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Frequently asked questions
Short answers to the questions homeowners ask most, each with a link to the guide that explains the details and the exceptions.
About Below the Sill
Can you look at my home or recommend a contractor?
No. We’re an independent publication, not a repair company. We don’t inspect homes, give quotes, take repair requests or refer contractors. If a problem needs someone on site, contact a qualified professional in your area. Many of our guides say which kind of professional fits which problem.
Who writes the guides?
Robert Stulpt, a licensed contractor in Virginia who specializes in moisture, water and mold, writes them. Daniel Mercer, a civil engineering graduate with more than 17 years evaluating existing homes in Virginia, reviews the structural information. We use AI tools to help prepare first drafts; Robert checks, corrects and approves every guide before it’s published.
How we research, review and correct our guides
Why do some guides quote Virginia rules?
Our author and reviewer work in Virginia, so when a guide gives code numbers, such as egress window sizes, yard grading or crawl space dehumidifiers, it uses the Virginia residential code and says so. Other places may use a different edition or local amendments. Your building department has the final word on what applies to your home.
Are the calculators free? Do you keep what I enter?
They’re free. The calculators and checklists run in your browser, and nothing you enter is stored or sent to us.
I found a mistake. How do I tell you?
Email us at [email protected] with the page and what looks wrong. We’ll check it against our sources and correct the guide if needed.
Crawl spaces
What is crawl space encapsulation?
It turns a vented crawl space into a closed one: the dirt floor and walls are lined with a sealed vapor barrier, the vents and air leaks are sealed, and a dehumidifier or conditioned air keeps the space dry. It’s usually worth it when a crawl space stays humid, has exposed dirt or keeps getting condensation. It won’t fix water coming in, and a closed crawl space isn’t recommended in flood-prone areas.
How much does crawl space encapsulation cost?
In Robert Stulpt’s Virginia work in 2026, on crawl spaces of about 1,000 to 1,500 square feet, prices run from about $4,500 for a dry crawl space to $15,000 or more when there are water problems and a drainage system or sump pump has to be added. Two crawl spaces of the same size can differ by double the price, depending on the work, access and location.
Crawl space encapsulation cost
Should crawl space vents be open or closed?
It depends on how the crawl space is designed, not on the season. A vented crawl space should keep its vents open year round, with a vapor barrier on the ground. If you want to close them, do it as part of a closed crawl space, with a sealed liner and a way to keep the air dry. Simply blocking the vents usually causes more problems.
Basements
What humidity should a basement be?
Keep it under 60% relative humidity, ideally between 30% and 50%, which is what the EPA recommends for indoor air. In Robert Stulpt’s experience in Virginia, basements commonly sit between 45% and 55% in summer. One reading of 60% isn’t a worry; several days above it is.
How do I get rid of a musty smell in the basement?
A musty smell almost always means something is damp: stored boxes, carpet or the back of drywall. Find and dry the source and bring the humidity down; an air freshener won’t solve it. A sewer smell is different and usually comes from a floor drain whose trap has dried out. If you smell gas or rotten eggs near a gas appliance or line, leave the house and call 911 from outside.
How much does it cost to waterproof a basement?
Published national figures (HomeGuide, updated January 2026) put an interior French drain for a whole basement at about $4,000 to $17,000 on average, and exterior waterproofing at about $10,000 to $18,000. Before paying for either, check the gutters, downspouts and grading: they solve many leaks on their own.
Interior vs. exterior waterproofing
What is an egress window?
A window big enough, low enough and easy enough to open that someone can climb out of a basement or bedroom in an emergency, and a firefighter can get in. In Virginia, the residential code calls for at least 5.7 square feet of clear opening (5 square feet at or below grade), at least 24 inches high and 20 inches wide, with the sill no more than 44 inches above the floor.
Foundations
Are foundation cracks normal?
Many are. A hairline vertical crack in a poured concrete wall is often shrinkage from when the concrete cured. What matters is the crack’s direction and shape, whether one side has shifted past the other and, above all, whether it’s moving. Have a structural engineer look at a horizontal crack, a crack that’s growing or one with displacement.
Foundation cracks: when to worry
What does a horizontal crack or a bowing basement wall mean?
Usually that soil and water have been pushing on the wall from outside. It needs a structural engineer’s evaluation: no need to panic, but don’t wait years either. Repairs range from carbon fiber straps for small movement to wall anchors, steel braces or rebuilding, and none of them will last if the water isn’t dealt with.
Horizontal cracks and bowing walls
Water and drainage
What does a sump pump do?
It sits in a pit at the lowest point of a basement or crawl space. Water from the drainage around or under the foundation collects in the pit; when the level rises, a float switch turns the pump on and it pushes the water out through a discharge pipe, away from the house.
How long does a sump pump last?
There’s no guaranteed lifespan. From his field experience, Robert Stulpt starts watching a pump more closely once it reaches seven to ten years; some last much longer, and others fail sooner because they run constantly or the pit is full of debris. He tests pumps three or four times a year, especially before storm season.
What is a French drain?
A trench, usually gravel around a perforated pipe, that intercepts water moving through the ground and carries it downhill to an outlet. It helps when water travels through the soil toward a place you don’t want it. It does little for a short downspout or a clogged gutter, and without an outlet it’s just a trench full of gravel and water.
How far from the house should downspouts drain?
The Department of Energy’s building guidance says at least 5 feet. Robert Stulpt starts at 5 to 10 feet, depending on where the ground takes the water from there. A simple extension is usually the first fix; bury a line only when you need to.
Moisture and mold
What size dehumidifier do I need for a basement?
For a basement under 2,000 square feet, ENERGY STAR’s sizing chart calls for a unit rated at about 20 to 50 pints per day, depending on how damp the space is. Use one when the basement stays above 60% humidity and no water is leaking in, and give it a continuous drain.
What should a basement dehumidifier be set at?
Robert Stulpt usually sets basement dehumidifiers at 50% to 55%. Don’t set it to 35% on the idea that lower is better.
How do I get rid of mold in my basement?
Fix the moisture first; mold in a basement always points to one. If the area is small, the water problem is corrected and the material can be cleaned, you can probably do it yourself: the EPA’s guideline is about 10 square feet. Larger areas, sewage or flood water call for a professional. Remove the mold rather than just spraying it, because dead mold can still cause reactions.
Is black mold in my basement dangerous?
You can’t tell what a mold is, or how harmful it is, from its color. Some people react to any mold, with a stuffy nose, coughing, wheezing, burning eyes or a skin rash, and people with weakened immune systems or chronic lung disease can develop lung infections. The CDC says all molds should be treated the same way: find the moisture, fix it and remove the mold. If someone in your home has health concerns, talk to a doctor.
Didn’t find your question? Search the guides or contact us.