Moisture Intrusion & Building Science Flashcards

In building science, the 'dew point' is defined as the temperature at which:

Answer: Relative humidity reaches 100% and condensation begins on a surface

The dew point is the temperature at which air becomes fully saturated (100% relative humidity) and water vapor begins condensing into liquid on surfaces. Capillary action is a separate liquid-water transport mechanism; HVAC mode changes are operational settings unrelated to dew point; and mold germination requires sustained moisture but has no fixed dew-point trigger.

Which type of vapor retarder is classified as 'Class I' per the International Residential Code (IRC)?

Answer: Polyethylene sheeting (6 mil)

Class I vapor retarders have a permeance of 0.1 perm or less; polyethylene sheeting (6 mil) meets this threshold. Kraft-faced insulation is Class II (0.1–1.0 perm), unfaced fiberglass has very high permeance and is not a vapor retarder, and standard latex paint is Class III (1.0–10 perms).

A homeowner reports recurring efflorescence on an interior concrete block basement wall. This primarily indicates:

Answer: Active water infiltration carrying soluble salts through the masonry

Efflorescence forms when water infiltrates masonry, dissolves soluble salts (calcium carbonate, sulfates), and then evaporates on the interior surface, leaving deposits behind. This requires active water movement through the wall, not just surface condensation or paint failure. Freeze-thaw damage and paint aging are separate failure modes that don't produce salt deposits on interior faces.

Which of the following is the primary driving force for bulk water intrusion through a below-grade foundation wall?

Answer: Hydrostatic pressure from saturated soil

Saturated soil exerts hydrostatic pressure perpendicular to the foundation wall, forcing water through cracks, joints, and porous masonry. Vapor pressure and capillary suction are secondary mechanisms that operate at a much smaller scale; wind-driven rain acts on above-grade surfaces and cannot directly cause below-grade intrusion.

Thermal bridging in a wall assembly causes localized cold spots that can lead to mold growth. What building component most commonly creates thermal bridges in wood-framed construction?

Answer: Wood studs themselves conducting heat out through the wall

Wood studs extend continuously from the interior to the exterior sheathing, conducting heat out of the wall at a much higher rate than the fiberglass insulation between them. Fiberglass batt is itself the insulating layer, not a bridge; house wrap and interior paint are thin surface materials with negligible thermal conductance.

An inspector finds elevated moisture readings on the interior face of an exterior wall only during summer months in a humid climate. The most probable cause is:

Answer: Inward vapor drive pushing humid outdoor air through the wall assembly toward the air-conditioned interior

In summer, the outdoor air is hot and humid while the interior is cool and dry; vapor pressure is higher outside, so moisture drives inward through the wall assembly toward the cold interior surface where it can condense. Roof flashing and plumbing leaks would show moisture year-round, not only in summer, and attic ventilation issues affect ceilings rather than exterior walls.