I've been thinking what would be the best and easiest (and cheapest) way to insulate my garage roof. I thought about foam but it is too expensive. I thought about installing a ceiling with loose insulation on top but I have too much junk in the way. So I thought what if I put a 1/2 by 1/2 piece of wood in the corners of the joist and roof deck and butt 3 inch rigid foam against it. This leaves the foam insulation flush with the joists and still leaves 1/2 inch under the roof deck for ventilation. It would also act as a vapour barrier if I coulk the edges. This would cost less then 1k. Wat do you guys think, would this be adequate for heating in the winter? Is there anything I am not considering?