I need to replace the roof on my house. It is a story and a half style built in the 1920's. There is no insulation in the roof, the attic is finished, and there is no venting. The current roof is Asphalt and I have not had any condensation issues.

When I tear off the old shingles I am thinking of putting 4" of foam insulation (R-20), a new layer of OSB or plywood, and then the shingles. This seems to be a good way to add insulation with out tearing out most of my ceilings.

Is there anything I am not seeing?

Would condensation on the 1st layer of OSB be an issue or will the faom keep that layer warm like the rest of the living space?