Heat, Air, and Moisture Flow
Heat moves three ways
Conduction through solids, convection by moving air or water, and radiation across air spaces and from warm surfaces. R-value measures resistance to conductive flow (higher is better). U-factor is its reciprocal, U = 1/R, and measures conductance (lower is better). A wall's whole-assembly R is always below its cavity R because framing, headers, and plates bridge around the insulation; in a 16 in. on-center wall roughly a quarter of the area is wood.
Air is the big mover
Air carries both heat and moisture. Air leakage typically transports an order of magnitude or more moisture than vapor diffusion through the same assembly, which is why air sealing precedes vapor control in most designs. A hole the size of a fingernail in a cold attic plane can deposit more water than an entire wall's diffusion load.
Moisture and dew point
Water moves by bulk water, capillary action, air transport, and vapor diffusion, roughly in that order of magnitude. The dew point is the temperature at which air reaches saturation; any surface colder than the dew point condenses. Relative humidity is temperature-dependent, so warm, moderately humid air leaking into a cold attic or cavity can create problems even when the room feels fine.
Traps
Confusing R and U. Assuming a vapor retarder is always needed, or that it belongs on the same side in every climate. Treating foil or vapor-barrier paint as an air barrier. Forgetting that the interior surface temperature, not the thermostat setting, decides condensation.
Worked example
At 70 F and 50% RH the dew point is about 50 F. Single-pane glass at a 35 F interior surface sweats; a double-pane unit at 55 F does not. Drop the indoor RH to 30% and the dew point falls to about 37 F, drying the same window without changing the glass.