I've hired a builder to build my new home and planned on using Spray Foam Insulation. A vendor mistakenly delivered Radiant Barrier to my homesite and decided it was less expensive to leave it then to come back out and pick it up. So my builder installed the Radiant Barrier on the roof trusses and will use the spray foam underneath. Did I just get lucky or is there something I should be concerned about with this combination? Thanks, Dave

Will there be an airspace between the radiant barrier an the roof decking?

Call wally too,

water/vapor barrier made with 1¼-in.,. 100% recycled ... Northwestern Ohio Foam packaging. 800-339-4850. www.nofp. com

he has easy to understand delivery of technical info since the 90's

No, the radiant barrier was installed on top of the roof trusses and the roof decking, I believe, was nailed directly on top of the radiant barrier. Should I be concerned?

If the "Austin" in AustinDave means the house is in Austin TX, it's not as likely to become a problem the way it might in much colder climates where the roof deck could stagnate for weeks/months under a snow load. But without an air gap on either side of the radiant barrier it's thermal effectiveness is zero, whereas if there had been a ventilation gap between the r.b. and roof deck there would have been at least a marginal benefit thermally, and give the roof deck better drying capacity. In an Austin TX climate in almost all roof stackups the decking can still dry quickly enough toward the exterior.

What is the roofing material/stackup above the roof deck?

Was it open cell or closed cell foam? (And how thick?)

Yes, the house is in Austin, TX

What is the roofing material/stackup above the roof deck?
- I think it is just black "paper" (don't know the official term) followed by shingles (Timberline Utra HD)

Was it open cell or closed cell foam? (And how thick?)
- Closed cell. It has not been sprayed yet, so I do not know how thick

I thought the spray foam needed to be sprayed directly against the wood to avoid air pockets. The air exchange could trap condensation. This might be more of a concern with colder climates.

Closed cell foam at R30 (code min for that part of TX) would not allow the roof deck to dry toward the interior in any event, so the presence of the radiant barrier foil doesn't change things much. The roofing felt is about ~2-5 perms (higher when wet), and the shingles close it down even tighter (they're essentially vapor-impermeable, but slightly ventilated.)

Open cell foam and no radiant-barrier would offer more than an order of magnitude higher drying capacity to the roof deck and making o.c. preferable from a drying point of view, but you'll be still OK here with closed cell.

Without the radiant barrier closed cell @ R30 would be structural, gluing the roof deck to the truss/rafters, making the roof deck less susceptible to being ripped apart in a hurricane. But simply by going with an unvented roof design (independent of insulation type) some protection against that damage is already built-in (at least until the first window breaks, allowing the wind to deliver an upward-force punch to the roof.)

Thanks all for the responses. Sounds like the only benefit I might get would be some minimal sound reduction from outside noise. -- Dave