Demilec Vapor Barrier Installation: The San Marcos Owner’s Guide
We stock and install Demilec-compatible vapor barrier systems for crawl space and slab-on-grade homes in San Marcos, with most completed jobs running between $1,800 and $3,400 depending on square footage and existing moisture damage. Same-day installation is available when we have your dimensions and access conditions confirmed by phone. Most San Marcos homeowners discover their vapor barrier has failed when they notice a persistent musty smell after spring rains, find condensation pooling on the barrier surface, or spot mold blooming on floor joists during our humid late-summer months.
Which Demilec Models This Applies To
Demilec produces vapor barrier materials primarily through their closed-cell spray foam and complementary moisture-management lines, including the SEALECTION 500 and Heatlok HFO High Lift series commonly specified for Texas crawl spaces. The critical compatibility point is mil thickness: Demilec-recommended barrier installations typically call for 10-mil or 12-mil reinforced polyethylene, not the 6-mil builder-grade sheeting found in most San Marcos subdivisions built between 2015 and 2020.
To identify what you have, check for a printed spec label on any existing barrier edge, or measure thickness with a micrometer if you can access a clean corner without disturbing sealed seams. Demilec’s own vapor barrier specifications are designed to integrate with their foam products at the rim joist, so the transition detail matters: foam-to-barrier overlap should run 6 inches minimum, and we find many San Marcos installations stop at 2 inches or skip the seam tape entirely.
We install OEM-spec Demilec-compatible materials and quality aftermarket equivalents from Johns Manville and Owens Corning where the application permits. Aftermarket 12-mil reinforced poly meets the same permeance ratings at lower cost, but we default to Demilec-spec seam tape and mastic at foam transitions to maintain the intended system warranty.
Signs Your Demilec Vapor Barrier Installation Is Failing
- Condensation beads on the barrier surface. This means warm, humid exterior air is reaching the cool crawl space and hitting the plastic at dew point. In San Marcos, we see this most often in homes near the Blanco River corridor where the water table sits high and seasonal humidity pushes past 80 percent.
- Musty odor rising through floor registers. A intact vapor barrier blocks soil gas and moisture; when seams split or the barrier pulls away from walls, that smell is mold metabolites and possibly radon precursors moving upward through construction gaps.
- Visible mold on floor joists or sill plates. Wood moisture content above 20 percent sustains fungal growth. We bring a pinless moisture meter on every crawl space inspection and find readings of 28 to 35 percent in failed barrier situations, especially in older slab homes near the Texas State University corridor where perimeter drainage was never properly addressed.
- Barrier tearing at seams or pulling from foundation walls. The San Marcos clay soil shifts seasonally, and inadequate fastening or wrong anchor type lets the barrier drift. We use masonry-drive fasteners with polyethylene washers, spaced 12 inches on center, because adhesive alone releases within two hot summers here.
- Increased cooling bills with no equipment change. Wet crawl space air carries more latent heat load. Your AC works harder to dehumidify air that a proper barrier would have kept out. We document this with blower-door numbers: the crawl space is part of the building envelope whether you condition it or not.
Before
AfterReplacement Cost & What’s Included
Most Demilec-compatible vapor barrier replacements in San Marcos run $1,800 to $3,400 for crawl spaces between 1,200 and 2,400 square feet. The range reflects three factors: existing barrier removal and disposal, subfloor moisture remediation if joists show active mold, and whether we need to install a sump pit or perimeter drain as part of the same scope.
Every job includes: full removal of old barrier material, surface prep and debris disposal, installation of new 10-mil or 12-mil reinforced polyethylene with sealed seams and wall terminations, mechanical fastening at foundation walls, and integration with existing or new rim-joist air sealing. We also run a blower-door test when the crawl space connects to conditioned space through floor penetrations, so you get a before-and-after air-changes-per-hour figure.
Under our 365-Day Done Right Promise, any seam separation or fastening failure in the first year is repaired at no charge. The Haven Standard written scope and warranty are delivered before anyone enters the crawl space.
DIY or Call Us?
You can safely check your crawl space entry point, photograph any visible barrier damage, and measure approximate square footage with a tape. Do not enter if you smell gas, see standing water over electrical, or find mold coverage exceeding a few square feet without respiratory protection.
Where DIY goes wrong: vapor barrier installation looks like spreading plastic, but performance lives in the details. Seam overlap must run 12 inches minimum with compatible tape rated for damp substrates; wall fastening must survive clay soil heave; and the rim-joist transition to any existing foam demands fire-rated mastic where code requires it. We’ve pulled out three homeowner installations in San Marcos this year where the barrier was laid flat but unsealed, trapping more moisture than it blocked.
We’re not affiliated with Demilec, but we install their specified systems correctly. Call (737) 337-8049 for a free estimate with written price before any work starts.
FAQs
A properly installed 10-mil or 12-mil reinforced barrier in a San Marcos crawl space typically lasts 15 to 20 years, though clay soil movement and high groundwater in neighborhoods near the Blanco River can shorten that to 10 to 12 years if drainage is poor. Annual visual inspection of seams and wall terminations catches early failure. Call (737) 337-8049 and we’ll walk you through what to look for, estimates are free.
Small punctures or seam gaps under 6 inches can be patched with compatible polyethylene tape if the surrounding material is still supple and the subfloor is dry. Widespread embrittlement, mold on joists, or multiple tear points mean replacement is more cost-effective than chasing individual failures. We assess this on every site visit and will tell you straight if a patch will hold.
Most residential crawl spaces in San Marcos take one full day for removal and replacement, or two days if we pair it with rim-joist spray foam or subfloor mold remediation. We arrive with materials on the truck, so there’s no mid-job supply run. You’ll get a text when the crew is en route and a Documented Photo Record before we leave.
We stock 10-mil and 12-mil reinforced polyethylene, Demilec-spec seam tape, masonry fasteners with poly washers, and rim-joist transition mastic on every truck serving San Marcos. For Demilec spray foam integration at the rim joist, we coordinate material delivery to match the vapor barrier schedule. Call (737) 337-8049 to confirm compatibility with your specific Demilec foam installation.
The R-value tells you how slow heat moves through the material - the blower-door number tells you whether any of that matters.
Written by Wes Okafor, Owner, Topside Attic Insulation San Marcos at Topside Attic Insulation San Marcos, serving San Marcos and nearby areas.