In more detail
A layer's R-value is its thickness divided by its thermal conductivity. US R-values are in ft²·°F·h/BTU; metric RSI values are in m²·K/W, and one RSI equals about 5.68 US R, so R-13 is RSI 2.29. R-values of layers in series add, including the thin air films on each surface, and the U-factor used for windows is simply 1 / R. Heat loss through an assembly is area × temperature difference / R.
Why it matters
Building codes set minimum R-values by climate zone, and the benefit shrinks as R grows: each extra inch saves less than the one before. A batt's printed rating is not the whole wall's: wooden studs, gaps and compressed insulation conduct heat around it, so the whole-wall value is lower. Mixing up US and metric values is a factor-of-5.68 error.
Example
100 ft² of R-13 wall, 40 °F difference: 100 × 40 / 13 = 307.7 BTU/h
1,000 ft² attic, 40 °F difference:
R-11: 1,000 × 40 / 11 = 3,636 BTU/h
R-49: 1,000 × 40 / 49 = 816 BTU/h (2,820 BTU/h less)
R-13 in metric: 13 / 5.678 = RSI 2.29