Hi GBA,

I am turning an attic into a room.   I have 2×5 rafters (old house), 16″ on centre.

The roof (which I am about to replace) will be steel on a double layer of strapping – so air can move easily under the steel.

Still, I would like to keep the insulation at least a half-inch off the underside of the roof sheathing.  

I have the idea to 1/2″ spacers, install them, and then install 1″ polyiso foam.  This would use up 1.5″ of my 5″ space.   

Then I thought I would put R14 Rockwool.  That would bring me up to the edge of the rafters. 

To eliminate the thermal bridging, I propose to use another layer of 1″ polyiso on the underside of the rockwool.

This gives me a Rockwool sandwich, with polyiso as the bread on both sides.   Yummy!

Do you think this is a good and safe method of insulating the ceiling?

Thank you in advance,

Eric in Ontario, Canada