Below the house in a hot climate
Full basements are less common in Fresno than in colder regions; many homes are on slabs or raised foundations with crawlspaces. Fresno summers are extremely hot, with National Weather Service records showing 69 days at or above 100°F in 2021, and crawlspaces and basements are often the coolest spaces in the house. That can work for you or against you: cool surfaces in a humid space can collect condensation.
Crawlspace moisture
Winter rain and tule fog keep crawlspaces damp for long stretches, and irrigation and plumbing leaks add more water. Moist crawlspaces invite mold, wood decay and pests, including termites. A heavy vapor barrier over the soil, sealed at seams, good drainage, and either clear vents or a conditioned space depending on design help keep crawlspaces dry.
Irrigation
Heavily irrigated landscaping next to the house saturates soil against the foundation. Aim sprinklers away from walls, fix leaks promptly and keep soil sloped away from the house.
Plumbing and slab leaks
Plumbing leaks under raised floors and slabs are a common source of below-grade moisture. Musty smells, damp flooring and a rising water bill are signs.
Evaporative coolers
Homes with evaporative coolers should make sure cooler drain lines don't discharge near the foundation or into crawlspace vents.
Finishing a basement
If your home has a basement, confirm it stays dry year-round, test for radon, plan egress windows for bedrooms, and plan cooling and dehumidification.
Before you hire
Ask Fresno contractors where the moisture is coming from, how they'll handle the crawlspace, and whether permits are needed. Look up the contractor's California license with the CSLB.
Termites and crawlspaces
Termites are present in the Central Valley, and damp crawlspaces with wood in contact with soil invite them. Keep wood debris out of the crawlspace, make sure posts sit on concrete piers, and keep the space dry. A termite inspection is worthwhile if the crawlspace has been damp for a long time.
Dehumidification and summer humidity
In summer, outdoor air in Fresno is hot and often dry, but crawlspaces and basements are cooler. If warm air reaches cool surfaces below the house, condensation can form. A sealed, conditioned crawlspace with a dehumidifier may be appropriate for some homes; ask a contractor what local code allows and what suits your house.
Signs to watch for
A musty smell in the house, damp or sagging insulation under the floor, rust on metal in the crawlspace, and higher indoor humidity all point to below-grade moisture. Check the crawlspace after the first big winter storm.
Drainage outside
Gutters, downspouts and grading are the first line of defense. Downspouts that empty right next to the foundation send water straight into crawlspaces.
Radon
The EPA recommends testing all homes for radon, which can enter through foundations and crawlspaces. A simple test kit tells you whether mitigation is needed.
Utilities under the house
Ducts and water lines often run through Fresno crawlspaces. Ducts that leak or sweat add moisture; sealing and insulating them helps both comfort and moisture control.