I have a two story workshop. 18x20. Perhaps 25 feet high.
Last fall I finished out the lower floor. Insulated the walls/ceiling. Drywalled.
For heat I'm using a kereosene heater. I plan to have a gas line installed before next winter.

here's pic of the building.

http://web.mac.com/jollybgood/iWeb/Photo%20Depot/Jolly%27s%20Sketch%20Book_files/IMG_0009.jpg

Anyway I put off doing the second story because I'm not sure how to go about insulating/ventalation and I'm concerned about shortening the life of the roof.

The rood joists are 2x4's. The roof design/slope is that of a barn. (see pic).

Can I just put rolled insulation between the roof joists and the drywall over that? That would mean no ventalation between the roof sheeting and the insulation.

Would rigid foam board insulatiion stacked to fill the space between the joists be better?

Again my concern is shortening the life of the shingles and/or unseen condensation/moisture problems behind the drywall once I finish it up.

With 2x4 joist I'm not sure putting in roof vents would really help since there's no room for air to circulate between the insulation and sheeting.

I want to make the second floor a game room/tv room for guests. as a 15' x 15' space it'd be nice to have it finished off and use the space. Eventually I'm going to build a deck with stairs leading up to the space with a second level door.

Currently access is via a 2x6 steps/ladder leading up from the first floor.

 

I would build walls in 6 inches from top of first roof slant then put a ceiling tie across the top of this. Effectively making a square box inside the area. Then you could insulate with 3 1/2 inch fiberglass all the way around it and not intrude on the roof rafters at all. This would leave plenty of room for airflow around your enclosed space to keep the roof cool and your insulated space conditioned.

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