Building Science Principles
The house as a system
Every BPI-style question about building science starts with the idea that a house is a set of coupled flows: heat, air, and moisture move through the enclosure, and a change to one changes the others. Candidates must be comfortable with the three heat-transfer modes (conduction, convection, radiation) and with the distinction between the building envelope (the thermal and pressure boundary) and the structural frame. Know that insulation resists conductive flow, air barriers stop convective flow, and neither of these is a vapor barrier unless it is designed to be.
Pressure and the stack effect
Understand the driving forces of air movement: wind, the stack effect (warm air rising in winter, neutral pressure plane near mid-height), and mechanical fans. In winter, the lower floors of a leaky house are typically depressurized relative to outdoors and the upper floors pressurized. Traps appear here: questions often reverse the direction, or confuse the neutral pressure plane with the thermal boundary. Remember that air carries far more moisture and heat per unit than conduction through the same assembly, which is why air sealing is prioritized.
Thermal and moisture fundamentals
Know R-value (resistance) versus U-factor (conductance), that U = 1/R, and that assembly R-values must account for framing, not just cavity insulation. Understand dew point, relative humidity, and that condensation happens where warm moist air meets a cold surface. Also know the difference between vapor diffusion (slow, driven by vapor pressure) and air leakage (fast, driven by pressure).
Worked example
An assembly has a 2x6 cavity at R-21 with 25% framing at R-6.5. Cavity contribution = 0.75 x 21 = 15.75; framing = 0.25 x 6.5 = 1.63; total = about R-17.4. That gap between nominal and whole-assembly R-value is a favorite exam trap.