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Insulating With 2x4 Rafters

Topic: 
Insulation and Energy
Question: 

We are in the process of refinishing the upstairs (attic space) of our 1920's Cape Cod style home. The rafters are 2x4s with no insulation of any type. There are no soffit vents either. I am considering using 2" thick foam insulation boards between each rafter (leaving an 1 1/2" air gap), adding soffit vents and a ridge vent. Does this sound like a wise plan? If so, how do I know what size venting to use? (your 1 sq. ft vent/300 sq. ft ceiling ratio applies?) Can you recommend any books or videos? Thanks very much,

Answer: 

I think that you are on the right track. I would use 2" of polyisocyanurate foam board between the rafter and then 1" or more attached to the bottom of the rafters. This should allow you to attach the drywall the the rafters with long screws, although you might want to go with 5/8" or ridged type drywall.

The 1.5" continuous air gap is important. You should have soffit vents at the bottom and a continuous ridge vent at the top. The total amount of venting can be reduced to 1/150 by installing a vapor barrier on the warm (inside) side of the insulation. The vapor barrier could be a plastic film under the drywall or you could tape the foil backing of the foam board.

I can't thin of a specific book to help you with the insulation details, but you will find several books on our list to be relevant and of good quality.

George