Building Science and Home Performance Fundamentals
The House as a System
Every audit question rests on the idea that a house is a coupled system: the building enclosure, the mechanical systems, and the occupants interact. Heat, air, and moisture move together. A candidate must be fluent in the three heat-transfer modes (conduction, convection, radiation) and in the distinction between the building enclosure (the pressure and thermal boundary) and the insulation boundary. Gaps between these boundaries create thermal bypasses and condensation risk.
Pressure and Driving Forces
Understand the stack effect (warm air rising, neutral pressure plane), wind (positive on windward, negative on leeward), and mechanical pressures from exhaust fans and duct leakage. These forces sum; a house can be depressurized on the lower floors and pressurized above the neutral plane. Know that air sealing changes pressure dynamics and can backdraft combustion appliances-this is a classic trap.
Moisture and Durability
Know the moisture sources (bulk, capillary, diffusion, air transport) and that air-transported moisture dominates in most assemblies. Understand vapor retarder placement relative to the dew point, and that adding interior vapor retarders in mixed climates can trap moisture. Questions often confuse vapor diffusion with air leakage; diffusion is small compared with convective transport.
Combustion Safety and Indoor Air
Recognize spillage, backdrafting, and the need for combustion appliance zone (CAZ) testing. Carbon monoxide is a life-safety issue; never trade safety for savings.
Worked rule of thumb: For a 2,000 ft² home with 8-ft ceilings, a 50 Pa blower-door reading of 3,000 CFM gives 3,000/16,000 = 0.19 CFM50 per ft² of envelope area-use envelope area, not floor area, for the CFM50/ft² metric. Trap: candidates often divide by floor area and get a wrong, inflated number.