From the insulation4less site:
Reflective insulation is the most effective when airspace exists between the insulation and the adjacent material. The R value of the system will vary dependent on the airspace size and the direction of the heat flow, and the conductive and convective properties of surrounding materials. We are not able to expand on the Characteristics, Values or Regulations shown in above chart. All pertinent technical information available is posted above. Please consult your local code official if concerned with an approval.
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There was a discussion about this insulation a while back and one of the folks called and asked what the airspace needed to be to achieve the R7.3 that they claim. IIRC: it was 5-1/2"…..
So, this would not be good for our purposes. Needs airspace.