Moisture Intrusion & Building Science Flashcards

The IRC requires that crawl space ground cover vapor barriers cover what minimum percentage of the ground surface?

Answer: 100%

The IRC requires ground-cover vapor barriers to cover 100% of the exposed crawl space floor to prevent soil moisture from evaporating into the crawl space. Partial coverage percentages (50%, 75%) would leave unprotected soil that continues to emit vapor, defeating the barrier's purpose.

A flat roof with interior drains begins showing mold growth on the ceiling decking of the top-floor offices. The most likely cause is:

Answer: Roof drain clogs causing ponding water and eventual infiltration

Clogged interior drains allow water to pond on a flat roof; sustained ponding eventually finds paths through membrane seams, penetrations, or degraded flashing and infiltrates the roof deck and ceiling below. HVAC condensation would appear near diffusers and not correlate with drain locations; skylight leaks are localized to skylight perimeters; and ridge vents are not present on flat roofs.

In a 'hot-humid' climate (IECC zones 1–3), exterior insulation on a wall assembly is preferred over interior insulation primarily because:

Answer: It keeps the sheathing warm and above the dew point, preventing condensation within the wall

Placing insulation on the exterior side of the sheathing keeps the sheathing warmer than the outdoor dew point, preventing condensation on that surface during cooling season. Interior insulation leaves the sheathing exposed to cool conditioned air, dropping it closer to or below the dew point. Cost, fire code, and rebate eligibility are not the primary building-science rationale.

Ice dams on a roof cause water intrusion into wall and ceiling cavities primarily by:

Answer: Meltwater backing up behind the ice dam and penetrating under shingles

Meltwater from snow higher on the roof runs down and refreezes at the cold eave, forming an ice dam; subsequent meltwater pools behind the dam and is forced upward under shingles by hydraulic pressure, entering the roof deck and wall cavity. Freezing expansion cracks shingles but does not drive liquid water inward; snow load compresses the deck but does not open penetration pathways; ridge vent blockage raises attic humidity but is not the primary mechanism of wall/ceiling water intrusion.

Which diagnostic tool allows an inspector to detect moisture behind finished wall surfaces without destructive sampling?

Answer: Non-contact infrared thermometer used in conjunction with a moisture meter

An infrared thermometer detects surface temperature differences that suggest evaporative cooling from moisture, and a moisture meter then confirms elevated moisture content at that location — together they locate hidden moisture without cutting into walls. A psychrometer measures ambient air humidity, not surface moisture; a pin meter alone requires contact with the suspect surface and cannot detect moisture behind a finished wall; a room-center datalogger records general air humidity, not localized wall moisture.

Negative air pressure in a basement relative to the outdoors during winter will most likely cause which moisture-related problem?

Answer: Inward infiltration of cold dry outdoor air, reducing basement humidity

Negative basement pressure relative to outdoors draws cold outdoor air inward; winter outdoor air is cold and therefore holds very little moisture, so infiltration actually reduces indoor relative humidity rather than raising it. Condensation would require warm humid interior air moving toward a cold surface (the opposite direction); radon entry is a separate concern from moisture; and outward exfiltration occurs with positive, not negative, interior pressure.