Garage (workshop) roof insulation using rigid foam

[ruthart]

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?

Thanks,
- George

[ArtMulder]

make the 1x1's out of styrofoam also...
That's how it was done in the vaulted section of our addition. (though we then sprayed foam over top of it all. we used 1/2" or 1" rigid, I forget which.)

Oh, and "caulk" the edges with spray foam in a can, to really seal it.
One thing to remember with your approach is that you will get thermal bridging on the rafters, since there will be no insulation there. I have no idea if that will make a big difference in a garage or not.