Advice

[kurt]

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.

[Jim Katen]

I'm not aware of any *need* for it.

But I'd encourage it because it's so cheap and so easy to install and because it will provide a small but constant benefit for the life of the roof.

[kurt]

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.

[Jim Katen]

Wouldn't it provide some leverage as a break in the thermal bridging of the roof framing?

[kurt]

We're going to have 1/2" expanded perlite board under the membrane.

The thermal bridging consideration is a good point, though.

[kurt]

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.

[Marc]

Insulation board on top might reduce the temps seen by the SPF during the summer season. You might inquire of BSC as to how closed cell will hold up to elevated temps over a period of 2 or 3 decades.

Marc