Are there any studies available involving radiant barrier ‘underlayment’ (i.e. Polaralum) when applied or sandwiched between roof decking and composition shingles?
Answer:
Reflective insulation and radiant barriers only work in the presence of air spaces. The application you describe does not have any air spaces facing the reflective surfaces. Therefore, the reflective material will be a direct conductor of heat — no thermal benefit at all.