R-value and U-factor: which number to use
R-value measures resistance to heat flow and is reported for materials and assemblies. U-factor measures the rate of heat flow and is what codes use for windows and whole assemblies. They are reciprocals, and mixing them up is a common specification error.
The two numbers
R-value is thermal resistance: higher is better. U-factor is thermal transmittance: lower is better. For a simple assembly U equals one divided by R.
Insulation products are sold in R. Windows are specified in U. Whole assemblies appear in the codes both ways, which is why a specification can contain both and still be consistent.
Why an R-value needs a temperature
The thermal conductivity of insulation changes with temperature, so an R-value is only meaningful alongside the mean temperature it was measured at. Standard test methods report it that way; marketing rarely does.
A specification that publishes two temperatures is telling you something useful: which figure to feed an energy model for a heating climate and which for a cooling one.
What Guardian publishes
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Guardian publishes each panel at two mean temperatures, 40 °F and 75 °F, rather than a single headline figure.
Source · 3-Part Specification 1.5 E.5 b
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Roof panels: 12 in R-50 at 40 °F and R-46 at 75 °F; 8 in R-33 and R-30; 6 in R-25 and R-23.
Source · 3-Part Specification 1.5 E.5 b
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Written by Data Subsystems, for Guardian Structural Technologies. Last updated .
Drafted from Guardian Structural product literature and published building-science sources. Awaiting Guardian technical review. Confirm any figure against the current product documents or ask us before you rely on it in a design.