Below grade
Sealing and Conditioning a Crawlspace
Encapsulating a crawlspace is dew-point work: every liner, foam board and dehumidifier decision either moves a condensing surface out of trouble or leaves it there.
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Read the Assembly Before You Touch It
Walk the space with a psychrometer and an infrared thermometer before a single roll of liner comes off the truck. Log dry-bulb and relative humidity at mid-span, convert to dew point, then shoot surfaces: foundation wall at mid-height and again at the footing, underside of the subfloor, exposed duct, cold water lines, pier faces, bare soil. Anything colder than that dew point is wetting right now, visible or not.
Two visits beat one. A December reading tells you where the heat is leaving; a July reading tells you where the moisture is landing, and those are rarely the same square foot. Note the gap between soil surface temperature and crawl air temperature — a wide spread in summer predicts exactly which joist bays will grow something before the encapsulation is finished.
Pin meters settle the argument. Drive probes into joists, girders, sill plate and subfloor sheathing at several depths, following the direct measurement methods of ASTM D4442; readings that hold near or above twenty percent point to an active source rather than a bad season. Mark those bays on a sketch — they become the punch list you check against next summer.
Summer Air on Cool Masonry
Foundation vents were drawn for a drying problem that only exists in a heating climate. Outdoor air at 80°F and 80 percent relative humidity carries a dew point near 73°F. Below-grade masonry and the soil beneath it track closer to the annual mean air temperature, commonly in the fifties or sixties across much of the temperate United States. Open a vent to that air and the wall becomes a condenser.
Damage reads like a map once you know what drew it. Staining and cupping concentrate downstream of vent openings; duct seams rust nearest the outside air; cold supply lines run wet from spring through September; fungal growth blooms on the joist faces that see the most air movement and the least warmth. Winter, by contrast, usually leaves the same crawlspace dry.
Model codes have followed the physics. The unvented, conditioned crawl space provisions of the International Residential Code allow a sealed space where a continuous ground vapor retarder is paired with an approved means of conditioning the air. Which edition applies, and what local amendments do to it, is set by the adopting jurisdiction — confirm with the building official before you spec the closure of a single vent.
Bulk Water Sets the Floor
No membrane outruns a downspout dumping at the foundation. Before anything gets sealed, chase the surface water: extend leaders well clear of the wall, restore positive grade away from the house, clear window wells, and cut any planting bed that traps runoff against the block. Liquid water under a liner holds the soil at saturation and pins the space's dew point to ground temperature.
Where the water table rises into the space, drainage comes before encapsulation. An interior perimeter drain tied to a sealed, lidded sump with a check valve and a dedicated circuit handles what grading cannot. Route the discharge far enough out that it does not return; a pump recycling its own water runs constantly and tells the owner nothing is working.
Efflorescence and a persistently damp block face signal capillary rise through the footing. Coatings applied to the inside of that wall rarely last, because the drive is coming from behind them. Plan to insulate and drain rather than to seal the masonry face, and let the liner and the dehumidifier carry the residual load.
The Ground Plane
Bare soil is the largest continuous vapor source in the assembly, and covering it is the single change that moves the whole space's dew point downward. Specify a reinforced polyethylene sheet meeting ASTM E1745, Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs; the classes in that standard separate products by tensile strength, puncture resistance and water vapor permeance, and puncture resistance matters more than nominal thickness on a rocky floor.
Prep earns the warranty. Rake out rubble, remove sharp stone, fill low spots that will pond under the sheet, and cut back debris around piers. Lap seams generously and tape the full run rather than spot-sealing; ASTM E1643, Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill under Concrete Slabs, is installation discipline worth borrowing here even though the concrete never arrives.
Terminate the liner up the wall and hold it with a termination bar bedded in sealant, not tape alone — tape on damp block releases in a season. Wrap piers and tape the collar. Where termite inspection is required, leave the exposed band the local code demands rather than covering it and arguing about it at final.
Sheet quantity, not sheet spec, is what stalls this stage on site — settle the roll count against the real floor area, wall run-up and lap allowance before the crew is standing on raked dirt.
The standard allowance most suppliers and estimating guides assume for ordinary work.
