Blown-in attic insulation for Bay Area homes

What blown-in insulation is

Blown-in (or loose-fill) insulation is exactly what it sounds like: insulation in small loose pieces, fed into a machine on the truck and blown through a long hose up into the attic. It lands as an even blanket across the attic floor, settles around wires, pipes and odd framing, and fills the corners that a roll of batting leaves half-covered. That's why it is the default for open attic floors, which is most attics in the Bay Area.

There are two common materials:

• Loose-fill fiberglass: spun glass fibers, light and fluffy, and it doesn't absorb water.
• Cellulose: recycled paper fiber treated with a fire retardant. It's denser, so each inch carries more R-value, and it settles a little after install.

Insulation sold in California has to appear in the state's insulation material directory, kept by the Bureau of Household Goods and Services, so whatever goes into your attic is a product the state has on file.

A thin attic, at a glance

If you can see the tops of the ceiling joists from the hatch, there are a few inches at most, well short of R-49, and gray streaks in fiberglass mean house air is leaking through it, an air sealing job before an insulation one. The full self-check, with the ruler method and the stop signs, is in how to check attic insulation; if the complaint is a hot upstairs, start there.

The code target, in brief

Under the 2025 Energy Code, adding or replacing attic insulation makes the ceiling an "altered ceiling", and depending on your climate zone and what's up there now, that can bring in R-49 and more. The zone-by-zone rules for the 30 cities we serve, and the exemptions, are on our Title 24 attic insulation page.

How we do it

1. Look first. At the free inspection we measure the depth that's there, check its condition, and look for anything that stops the job (more on that below). If you'd rather not have a visit, photos or video of the attic get you an estimate too.
2. Prep before fill. Gaps in the attic floor get sealed where the code or the attic calls for it. Recessed lights that aren't rated for contact with insulation get covers or replacements. The hatch gets a dam so the new fill doesn't spill out every time you open it, and baffles go in at the eaves so the soffit vents keep breathing.
3. Blow in to the number on the quote. We work from the far eaves back toward the hatch, to the R-value and depth written on your quote, keeping the blanket even across the whole floor.
4. Clean up and close up. Hoses out, hatch insulated and closed, work area cleaned.

The Energy Code has its own paperwork for an insulation alteration, and the building department does the final check. There's no third-party rater for an insulation-only job.

Which loose fill we recommend, and why

Neither material is a mistake on an open attic floor. Where the old insulation is staying and getting topped up, matching what's already there keeps the coverage chart accurate. Where the eaves are tight under a low-pitched roof, the fill that reaches the target at a shallower depth gets the nod, and where live knob-and-tube stays put, the code's noncombustible rule decides it. We'll recommend one at the inspection and say why on the quote. For the material-by-material comparison, see fiberglass vs cellulose.

What a good blown-in quote spells out

Blown-in is easy to do badly and hard to check once it's done, so the quote is where you protect yourself. Whoever you hire, it should name:

• The product, by manufacturer and name, so you can look up its coverage chart and its manufacturer warranty.
• The R-value and the installed depth in inches. Both, not one or the other. The depth is what you can measure later with a ruler.
• The number of bags for your attic's square footage, taken from the same chart. Too few bags for the area means the blanket comes out thinner than promised.
• What happens before the fill: which gaps get sealed, which recessed lights get covers, whether baffles go in at the eaves, and how the hatch gets dammed.
• What comes out first, if anything, and how it leaves the house.

Blown-in in typical Bay Area attics

Bay Area housing stock gives blown-in a few recurring puzzles:

• Low-pitched postwar roofs. Many ranch and tract houses from the 1950s and '60s have hip roofs that come down close to the attic floor at the edges. The far eaves are where a blanket tends to come up short and where vents get buried, so baffles go in first and the fill is worked from the edges back.
• Pre-war houses with tall attics. Plenty of headroom, often very little insulation, and sometimes knob-and-tube wiring or old fill that has to be identified before anything else happens.
• Ducts across the attic floor. Common in inland houses with forced-air heating. Loose fill flows under and around the ducts, which batts can't, but crushed or leaky ducts are worth fixing before they're buried; see ductwork.
• Storage platforms. Plywood laid over the joists for boxes leaves the bays below it thin or empty. Whether to lift it, fill under it or leave a marked storage area is a decision for the quote, not for install day.

Safety stops: knob-and-tube, vermiculite and old light fixtures

Some things end a blown-in job before it starts, so here they are up front:

• Knob-and-tube wiring. Insulation can go over live knob-and-tube only under the electrical code's strict exception, so we stop and refer you to an electrician; see the knob-and-tube guide.
• Vermiculite. The EPA says to assume it contains asbestos and not to disturb it, so we don't remove it or blow over it; see the vermiculite guide.
• Recessed lights. Cans not rated for insulation contact get covers or new fixtures before the fill, never after; insulation over recessed lights explains the labels.