After doing lots of research and reading lots of different opinions I would like to know what is the best way to insulate and vent a 1 1/2 story house that has a bedroom upstairs in the roof/attic area. I live in Minnesota and want to know the proper way to do this so the upstairs bedroom stays warm in the winter and cool in the summer without having to worry about ice dams or lost heating/cooling air.
Here is an image i found that diagrams two different methods
(source: dspinspections.com)
The picture isn't really two different methods, but two different plans. It's one method (soffit at the bottom and ridge at the top). The only difference is what you actually insulate - and it's the same thing. Insulate anything that you want kept at "livable" temperature, since it will be part of the conditioned envelope of the house, keeping in mind that the larger you make that area, the more heating and cooling you're doing. We did a blog post on this subject as well, check out http://diy.blogoverflow.com/2011/12/resuscitating-the-roof-providing-adequate-roof-ventilation/
It seems you answered your own question by the diagram. What's not answered is how much insulation is needed. Your local jurisdiction may have specific requirements. At a bare minimum you're probably going to need R-19 wall and R-40 roof, if not more. It's quite likely that there is not enough framing space to achieve this with fiberglass batts alone. You'll need to consider either or both more efficient insulation and some soffiting to get enough room for proper insulation.
The air gap under the roof sheathing is very important, and can be easily compromised by hasty insulation installation unless some sort of baffles are installed to preserve the space and protect it from intrusion by heavy handed installers.
We all create water vapor, by washing, cooking, breathing, sweating. Water vapor is programed to move from warm to cold. If it is trapped, it will condense and form mold, condensation, frost or ice possibly leading to wood rot. Therefore. the first thing to do is to decide how you will deal with water vapor rising from downstairs, the second the water vapor created in the bedroom. You can leave the walls covered with drywall and fibreglass...the water vapor will merely pass through these on its way to the cold outside...no problem. If you create a water vapor proof room, no holes or cracks, in effect a plastic box, it will also be airtight and you will need fresh air. How much heat will you save, by sealing the room, then providing ventilation?
