Since I always put furring strips on the ceiling to level the drywall, would there be enough benefit to be worth putting a layer of rigid foam on the trusses, then furring? It could be taped as a good air barrier, and would help isolate the thermal transfer through the trusses. Or am I getting a little crazy here? I can't say as I've ever seen it done. Thoughts?

You may be crazy there, or wherever you go. But I think it is a good idea. I should have done that. In my own place I put up 2"of XPS to hold in temp heat, then took it all down, hung MgO board, spray foamed and then blew in. Nudura makes ceiling technology, there is InSoFast and others that have furring strips.

Why do you have to level out the drywall, what happened?

More thermal break is always better from a thermal performance point of view, but if your truss chords are to be buried in 12" of cellulose adding another R2-R6 in foam isn't going to show up in the heating/cooling bills, and adding an inch or two to the cellulose would have at least as much effect, at lower overall cost. It's just as easy to air-seal the gypsum as it is to air-seal the rigid foam.

As I said- I always apply drywall over furring strips for the ceiling. Trusses aren't the straightest things in the world, and you can wind up with a wavy ceiling. I was taught to pull a string, and shim the low (Actually, high) spots of the furring to create a true plane. I've seen it done on walls, too. Judging from the lumber I bought for my barn, the chances of getting a true, straight surface range from zerol to none. Now days, contractors just shoot popcorn all over the ceiling and call it good. This will be my own home, so I'll take whatever trouble necessary to get good ceilings and walls.

Dana- I see your point about piling on more cellulose in the attic. Everyone is so fired up about thermal transfer that even the braces in the trusses would conduct some heat, although I think it's probably overkill.

The braces of the trusses DO conduct some heat, even more so with the metal gussets, but as a fraction of the total area it's getting down there- sub-1% of the total ceiling area. At R1/inch if the brace or king-pin is going through 12" of cellulose, the R value of the penetrating wood is about R12. Are we to believe that a half-inch of XPS (R2.5) raising that 1% fraction to an effective from R12 to about R15 is going to show up in a meaningful way? Assuming a 2x4 chord, the 8.5" of cellulose above the chord is a huge thermal break already over the chords, making the contribution of the foam even less. Going an inch deeper in cellulose over the whole shebang would also be a pretty small difference, but more than doing half inch foam under the trusses.

Dana -

When would an application for these foam boards on the ceiling be good for?

On a steel SIP roof, I need to have the ceiling furred to provide a sound break, would panels like this work well for that?