Im in the process of converting an attached garage into a living room. Were going to have a cathederal ceiling in the new room. The question I have is: What is the recommended way to insulate a cathederal ceiling on a home that has a hip roof ? The room is on the end of the house ( were the hips come together) I am concerned about adequete air flow. I have plenty of suffit vents to let air in, but no where for the air to escape (due to the hip roof). The room size is 16 feet wide by 32 feet long, the lenght of the house. It is approx 16' from the top of the walls to the peak of the roof. If you need more info let me know. Any help or suggestions would be great.

 

http://www.eteknic.com/HipRafter.html

This site gives a diagram to illustrate what I am discussing.

A "Common Rafter" runs from the top plate of the wall to the Ridge.

A "Hip Rafter" runs from a corner of a wall top plate to the Ridge.

"Hip Jacks" run from the top plate of a wall to the "Hip Rafters".

When you insulate between the rafters of a Hip Roof, even though you use baffles to keep the insulation away from the roof sheathing, the cavities between the "Hip Jacks" are not ventilated properly. The end result is moisture forms in these areas.

There are basically two ways to properly ventilate those areas, build out or build down. Build out is the preferred way because of the bouyancy of warm air, but building down is the most common.

Building out is when 1x2 wood slats are nailed perdendicular to the rafters from the top plate to the ridge in rows 16 inches on center. Then the roof decking/sheathing is nailed on top of the slats. The gap created by the the slats between the roof decking/sheathing and the rafters allows the warm air to move upwards towards the vents near the ridge.

Building down are sometimes called "Trade Ceilings". This is where the Rafters are not used for sheet rock nailing. The ceiling framing is done to mimic the form of the rafters. This is not as easy as it sounds. You have to be a fairly good carpenter to accomplish it. The easiest way to do it is to go to a lumber yard and ask for trusses for the ceiling to be made.

 

If you're curious about truss pricing, it's generally priced per the amount of lumber involved. For reference, the 28' span scissor trusses I'm buying for my addition will run about $100 apiece and will go 24 OC. Your local truss yard can give you an exact quote of price and weight for what you're looking at.

Here's a good rundown of the different types of trusses availible:
http://www.cwc.ca/products/trusses/shapes.php

 

could I install a ridge vent on the hip rafters? Cobra, etc. ? Im going to install tongue / Grove birchwood on the ceiling so I have to have something to nail to that runs verticle from wall to top of the room.