Condensation on the Crawlspace Joists
A vented crawlspace under a 1,200 ft² single-story house has fiberglass batts between the floor joists with the kraft facing up against the subfloor. In summer, the homeowner notices condensation dripping from the joists and a musty odor. The crawlspace has foundation vents that are open year-round. The soil is bare and damp; a hygrometer reads 78% RH in the crawlspace while outdoor air is 88°F and 70% RH. The floor above feels cold and the hardwood floors are cupping. The HVAC ductwork runs through the crawlspace and is insulated but shows condensation on the boots.
1. Explain the moisture mechanism and why the current vented-crawlspace-plus-batts approach is failing.
Read the worked answer
In summer, hot humid outdoor air enters through the open foundation vents and contacts cool surfaces - the joists, the duct boots, and the subfloor cooled by air conditioning above. The air is cooled below its dew point and condenses. The kraft facing against the subfloor is on the wrong side for a vented crawlspace in a cooling-dominated climate; it traps moisture and prevents drying. The bare damp soil adds a large vapor load. The result is condensation, musty odor, and cupped floors. Vented crawlspaces work only where outdoor air is reliably drier than the crawlspace; here it is not. The fix is to close and condition the crawlspace: seal the vents, install a continuous ground vapor retarder (at least 6-mil, lapped and sealed), insulate the walls rather than the floor, air-seal the floor above, and provide conditioned air or a small dehumidifier to hold RH below 60%.
2. The homeowner proposes simply closing the vents. Evaluate that idea and describe the complete retrofit needed.
Read the worked answer
Closing the vents alone is not enough and can make things worse by trapping the existing moisture. A complete retrofit requires: (1) a sealed, continuous ground vapor retarder over the entire soil, lapped 12 inches and sealed at penetrations and walls; (2) sealing all foundation vents and any other openings to outdoors; (3) insulating the crawlspace walls (rigid foam or closed-cell spray foam) down to the footing and protecting the foam; (4) air-sealing the floor above - rim joists, penetrations, and the duct boots; (5) supplying conditioned air or a dedicated dehumidifier to control humidity, targeting under 60% RH; and (6) verifying with a hygrometer over time. The ducts in the crawlspace should be sealed and insulated, and the boots detailed. This converts the crawlspace into a semi-conditioned space inside the envelope, eliminating the condensation surface and the vapor drive.