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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?

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  • $\begingroup$ 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. $\endgroup$ – CuriousOne Sep 12 '14 at 3:52
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If you coat the inside, the metal will get hot and this heat will conduct into the building. If you coat the outside (assuming it remains clean…) you will never get hot to begin with.

I think this means that case 2 will be better for you. I have often wondered about simply having a secondary roof with a standoff - in essence, a space where air can flow unimpeded. Because that's really what you need - a roof that itself stays cool. Otherwise, your house is still under a "hot cover". So focus on getting the heat away - first by reflecting as much as possible back into space, and next by conducting away heat. Ventilation under the roof is the key.

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    $\begingroup$ Absolutely: a proper installation consists of a well-insulated attic floor, and plenty of soffit ventilation to allow air to flow along the underside of the roof(which will help keep the top and bottom side of the roof at approximately the atmospheric temperature). $\endgroup$ – Carl Witthoft Sep 12 '14 at 11:41
  • $\begingroup$ I am impressed that three people have responded so quickly! The answers are all good. But don't address my hope to understand the physics of radiation and conduction that occur inside the metal. What is happening and what is the relative importance of each if the radiation barrier is applied to the upper vs lower surface. $\endgroup$ – mikeclarkusa Sep 13 '14 at 15:35
  • $\begingroup$ @mikeclarkusa - the problem is complicated by the fact that emission and reflection properties are a function of wavelength - and that the degree to which a hot metal surface conducts heat to the structure below depends entirely on the details of the construction (if you like on the relative ease with which heat is conducted to the environment vs. the air). As such it is hard to give a detailed answer and I stuck to generalities. How can I improve this answer for you given the above thoughts? $\endgroup$ – Floris Sep 13 '14 at 15:39
  • $\begingroup$ One fact is that no external radiant coating reflects 100% of the solar radiation. So conduction is a factor in both cases. But what is its importance in each case? $\endgroup$ – mikeclarkusa Sep 13 '14 at 15:42
  • $\begingroup$ Conduction is more important with the coating facing inwards because the metal will be much hotter (it absorbs more of the incident radiation). $\endgroup$ – Floris Sep 13 '14 at 15:53

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