Believe it or not, I cannot find an answer to this question on the internet. Reflective sheathing/decking for a roof – although manufacturer guidelines states to install shiny side in, why? What happens if it is installed outward then felt and shingles on top of it? I cannot find what difference it makes.
When aluminum foil faces an air space it will reflect 97% of the heat rays directed toward it, or conversely, it will not emit more than 3% of the head that is conducted to it from the other side. When aluminum foil is in direct contact with another surface, (like shingles), it will become a direct conductor of heat. Having the aluminum foil surface in direct contact with the roof shingles will provide no thermal benefit whatsoever.