Mold Does Not Just Happen - It Requires Moisture, and Moisture Sources Can Be Designed Out
Mold growth in buildings is a moisture problem - remove the moisture source and mold cannot establish itself. Mold spores are everywhere in the natural environment and will grow on virtually any organic material (wood, paper, drywall facing) when moisture content exceeds approximately 20%. Building moisture problems arise from three primary sources: bulk water intrusion (leaks through the envelope - roof, wall, window, or below-grade infiltration); condensation (water vapor that condenses when warm humid air contacts a surface below its dew point temperature); and construction moisture (concrete and masonry that are not fully cured before enclosure, or wet materials used in construction). Each requires different design and construction strategies to eliminate. Understanding mold prevention as a building science topic is relevant to the ARE PPD exam's building envelope and environmental quality content areas.
Preventing Bulk Water Intrusion
Bulk water intrusion is prevented by proper design and construction of the building envelope: continuous waterproofing membranes at all below-grade surfaces; proper flashing at all envelope penetrations (windows, doors, roof-wall intersections, penetrations for utilities and equipment); drainage planes that direct any water that does penetrate the outer surface away from the building interior; and weep holes and drainage composites in masonry cavity walls that allow water to exit the drainage cavity. The most common sources of bulk water intrusion are: failed window and door perimeter flashing; inadequate roof membrane lap at penetrations; and improper termination of below-grade waterproofing at the grade level transition.
Preventing Condensation
Condensation in wall assemblies is prevented by correct vapor retarder placement (on the warm-in-winter side of the insulation, preventing warm moist air from contacting cold surfaces within the wall), adequate insulation levels (keeping the dewpoint surface closer to the exterior face where temperatures are already cold rather than within the assembly), and continuous air barrier that prevents air-carried moisture from infiltrating into the assembly. Condensation on interior surfaces (cold windows in winter, sweating pipes) is prevented by adequate surface temperature - insulating pipes, using thermally broken window frames, and maintaining adequate indoor humidity levels to reduce the likelihood of surface condensation.
Construction Moisture Control
Wet materials installed in construction can be a source of mold years after occupancy. Best practices: do not install gypsum board or insulation that has been wet; protect framing lumber and sheathing from prolonged rain exposure during construction; allow concrete and masonry adequate time to cure before enclosing; and verify moisture content of wood framing with a moisture meter before installing vapor-sensitive interior finishes. On critical projects, a building enclosure commissioning process verifies that moisture levels in the construction assembly are within acceptable ranges before installation of finishes.
Key Exam Points
- Mold requires moisture - eliminate moisture sources (bulk water, condensation, construction moisture) to prevent mold.
- Bulk water: proper flashing, continuous waterproofing, drainage planes, weep holes.
- Condensation: vapor retarder on warm-in-winter side; adequate insulation; continuous air barrier.
- Construction moisture: protect materials from rain; don't enclose wet lumber or gypsum; verify moisture content before finishing.
- LEED Indoor Environmental Quality: low-emitting materials and construction IAQ management plan address construction moisture and material off-gassing.
AREprep's PPD building envelope module covers moisture management - vapor control, waterproofing, drainage, and condensation prevention - as an integrated building science topic, giving ARE PPD candidates the moisture control knowledge they need to evaluate envelope assembly decisions and to answer building science questions on the PPD exam that address indoor air quality and envelope performance.
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