Building Science and Diagnostic Testing
The four control layers
A building enclosure manages four flows: bulk water, air, vapor, and heat. The single most tested concept is that air leakage carries far more moisture than vapor diffusion in most assemblies, so air sealing usually outranks vapor control. Traps ask you to choose a vapor retarder when the real defect is an air barrier discontinuity.
Pressure, temperature, and psychrometrics
Understand that air moves from high pressure to low pressure, and that warm air holds more moisture. Condensation occurs on the surface or within the assembly that drops below dew point. Stack effect, wind, and mechanical imbalance drive pressure differences. Questions often present a winter condition and ask where moisture accumulates: the answer is typically the cold side of the insulation or a cold interior surface, not the warm side.
Diagnostic tools and what each proves
Know the purpose and limits of each test. A blower door quantifies whole-building air leakage at a reference pressure and can help locate leaks with smoke or infrared. Infrared thermography reveals surface temperature differences, which suggest but do not prove moisture or missing insulation; it requires confirmation. A duct blaster measures duct leakage. Moisture meters give relative readings that vary by material and require calibration and interpretation. The trap is treating any single instrument's output as conclusive.
Worked rule of thumb
For infrared, require a minimum indoor-outdoor temperature difference of roughly 10 degrees Celsius (about 18 Fahrenheit) and low solar loading for meaningful contrast. Document conditions alongside images, because the image alone is not evidence without the driving conditions.
Reporting diagnostics
Always state the test standard, conditions, equipment, and uncertainty. A number without its conditions is not a finding.