Does Sealing The Crawl Space Keep Termites Out?

Sealing a crawl space is moisture work. It takes the damp out of a space that camel crickets, sowbugs and wood decay fungi depend on, and it makes the floor above it a better place to stand in February. What it does not do is stop eastern subterranean termites, and a liner run tight against the wood can hide the first evidence that they are there.

Both halves are worth knowing before anyone unrolls plastic under your house, because the second one tends to surface when a buyer's inspector is writing up the report. Here is what the building code asks for, what the damp was feeding, and where termite control sits in all of it.

What Sealing A Crawl Space Actually Changes

It changes where the water in the air comes from. Bare soil gives off moisture all year, and in an Ohio summer the humid air coming through the vents meets cool framing and condenses on it. A ground cover cuts off the first source. Sealing the vents and conditioning the space cuts off the second.

The code treats those as two different buildings. A vented crawl space under the Residential Code of Ohio, Section R408, needs 1 square foot of net free vent area for every 150 square feet of under-floor area. Cover the exposed earth with an approved Class I vapor retarder and that requirement drops to 1 square foot for every 1,500, provided the openings still give cross ventilation. The sealed version is its own section, R408.3, and it is more demanding: a continuous Class I vapor retarder over the soil with joints overlapped 6 inches and sealed or taped, edges carried at least 6 inches up the stem wall and attached there, plus a way of drying the space, either continuously operated mechanical exhaust at 1 cubic foot per minute for every 50 square feet of floor area or a supply of conditioned air sized the same way.

What contractors call encapsulation is the heavy duty end of that second option. The code also carries a housekeeping line that gets skipped, R408.5: the grade has to be cleaned of vegetation and organic material first. Scrap lumber, form boards and a decade of leaves blown in through an open vent are not something to roll plastic over.

The Pests That Do Leave When The Damp Does

A dry crawl space is genuinely hostile to a specific crowd: the animals that are down there for the humidity rather than for the house.

• Camel crickets. The jumping insect people find on the hatch door is a humidity specialist, and a dried out crawl space stops supporting a population. We went through the two different ones in camel crickets in Ohio basements.
• Sowbugs, pillbugs and millipedes. All three breathe in a way that ties them to damp ground, which is why they die within a day or two once they wander indoors. More on that in sowbugs and pillbugs in the house.
• Wood decay fungi. Not an insect at all, and often the real reason a joist is soft. Telling rot from insect damage is its own job, covered in termite damage or wood rot.
• Nothing with fur. A liner is not a seal against mice. A torn vent screen or a gap at the rim joist works exactly as well after the plastic goes down, as in where mice actually get in.

Why Termites Are Not On That List

Moisture matters to termites. University of Kentucky Entomology's guide to protecting a home against termites says they are attracted to moisture and are more likely to infest a building when the soil next to the foundation stays consistently moist, and it recommends a soil cover for exactly that reason, noting that the cover acts as a vapor barrier to reduce evaporation from the soil and condensation on joists and subflooring. So drying the space moves the odds, and the same guide still treats professional treatment as the prudent step for a structure with no history of it.

The clearest way to see the difference is to read what the code counts as termite protection. Section R318 lists the accepted methods: chemical termiticide treatment, termite baiting, pressure preservative treated wood, naturally durable wood, and physical barriers. A sheet of polyethylene over the dirt is not on that list, and it is not built like something that is. It is lapped at the seams, cut around every pier and pipe, and set on the soil rather than bonded into the structure.

The insect, meanwhile, does not need to chew through it. Subterranean termites travel underground and build shelter tubes over any surface they cannot eat, which is how they already cross poured concrete, block and steel. Ohio State University Extension's BYGL bulletin on the eastern subterranean termite rates Ohio's infestation probability as moderate to heavy and names it the most common and widely distributed termite in the continental United States. The same bulletin notes that a spring swarm is the most common way an infestation gets noticed at all, which tells you how quiet the rest of the year is.

What A Liner Hides From The Next Inspection

This is the part that turns up at a closing. The evidence an inspector reads in a crawl space sits mostly on vertical surfaces: shelter tubes climbing a foundation wall or a pier, damaged wood at the sill, and the band of framing where the floor meets the foundation. Encapsulation covers a good share of that on purpose, since carrying the liner up the stem wall is what the sealed detail calls for, and foam board or batts over the rim finish the job.

A wood destroying insect report has a place to record areas that were obstructed or inaccessible, and a crawl space finished without a thought for this can fill that section up. Our walk through what the NPMA-33 report says covers how that reads to a lender. The fix is simple, and it has to happen before the work rather than after: ask for a visible strip of foundation wall left uncovered below the sill, keep the access hatch usable, and keep the liner unsealed where it can be lifted for a look.

Before The Plastic Goes Down

Those clearance numbers are worth a look even if you never seal anything. The code commentary describes them as the minimum needed for adequate circulation under the floor, which is a statement about airflow rather than a promise that wood sitting at 19 inches is fine. Plenty of older houses around here were never built to that line, as we went through in whether older Ohio houses really get more pests.

So Is It Worth Doing?

For a crawl space that is genuinely wet, usually yes, as building work. Drier framing, fewer humidity pests, a warmer floor and a dehumidifier that is not running all summer are all real. Just file it where it belongs: a moisture project that happens to reduce one risk factor for termites, and no substitute for protection against them.

If your crawl space is on the list this winter, get a look at the wood before it disappears behind plastic. State Termite has worked on Northeast Ohio houses since 1933, out of North Canton and Hartville, and a termite inspection is the right first step on a space about to be closed up. Inspection fee waived with treatment. Start on our request a quote page or call 330-494-6199.