JuanMoore said:Sounds fine to me. A lot of people here use just a 2x without any insulation at all (~1.5 R value?). I haven't heard any complaints of major inefficiency issues, and what you're describing would have 2-3 times the R value. I'm no expert, but I'd think making it airtight is just as if not more important than the R value of the collar.
If you really wanted to you could use some aluminum foil. It would help reduce radiant heat transfer, and might help a little. You could staple it to the poplar and/or plywood, or tape it to the foam before assembly.
Ah yes, I do recall that there is a coefficient for reflectivity in that equation for heat loss from my thermo class in college. Although just thinking about it makes me cringe. Thanks for the tip. I'll definitely do that.