Radiant barriers are frequently sold with claims like "equivalent to R-19" or "performs like R-8". These claims are not quite lies, but they describe something other than what a buyer reasonably assumes. A radiant barrier does not have a meaningful R-value, and the number you should be asking for is emittance.
What R-value measures
R-value is resistance to conductive heat flow through a material. It is a property of the material and its thickness: six inches of fibreglass has roughly twice the R-value of three inches. It is measured in a guarded hot plate apparatus with the sample sandwiched between two surfaces, which is to say with no air gap.
A sheet of aluminium foil a few thousandths of an inch thick resists conduction almost not at all. Its intrinsic R-value is close to zero, and aluminium is in fact an excellent conductor.
What a radiant barrier actually does
It reduces radiant heat transfer, which R-value does not describe at all. The relevant property is emittance: the fraction of heat a surface radiates compared to a perfect black body. A true radiant barrier has emittance of 0.05 or lower under ASTM C1313, measured per ASTM E408.
Low emittance only works with an air gap facing the reflective surface. Press foil flat against a solid and the radiant mechanism is defeated, which is precisely the condition an R-value test creates.
So where do the R-numbers come from?
They come from testing an assembly, not the foil. Build a wall or roof section that includes a foil layer and an air space, measure the whole thing, and you get a number that can be quoted in R-units. That number is real for that assembly under those conditions. It is not a property of the product in the roll, and it will not transfer to your attic if your air gap, orientation or temperature differ.
This is why the Federal Trade Commission's R-value Rule requires specific substantiation and disclosure for insulation claims. If a seller quotes an R-equivalent, the fair questions are: what assembly, what air gap, what temperature difference, and what direction was the heat flowing.
What to ask for instead
- Emittance. Should be 0.05 or lower. This is the number that matters.
- Reflectivity. Typically quoted around 95% or higher, and effectively the mirror of emittance.
- Perm rating, if you need perforated. See perforated vs. non-perforated.
- ASTM C1313 conformance, which is the product standard for radiant barrier material.
The exception: foam-core products
Some products marketed as radiant barriers are foil-faced foam or bubble laminates. Those do have a genuine R-value, because the foam or bubble layer resists conduction. It is usually modest, in the R-1 to R-6 range, and it costs several times more per square foot than plain foil. That is a legitimate product with a legitimate R-value; just be clear you are buying two things and paying for both.
If R-value is what you want, blown-in insulation buys far more of it per dollar.
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Full background in the complete radiant barrier guide.