Demilec Vapor Barrier Installation: The San Antonio Owner’s Guide
Demilec vapor barrier installation in San Antonio typically runs $1,100 – $2,800 depending on square footage, existing insulation condition, and whether the barrier is being laid in a crawlspace or an attic. We stock Demilec-compatible vapor barrier material on our trucks and can usually install same-day or next-day for most San Antonio homes. Most owners discover a vapor barrier problem the same way: a musty smell that won’t quit, higher-than-normal humidity inside, or actual moisture beading on surfaces where it never used to. If you’ve got a Demilec spray foam job in your attic or walls and the vapor barrier layer has failed, this page will tell you exactly what to do about it.
Which Demilec Models This Applies To
This guide covers vapor barrier installation for homes with Demilec spray foam insulation, most commonly the Heatlok HFO and Sealection 500 series products. Those are the two Demilec formulas we run into most often in San Antonio attics and crawlspaces. The vapor barrier function on these systems comes from the closed-cell foam itself acting as a Class II vapor retarder, but in many San Antonio homes, a separate polyethylene or foil-faced barrier is installed over the foam or under it, depending on the assembly.
The confusion we see a lot: Demilec doesn’t sell a standalone “vapor barrier” product line with its own model numbers. The vapor barrier is either the foam itself or a complementary membrane installed alongside it. If you have a foil-faced product, it’s often a Reflectix-style radiant barrier performing double duty as a vapor retarder, which is why we keep both Demilec and Reflectix materials on hand. The model-specific question is whether your system uses open-cell (Sealection 500) or closed-cell (Heatlok HFO) foam. Closed-cell is its own vapor barrier at two inches of depth. Open-cell needs a separate membrane, because it will let moisture pass through. That’s a distinction that drives a lot of the cost difference on these jobs, and it’s the first thing we check when you call.
Signs Your Demilec Vapor Barrier Installation Is Failing
- Musty or earthy smell in the attic or crawlspace. This is moisture getting past the barrier and into fiberglass batts, wood, or drywall. Once it smells, the barrier has already been compromised for weeks or months.
- Condensation on attic sheathing or rafters. Warm, humid San Antonio air is reaching the cold side of the assembly. That means the vapor retarder isn’t stopping what it should. Summer is when this shows up most clearly.
- Water stains on ceilings below the attic. Moisture has pooled and dripped through. This is a late-stage sign, and by now the barrier has been failing long enough to move liquid water, not just vapor.
- Insulation that feels damp or clumped. Fiberglass loses its R-value when it gets wet. Open-cell foam holds water like a sponge. Either way, you’re losing thermal performance and gaining mold-friendly conditions.
- Higher cooling bills without an obvious cause. A failed vapor barrier means your air conditioner is fighting latent heat and humidity, not just dry heat. Your unit works harder, and the bill shows it.
Replacement Cost & What’s Included
For San Antonio homes, here’s what we typically see on Demilec vapor barrier installation work:
- Crawlspace vapor barrier (6-mil poly, sealed at seams and edges): $1,100 – $1,800 for a standard 800 – 1,200 square foot crawlspace.
- Attic vapor barrier retrofit over existing insulation: $1,400 – $2,400 depending on attic accessibility, joist spacing, and whether old insulation needs removal first.
- Open-cell foam area where a separate membrane is needed: $1,800 – $2,800, which includes membrane material, seam taping, and fasteners.
Every quote we write includes the material, labor, disposal of any failed barrier material, and a written workmanship guarantee. You get a fixed price before we start, and the Relay Standard means you’ll see photo proof of the finished barrier before you pay. If it fails within 365 days, we come back and fix it at no charge. That’s not a maybe. It’s in writing.
DIY or Call Us?
You can inspect your own vapor barrier for visible tears, gaps at seams, or areas where it’s pulled away from the foundation wall. Those are safe things to look at. The risk in DIY comes when you try to install a new barrier in a crawlspace with San Antonio’s heat and humidity, or when you’re working around exposed nails, wiring, and potentially compressed fiberglass. A poorly sealed barrier will keep letting moisture through, and you won’t know it failed until the smell comes back. There’s also the question of getting the right material for your specific Demilec system: open-cell and closed-cell take different approaches. If you want us to handle it, call (254) 279-0971 and we’ll get you a written quote, usually within one visit.
FAQs
How long does a Demilec vapor barrier installation last? A properly installed vapor barrier alongside a closed-cell Demilec system should last the life of the foam, which in a San Antonio attic is realistically 20 to 30 years. Separate polyethylene membranes in crawlspaces usually last 10 to 15 years before punctures or seam failures add up. If yours is failing sooner than that, installation was likely the problem.
Can you repair a vapor barrier instead of replacing the whole thing? Small tears and seam gaps can be patched with the right tape and a compatible patch material, and we do that regularly when the barrier is mostly intact. If moisture has already gotten into the insulation or the barrier is brittle and coming apart, patching won’t hold. We’ll tell you honestly which one you’re dealing with. Call (254) 279-0971 for a straight answer.
How long does the installation take? Most crawlspace vapor barrier installations in San Antonio take one day, four to six hours on site. Attic retrofits usually run half a day to a full day depending on access. We’ll give you a live arrival window when you book, and you won’t be sitting around waiting on a four-hour guess.
Do you stock Demilec vapor barrier material or have to order it? Our trucks carry Demilec-compatible sealants, tapes, and 6-mil and 10-mil poly material, plus Reflectix for foil-faced applications. For most jobs, we can start the same day we give you the quote. If a specific Demilec accessory needs to be sourced, we’ll tell you before you commit, not after.
Written by Chase Whitaker, Owner, Laurel Attic Insulation San Antonio, serving San Antonio and nearby areas.