now, ten or more years later we get to see the nasty ramifications of venting your roof. yuck!
There was a lot of other information which I may add in here over the next few days and some of which was rather techy involving cost analysis calculations and BTU's and therms, (oh My!)
What disturbed me as it often does at these events is the lack of local builders in attendance. Of the 10% of local builders who care about building science and sustainbility issues, very few will go much further than a subscription to JLC. I'm afraid that if I go out there and draw up plans for a house with an unvented roof or create specifications for enhanced air sealing, builders will simply refuse to follow the plans and convince the homeowners that the architect (me) is full of it. It's an issue that I'm sure a lot of other architects face as well. Although I suspect that 90% of architects don't really care about such things either.
Added the next day in response to a comment.
The presenter showed lots of slides of what happens when you leave a pencil size hole in the sealing of the attic before adding insulation. Basically air pressure turns it into an moisture laden air nozzle. All the moisture then condenses (dew point) on the sheathing and rots it through fast. Or it freezes on the underside of the sheathing then rains down on the insulation when it thaws. Ventilation compounds this pressure effect. The best method seams to be to bypass all these issues and spray the underside of the sheathing with closed cell foam which is what I specify on new construction and treat the attic as conditioned space. Loose fill cellulose or fiberglass batts lose much of their insulating value when exposed to air movement. Many independent tests in recent years have shown the temperature on the underside of the roof does not vary due to ventilation or no ventilation. There were also slides of what happens when insulation is added to an existing house attic that previously had no problems other than high heat bills. Suddenly the attic was cooler, the dew point moved to inside the attic and rot set in almost immediately. board sheathing holds up better than plywood which holds up better than osb.
I get the feeling that as building science matures, ventilated roofs will become a way for architectural historians to date houses to a specific time period in the late 20th century and early 21st century.