Insulation, Ventilation, and Condensation Control
This domain tests why metal roof assemblies condense and how insulation, air sealing, and ventilation prevent it. Questions usually present a climate, an assembly, and a symptom.
Condensation
Surface condensation occurs when the panel is below the dew point. Concealed condensation comes from moist air leaking into the cavity and hitting a cold surface. Air sealing matters more than the perm rating of the retarder.
Ventilation
Attic vent area ratio is 1:150 of floor area; 1:300 when a balanced ridge and eave system is used or a vapor retarder is installed. Balance vent area about equally between ridge and eave, and keep an insulation baffle at the eave so the vent stays open.
Insulation
Rigid board above the deck: polyiso R-5.7 to R-6.5 per inch, XPS about R-5, EPS about R-3.6–4.0. Thermal blocks at clip lines reduce point compression and thermal bridging. Never compress batt insulation under the panel.
Worked example
A 2,000 sq ft attic floor: net free vent area = 2,000 ÷ 150 = 13.3 sq ft. Balanced means about 6.7 sq ft at the ridge and 6.7 sq ft at the eaves. A ridge vent rated 18 sq in. net free per foot over 4 ft gives only 0.5 sq ft per foot × 4 = 2 sq ft, so the design needs a longer ridge or additional vents.
Traps
Installing vapor retarders on both sides of the assembly; blocking the eave vent with insulation; and assuming the metal panel is an air barrier that stops moisture movement.