I hope you find the following answer satisfactory; if you have any
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my best to resolve any outstanding issues.
I looked up some information on ASTM C518, which is a thermal
resistance testing standard, and it seems that the numbers you
provided are probably indicating the thermal resistance of an inch
thickness of material. This is supported by the Imperial units given
for thermal resistance, which seem to be a "per-inch" unit as you
indicated. Also see note at the end of this answer.
It turns out that the numbers you have given for thermal resitance are
exactly the R-values of the material in both Imperial and SI units.
According to several web sources I have found, R-values in metric are
given in units of m^2*C/W, and in English units of (ft^2*h*F)/(BTU).
So that means that the English/US R-value is about 3.8 per inch of
material, and the metric R-value is about .671 per inch of material.
This also coincides with the known conversion factor between US and
metric R-values: when we convert .671 metric R to Imperial R, we get
Now an inch is approximately 25 mm; if we have 90mm of insulation we
have about 3.6 inches, so the R-value of a 90mm thickness is 3.6 times
the R of a single inch. So a 90mm thickness of the material has a
metric R of approximately 2.416.
I hope you find this answer satisfactory; if not, please feel free to
request a clarification, and I will do my best to answer any questions
Here is some information on the units of R-value in both English and metric units:
Metric and Imperial R-value:
General information on Imperial R-value and calculating total R-value
This website gives the units of R-value as (°F?hr?ft2)/Btu:
Having inches in the denominator of the listed thermal resistance on
your product seems to indicate that each inch of the insulation
provides an (Imperial) R-value of 3.8.
Google searches performed:
<"r value" insulation equation>