Step by Step ProjectsTips from the ProsAffordable Remodel
You may have already explored these areas and this may or may not be helpful info... but Green Building Advisor is a helpful resource. I recently went through some questions on a larger attic - the vented versus unvented situation - in a 1890 house. I did the trial for the Q&A on that green building advisor site and got some helpful advice. The site is active and you will normally get a quick and helpful responses. There are also good articles from building science if you want to learn more on insulation, moisture, etc. We didn't go through with the purchase of the house I was exploring. The International Residential Code does allow for an "unvented assembly" ("unvented attic", "hot roof", etc) and localities either adopt that or amend it. I would be surprised if they simply didn't allow it given it's now in the international code. The issue is usually more about your zone, cost, etc. I hesitated because of the cost - whether I had to use foam (e.g. closed cell) or rigid panels to meet code, it was expensive. In the end I was going to spray foam a smaller section of the attic we would later finish and then do blown insulation (after air sealing) in the floor of the other section. It's otherwise hard to justify the cost of foam. The idea of foam made sense for the house - it was a complex roof, HVAC was in the attic, and it was build as an unvented attic.
At this point I have basically decided to spray foam the slants in order to use the space and cut down on ice dams, and to just leave all the old insulation for the walls and cellulose in the attic ceiling. The city wants us to fir out every 2x4 exterior wall to allow more insulation space. The cost doesn't justify the return when they add all kinds of pointless requirements.