Case file — Gaithersburg, Montgomery County, MD. One of the 25 jobs in our 250-home series. New here? Start with our open letter.
At a glance
| Location | Gaithersburg, Montgomery County, MD |
| The problem | A musty smell in the house that cleaning never fixed |
| Real cause | Mold growing on the framing in an open, humid crawl space — and openings letting outside air keep it that way |
| The fix | Mold treated and removed, penetrations sealed, ground lined and taped, insulation re-hung, dehumidifier with a dedicated drain line, and a humidity gauge left behind |
A smell is not a smell
When a house smells musty and no amount of cleaning changes it, the smell is not in the house. It is being delivered to the house.
Air moves upward through a home — warm air rises, leaves through the top, and pulls replacement air in from the lowest point it can find. In a house over a crawl space, that lowest point is the crawl space. Whatever the air down there smells like is what the first floor smells like a few hours later.
So the question is never “how do we get rid of the smell.” It is “what is the crawl space air doing, and why.”
What we found
Mold on the framing. Enough of it that this was not a wipe-and-go job: the crew worked in disposable protective suits, and the treatment was a two-part system — a cleaner followed by an activator — applied across the affected surfaces rather than spot-treated where it was easiest to reach.
Mold is not the disease. Mold is what grows when a surface stays damp long enough, which means the real finding was everything that was keeping it damp: an open crawl space taking in outside air, and unsealed penetrations where pipes, wires and ducts pass through.
What we did about it
Treated and removed the mold first. Before anything was sealed or lined. You do not close up a crawl space over active mold growth — sealing it in is how a small problem becomes a permanent one.
Sealed the openings. Foam board and expanding foam at the penetrations: the gaps around pipes, vents and wiring that let humid outside air in. These are the holes nobody looks at, and they are the reason a crawl space never dries out on its own.
Lined and taped the ground. A vapor barrier across the dirt floor, seams taped rather than overlapped and left loose, pinned down so it stays where it was put. Bare soil releases moisture into the air continuously. Covering it is the single cheapest thing you can do for crawl space humidity, and it is the step most often done badly — laid loose, untapped, and peeled back within a year.
Re-hung the insulation. Fiberglass batts supported properly with hangers rather than left sagging between joists. Insulation that has fallen is not insulating anything, and the gaps where it hangs down are where air moves.
Installed a dehumidifier with a real drain. Not a unit sitting on the floor with a bucket someone has to empty. It sits up on block, off the ground, with a condensate pump and a dedicated PVC line carrying the water out — so it runs unattended and the water it pulls out of the air actually leaves the building.
The part we like best: we left a gauge
A humidity gauge went in before we left.
Most contractors finish a crawl space job and hand the homeowner a feeling. A gauge hands them a number. They can look at it next week, next August, and two years from now, and know whether the space is still dry — without calling us, and without taking our word for anything.
If you want to know whether a crawl space job worked, that is how you find out.
What this job was not
This was not a full encapsulation. There is a version of this work with a heavier barrier carried up the walls, insulated wall liners and rim joist insulation, and it costs considerably more. This crawl space did not need it.
The diagnosis said the problem was moisture and mold, not heat loss — so the scope answered moisture and mold. We would rather solve the problem in front of us than sell the biggest system in the catalog to a house that does not need it.