Liner needed
800.2 ft²
- Wall area
- 297.5 ft²
Running these inputs gives 800 ft² as the liner needed. Wall area carries the most weight in this calculation, at 298 ft². Currently reading for United States — pick a different market above and the figures re-cast accordingly.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Moving the Condensing Plane Into the Foam
Fiberglass hung against a foundation wall does nothing to relocate the dew point. Air-permeable insulation lets crawlspace air reach the cold concrete behind it, so the condensing surface stays exactly where it always was — on masonry, under a wet batt, out of sight. Board foam or closed-cell spray applied tight to the wall changes the geometry: the cold face is now foam, and the interior face runs warm enough to stay above the space's dew point.
Thickness is a climate-zone decision. Minimum R-values for below-grade walls come from the International Energy Conservation Code and the energy provisions of the International Residential Code as adopted and amended locally, so the number governing your job is whatever the authority having jurisdiction enforces. Run the assembly continuous from sill down to the liner, seal every board edge, and avoid leaving a thin lap at the top of the wall.
Resist adding an interior Class I vapor retarder over the foam in most climates. Foundation walls dry inward; a second low-perm layer on the warm side leaves incidental moisture no exit. The foam already sits low on the permeance scale measured by ASTM E96/E96M, Standard Test Methods for Water Vapor Transmission of Materials, and that is enough.
Wall foam earns its place only if the interior face lands above crawlspace dew point at design conditions, and that is a number to settle before the boards are cut, not after the batts are already wet.
Temperature at vapor barrier location
55.85 °F
PASSES — the temperature at this location (13.3°C) stays above the interior dew point (10°C), so condensation risk at the vapor barrier is low under these design conditions.
- Interior air dew point
- 50 °F
For the dimensions entered, expect a temperature at vapor barrier location of 55.9 °F. Of the working steps, interior air dew point dominates at 50 °F. Set for United States. The market selector changes the units and the trade terminology; any standard behind the formula is cited under sources.
Rim Joist and Sill: the Coldest Wood in the Box
The rim joist and sill sit at the top of the wall where warm interior air meets an uninsulated wood band with siding on the far side. Colder than the foundation below it during the heating season and rarely conditioned, that band collects condensation from indoor air pushing outward through the floor system, and it is usually the first framing to rot in an otherwise dry crawlspace.
Detail it with closed-cell spray foam or cut rigid board sealed on all four edges, bearing hard on the sill and lapping the top of the wall insulation. Thin foam with a batt stuffed over it recreates the original failure: the batt keeps interior heat off the wood while the foam is too thin to hold that face above dew point. Continuity beats nominal R-value at this joint.
Check two things before foam goes in — that the sill plate is dry and not already in decay, and that nothing gets buried that must stay accessible. Junction boxes, cleanouts, gas line unions and any required termite inspection band all need to remain visible after the crew leaves.
Air Leakage Carries the Water
Diffusion through materials moves a trickle; air carrying vapor through a hole moves a flood. A crawlspace connected to outdoors by open vents, a loose hatch and an unsealed band imports summer dew point faster than any dehumidifier can strip it. Tightening the enclosure is what makes conditioning affordable instead of a permanent line on the power bill.
Order matters on site. Close vents with cut rigid foam bedded in sealant or with masonry infill, gasket and latch the access hatch, seal plumbing and wiring penetrations through the subfloor, then chase duct seams and boot connections. Leave the hatch itself for last so the crew keeps a working opening while the messy sealing happens.
Verify rather than assume. A blower door run under ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, or ASTM E1827 confirms the enclosure actually tightened; a digital manometer reading between crawl and living space confirms the crawlspace now sits inside the pressure boundary instead of acting as a supply plenum for the house above it.
Holding Dew Point Under the Coldest Surface
One target governs the mechanical design: crawlspace air dew point must stay below the coldest surface in the space, in every season. That goal moves — the coldest surface is the rim joist in January and the foundation wall in August — so the control strategy has to hold year-round, not merely during the months the owner notices an odor.
Three approaches appear in code-recognised practice: continuously operating exhaust to outdoors, supply air delivered from the conditioning system with a path back to the house, or a dedicated dehumidifier. Exhaust works in dry climates and backfires in humid ones by drawing in the very air causing the trouble. Supply air needs a real return path, not a gap around a duct. Across mixed and humid climates the dehumidifier usually does the actual work.
Set the humidistat to hold in the low fifties percent relative humidity, then confirm the resulting dew point sits under your measured minimum surface temperature. Drain condensate by gravity where grade allows and by pump where it does not, run the line to daylight or a trapped fixture drain, stand the unit off the liner with filter access, and label the service interval on the housing. Whole-house ventilation rates remain governed by ANSI/ASHRAE Standard 62.2.
Soil Gas, Combustion Air, and Fire Protection
Sealing an enclosure changes the combustion picture. An atmospherically vented water heater or furnace sitting in a crawlspace loses its dilution and makeup air the moment the vents close, and a backdrafting appliance in a space directly connected to the floor above is a life-safety failure, not a callback. Test worst-case depressurization in the combustion appliance zone; where the appliance cannot be converted to sealed combustion or relocated, the encapsulation scope has to change.
Soil gas concentrates under a continuous membrane. Test for radon after the work as well as before, because the pre-existing number describes a space that no longer exists. Where results exceed the 4 pCi/L action level used by the U.S. EPA, a sub-membrane depressurization system installed to ASTM E2121, Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings, is the remedy — and roughing in the suction pit while the liner is open beats cutting it open later.
Foam plastics generally require a thermal or ignition barrier unless the specific product carries an approval for exposed installation, with surface burning characteristics established by ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials. Both the limits and the acceptable coverings come from the adopted building code and the local inspector, so confirm the detail before the wall gets closed in.
Commissioning and the First Cooling Season
Leave instrumentation behind. A logging thermo-hygrometer in the middle of the space plus a second sensor taped to the coldest surface identified on day one turns the next twelve months into evidence rather than opinion. Baseline the wood moisture readings at the same joist bays marked during the survey and hand the owner that sketch with the closeout package.
Return during the first humid stretch, not in the fall. Pull the log, compare recorded dew point against surface temperatures, and read the signatures of an incomplete job: a wet band along one wall means a liner termination let go; condensation on ductwork means the duct is uninsulated or leaking cold air into a space still running humid; a dehumidifier that never cycles means the enclosure is still open somewhere.
Documentation belongs in the deliverable. Photograph the liner before terminations are covered, record foam product and thickness, note the humidistat setpoint and condensate route, and write down which vents were sealed and by what method. The next trade into that space — plumber, pest inspector, HVAC tech — will cut the liner if nobody tells them what it does.
Crawlspace Encapsulation Takeoff
Quantities and checks to settle before the truck is loaded, each one tied to a surface whose temperature you already measured.
- Liner area: floor, wall run-up, pier wraps — Add lap allowance at every seam and the vertical run to the termination bar; piers and stub walls eat more sheet than the floor plan suggests.
- Seam tape, termination bar, sealant — Tape alone fails on damp block — mechanical termination bedded in sealant is the joint that survives.
- Rigid or closed-cell foam, wall and rim band — Thickness follows the locally adopted energy code; continuity from sill to liner matters more than the last half-inch of R-value.
- Vent and hatch closures — Count openings including crawl-to-garage and old coal doors; the hatch needs a gasket and a latch, not a leaned panel.
- Dehumidifier, stand and condensate route — Confirm a gravity drain or pump path and a serviceable filter before the unit is set; verify setpoint dew point against the coldest measured surface.
- Combustion and radon checks — Worst-case depressurization test on any atmospheric appliance, and a post-encapsulation radon test with the suction pit roughed in while the liner is open.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.
Drawn from
- ASTM E1745 — Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs
- ASTM E1643 — Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill under Concrete Slabs
- ASTM E96/E96M — Standard Test Methods for Water Vapor Transmission of Materials
- ASTM E779 — Standard Test Method for Determining Air Leakage Rate by Fan Pressurization
- ASTM E1827 — Standard Test Methods for Determining Airtightness of Buildings Using an Orifice Blower Door
- ASTM E2178 — Standard Test Method for Air Permeance of Building Materials
- ASTM E84 — Standard Test Method for Surface Burning Characteristics of Building Materials
- ASTM E2121 — Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings
- ASTM D4442 — Standard Test Methods for Direct Moisture Content Measurement of Wood and Wood-Based Materials
- ANSI/ASHRAE Standard 160 — Criteria for Moisture-Control Design Analysis in Buildings
- ANSI/ASHRAE Standard 62.2 — Ventilation and Acceptable Indoor Air Quality in Residential Buildings
- ASHRAE Handbook—Fundamentals, Psychrometrics chapter
- International Residential Code (as adopted and amended by the local jurisdiction)
- International Energy Conservation Code (as adopted and amended by the local jurisdiction)
- U.S. Environmental Protection Agency — indoor radon action level guidance
Guidance, not a specification. Local codes, the engineer of record and the product manufacturer’s instructions govern where they differ from anything written here.