Demilec Vapor Barrier Installation: The San Antonio Owner’s Guide
We install Demilec vapor barriers in San Antonio attics and crawl spaces, with most jobs running between $1.40 and $2.60 per square foot installed, including removal of the old failed barrier and a written price before we touch anything. Most Demilec vapor barrier work we schedule is quoted within 24 hours and installed within the same week.
Most homeowners call us after they have spotted the telltale signs: condensation on attic ductwork, a musty smell coming from the crawl space, or peeling paint along ceiling edges where humid air is pressing through. The vapor barrier was doing its job quietly for years, and then one afternoon it stopped. That is usually how it goes: nobody thinks about the barrier until moisture shows up where it should not be.
Here is what you need to know about your Demilec vapor barrier, what fails first in a Texas climate, and what a proper replacement in San Antonio should cost.
Which Demilec Models This Applies To
Demilec does not make a standalone “vapor barrier” product in the way some manufacturers do. Their Heatlok HFO and Sealection 500 lines act as air and vapor barriers when sprayed to the right thickness, typically 2 inches or more of closed-cell foam depending on the assembly. If your attic or crawl space was insulated with Demilec spray foam, the vapor barrier is the foam itself, and a “failed vapor barrier” usually means the foam was installed too thin, has pulled away from the substrate, or was never thick enough for San Antonio’s humidity loads in the first place.
For homeowners with a separate polyethylene or reinforced vapor barrier paired with Demilec open-cell foam (like Sealection 500 in a vented attic), the failure point is almost always the poly sheeting: it gets punctured, torn at seams, or degraded by heat. Polyethylene breaks down faster in San Antonio attics than in cooler climates. A 6-mil poly barrier in a 140-degree Texas attic will become brittle within five to seven years, especially where it is exposed to direct sunlight or UV through vents. That is a lifespan reality most national spec sheets do not tell you.
When we quote a Demilec vapor barrier replacement, we match the barrier to the foam product already in your home. Heatlok HFO gets paired with a compatible substrate primer and a poly barrier rated for contact with closed-cell foam. Sealection 500 open-cell jobs get a Class I or Class II vapor retarder based on the attic’s ventilation. Tell us what is on your walls or roof deck, and we will match it. Neil Barron put it well once, walking a job on McCullough Avenue: “Tell me what’s in that room, and I’ll tell you exactly what goes on the other side of the wall.”
Signs Your Demilec Vapor Barrier Installation Is Failing
- Condensation on ductwork or roof decking in the attic. This is the most common first sign in San Antonio. Warm, humid air is passing through a gap in the barrier and hitting a cooler surface. If you see water beading on foil-faced ducts in July, the barrier is not doing its job.
- Musty or damp smell coming from the crawl space or attic hatch. A punctured vapor barrier lets ground moisture migrate upward. In San Antonio’s clay-heavy soil, crawl space humidity climbs fast after rain, and the smell follows within days.
- Peeling paint or staining on ceilings below the attic. Moisture trapped against the ceiling drywall will soften paint adhesion. This often shows up in hallways and closets first, where attic airflow is weakest.
- Visible tears, gaps, or sagging in the poly sheeting. If you can see daylight through a seam or the plastic has pulled away from a rafter, the barrier is compromised. Sagging means it was never stapled or sealed correctly.
- Insulation feels damp or has visible mold spots. Once moisture reaches the Demilec foam or fiberglass batt behind the barrier, you have a bigger problem. Mold on insulation is a removal job, not a patch job.
Replacement Cost & What’s Included
In San Antonio, Demilec-compatible vapor barrier installation runs from $1.40 to $2.60 per square foot for most attics and crawl spaces. A typical 1,500-square-foot attic comes in between $2,100 and $3,900. That price includes: removal and disposal of the old failed barrier, surface prep (wiping down contact points, checking for mold before we re-cover), the new barrier material matched to your Demilec foam, seam sealing with compatible tape or adhesive, and full clean-up with a photo of the finished work before we leave.
What moves the price? Attic height and access. An attic with standing room and a permanent staircase is faster than one with a scuttle hole and tight clearances along McCullough Avenue bungalow rooflines. Crawl spaces with low clearance add labor. If mold remediation is needed before the new barrier goes in, that is quoted separately, and we will tell you before any tool comes off the truck.
Every job gets the 365-Day Done Right Promise. If the barrier fails or pulls away within a year, we make it right. No debate, no fine print. And you will have the written price in hand before we start, never after.
DIY or Call Us?
What you can safely do: inspect the visible barrier for tears, check for condensation after a humid night, and take photos to document what you see. That helps us quote accurately over the phone. What gets homeowners into trouble: pulling back old poly sheeting without knowing what is behind it. If the Demilec foam has pulled away from the roof deck, you are not looking at a barrier problem anymore; you are looking at a spray foam adhesion problem that needs a trained eye. Same caution for mold: disturbing moldy insulation without containment spreads spores through the house. That is a health risk for kids and pets, not just a weekend inconvenience.
If the tear is smaller than a dinner plate and the surrounding foam is dry, a carefully taped patch can buy you a season. But in a San Antonio attic, that patch is a bandage. The heat cycles will stress the seam, and you will be back up there in six months. Call (903) 437-9164 and we will give you an honest read on whether it is patch-worthy or replacement time. The estimate is free.
FAQs
A properly installed vapor barrier paired with Demilec spray foam should last 10 to 15 years in San Antonio’s heat and humidity, though polyethylene sheeting in unconditioned attics may need replacement sooner, often in the 7-to-10-year range depending on UV exposure.
Small tears under roughly 12 inches can sometimes be taped and sealed, but the repair rarely matches the original barrier’s integrity. If the tear is larger, the seams are pulling, or moisture has already reached the insulation, replacement is the honest fix. Call (903) 437-9164 and describe what you see; we will give you a straight answer on repair versus replacement before we ever schedule a visit.
Most San Antonio attics and crawl spaces take one day, sometimes two for larger homes or when old barrier removal and mold inspection add time. We will give you a specific time window in your written quote.
Yes. Our trucks carry Demilec-compatible vapor barrier materials, seam tapes, and adhesives sized for typical residential attics and crawl spaces. We also stock Reflectix radiant barrier for homes that need a supplemental heat block alongside the vapor barrier. If your job requires a specific-mil poly or a specialty primer, we source it before installation day, not after.
If you are seeing moisture where it should not be, or your Demilec foam and barrier are not holding up to a San Antonio summer, call Thistledown Insulation San Antonio at (903) 437-9164. You will get a live person on the line, a written price before any work starts, and a crew that treats your home the way we would want ours treated: shoe covers on, drop cloths down, and a photo of the clean-up before the van pulls away.
Written by Neil Barron, Owner, Thistledown Insulation San Antonio at Thistledown Insulation San Antonio, serving San Antonio and nearby areas.