R-Value Calculator

Most of the US lands between R-49 and R-60 for attics. Warm southern zones can stop at R-30 to R-49; anything from the Midwest north should target R-60.

For batts and boards, essentially yes. Loose-fill settles, so blown products lose a little R over time — buy to the coverage chart depth, not the bare minimum.

In series through an assembly, yes — a layer of R-13 behind a layer of R-5 gives R-18 through that path. What does not add up is the whole wall, because studs run straight through the insulation and conduct around it. A wall with R-13 batts between wood studs performs closer to R-11 overall once the framing is accounted for.

They are reciprocals of each other. R-value measures resistance to heat flow, so higher is better; U-value measures the rate of heat flow, so lower is better. U equals 1 divided by R. Windows and doors are normally specified in U, and insulation in R, which is why comparing the two directly needs a conversion.

Yes, and for some materials noticeably. Polyiso loses R-value as it gets very cold, which matters in a cold-climate roof where its rated performance is measured at a warmer mean temperature than it will actually see. Fibreglass and cellulose are comparatively stable. This is one reason published R per inch should be treated as a comparison figure rather than a guarantee.