Practical guidance, design questions and sources: Envelope & Building Physics.
The Thermal Envelope: How to Define It Correctly
The thermal envelope separates spaces maintained at a controlled temperature from outdoors or unconditioned areas. Define that boundary on plans and sections before selecting insulation.
R-Value and U-Value: How to Read Thermal Performance
R expresses thermal resistance; U expresses heat transfer per unit area and temperature difference. Values are comparable only when they describe the same assembly boundaries and use consistent units.