Demilec Vapor Barrier Installation: The Seguin Owner’s Guide
Demilec vapor barrier installation in Seguin typically runs $1,800-$3,400 for a full crawl space or basement application, depending on square footage and whether the existing barrier needs removal first. We stock Demilec-compatible 10-mil and 15-mil reinforced polyethylene barriers on our trucks, and most Seguin jobs can start same-day or next-day once we have your written price approved. Most homeowners realize something’s wrong when they notice a persistent musty smell coming up through the floors or see condensation droplets collecting on ductwork in the crawl space during our humid spring months.
Which Demilec Models This Applies To
Demilec doesn’t sell vapor barrier as a standalone branded kit the way they package spray foam systems. What we’re talking about here is the vapor barrier component installed alongside Demilec open-cell or closed-cell spray foam jobs, particularly in crawl space encapsulations where the foam seals the rim joist and the barrier handles ground moisture. The key identifier is the existing barrier’s mil rating and seam tape - Demilec-specified jobs in Seguin typically use 10-mil cross-woven polyethylene with butyl-backed seam tape, not the 6-mil big-box variety that degrades in our alkaline Hill Country soil.
To identify what you have, look for a white or silver label on the barrier edge noting the mil rating, or check your original job documentation for “Vapor Barrier: 10-mil reinforced” or “15-mil string-reinforced.” If your home was built between 2015 and 2022 in the Park Heights or River Oaks areas near Guadalupe Regional Medical Center, there’s a decent chance we installed the original barrier and have your spec on file. OEM-equivalent barriers from Poly-America or Raven match Demilec’s permeance specs (.03 perms or lower) without the proprietary markup. We stock both on the truck because the 15-mil holds up better where crawl space access requires dragging tools across the surface.
Signs Your Demilec Vapor Barrier Installation Is Failing
- Musty odors rising through floor registers. When the barrier tears or the seam tape fails, soil gas and moisture bypass the seal and get drawn into your HVAC return. In Seguin’s clay-heavy soils, this usually starts at the perimeter where the barrier meets the foundation wall.
- Condensation on ductwork or plumbing lines. A properly installed vapor barrier keeps relative humidity below 60% in the crawl space. If you’re seeing sweat on cold ducts in July, the barrier has likely pulled away from piers or developed pinholes from rodent traffic.
- Soft or cupping hardwood floors above the crawl space. Moisture migration through a compromised barrier raises the wood moisture content in your subfloor. We’ve measured 18% moisture content in Oak Park area homes where the barrier failed - 12% is the target.
- Visible mold on floor joists or insulation. Demilec spray foam on rim joists is mold-resistant, but organic debris on a wet barrier surface isn’t. White or green spotting on the barrier itself means water is pooling underneath from a puncture or inadequate overlap at seams.
- Increased energy bills with no HVAC changes. A failed barrier forces your system to dehumidify air it shouldn’t be touching. We’ve seen blower-door readings jump 300-500 CFM50 in Seguin homes where crawl space moisture load increased due to barrier failure - the envelope is leaking humidity, not just air.
Before
AfterReplacement Cost & What’s Included
Most full vapor barrier replacements in Seguin run $1,800-$3,400, with smaller partial repairs starting around $650. The range depends on three things: crawl space square footage (800 sq ft vs. 2,000+), whether we need to remove and dispose of the old barrier, and how many piers or penetrations require custom cutting and sealing. A typical job includes the reinforced polyethylene barrier, butyl seam tape, mechanical fasteners at the foundation wall, and sealing around all piers and ductwork penetrations.
We don’t start work without a written price - Haven Standard Clause 1 - and that written scope includes a 365-Day Done Right Promise on the installation. You’ll get a Documented Photo Record showing the old barrier condition, the prepped surface, and the completed installation. On encapsulation jobs where we’re also sealing rim joists, we run a blower-door test before and after so you can see the actual air leakage reduction. The report goes home with you - the numbers do the talking.
DIY or Call Us?
There’s nothing inherently dangerous about laying polyethylene sheeting, but there’s plenty that goes wrong. A homeowner can safely check for obvious tears, pulled fasteners, or failed seam tape - crawl in with a flashlight and look for gaps where the barrier meets the wall or around piers. What you can’t see is whether the soil beneath is properly graded to drain, whether your dehumidifier is sized correctly for the space, or whether that “small tear” is actually a symptom of hydrostatic pressure pushing water through the foundation.
We’ve found dead snakes, active wasp nests, and structural rot that homeowners missed because they were focused on the barrier surface. If you’re in Seguin and suspect failure, call (830) 549-6646. We’ll give you a free second opinion on any written estimate you’ve already received, and we’ll put our own written price in your hand before anyone grabs a tool.
FAQs
A properly installed 10-mil or 15-mil reinforced barrier typically lasts 15-20 years in Seguin’s climate, though our alkaline soil and occasional flooding along the Guadalupe River watershed can shorten that to 10-12 years if drainage is poor. Call (830) 549-6646 for an exact assessment of your crawl space conditions - estimates are free.
Small punctures or isolated seam failures can be patched with compatible butyl tape and additional polyethylene, but widespread embrittlement, rodent damage, or multiple tears mean replacement is more cost-effective than chasing individual failures. We carry patch kits on the truck for minor fixes, but we’ll tell you honestly if you’re throwing good money at bad material.
Most residential crawl spaces in Seguin take one full day to remove the old barrier, prep the surface, and install the new one with proper sealing at all penetrations. Larger homes or those with restricted access - common in the older neighborhoods near downtown - may extend to a second day.
Yes - we carry 10-mil and 15-mil reinforced polyethylene barriers, butyl seam tape, and mechanical fastening systems on every truck, along with Reflectix radiant barrier materials for jobs that need both moisture and heat control. We’re an independent service provider, not affiliated with Demilec, but we match their published permeance and durability specs on every installation.
Written by Wes Okafor, Owner & Lead Insulation Technician, Topside Attic Insulation Seguin at Topside Attic Insulation Seguin, serving Seguin and nearby areas.