Building Enclosure Design
Building Enclosure Design
This domain tests the control-layer logic of walls, roofs, slabs, windows, and foundations: keeping water out, air sealed, vapor managed, and heat flow continuous. Exam items often ask which layer belongs where or what detail breaks a thermal bridge.
Core rules and concepts
- Four control layers: water/rain, air, vapor, thermal. Air barriers must be continuous and supported; vapor control must suit the climate and assembly, not automatically be a polyethylene sheet.
- Keep insulation continuous across transitions: foundation-to-wall, wall-to-roof, window jambs, balconies, and service penetrations. Interruptions are thermal bridges and condensation risks.
- In cold climates, favor exterior insulation and vapor-open interiors when drying potential is needed; in hot-humid climates, control inward vapor drive and interior moisture.
- Windows are the weakest thermal and solar element: specify U-value, SHGC, frame, spacer, and installation detail together.
- Durability follows drying: assemblies should be able to dry in at least one direction.
Worked example
A wall is 80% clear field with U = 0.15 W/m²K and 20% framing and penetrations with U = 0.45 W/m²K. Area-weighted U = 0.8(0.15) + 0.2(0.45) = 0.21 W/m²K, so R = 1/0.21 = 4.76 m²K/W. If the framing fraction rises to 30%, U = 0.7(0.15) + 0.3(0.45) = 0.24 W/m²K, a 14% worse wall. That is why framing layout and continuous insulation are enclosure decisions, not finish details.
Traps
- Putting vapor barriers on both sides of an assembly.
- Drawing an air barrier that is interrupted at floor slabs or window frames.
- Ignoring foundation and roof junctions in the thermal-bridge calculation.
- Specifying window U-value without the installation thermal bridge.
- Treating a rainscreen as the water barrier.