A reader wants to know how to eliminate a musty smell coming from underneath a 1928 rowhouse. The crawl space, as shown above, probably needs to be encapsulated. (Reader photo)

Q: We are trying to figure out the next step in dealing with a musty odor in our 1928 brick rowhouse. It was originally built as a corner store and does not have a full cellar. The odor is most distinct in the living room. When we purchased the house, the inspector thought the smell was from the wet earth under the living room, drawn in by the dehumidifier. We moved the dehumidifier to the cellar and began using it only when it rains extensively, but that did not reduce the smell. We've also tried airing out the house, scrubbing surfaces and using odor-eliminator products. Nothing has worked.

Friends have suggested installing a vapor barrier under the hardwood flooring in the living room, but that would be expensive and difficult because the crawl space underneath is not really accessible. Would ceramic tile or cork tiles provide a vapor barrier?

A: The solution is probably to “encapsulate” the crawl space — a treatment that includes sealing the dirt floor and outside edges of the space and then providing a way to keep the air temperature and humidity level there similar to what is in your living space. There is no way to eliminate the mildew problem by installing different flooring because the problems are rooted in what is going on within the crawl space.

Builders used to think that the way to avoid musty smells in and over a crawl space was to provide plenty of ventilation under the house. To keep a house warm, they insulated the floor from underneath, often with a vapor barrier intended to block moisture in the crawl space from getting into the house.

The thinking has almost completely changed. Having exterior vents in a crawl space actually allows warm, humid air to flow in during the summer. When air in the living space is cooled by air conditioning, that moisture condenses on the underside of the cold flooring, and mildew grows, especially when there is a moisture barrier to keep the condensation from evaporating. In the winter, the opposite happens, with the same effect.

The solution is to eliminate the conditions that allow condensation.

Access into crawl spaces often is pretty tight, but don’t assume there is no way to retrofit your crawl space. The picture you sent shows what is probably just a foundation vent, and there is probably an actual access door somewhere else, said Chris McLaughlin, a sales manager at JES (877-537-9675; jeswork.com ), a company based in Virginia Beach that does crawl space encapsulation and other work throughout the D.C. area. “Sometimes there is a door in a closet and someone has put carpet over it, so you don’t even know,” he said. If there really is no opening, his company can cut one in, either on the outside (with a steel lintel to support the bricks over the opening, in a case such as yours) or in an out-of-the-way place such as a closet, which would then be outfitted with a trap door. Call at least two companies that specialize in crawl space encapsulation and ask for an on-site evaluation and recommendation, with an estimate of costs.

Mohammed El-Ghoul, owner of Home Energy Saving Solutions in Rockville (301-842-8818; marylandenergyaudit.net ), said encapsulating a crawl space almost always eliminates musty smells. The exception, he said, is when there is “bulk water” flowing into the space. So the first step in fixing your problem is to make sure there is no obvious source of water, for example from gutters that dump next to the foundation or sprinklers that splash against the walls.

Then, a company, such as El-Ghoul’s, will send a crew to clean up any debris in the crawl space. This includes removing any insulation with a moisture barrier installed on the underside of the flooring. Insulation without a moisture barrier can be left in place as long as it is isn’t moldy or falling down in places.

Once the space is clean, the crew spreads a moisture barrier, such as thick six-millimeter plastic, across the dirt floor. They also install a moisture barrier up the walls and seal all seams. Fresh insulation goes against the walls, over the moisture barrier. The final steps involve closing off outside vents and adding a vent or a register for the heating and air-conditioning system. This makes the crawl space, in effect, part of the heated and air-conditioned space of your house. Sometimes, El-Ghoul said, a dehumidifier in the crawl space substitutes for a heating and air-conditioning register.

This work isn’t cheap, but in addition to solving the odor issue, it should reduce drafts. And by keeping the floor framing drier, it also may help preserve your house, because pests such as some kinds of powderpost beetles are more likely to infest moist wood. El-Ghoul estimated that for a typical Washington rowhouse, the bill often comes to $1,700 or $1,800. But for complicated situations, including ones with extensive mildew that needs to be cleaned, it could go into the thousands.

McLaughlin said the minimum cost is more likely to be around $4,000 to $5,000, which would include a more robust dehumidifier than ones typically sold to homeowners, and the electrical work needed to run it. The upper limit could be tens of thousands of dollars if the floor structure is rotted and needs to be replaced, he said.

Retrofitting a rowhouse is usually similar to working on a detached house, El-Ghoul said, because the crew treats the neighbor’s crawl space as if it is an exterior wall. But rarely, rowhouses have shared crawl spaces. That presents additional issues — and cost.