The Vented Crawl Space Myth
For decades, builders put vents in crawl space walls because the theory said outside air would carry moisture away. In dry climates, that sort of works. In Washington, DC, it backfires. All summer, those vents pull hot, humid air into your cool crawl space. The moisture condenses on the cold ducts, pipes, and joists — like a cold glass sweating on a July day. The vents are not drying your crawl space. They are wetting it.
What Sealing Involves
We seal every vent from the inside with rigid foam board, cut to fit and sealed airtight at the edges — not stuffed with fiberglass, which just gets damp. We also seal the other air leaks people forget: gaps around pipes and wires, the crawl space access door, and any holes in the foundation wall. The goal is simple: stop uncontrolled outside air from entering the space.
Sealing Is Half the System
Sealing the vents without controlling humidity just traps damp air inside. Vent sealing works as part of a sealed system: vents sealed, vapor barrier down, and a dehumidifier managing the air. That is the full encapsulation package. If you are doing it in stages, seal the vents and add the dehumidifier together — that pairing alone transforms most DC crawl spaces.
What About Combustion Appliances?
Good question, and we check it on every job. If your furnace or water heater is in the crawl space and burns gas or oil, it needs combustion air. Sealing the vents changes how that air gets in. We make sure any fuel-burning appliance still gets the air it needs safely before we seal anything — this is a standard part of our inspection.
How It Works: Our Process
Step 1 — Free inspection. We count the vents, check for combustion appliances, and find the other air leaks.
Step 2 — Seal. Rigid foam in every vent, sealed gaps, weatherstripped access door.
Step 3 — Pair with drying. We confirm the humidity plan — usually a dehumidifier — so the sealed space stays dry.
Foundation Vents vs. Everything Else: What Gets Sealed
We seal the foundation vents — the rectangular openings around the crawl space walls that were supposed to ventilate it. We do not seal gable vents, roof vents, or anything in the attic; those belong to a different system. We also do not seal the dryer vent or any exhaust fan duct — those must vent outside, and if a previous owner dumped the dryer into the crawl space (it happens more than you would think), we reroute it outside as part of the job. Sealing is surgical: the crawl space vents close, everything else stays exactly as it should.
The Access Door: The Biggest Leak
After the vents themselves, the leakiest opening in most crawl spaces is the access door or hatch — often a loose piece of plywood with daylight visible around the edges. We treat it like an exterior door: weatherstripping, a proper fit, and insulation on the inside face. On outdoor access doors we make sure the seal keeps both air and bulk water out. It is a small piece of the job that makes a real difference in how tight the final system is.
Should I Open the Vents in Summer and Close Them in Winter?
This was the old advice, and it is backwards for DC. Summer is when outside air is wettest — opening vents in July invites the most moisture of the year. Winter air is dry, so winter ventilation was never the problem. The old open-in-summer routine guaranteed maximum dampness exactly when the crawl space was most vulnerable. Sealed year-round with humidity control beats seasonal vent-fiddling in every measurement that matters.
Call (301) 679-9193 for a free crawl space inspection.