Better Air Quality Starts in Your Crawl Space

If your home sits over a crawl space, as most homes in Cary, Raleigh and the rest of the Triangle do, the air under the floor does not stay down there. A large share of the air you breathe on the first floor started underneath it. If there is moisture, mildew or mold under the house, that smell and those spores work their way up into your living space through gaps in the floor, around pipes and wiring, and through the ductwork.

So the cleaner and drier your crawl space, the better the air in the rest of the house. Here is how that air moves, why humidity is the real problem, and what actually fixes it.

How Crawl Space Air Gets Into Your House

Warm air rises. In a house, that means warm air is always trying to escape through the upper floors and the attic: around recessed lights, attic hatches, chimneys and plumbing vents. Air that leaves at the top has to be replaced, and the replacement is pulled in at the bottom of the house. In a home built over a crawl space, the bottom of the house is the crawl space.

This is called the stack effect. It is strongest in winter, when the inside of the house is much warmer than the outside, and it is why a crawl space problem can show up as a smell in an upstairs bedroom.

Ductwork adds a second path. Many Triangle homes have their heating and cooling ducts running through the crawl space, and any leak on the return side of that system pulls crawl space air straight in and delivers it to every room in the house.

Why Humidity Is the Real Problem

Air from a clean, dry crawl space is not much of a concern. The trouble is that crawl spaces here are rarely dry. Bare soil gives off water vapor all year. Foundation vents let in warm, humid summer air, and from June into September the outdoor air here routinely carries more moisture than a cool crawl space can hold. When that air meets cool ducts, pipes and framing, it condenses on them.

Mold and wood-rot fungi need that moisture. Hold a crawl space's relative humidity in the 50–55% range, the usual target for a crawl space dehumidifier, and they do not get established. Let it sit high for weeks at a time and they will, on the joists, the subfloor and the paper facing on fiberglass insulation. That growth is what you are smelling upstairs.

What That Musty Smell Is Telling You

A musty smell on the first floor is the most common sign that crawl space air is reaching your living space. It is often strongest near floor registers, in closets on outside walls, or in the room over the dampest part of the crawl space. It tends to be worse in summer and for a day or two after heavy rain.

Candles and air fresheners cover it for a while. They do nothing about where it comes from. The smell is a symptom of moisture and growth under the house, and it keeps coming back until the moisture is dealt with.

Signs Your Crawl Space Is Affecting Your Indoor Air

• A musty or mildew smell on the first floor, especially near vents or floor registers
• Condensation on windows or cool surfaces, even when the AC is running
• Allergy or respiratory symptoms that improve noticeably when you're away from home
• Visible mold growth on crawl space framing, insulation, or ductwork
• Insulation sagging or falling out from between the floor joists, which usually means it is wet
• Ductwork under the house that sweats in summer
• Hardwood floors that cup or feel soft in late summer

Better Air Starts Below the Floor

Because so much of the air upstairs starts below, the most direct way to improve it is to change what is down there. In practice that means three things working together:

• Stop the ground moisture. A sealed vapor barrier over the soil and up the foundation walls keeps water vapor in the ground from becoming humidity in the air.
• Stop the humid outdoor air. Sealing the foundation vents and rim openings keeps summer air from coming in and condensing on cool surfaces.
• Take out the moisture that remains. Even a sealed crawl space picks up moisture from the house above, from small air leaks and from the masonry itself. A crawl space dehumidifier, sized for the space and properly drained, holds the humidity where mold cannot grow.

Together, those three are crawl space encapsulation. Any one of them alone underperforms. A liner with the vents still open lets the summer air in, and a dehumidifier with the vents open is fighting the weather. If mold is already growing on the framing, it is removed before anything is sealed, because sealing over active growth just traps it in.

Why Not Just Run a Household Dehumidifier Down There?

It is the obvious first move, and it usually disappoints. A portable unit collects water in a bucket that nobody under a house is going to empty, so it fills up and shuts off while everyone assumes it is still working. Many household units lose efficiency in cool air, and a crawl space runs cooler than a living room for most of the year. And they are built to run on and off in a conditioned room, not around the clock in a damp one. A dedicated crawl space unit drains on its own, works at low temperatures, and is rated for continuous duty.

What You Can Check Yourself

• Put an inexpensive hygrometer just inside the crawl space access door and read it after a stretch of humid weather.
• Look at the insulation from the access door. Batts drooping out of the joists are usually wet.
• Check whether the foundation vents are open, and whether the ground is bare soil or covered by plastic, and if so whether that plastic is torn or pulled back.
• Notice when the smell is worst. A smell that follows the weather points to the crawl space.

If you have already found growth, or are wondering whether the smell is mold, our post Is Mold in Your Home? covers how to tell crawl space mold from surface mildew and why moisture is always the cause.

Triangle Reconstruction has been sealing and drying crawl spaces across Wake and Durham counties and every county bordering them since 2012. If your house smells musty or your crawl space is damp, call (919) 924-7600.