First off this is for my own house. I have read numerous posts on spray foam, cellulose and fiberglass blow in insulation, and to be honest I only have hands on with cellulose. I know that I need more insulation in my attic, currently there is only R-15 batt insulation installed on 24" o.c. We have a whole house fan that we use early in the mornings and late evening to pull cool air in with, vented soffit, I would replace the soffit baffles with better ones but the originals are still there currently, and box vents near the ridge so I think I have adequate ventilation. There is one house near by that had spray foam installed on the underside of the roof sheeting when it was built new and the owner is just crowing about the low utility bills. I talked with the installer and he said it was very do able to install spray foam an existing house. He also talked about making it a "closed" attic, to me it sounded as if he would take all the ventilation sources out and spraying the foam on the underside of the sheeting. Is this correct? I also asked about spraying the foam on the joist cavities and he said that was an option also if we took the batt insulation out and then we could lay it back in afterwards. I am thinking that I would like to get to an R-60 factor. Lets hear the pros/cons and do's and don'ts.