Case file — Bethesda, Montgomery County, MD. One of the 25 jobs in our 250-home series. New here? Start with our open letter.
At a glance
| Location | Bethesda, Montgomery County, MD |
| Crawl space | 1,300 sq ft |
| The problem | Cold floors over the crawl space, and a house that never heated evenly |
| Real cause | A vented crawl space with bare walls and an open rim joist band — the floor was losing heat straight down, and outside air was carrying moisture in with it |
| The fix | Full encapsulation: sealed vapor barrier, insulated wall liner, insulated rim joists, two condensate pumps, and a dehumidifier |
| Time on site | Three days, Monday through Wednesday |
What we installed
The floor and walls — 1,300 square feet. A 20 mil vapor barrier, Class A fire rated, laid across the entire crawl space floor and carried up onto the walls, where it attaches to an insulated wall liner: cross-woven polyethylene with a closed-cell foam core and an aluminum-faced blanket on the back. The liner is mechanically fastened, not glued, so it cannot peel away from the block over time. It is set on thermal spacers rather than flat against the wall, so the insulation actually works instead of being pinned tight to cold block. Every seam — floor to floor, floor to wall, and around every pier — is sealed with waterproofing tape.
The rim joists — 230 linear feet. The rim joist is the band of framing that sits on top of the foundation wall, and in most DMV crawl spaces it is the single largest uninsulated surface in the house. We cut insulation to fit each joist pocket individually rather than stuffing a continuous roll across them, which is what leaves the gaps that make rim joists leak air in the first place.
Water management — two condensate pumps. Encapsulating a crawl space does not make water disappear; it changes where the water goes. Two condensate pumps move accumulated moisture out to a drain and to the exterior, so nothing collects on top of the new barrier.
Air management — one dehumidifier. A 70-pint-per-day unit with a MERV 11 filter. It is rated to serve 2,200 square feet against a 1,300 square foot crawl space — deliberately oversized, so it runs in shorter cycles instead of running continuously at its limit. That is the difference between a dehumidifier that lasts and one that burns out in three summers.
Why all four, and not just the plastic
Most crawl space quotes in this region are for one thing: a sheet of plastic on the dirt. That stops moisture coming up out of the ground, and nothing else. In a crawl space like this one, two other things are going on at the same time.
Heat is leaving the floor above. In an unfinished crawl space the joist bays and the block wall are the last uninsulated surfaces left in an otherwise insulated house, so the floor sits at crawl space temperature, not room temperature. You feel that in your feet in February and you pay for it every month.
And air is arriving from outside. A vented crawl space pulls in whatever the weather is doing. In a DMV summer that means warm, humid air landing on surfaces that are cooler than it is — which is the definition of condensation. Plastic on the floor does not stop that.
So the four parts each answer a different question. The barrier handles the ground. The insulated wall liner and the rim joist insulation handle the heat loss. The dehumidifier handles the air. The condensate pumps handle the water those last two produce. Leave one out and the other three work harder for a worse result.
One crawl space, three different phone calls
Homeowners call us about three separate things, and underneath they are almost always describing the same crawl space.
If it is not insulated, the floor is cold. That is the call we got here.
The same bare, vented space is also open to outside air and to ground water, so it is damp — and damp is what a musty smell actually is. That is a different homeowner, same crawl space.
And whatever opening let the water in will let a mouse in. Water and mice use the same gaps. That is a third homeowner, still the same crawl space.
So we do not price a crawl space by the complaint. We price it by what the space is. This homeowner called about cold floors; if they had called about the smell instead, the scope of work would have looked nearly identical.
What changes after
The floor above stops being the coldest surface in the house, which is the part homeowners notice first. The space itself goes from a dirt-floored, outside-air room under your house to a sealed, conditioned one — dry enough to store things in, and dry enough that the framing above it stays dry too.
Three days, start to finish.