Building Envelope
Core concepts
Understand heat transfer mechanisms: conduction, convection, and radiation. Know R-value (thermal resistance) and U-factor (thermal transmittance), where U = 1/R. Understand thermal bridging, infiltration versus ventilation, air barriers versus vapor barriers, and solar heat gain coefficient (SHGC) and visible transmittance for glazing. Candidates should understand cool roofs, emissivity, and albedo.
Traps to watch
A frequent trap is mixing units - R-value in IP units (ft²·°F·h/Btu) versus metric. Another is assuming adding insulation always has proportional returns; diminishing returns apply. Watch for confusion between air barrier (controls air leakage) and vapor retarder (controls moisture diffusion); placing a vapor retarder on the wrong side can trap moisture. Questions may ask about thermal bridging at studs or slab edges, which reduces effective R-value. Also beware of assuming low SHGC is always good - in heating-dominated climates, winter solar gain can be beneficial.
Worked example
A wall has a U-factor of 0.10 Btu/(h·ft²·°F) over 1,000 ft². With a 30°F indoor-outdoor temperature difference, heat loss = U × A × ΔT = 0.10 × 1,000 × 30 = 3,000 Btu/h. Adding insulation to halve U saves 1,500 Btu/h. Over 5,000 heating hours, that is 7.5 million Btu, or about 7.5 MMBtu per year.
Practical guidance
Prioritize air sealing before adding insulation, verify continuous air and thermal barriers, and use blower-door testing to quantify infiltration.