# Estimating the effect of radiant barrier on the radiant and conductive heat transfer through a metal roof

The basic question is -- Will a radiant reflective coating be more effective applied to the upper surface or the lower surface of a metal?

Case 1: Imagine a metal roof subject to solar heating. The solar heat load is primarily visible and IR radiation. The roof reflects some portion of the radiation back up into the atmosphere. The remaining radiation heats the metal roof which then re-radiates in the IR, presumably both up into the atmosphere and down into the structure below. The atmosphere is opaque to some fraction of the IR, so the roof heats up above the ambient temperature. The solar radiation also increases the thermal motion of the roof metal which transfers heat by conduction into the air above and the structure below.

Case 2: Now imagine the upper surface of the metal roof is coated with a radiant reflective coating that reflects 95% of the solar visible and IR radiation. Presumably, the remaining 5% will reach the metal, with the same results as in the paragraph above for this 5%. A small amount of radiant and conductive heat will pass into the structure below.

Case 3: Now imagine that instead, the lower surface of the metal is roof is coated with this same reflective coating. In this version, much of the solar radiation will heat the metal as in Case 1. But this time, the IR radiation emitted by the metal is 95% reflected by the reflective coating. Presumably, some of this reflected IR re-heats the metal even more and some passes through the metal into the atmosphere. The conductive heat, on the other hand, passes through the reflective coating into the structure below.

Question 1: Are there any basic principles that allow one to estimate which Case, 2 or 3, will result in the least heat transferred into the structure below from radiation and conduction (ie, most radiation reflected)?

Question 2: For all three Cases, is there any way to estimate the relative fraction of radiant vs conductive heat that the metal will generate in either direction, both up and down?

Question 3: Are these results different if the metal roof is replace with conventional asphalt shingles or something similar?

• One can theoretically estimate all of these things, but why? In practice you aren't even interested in the result. What you are really interested in is how the reflective roof performs 12 months after installation, when it has gotten dirty from dust, rain, leaves falling on it etc.. The way to do that is by installing temperature sensors on different roof types and and by measuring the temperatures of both the roof as well as the ceiling surfaces (and the internal and external air temperatures) over the course of a year. That kind of dataset will give useful technical results. Sep 12, 2014 at 3:52