How Is Condensation Risk Analyzed in a Wall or Roof Assembly?

Condensation analysis calculates temperature at each interface of the assembly and compares it to the dew point of adjacent air — surfaces below dew point condense. Simple steady-state methods screen assemblies; transient hygrothermal modeling evaluates complex, high-risk, or unconventional assemblies under real climate data. The analysis must use realistic indoor humidity, not optimistic defaults. The discipline is straightforward but the inputs decide everything: get the indoor humidity right, get the coldest realistic outdoor condition, model the actual layer sequence with real material properties, and the math will tell you where the water forms. Every condensation failure I have investigated would have been predicted by an honest analysis.

The analysis is only useful if it changes the design. When the model shows condensation, the fixes follow a hierarchy: warm the cold surface (move insulation outward, add exterior CI), reduce the humidity (ventilation, dehumidification, source control), or add vapor control to keep humid air from reaching the cold surface. Often the cheapest fix is relocating insulation — exterior CI that keeps the sheathing warm eliminates a whole class of winter condensation problems. I also check the reverse: assemblies that the analysis clears but that have air leakage paths, because air-transported moisture dwarfs diffusion. A perfect vapor analysis on a leaky assembly is fiction — humid air bypassing the retarder through gaps will condense regardless of what the diffusion math says. Air barrier continuity is a precondition for the condensation analysis to mean anything.

The analysis is a package: inputs, method, results, and the design response. I expect to see all four. My condensation analysis requirements:

• Real layer sequence: actual materials with real thermal and vapor properties, in the order they will be built
• Honest humidity inputs: realistic indoor conditions for the occupancy, with a sensitivity check at higher humidity
• Full-year consideration: winter design day plus summer inward-drive and shoulder-season conditions where relevant
• Air leakage addressed: the assembly's air barrier continuity verified as a precondition, not assumed
• Design response documented: where risk was found, the assembly change that resolved it — and re-analysis confirming it