Demilec Attic Air Sealing Nationwide

Demilec Attic Air Sealing: The San Antonio Owner’s Guide

Demilec attic air sealing in San Antonio typically runs $450 – $1,100 installed, depending on how many linear feet of sealant and foam backer rod your attic needs. We stock Demilec Sealection 500, Heatlok HFO, and Demilec’s air-seal accessory kits on our trucks, so most jobs are done same day or next morning. The giveaway that your air seal has failed is almost always the same: an attic that smells like the garage, a bedroom that won’t hold temperature, or a utility bill that jumped 30% between June and August for no apparent reason.

Call (254) 279-0971

Demilec’s spray foam is durable stuff. But the air sealing around it, the canister-applied beads and membrane transitions at top plates, plumbing chases, and recessed light housings, breaks down faster than the foam itself. In San Antonio’s attic environment, where summer surface temps routinely push 160°F on the underside of the roof deck, expansion and contraction cycles work on every seam. What was tight in March can be a pencil-width gap by September. That gap is what we fix.

Key takeaways:

• Demilec air sealing installed in San Antonio: $450 – $1,100 typical, finalized with a written price before we start.
• We stock Demilec-compatible sealant and backer rod on the truck, same day in most San Antonio ZIP codes.
• The 365-Day Done Right Promise covers the work for a full year, with a free second visit if it fails.
• We’re an independent San Antonio insulation company, not affiliated with Demilec, and we’ll tell you honestly whether a section needs replacement or just touch-up.

Which Demilec Models This Applies To

Demilec is the brand behind two closed-cell and open-cell foam families; Sealection (open-cell, including 500 and Agribalance) and Heatlok (closed-cell, including HFO and XT). The attic air sealing component we’re talking about is not the full foam application. It’s the targeted sealing work at the attic floor plane: the perimeter of the attic, top plates, electrical penetrations, HVAC boot connections, and the junction where drywall meets framing. The material itself is Demilec’s low-pressure one-component foam, the same chemistry as their spray systems, but dispensed from cans or small-batch kits for precision Work.

If you’re matching a part, look for a label on the existing canister or invoice from the original install. Demilec’s air-seal foam is typically sold under the Demilec brand with a fire-resistance rating printed on the can. The compatibility gotcha is bead diameter: we see a lot of San Antonio attics where someone used a generic window-and-door foam instead of Demilec’s attic-specific low-expansion formula. The generic stuff over-expands and pulls away from the joint within two or three San Antonio summers. If your air seal foam is crumbling, cracking, or flaking when you touch it, it’s the wrong product for the application.

We stock both Demilec-branded sealant and a US-made aftermarket equivalent that meets the same ASTM E84 flammability rating and expansion profile. For attic air sealing in unconditioned spaces, the aftermarket is often the smarter buy: same performance, about 15% less cost, and done in one visit. We’ll match to your existing foam so the chemical bond is correct at every joint.

Signs Your Demilec Attic Air Sealing Is Failing

• Dust and fiberglass particles on top of your canister lights or bathroom fans. When attic air is sealed, no dust migrates into the living space. If you can write your name in the dust on a ceiling vent cover, your air seal has a breach somewhere nearby. Air pulls dust through any gap when you run a bathroom exhaust fan or the AC creates negative pressure in the attic.
• Upper-floor rooms in your San Antonio home run 5 – 8°F warmer than the rest of the house in July and August. Hot attic air stacks up at the top of the house and pushes through gaps at ceiling level. Sealed attics don’t produce that kind of room-to-room temperature difference.
• You smell the garage when the AC kicks on. The wall between your garage and the attic is a common air leak path. If Demilec’s air sealing was applied correctly, that path is closed. If you smell brake cleaner or lawnmower gas inside the house, it’s not.
• Your AC runs 45 hours per week or more during a hot June in San Antonio, and the house still hits 78° by 5:00 PM. Air sealing plus adequate insulation is what makes an AC cycle normally. Without it, you’re air-conditioning the attic alongside the living room.
• Fiberglass batts in the attic are turning dark at the edges. That dark staining is filtration dust; air moving through the fiberglass filters out fine particles. A properly sealed attic floor shows clean batts, even after ten years.

Replacement Cost & What’s Included

Attic air sealing in San Antonio, using Demilec-compatible sealant and backer rod, runs $450 to $1,100 for most single-story and two-story homes. Here’s how that range breaks down in the real world:

• $450 – $650: Small attics or spot-correction. Sealing 150 – 300 linear feet of top-plate seams, plumbing penetrations, and a handful of electrical penetrations.
• $700 – $900: Typical San Antonio ranch or two-story. Full attic-floor perimeter, 300 – 500 linear feet of sealing, plus foam gaskets at top plates and sealing around can lights, HVAC boots, and bath fan housings.
• $1,000 – $1,100: Large or leaky attics. Additional penetrations found, attic access hatch sealed and insulated, and multiple rooms needing correction from prior improper work.

Every estimate includes labor, material, and the written price before anything starts. We deliver that price by text or email from our truck, so there’s no door-step revision. The 365-Day Done Right Promise covers workmanship for a full year, and if the repair fails within that window, the second visit is free. That’s the whole deal.

DIY or Call Us?

Here’s what you can safely do: look. Pull down the attic ladder or peek through the scuttle hole. Look at the existing foam beads at the corners of the attic. If they’re intact, tawny-colored, and still spongy, you may not need us. If they’re cracked, separated from the wood, or someone used window-and-door foam and it’s shrunk to nothing, you need a professional touch.

We caution against DIY on this for one specific reason: using the wrong foam chemistry in a San Antonio attic creates worse problems than it solves. Over-expanding foam can bow your drywall from above. Under-expanding foam can leave a gap you can’t see from below because the recessed light housing blocks your sight line. And once the wrong product is applied, removal costs more than the original fix. Our crew does this work with the right Demilec-spec chemistry, and we can usually finish in a single afternoon. Call (254) 279-0971 and we’ll give you an honest read on whether it’s worth doing at all.

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