Case file — Germantown, Montgomery County, MD. One of the 25 jobs in our 250-home series. New here? Start with our open letter.
At a glance
| Location | Germantown, Montgomery County, MD |
| Crawl space | About 540 square feet |
| The problem | Air moving into the house from the crawl space — drafts, uneven temperatures, and a space that never stabilised |
| The fix | Spray foam air-sealing on the walls, sealed encapsulation floor to wall, rim joist insulation, and foam inserts in both vents |
| Time on site | One day |
Insulation and air sealing are not the same thing
This is the distinction that costs homeowners the most money, so it is worth being plain about.
Insulation slows heat. It is a resistance value. It does nothing about air that is physically moving from one place to another.
Air sealing stops movement. If air can travel from the crawl space into the living space, it carries temperature, humidity and smell with it, and no amount of insulation in its path changes that. You can insulate a crawl space thoroughly and still feel it in the room above, because the air is going around your insulation, not through it.
Most crawl spaces are sold insulation. This one needed sealing first.
Spray foam on the walls
About 120 linear feet of spray foam along the crawl space walls.
Spray foam is used here for one reason above the others: it expands into the irregular gaps that a batt or a board cannot follow. Block joints, the seam where the wall meets the sill, the places where the foundation is out of plumb or the mortar has crumbled — those are the leak paths, and they are all different shapes. Foam goes where they go.
It insulates as well. But it was bought for the sealing.
Both vents, plugged
Two foam vent inserts, one in each vent opening.
A flexible foam panel cut to the opening and pushed in. It is a small line item that changes the physics of the whole space: a crawl space with open vents is outdoors, and a crawl space with sealed vents is indoors. Every other thing on this job depends on that being true first.
Then the barrier — and why the wall gets the thicker one
The floor of this crawl space was covered in a 20 mil vapor barrier. The walls were covered in a 90 mil one — four and a half times thicker.
People assume it should be the other way around. The floor is what gets walked on, so surely the floor needs the heavy material?
The two barriers are doing different jobs.
The floor barrier’s job is moisture. It stops water vapour rising out of the soil. It needs to be tough enough not to tear during installation and thick enough to stay sealed, and 20 mil does that.
The wall barrier’s job is heat. The heavy wall material here is foil faced: it reflects heat back into the space instead of letting it pass out through cold block. That is why it is built up thicker and finished differently. It is closer to insulation with a vapor barrier attached than to plastic sheeting.
Every seam — floor, wall, and where the two meet — sealed with waterproofing tape. A barrier is only as good as its joints.
Rim joists
About 120 linear feet, insulated into each pocket. Same reasoning as every other crawl space we do: the rim joist band is where the framing meets the foundation, and it is the largest uninsulated, un-sealed surface left in most houses.
One day
That is what a crawl space of this size looks like when the diagnosis is clean and the scope matches it: one crew, one day, no surprises.
The jobs that run three days are not bigger jobs. They are jobs with mold to treat first, insulation to remove first, or water that needs somewhere to go before anything can be sealed. This crawl space had none of those. It had air moving through it, and we stopped the air.