Underhill Crawlspace & Base · Fennimore Flats & the Reedmere · Est. 2002
Half the air in your house comes from under it. We make it worth breathing.
Stack effect draws crawlspace air up through every floor of the house — moisture, spores and the smell of wet clay included. Underhill seals the floor of that air with an eleven-mil liner, conditions it to a dialled humidity, and repairs the walls down there only when the readings insist: staples for bending courses, piers for quitting footings, drains for water that arrives anyway.
The liner — $4.60 the square foot
An eleven-mil reinforced vapor barrier, seams overlapped and sealed, mechanically fastened a foot up the walls, piers wrapped, ground sealed to the footing. The earth stops exhaling into your floor joists, and the crawlspace becomes a room instead of a hole.
The air — from $1,850 installed
A sealed crawlspace wants conditioning: a dehumidifier sized to the volume, drained to the sump, holding the dial at fifty-five percent or lower. Stack effect keeps drawing — but now it draws air you chose, at a humidity you set, through a filter you can change.
The walls — when the readings insist
Carbon-fiber staples at $154 across a bending course, helical piers at $1,875 under a footing the profile says is leaving, interior drain tile at $52 the foot where the water still arrives. Sealed air over a failing wall is a nice-smelling problem, so we measure before we liner.
The under-house, in figures
Sealed, conditioned, repaired
The rate sheet, read from the hatch
Prices from last season’s crawlspaces across the Flats and the Reedmere. The $290 under-house survey — moisture map, wall strings, slab where there is one — comes off any work the verdict names, and every line includes the cleanup, which down there is half the craft.
The Underhill sheet — 2026 season
The under-house survey$290 flat
Moisture map, wall strings, vent and hatch audit
Full encapsulation, 11-mil$4.60 / sq ft
Seamed, fastened, piers wrapped, sealed to footing
Crawlspace dehumidifier, sized & drainedfrom $1,850
Dialed to 55%, plumbed to the sump
Interior drain tile, crawl perimeter$52 / lin ft
At the footing, daylit to the 2+1 basin
Carbon-fiber staple, crawl wall$154 each
Across a bending course, skimmed flush
Helical pier, crawl access$1,875 each
Bracketed, torqued to refusal, logged
The under-the-house sequence
How a sealing runs
Five movements, headlamp to hatch — the same on a weekend cabin as on the Reedmere grain loft, and the wall work only happens if the strings say it must.
Day 0
The survey, on hands and knees
Moisture map of the ground and every wall, strings on anything that leans, vents and hatches audited, and the water path traced — ponding, wicking, or arriving through the wall. The verdict names the liner, the air, the walls, or a combination.
Days 1–2
The water first
Standing water managed, drain tile laid at the footing where the map says so, the basin fitted with the 2+1 treatment: twin pumps, battery, alarm. Sealing a wet hole is wrapping a leak, and we do not.
Days 3–4
The liner
Eleven-mil reinforced barrier, seams overlapped and taped, fastened up the walls, every pier wrapped and sealed, the hatch insulated and latched. The earth is taken out of your air supply in two days of crawling.
Day 5
The air
The dehumidifier sized to the volume, drained to the sump, the dial set to fifty-five. Vents sealed or converted, and a sensor readout where the household can see it — because a number on the wall is trust that never needs the hatch opened.
Month 12
The hatch visit
Back down with the same meters: ground moisture, wall readings, the dial’s log. The closing sheet goes beside the survey, the warranty renews against numbers, and the only thing that has changed is that nobody has thought about the crawlspace all year.
Fennimore Flats & the Reedmere, 2025 — 2026
Crawls we have sealed
Six under-houses from last season, photographed at the hatch visit a year on — dry, dialled and boring, which is the goal.
The smell went first, then the sneezing, then the inexplicable winter condensation on our floorboards — all of it one crawlspace, all of it one week of work. The sensor reads fifty-three on the kitchen wall and my wife checks it the way other people check the weather.
They found the wall bowing before they found the moisture — postponed the liner two weeks, stapled the course, and only then sealed. Two winters on, the closing sheets stack: wall unchanged, ground dry, dial steady. The hatch visit each year is the only time anyone goes down there at all.
Our “crawlspace” was eighteen inches of black mud and a pump that ran all March. They drained it, tiled it, sealed it and conditioned it in nine days, and the dehumidifier has not kicked on since October. I went down there last month with a torch out of pure curiosity — it looked like a laboratory.
Every other quote wanted to sell me the liner on day one. Underhill spent the first visit on their knees with a moisture gun, found the water arriving through the west wall, and laid drain tile before a single metre of barrier went down — then stapled the course the strings flagged, wrapped the piers, and sealed the lot. The hatch visit last spring read: ground 11%, wall unchanged, dial 54. Our upstairs windows stopped sweating the month they finished.
Before you open the hatch
Fair questions
The five the shop line hears most, seven to five, Flats-wide. Anything else, crawl@underhillbase.example reaches the crew between jobs. The $290 survey comes off whatever the verdict names.
Vents open or vents sealed?
Sealed, in this climate, nearly always. The old logic — let the crawlspace breathe — predates conditioned sealings: open vents down here admit humid summer air that condenses on the cool ground and becomes the moisture you were trying to vent out. A sealed, conditioned crawl holds a set humidity year-round instead of tracking the weather. The survey audits yours before anything changes.
What is stack effect, exactly?
Warm air rises and leaves the house at the top, and the replacement air is drawn in at the bottom — through the crawlspace, which is why a damp crawl announces itself in second-floor bedrooms. Sealing the ground and conditioning the air means the house inhales fifty-five percent humidity you set, instead of whatever the clay is exhaling. Half your air is the understatement; on still winter days it is more.
Can you repair walls down there, or just seal around them?
Repair them, and we measure before we liner so nothing gets sealed over. Staples at $154 suit crawl walls bending on sound footings; piers at $1,875 suit the ones following the ground down — the machine breaks down to fit a twenty-four-inch access, which is most Flats hatches. A sealed crawl over a failing wall is a nice-smelling problem, and we decline to build those.
How long does a sealing hold?
The liner is warranted ten years and typically outlives it; the dehumidifier is serviceable hardware with a five-year warranty and an annual check built into the hatch visit. What ages a sealing is water nobody managed — which is why the survey traces the water path before anything is ordered and the drain goes in first where the map says so.
Will it lower my bills?
Somewhat, and we will not promise a number — the floors stop conducting ground damp and cold, and the HVAC stops drying air the crawlspace keeps re-wetting. The survey’s before readings and the hatch visit’s after readings will show you the delta your house actually got, which is the only figure we would put our name on.
Surveys within the week · the sealing season runs April to October · (555) 037-7781 · crawl@underhillbase.example
Open the hatch on purpose
Tell us the age of the house, what the crawlspace smells like and whether anyone has been down there this decade. The $290 under-house survey maps the moisture and the walls, comes off any work the verdict names, and the map is yours either way. crawl@underhillbase.example