Attic air sealing

Insulation slows heat. Air sealing stops air.

Think of attic insulation as a sweater and air sealing as a windbreaker. A thick sweater does a lot on a still day and very little in a stiff wind, because the wind goes right through it. Your attic works the same way. Warm air in a house rises and pushes out through every gap in the ceiling, and outdoor air gets pulled in low to replace it. That flow carries heat and dust straight through loose insulation, no matter how deep it is.

Air sealing closes those gaps on the attic floor, so the insulation on top can do its job. It's quiet, unglamorous work done with caulk, foam, sheet metal and rigid board, and once the insulation goes back down, nobody will ever see it. That's also why it has to happen first.

What it isn't: sealing the attic itself. The attic above the insulation stays ventilated. We seal the line between your rooms and the attic, not the vents that let the attic breathe.

Where the leaks are

ENERGY STAR's attic air sealing guide lists the usual suspects, and Bay Area attics have all of them:

• Top plates and wiring holes. The tops of interior walls are open to the attic, and every wire and pipe that drops into a wall leaves a gap around it.
• Plumbing vents and pipe chases. The stack from your bathroom and kitchen often has a gap you could put a hand through.
• Dropped soffits. Boxed-out ceilings over kitchen cabinets or a hallway are often open to the attic on top, and they're frequently the biggest single hole in the ceiling.
• Recessed lights. Older "can" lights are basically chimneys. Kitchen and living-room remodels tend to add them by the dozen, and the older ones were rarely rated for contact with insulation.
• Furnace flue and duct chases. The space around a flue or a run of ductwork going up through the house.
• The attic hatch. An unweatherstripped square of drywall that lets air through on all four sides.
• Kneewalls. In two-story homes and bonus rooms, the short walls under a sloped ceiling are often open behind.

In older, pre-war houses, wall cavities can run open from the crawl space all the way to the attic, which turns the whole wall into a flue. Those get blocked at the top.

Does the code require it where you live?

Under the 2025 California Energy Code, air sealing is one of the four possible requirements for an altered ceiling, meaning one where attic insulation is added or replaced. Whether it applies depends on your climate zone and on how much insulation the attic had before:

• Climate zones 2, 4 and 12 (North Bay, Santa Clara Valley and inland East Bay): required when the existing insulation is below R-19.
• Climate zone 3 (San Francisco and the cities along the Bay): not required for an altered ceiling, though the gaps are still there and still worth closing while the attic floor is open.

The fog side is a good example of why "not required" isn't the same as "not useful." Summers there are cool and the heating season is long, so the house is warmer than the outdoors for most of the year, and that steady difference keeps warm air rising through every gap in the ceiling. Sea breezes that pick up in the afternoon add a push of their own on the windward side of the house. In the hot inland zones the same leaks work in reverse on summer afternoons, pulling superheated attic air down into the rooms.

There's an important exception: homes with gas appliances that vent naturally, located inside the house's pressure boundary, are exempt from the air-sealing requirement. That ties directly to the safety section below. The building department verifies the work; no outside rater is needed for an insulation-only job. The rest of the altered-ceiling rules are on our Title 24 attic insulation page.

How we do it

1. Find the leaks. At the inspection we look for the tell-tale dark streaks in fiberglass, gaps around penetrations, open soffits and chases, and the condition of the hatch. Photos or video you send can get you started too.
2. Clear the way. Clean insulation is pulled back from the areas being sealed. Soiled insulation comes out completely: see insulation removal.
3. Seal by gap size. Following ENERGY STAR's method: caulk for small gaps, expanding foam for gaps up to about 3 inches, rigid board cut to fit and sealed at the edges for open soffits and chases.
4. Treat the heat sources right. Around flues, sheet metal and a high-temperature sealant, never foam. Recessed lights that aren't rated for contact with insulation get covers or replacement.
5. Fix the hatch. Weatherstripped, insulated on top and dammed so the new fill stays where it belongs.
6. Insulate over it. The insulation goes back, usually with new blown-in insulation on top to the R-value on your quote.

Safety: gas appliances, flues and old wiring

Natural-draft gas appliances. ENERGY STAR warns that sealing a house can, in some cases, cause gas appliances that vent naturally to backdraft their exhaust into the home, and recommends having them tested to make sure they draft properly before and after sealing. If you have an older gas furnace or water heater inside the house, tell us at the inspection, and plan on that check by an HVAC technician as part of the project.

Flues and chimneys. They run hot, so they get clearance and metal, never foam or insulation packed against them; see is attic insulation flammable?

Knob-and-tube wiring. ENERGY STAR flags it as a fire hazard in contact with insulation. If your attic has live knob-and-tube wiring, the work stops and we refer you to an electrician first.