CMI Moisture Intrusion & Building Science 2 — Questions and Answers
Question 1: In building science, the 'dew point' is defined as the temperature at which:
- Relative humidity reaches 100% and condensation begins on a surface (Correct answer)
- A material becomes saturated with liquid water through capillary action
- HVAC systems switch from cooling to heating mode
- Mold spore germination becomes possible on a substrate
Correct 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.
Question 2: Which type of vapor retarder is classified as 'Class I' per the International Residential Code (IRC)?
- Kraft-faced insulation
- Unfaced fiberglass batt insulation
- Polyethylene sheeting (6 mil) (Correct answer)
- Standard latex paint
Correct 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).
Question 3: A homeowner reports recurring efflorescence on an interior concrete block basement wall. This primarily indicates:
- Active water infiltration carrying soluble salts through the masonry (Correct answer)
- Normal paint failure due to aging
- Poor ventilation causing condensation on the surface only
- Freeze-thaw damage to the mortar joints without water migration
Correct 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.
Question 4: Which of the following is the primary driving force for bulk water intrusion through a below-grade foundation wall?
- Vapor pressure differential
- Hydrostatic pressure from saturated soil (Correct answer)
- Capillary suction from dry interior masonry
- Wind-driven rain striking above-grade surfaces
Correct 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.
Question 5: 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?
- Fiberglass batt insulation between studs
- Wood studs themselves conducting heat out through the wall (Correct answer)
- Exterior house wrap membrane
- Interior latex paint on drywall
Correct 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.
Question 6: 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:
- Roof flashing failure allowing rain penetration
- Inward vapor drive pushing humid outdoor air through the wall assembly toward the air-conditioned interior (Correct answer)
- Plumbing condensation from a supply line in the wall
- Inadequate attic ventilation causing heat buildup
Correct 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.
CMI Moisture Intrusion & Building Science 2
In building science, the 'dew point' is defined as the temperature at which: