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Flat (low slope) roof, 6" closed cell on the underside of the roof sheathing.

Should we put down insulation board under the (new) mod bit roof system, or just go with the fiberboard underlayment?

The extra insulation is worthwhile, I suppose, but is there a "need" for the insulation board on top of the sheathing? I'm checking @ BSC, but want opinions.

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I agree. But, this is that LEED house where we're squeaking any additional R value out of the mix that's possible.

We're in the "balance budget against benefit" component of everything right now.

I just couldn't remember if there was some thermal problem with omitting it. We'll have fiberboard under the roof to provide a good substrate.

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Thanks for the reminder about bridging. We're so tangled up about so many details, it slipped my mind. Which is bizarre, in as much as we've been absorbed with eliminating thermal bridging everywhere else.

Just goes to show more heads are better than fewer.

In the final analysis, we've done so many other things to combat heat loss, we decided to go with a layer of 3/4" polyiso board, then Fesco/expanded perlite, then membrane.

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