I would think you would want a open cell foam.

Maybe foam rafters and create a hot roof on top
Many options here:
http://www.buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/

or

Continuous baffles to the peak and use a roof 2-wall vent.

http://www.roof-2-wall.com/

dielectricunion said:

I'm trying to figure out how to deal with a small section of roof that extends off a 12-12 gable roof section. It used to be a porch and has 2x6 rafters. I would fur out the rafters to add room for insulating and venting but 1. The low point of ceiling is already only 73" and 2. Clearance to edge of the window is only a few inches beyond rafters edge. Should I look into closed cell spray foam and keep this roof unvented as highest R value I can pack in? Previously, it was not vented and had r13 insulation. I'm in south/central Indiana.

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I've looked at building science's info, it's really great. I'm just so indecisive when I don't see one obviously "right" or "best" way to deal with something. I'll look at that other link, it's something I've never heard of. Thanks

Let me throw in some food for thought... my brother spent about $8 grand having his attic completely reworked. They came in, sucked out all the insulation, sealed off all gable, soffit and ridge vents, then blew on the expanding foam on all exterior surfaces including the gables and the underside of the roof.

We live in South Carolina and our summer days are typically in the mid 90's. If you go into a typical attic in the middle of the day here, it will be in the 130+ degree range.

He told me the temperature in his attic never varies more (or less during winter) than 7 or 8 degrees from his living space. He is very happy with the outcome and his energy bills are down considerably. So, with that, I recommend spraying the underside of the roof.

Foam on top of the roof with a vent space for shingles above foam. Air seal it really well and blower door test it. Then you can put whatever you want in the rafter cavity as long as the dew point isn't in the rafter bay. Easiest approach is to spray with closed cell and call it a day.

I would definitely do 1 or 2" foam over roof sheathing but the roof was redone less than 2 years ago and I have no interest in tearing it off. If I were re roofing I'd consider jacking the roof up to gain more interior space and more space for insulation

Morning Wood said:

Foam on top of the roof with a vent space for shingles above foam. Air seal it really well and blower door test it. Then you can put whatever you want in the rafter cavity as long as the dew point isn't in the rafter bay. Easiest approach is to spray with closed cell and call it a day.

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