Demilec Attic Air Sealing Nationwide

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

Demilec attic air sealing in San Marcos typically runs $1,800-$3,400 for a complete top-plate and penetration seal on a 1,500-2,500 square foot home, and we stock Demilec-compatible open-cell foam, caulk, and gasket materials on every truck for same-day starts. Most homeowners figure out something’s wrong when their summer electric bill jumps $80-$140 over the same month last year, or when they notice the hallway near the attic hatch is noticeably warmer than the bedroom two rooms over. The R-value tells you how slow heat moves through the material - the blower-door number tells you whether any of that matters.

Call (737) 337-8049

Which Demilec Models This Applies To

Demilec’s product line for residential attic air sealing centers on their open-cell spray foam formulations, primarily the SEALECTION 500 and HEATLOK SOY 200 series, plus their companion one-component canned foam and acoustic sealant products used for detail work around top plates, electrical penetrations, and chimney chases. If your attic was sealed between 2012 and 2022, there’s a strong chance the original crew used Demilec foam applied at 1.5-3.0 inches on the attic floor perimeter, with cut-and-cobble rigid board over the top in some hybrid jobs we see around the San Marcos Historic District.

To identify what you’ve got, check the spray foam surface for a Demilec lot code stamped or stickered on an exposed edge, usually near the attic access hatch where inspectors look first. The foam itself should read 0.5-1.2 lbs per cubic foot density for open-cell Demilec products. If you see yellowed, crumbly foam or gaps where the foam has pulled away from the top plate, you’re looking at degradation that happens faster in Texas attics that hit 140°F in July and August. We also carry Reflectix radiant barrier tape and gasket materials for hybrid systems where foam and reflective insulation were combined.

Topside Attic Insulation San Marcos is an independent service provider, not affiliated with Demilec. We source OEM-spec Demilec foam and also stock equivalent-density open-cell products from Icynene and Johns Manville when the original formulation has been discontinued or when a homeowner wants to match existing foam without a full strip-out.

Signs Your Demilec Attic Air Sealing Is Failing

• Hot spots on the ceiling below the attic. When Demilec foam shrinks or detaches from the top plate, conditioned air leaks straight through the gap and warms the drywall from above. We find this most often in San Marcos homes built 1985-2005 where the original builder stuffed fiberglass batts and called it done.
• Dust streaking around ceiling registers and can lights. Air moving through leaks carries attic dust into your living space. If you’re wiping dark lines every six weeks, your attic is breathing into your house, not just losing air.
• Ice damming or heavy frost on nails protruding through the roof deck. Warm, moist air hits cold steel and condenses. In San Marcos, we don’t see ice often, but the rust rings on drywall around nail pops tell the same story - humid attic air is migrating where it shouldn’t.
• Your HVAC runs 20-30 minutes longer to recover from setback. The equipment isn’t broken; the building envelope is. We measure this with a blower-door test - a pressurization reading that shows, in one number, how much air your building envelope is losing.
• Neighbors with similar homes report bills $60-$100 lower. This one’s not technical, but it’s reliable. When three homes on the same Hays County cul-de-sac built by the same developer in 2014 show wildly different usage, the variable is almost always whether the attic was sealed before insulation went in.

Before

After

Replacement Cost & What’s Included

Most Demilec attic air sealing jobs in San Marcos run between $1,800 and $3,400 installed, with the spread driven by three factors: attic square footage, accessibility (truss vs. stick-framed, decked vs. open floor), and whether we’re sealing over existing insulation or removing it first to get to the top plates. A 1,200 square foot ranch with open attic floor and clear access to all top plates sits at the low end. A 2,800 square foot two-story with engineered trusses, a partial plywood deck for storage, and blown cellulose covering everything - common in the Blanco Vista area - pushes toward the higher figure because removal and reinstallation of insulation is labor-heavy.

Every quote under Haven Standard Clause 1 is a flat price with a written scope and written warranty before anyone lifts a tool. The price includes Demilec-compatible open-cell foam or equivalent, acoustic sealant for electrical and plumbing penetrations, gasket material for the attic hatch, and before-and-after blower-door numbers published on the job. We don’t start until you have that written price in hand.

DIY or Call Us?

There’s a short list of checks any homeowner can do safely: verify the attic hatch gasket is intact, look for daylight through the top plate from inside the attic, and confirm bathroom exhaust fans actually vent outside, not into the soffit. These take twenty minutes and a flashlight.

Where DIY goes wrong is applying spray foam overhead on a 110°F San Marcos afternoon, working around energized electrical boxes, or assuming that Great Stuff from the hardware store matches the density and adhesion of the original Demilec application. We’ve found gaps in homeowner foam jobs within six months because the product was incompatible with the substrate or applied too thin to achieve the rated air permeance. If your blower-door number was 3,800 CFM50 before and still reads 3,400 after your weekend project, you’ve spent money and time for a result that doesn’t show up on the bill.

We carry Demilec-compatible materials on every truck, run the blower-door test before and after, and hand you a single printed page with your air-changes-per-hour figure so the improvement is a number, not a claim. Call (737) 337-8049 for a free estimate - estimates include a written price before any work starts, always.

FAQs