Building Envelope Requirements
The envelope as a system
Envelope requirements control heat flow through opaque assemblies, fenestration, and air leakage. Candidates must know the metrics: U-factor (thermal transmittance, lower is better), R-value (thermal resistance, higher is better), SHGC (solar heat gain coefficient for glazing), and air leakage rates. U-factor and R-value are reciprocals; questions frequently bait you by treating them as interchangeable.
Climate zones and tables
Requirements are climate-zone dependent. Know how to identify the zone and read the correct column: roofs, walls (above and below grade), floors, slabs, and fenestration each have separate criteria. Traps include applying the wrong climate column, using the prescriptive table when a trade-off path was declared, and forgetting that fenestration limits are often expressed as a maximum percentage of gross wall area plus a U-factor and SHGC cap.
Continuity and thermal bridging
Insulation must be continuous; thermal bridging at framing, balconies, and shelf angles degrades performance. Air barriers must be continuous and sealed at penetrations and junctions. Vapor retarders are climate- and assembly-dependent - installing one in the wrong climate can trap moisture.
Worked example
A wall assembly is described as R-13 cavity insulation plus continuous R-5 sheathing. Rule of thumb: compute the effective assembly U-factor by summing resistances (R_total = R_cavity path and R_framing path in parallel, plus continuous layers), then compare to the table's maximum U-factor - not to the nominal cavity R-value alone. Continuous insulation counts fully; cavity insulation is derated by framing fraction.
Candidate checklist
Identify climate zone, assembly type, and required metric before computing.