Back in 2010 I contacted RIMA regarding the effectiveness of installing radiant barrier directly over spaced sheeting vs. installing on rafters which would be the case in new construction and room additions. At that time, the person responding to my email stated that RIMA did not recommend using radiant barrier over spaced sheeting.
I no longer have that email, but if memory serves me correctly, the reason RIMA did not recommend it was because any point where the radiant sheeting came into direct contact with the nailing strips (spaced sheeting) the energy would be conducted vs. radiated.
Now that Title 24 is be enforced throughout California, I would greatly appreciate any information you could offer on this subject. This is a big topic of discussion that our prospective residential customers are asking, and they are receiving confusing information from various roofing contractors they are receiving proposals from.
I believe my recommendation was made based upon the closeness of the nailing strips — not that much air space was exposed. I don’t believe there is enough benefit to be gained to justify the cost of labor and materials. If there is a way to install a radiant barrier on the underside of the roof in between the rafters, that would be a good option. Or, if you could attach a radiant barrier to the face of the rafters, that would be another and the best option. One could also attach the radiant barrier to the joists and get essentially the same benefit. Once the roof is finished, there’s not much that can be done in that part of the system that would be cost effective.
I hope this answers your question. As to roofing dealers that are suggesting that there would be a good benefit of putting a radiant barrier directly under the roof deck, ask them to run the application by the RIMA EXPERT and I’ll respond to them.