We are often called to a Del Mar home because of a smell. The owner opens the crawl hatch or walks into a closet on the north side and notices that heavy, earthy air. The first question is usually where the leak is. Sometimes there is one. Often there is not, and the moisture is arriving the same way the morning clouds do.
This entry is about that second case: airborne moisture under raised floors, why it is common here, and how we approach it without overreacting.
The weather under the house
The National Weather Service’s forecasters’ handbook for extreme southwestern California describes the marine layer as a dominant feature of the coastal climate. Cool, moist air off the ocean sits under a warmer layer above it, producing low clouds and fog that are most persistent at night and in the morning. The handbook notes that relative humidity is higher at night and in the morning, and that deeper marine layers can bring drizzle.
A crawl space in Del Mar breathes that air. Vents that are meant to dry the space can, on many mornings, bring in air that is already close to saturated. When that air meets cool soil, cool concrete or a cold pipe, it can condense. The result is damp wood and damp steel without any water ever running in from outside.
Why our housing stock feels it
Del Mar’s housing is older than most of the county. The Census Bureau’s 2024 five-year estimates count about 2,550 homes in the city, and roughly three in four of them were built before 1980. Many older cottages and hillside homes sit on raised floors with crawl spaces, sometimes with little clearance and modest venting.
The County’s hazard plan annex for Del Mar rates expansive soil as a medium hazard, higher than several neighboring cities. Clay under a crawl space holds moisture longer, so the ground itself stays cool and damp, adding to the humidity problem.
What humidity does over time
Humidity alone does not rot a house quickly. It does a few things slowly:
- Raises the moisture content of joists, girders and sill plates, which invites decay fungi in the wettest spots.
- Encourages surface mold on wood, paper-faced insulation and stored items.
- Speeds corrosion of steel connectors, anchor bolts and nails.
- Lets fiberglass insulation sag and hold water against the floor.
Caltrans’ corrosion guidelines treat sites within 1,000 feet of the ocean as a marine atmosphere for exposure purposes. Under homes near the beach, the air carries salt as well as moisture, and that combination is hard on galvanized hardware.
How we measure before we decide
We try not to recommend a fix based on one visit on a foggy morning. Our approach is quieter:
- Place a small temperature and humidity logger in the crawl space for a few weeks, ideally across clear and overcast stretches.
- Take moisture-meter readings on joists, girders and sills at several locations.
- Note the condition of the soil surface, any vapor retarder, and the insulation.
- Check vents for blockage, screening and placement.
- Rule out plumbing and drainage sources, so we are not blaming the air for a leak.
The EPA’s mold guide recommends keeping indoor relative humidity below 60 percent, and ideally between 30 and 50 percent. Those numbers are written for living space, not crawl spaces, but they give a reference. A crawl space that sits well above that range for long stretches is worth addressing.
The two code paths for a crawl space
The California Residential Code allows two broad approaches under section R408.
Vented. The code calls for ventilation openings totaling at least 1 square foot per 150 square feet of under-floor area, with one opening within 3 feet of each corner. That can drop to 1 square foot per 1,500 square feet when the ground is covered with a Class I vapor retarder. Clearing blocked vents and covering bare soil with a retarder are often where we start.
Unvented. The code also allows a closed crawl space, with a continuous Class I vapor retarder over the ground, lapped at least 6 inches and run up the stem wall at least 6 inches. That space then needs one of several moisture-control measures, such as continuous mechanical exhaust at 1 cfm per 50 square feet, conditioned air supply, or dehumidification.
Neither is automatically better. In Del Mar, a vented crawl space can be fighting moist intake air on many days, while an unvented one needs ongoing equipment and attention. The choice depends on the logger data, the house, the owner’s plans and how often someone is on site.
Owners who are not here full time
Many Del Mar homes are second homes or rentals for part of the year. That changes the calculus. A dehumidifier with a drain line and an alarm is only helpful if someone checks it. A logger that reports remotely, or a simple seasonal visit, may be the more practical choice. We discuss that openly rather than assuming one answer. For a house that sits empty through part of the year, closing it up with the heat off can also raise humidity indoors and below, so we look at how the home is left between visits as part of the picture.
When it becomes structural
Most humidity issues are maintenance and envelope work. They become structural when we find:
- Decay that has softened sill plates, joist ends or girders.
- Connectors or anchor bolts with significant section loss from rust.
- Posts on wet soil that have settled or rotted at the base.
Repairs to framing or foundations are a different scope. When structural design is needed, we coordinate with licensed engineers as needed.
What sets the scope
We do not quote online. The work is shaped by:
- How long and how high humidity runs, based on measurement.
- Whether ground cover, vents or both need work.
- Whether the owner chooses a vented or unvented approach.
- Any decay or corrosion repairs found along the way.
- Access and clearance under the floor.
If your home is anywhere in Del Mar and the crawl space smells or feels damp, request a Free Quote. Related reading on this site covers raised foundation repair and foundation drainage.