Demilec Attic Air Sealing: The San Antonio Owner’s Guide
If you’ve got a Demilec spray foam attic that’s leaking air around penetrations, can lights, or top plates, we stock the sealants and foam kits that match Demilec’s open-cell and closed-cell chemistry, and we install them for $350 to $1,200 in San Antonio depending on how many bypasses need attention. Most jobs are done in a half day, and we keep the common materials on the truck, so same-week scheduling is the norm, not the exception. You’ll usually know something’s wrong when you get a utility bill that doesn’t match the thermostat setting, or you feel a draft near the attic hatch in January, or you notice dust collecting faster than it used to around ceiling fixtures. That’s air moving through the attic when it shouldn’t be.
One thing worth saying up front: Walt’s Insulation is an independent San Antonio contractor. We’re not a Demilec dealer and not affiliated with the company directly, but we’ve worked with their Heatlok and Sealection product lines for years and we know how they behave in South Texas attics.
Which Demilec Models This Applies To
The air sealing work we do ties most closely to two Demilec families. Sealection 500 is the open-cell, low-density foam you’ll find in vented attics where the installer foamed the floor or the roofline to cut radiant and convective heat. Heatlok HFO is the closed-cell product used where an unvented attic needs an air and vapor barrier in one pass. Both create their air seal by expanding into gaps, but they fail differently, and the fix isn’t interchangeable.
Here’s the compatibility gotcha most people miss: you cannot topcoat a closed-cell Demilec failure with an open-cell foam and call it sealed. The closed-cell foam acts as a vapor retarder, and layering open-cell over it can trap moisture at the seam, which in a San Antonio attic means mold conditions inside a year. If you’ve got an older Heatlok installation that’s cracked or pulled away from the top plate, the repair usually involves reefing out the failed section and re-spraying closed-cell, or using a Demilec-approved sealant for smaller gaps. We match the chemistry to what’s already in your attic. For small penetrations, a quality polyurethane or latex air sealing product does the job for under $50 in materials. For a failed foam section larger than a dinner plate, you’re looking at a re-spray, which is a different conversation.
Signs Your Demilec Attic Air Sealing Is Failing
- Ceiling stains around can lights or ceiling fans. This is the big one. Air leaking from the house into the attic carries humidity, and in winter that moist air condenses on the cold underside of the roof deck. The stain on your ceiling is where the air is leaving the room below, not where the moisture ends up.
- A bedroom that won’t hold temperature. If one room runs five degrees warmer than the rest of the house in August, there’s a good chance the top plates above that room were never sealed, or the Demilec has pulled away from the framing. Foam shrinks slightly as it cures, and on a hot San Antonio afternoon that gap opens just enough to matter.
- Dust accumulating faster than it used to. Air moving through the attic carries insulation fibers and decades of accumulated dust with it. If your ceiling fixtures have a gray halo around them, air is being pulled through the attic and into living space.
- A spike in your CPS Energy bill with no change in usage. The attic floor is the single biggest air exchange surface in most San Antonio homes. When the seal goes, your conditioned air has a freeway out the top.
- Cold drafts in winter near the attic hatch or pull-down stairs. The hatch is the most overlooked bypass in the house. Even a well-sealed Demilec attic leaks if the hatch isn’t weatherstripped and insulated. We check this on every air sealing job, whether the foam itself is failing or not.
Replacement Cost & What’s Included
For a typical San Antonio single-story home between 1,500 and 2,500 square feet, expect to pay $350 to $600 for targeted air sealing around penetrations, top plates, and the attic hatch. That includes materials, labor, and a full inspection of the attic floor. If we’re opening up sections of failed Demilec and re-spraying, the range moves to $700 to $1,200 depending on how much square footage needs to come out. You’re mostly paying for the labor and the prep work, because removing old foam without damaging the drywall below is slow, careful work. We don’t rush it.
Here’s the breakdown in plain numbers:
- Targeted air sealing (penetrations, top plates, hatch): $350-$600 installed
- Failed Demilec removal and re-spray (small section): $700-$900 installed
- Failed Demilec removal and re-spray (larger area or multiple sections): $900-$1,200 installed
- Materials-only for a capable DIYer doing small gap sealing: $30-$75
Every job we do comes with the Done Right Promise, which is a lifetime workmanship guarantee. If the air sealing fails because of how we installed it, we come back and make it right. No runaround, no fine print. And right now, there’s $40 off any repair over $200, and a permanent 10% discount for seniors and veterans on every job, every time.
DIY or Call Us?
There’s a narrow band of this work that a careful homeowner can handle safely. If you’ve got a small gap around a plumbing vent or a ceiling fixture in an accessible part of the attic, and you can reach it without walking on the joists, a can of foam sealant from the hardware store will do the job. The key is to read the Demilec product label on what’s already in your attic and match the foam type. Open-cell over closed-cell is a mistake that causes moisture problems down the road, and in San Antonio’s humidity, that mistake compounds fast.
What we wouldn’t suggest you tackle yourself is anything involving a re-spray, any work that requires moving insulation to reach top plates, or anything anywhere near recessed can lights that aren’t IC-rated. Foam around the wrong fixture is a fire hazard, and walking a truss system you don’t know well is how people put a foot through the ceiling. If you’re not sure what you’re looking at, call us at (866) 751-4428 and describe what you see. We’ll tell you over the phone whether it’s worth a visit.
FAQs
Properly installed Demilec open-cell and closed-cell foam should hold its air seal for 20 to 30 years, but the weak points in a San Antonio attic are usually the transitions where foam meets framing, and those can start to fail in 10 to 15 years as the wood expands and contracts through our hot-humid summers and short cold snaps.
If you’re seeing symptoms earlier than that, it’s usually an installation issue, not a product issue. Most of the Demilec failures we see aren’t the foam itself, they’re where someone skipped a corner or sprayed too thin. Call (866) 751-4428 and we’ll inspect it and give you an honest read on what’s actually wrong.
Yes, and most of the time repair is exactly what’s needed. Small gaps and cracks can be sealed with a compatible sealant or a targeted foam fill without touching the rest of the attic. Full replacement is rare, and we’ll never sell you more than the problem calls for. Call (866) 751-4428 for a straight answer and a free estimate, and we’ll also give you a no-obligation second opinion on any written estimate you’ve already received.
Most Demilec attic air sealing repairs in San Antonio are done in 3 to 5 hours, which means we’re typically in and out the same day. Larger re-spray jobs or work that requires removing significant amounts of existing foam can stretch into a two-day job, but we’ll tell you the honest timeframe before we start. You can call (866) 751-4428 to schedule, and a live person will answer, 24/7, not a voicemail.
We keep the common Demilec-compatible sealants, foam kits, and spray equipment stock on our trucks when we come out to San Antonio homes, along with materials from other manufacturers we work with like Owens Corning, Icynene, and Reflectix. That means most jobs can start the day we arrive. If we need something specific to your attic, we’ll tell you before we come out, so there’s no wasted trip. We answer the phone live, talk to a real person, get a straight answer.
Let’s Get Your Attic Sealed Right
If your Demilec attic air sealing is failing, or you suspect it is, the fastest way to a straight answer is a phone call. We’re the one down the road, locally owned and owner-operated in San Antonio since 2000, and we don’t outsource anything to a call center. A live person answers every call, 24/7, and we’ll talk through what you’re seeing before anyone shows up at your door. Call (866) 751-4428 for a free estimate, and ask about the $40-off repair special and the 10% senior and veteran discount while you have us on the line.
Written by Walt Brenner, Owner, Walt’s Insulation at Walt’s Insulation, serving San Antonio and nearby areas.