Demilec Attic Air Sealing Nationwide

Demilec Attic Air Sealing: The Palo Alto Owner’s Guide

Demilec attic air sealing in Palo Alto typically runs $1,800-$3,400 installed for a standard single-story attic, and we stock the common Demilec-compatible spray foam kits and two-component sealants on our trucks for same-day starts. Most homeowners notice the problem when their PG&E bill spikes in a winter month or when a home energy audit flags thermal bypasses at the top plates. Seal the leaks first. The insulation has nowhere to work if the air is moving around it.

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Which Demilec Models This Applies To

Demilec’s attic air sealing line centers on their Heatlok HFO Pro and Heatlok XT series, both closed-cell spray foams used to seal top plates, electrical penetrations, and chase openings in attic floors. The HFO Pro is the lower-density formulation (2.0 lbs/ft³) most commonly specified for residential attic air sealing in Palo Alto’s climate zone, while the XT variant runs denser (2.5 lbs/ft³) and gets called for cathedral ceiling assemblies or where local code requires a higher R-value per inch. You can identify which was used in your home by the product stamp on any exposed foam surface, or by the job documentation if the previous contractor left it. The label reads “Demilec Heatlok” followed by the series suffix and a date code.

We are an independent service provider with no affiliation to Demilec. We source OEM Demilec material when the specification calls for it, though for most attic air sealing repairs we use equivalent two-component closed-cell foams from our stocked supply, including Icynene and Johns Manville formulations that match the 2.0-2.5 lb density range and achieve the same air permeance rating (less than 0.02 L/s·m² at 75 Pa). The compatibility gotcha in Palo Alto’s older stock, particularly the ranch homes built 1950-1975 near Midtown, is that some original Demilec applications were done as thin flash coats (1/2 inch or less) over unsealed joints. Those flash coats crack at the stud-to-plate junction after 10-15 years of thermal cycling. A proper repair removes the failed flash and builds back to a minimum 1-inch pass that bridges the framing member fully.

Signs Your Demilec Attic Air Sealing Is Failing

• Drafty rooms below the attic, especially in January. Warm air rises through gaps in the top plate, pulling replacement air through windows and doors. The Demilec seal was supposed to stop that stack-driven exfiltration, and when it cracks, the house breathes at the wrong height.
• Insulation that looks stained or compressed directly above exterior walls. Air moving through a failed seal carries humidity and dust; over time it mats down fiberglass batts or discolbles blown-in cellulose. The insulation is not the problem. The leak above it is.
• Ice damming or concentrated frost on the underside of roof sheathing. In Palo Alto’s occasional hard freezes, warm attic air escaping through a failed Demilec seal condenses against cold plywood. The frost melts in the morning sun and wets the sheathing. We see this pattern most in the older Eichler and ranch neighborhoods where attic ventilation is already marginal.
• A blower-door test that shows little improvement after insulation was added. If a contractor blew in new insulation without air sealing first, the Demilec (or its absence) is the likely culprit. The house still leaks; it just leaks through a thicker layer of material.
• Hot ceiling patches in summer afternoon. Infrared cameras show warm stripes along the top of exterior walls where attic air is convecting around a failed seal and heating the drywall from above. The Demilec was meant to break that thermal bypass.

Replacement Cost & What’s Included

Most Demilec attic air sealing repairs in Palo Alto fall between $1,800 and $3,400 for a completed job, with the range driven by attic square footage, accessibility, and whether we are sealing over existing insulation or removing it first to get to the deck. A typical scope includes: removal of failed or inadequate existing foam, mechanical cleaning of the substrate, application of new closed-cell spray foam to all top plates, electrical and plumbing penetrations, chase openings, and the attic access hatch perimeter. We also seal can light housings with rated covers where accessible, and we rebuild attic access panels that do not close tight against weatherstrip.

Every job gets a written price before work starts, per Haven Standard Clause 1. The price you approve is the price you pay. We include a Documented Photo Record showing every sealed location, and on jobs where a blower-door test is scoped, we provide the pre- and post-installation CFM50 readings so you can verify the reduction in envelope leakage. The 365-Day Done Right Promise covers all completed air sealing work.

DIY or Call Us?

A homeowner can safely check the attic access panel for gaps, look for cracked or yellowed foam at visible penetrations, and note any stained insulation. Do not disturb vermiculite insulation, and do not attempt to apply two-component spray foam yourself. The chemicals require respirator-rated PPE, and the expansion pressure of closed-cell foam can bow framing or displace wiring if the pass is too thick. More critically, any attic work near gas-fired equipment, including furnaces and water heaters, risks depressurizing the combustion zone. Wes Okafor runs a combustion-safety check on every gas appliance in the attic space before the job closes, because depressurizing a house without verifying flue draft first is how carbon monoxide problems start.

For a written price and a scope that addresses what we actually find, call (650) 352-3577. Estimates are free, and we answer live 24/7.

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