That foil roll is not “insulation” per se. First, I would have to be air tight to make any difference at all, it has no warming capabilities. The pink stuff works by taking the inevitable flow of air through a “loose” structure, and warming it by forcing it to pass through the pink layer, effectively “exciting” the air as it bumps and squeezes through the thickness of the pink stuff.The paper is just a vapor barrier. They use the foil roll here in Michigan only for a cold air return when it has to be in an outside wall, and not to warm the air, only to pass code. You will also see it under a garage slab that has radiant infloor heat tubes running in the concrete. I suppose if you taped it or caulked all around it when you were putting it in, it would do something.
The spray foam mentioned above is the state of the art way of insulating. It allows no air to flow in or out, so instead of fighting against cold air coming in and warm air leaking out, it is a completely sealed barrier. They spray that right on the underside of the roof plywood, that’s called a hot roof. In an attic, there has to be air movement in traditional construction. If there isn’t, the underside of the roof plywood would condensate when it was colder outside, and I’ve seen it so bad that it actually frosts on the inside of the roof, in a hot roof, that doesn’t happen. It’s the way to go, but also the most expensive.
I worked at a townhouse complex where they used the smooth side of OSB facing the workshop space, and used high gloss polyurethane as a sealer. They rolled it right on, and it dried shiny. You could wipe it clean, drill and put screws and shelves wherever you wanted. I thought it was a pretty smart setup.
