Building Science Fundamentals and the House as a System
Think in systems, not parts
BPI's core premise is that a house is an interconnected system: the building enclosure (roof, walls, foundation, windows), the mechanical systems (heating, cooling, ventilation, water heating, ducts), and the occupants (behavior, moisture and pollutant generation) all influence one another. A change to any one produces pressure, thermal, and moisture consequences elsewhere. Tightening a house lowers infiltration, which saves energy but also reduces dilution of indoor pollutants and can depress pressures in the combustion appliance zone. Adding insulation changes surface temperatures and can move where condensation occurs.
Boundaries and drivers
Candidates must be able to locate the pressure boundary, thermal boundary, and moisture boundary, and recognize that in a well-built house these align. Three forces drive air: stack effect (buoyant warm air rising; strongest in tall houses during cold weather), wind (positive on the windward side, negative on the leeward), and mechanical fans (bath, kitchen, dryers, and duct leakage). The neutral pressure plane is the height where indoor and outdoor pressure are equal; above it the house exfiltrates, below it the house infiltrates.
Traps
Questions love "one fix solves everything" thinking: that air sealing is always safe, or that adding insulation without addressing air leakage and moisture cannot cause problems. Also watch for conflation of enclosure and equipment: ducts are part of the distribution system, and duct leakage outside the pressure boundary is both an energy problem and a pressure problem.
Worked rule of thumb
Air changes per hour at 50 Pa: ACH50 = (CFM50 x 60) / conditioned volume. A 2,000 ft2 house with 9 ft ceilings has 18,000 ft3. A blower door reading of 900 CFM50 gives 900 x 60 / 18,000 = 3.0 ACH50. Many efficient-home programs target roughly 3 ACH50 or tighter, while older homes often test well above 7.