Series thermal resistances of flat layers (same area)—rough total R in the same units (e.g. h·ft²·°F/Btu).
Each layer field that contains a positive number is summed; blanks, zero, or non‑finite values are treated as 0 for that layer. The third layer is optional. This is a simple series‑sum of nominal R‑values for quick budgeting, not a full assembly U‑factor calculator.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.