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I know that radiant barriers (i.e., reflective films applied over or under insulated spaces) don't technically have R-values (because they primarily reflect heat, instead of primarily slowing convection and conduction as does traditional insulation), but they do have some insulating effect, which could be quantified as equivalent to an R-value.

E.g., if I put a radiant barrier under R-19 insulation, what equivalent insulation will that space experience? Holding conditions constant, an HVAC system will see a reduced load after the addition of the radiant barrier. One could find the pure insulation level that matches that load reduction – perhaps it's R-30? – and based on that say the radiant barrier is "functionally equivalent to R-11."

Amendment: Yes, there are good theoretical reasons why radiant barriers are not assigned an R-value. However, as one user notes, we only ever talk about "R". For example, where I live the standard for home insulation is R-30. Can I reliably achieve that through a combination of blanket insulation and a radiant barrier? MIMA, hardly an objective trade organization, did a test in Florida 30 years ago which, while highlighting shortcomings of radiant barriers, did note reasonably consistent heat flow reduction from properly installed barriers. I would assume that with the benefit of 30 years of knowledge and increased emphasis on insulation we can say something useful about the functional value of adding a radiant barrier to an insulation stack.

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    $\begingroup$ Wikipedia: The limitation of R values in evaluating radiant barriers. The article notes that "emissivity values are the appropriate metric for radiant barriers". $\endgroup$ – Chris Mueller Oct 30 '15 at 11:57
  • $\begingroup$ I think that @ChrisMueller's link to Wikipedia explains some of the background confusion. The problem is that different insulating works in different ways, but we only ever talk about "R". Maybe the real question is how to define amount of insulation between different types? $\endgroup$ – hazzey Oct 30 '15 at 13:56
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    $\begingroup$ The U-factor/U-value is used as the quantifier for insulating from all types of heat flow including radiant. It is particularly useful for comparing transparent barriers such as glass. As the article states however, it is hard to measure. $\endgroup$ – Cohen_the_Librarian Oct 30 '15 at 20:02
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Insulating against thermal conduction can be represented as an R-value because it is a linear calculation. I can add two or more R-values together and I will see a linear benefit.

Insulating against thermal radiation is more complex: You can't add two radiation barriers and get one that is twice as good. Insulation is based on the temperature of the barrier itself and the temperature of the environment radiating to it (and occurs on both sides of the barrier). It gets even more complicated when it is placed between insulation or when there are materials against it on either side. Now only the radiation that permeates the surrounding material is being reflected. The absorption of the material is doubled because the radiation is absorbed again on the way back out. Thus radiant barriers are always more effective when at least one side of them faces an air gap.

Though it can't be represented by an R-value, you can reasonably calculate its effectiveness when against an air gap for a specific temperature differential. Just use the equations in the thermal radiation link above.

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No, the radiant effect cannot be quantified as an equivalent R-value.

It doesn't work like that.

You're trying to co-opt a measurement that it's completely unsuited for.

So don't.

ericnutsch has explained one of the main reasons why not, and I'll repeat it: R-values sum; radiant barriers do not.

R-values measure thermal insulation against heat-transfer by conduction. Radiant barriers offer little to no thermal insulation against heat-transfer by conduction. That's why the whole multi-foil thing failed.

if I put a radiant barrier under R-19 insulation, what equivalent insulation will that space experience?

Do it and find out. But remember that you'll have to account for lots of factors in different ways to if you were using better insulation, such as the emissivity of stuff around it. They are not swappable.

Holding conditions constant

They never are. If you are interested in a particular real-world situation, do a real-world test.

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  • $\begingroup$ Fair engineering response: Test it to find out. But one could say the same thing for traditional blanket insulation. I just amended the question to note that real-world tests have been done, but I haven't yet found anything authoritative and recent. $\endgroup$ – feetwet Oct 30 '15 at 14:27
  • $\begingroup$ the point is that there's nothing authoritative because it depends on a bunch of situation-specific stuff, in a way that simply doesn't apply to proper insulation $\endgroup$ – 410 gone Oct 30 '15 at 14:38

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