Fiberglass vs cellulose for a Bay Area attic floor

Most Bay Area attics end up with one of two loose fills: fiberglass, which is spun glass, or cellulose, which is shredded recycled paper treated with fire retardant. Both get blown through a hose onto the attic floor, both reach the same R-value when installed to their own coverage chart, and both have loyal fans who will tell you the other one is a mistake. The real answer is less exciting. On most attic floors either one does the job, and the choice comes down to a handful of facts about your particular attic: its wiring, its history with water, its ceiling and how much room there is at the eaves.

This page lays those facts out from the Department of Energy's Building America material, the federal cellulose rules and California's electrical code exception for old wiring.

Side by side

R-values, settling, weight and air figures are from the Building America Solution Center's loose-fill page, which draws on DOE's own loose-fill data. Water and fire notes combine that page and DOE's homeowner insulation guide.

Per inch, and why the attic cares less than you'd think

Cellulose packs more R-value into each inch. That sounds decisive until you remember what the code actually asks for. California's Energy Code sets a target R-value for the ceiling, not a depth, so fiberglass simply goes in deeper to reach the same number. For the depth each one needs to hit R-38 or R-49, see the guide to checking your attic insulation, which turns a ruler reading into an R-value.

Where the inches do matter is at the edges. Many Bay Area ranch houses and postwar tracts have low-pitched roofs, and the space where the roof meets the wall at the eaves is tight. A fill that reaches the target in fewer inches leaves more room for the baffles that keep the soffit vents open. In a steep attic with plenty of headroom, the difference barely registers.

Settling is the other half. Cellulose settles noticeably after install, fiberglass much less. That's expected, and the bag's coverage chart already lists both an installed thickness and a minimum settled thickness for each R-value. If someone quotes you R-49 in cellulose, the settled number is what to hold them to, and the federal R-value rule requires the installer's receipt to show it.

Air: the quiet advantage of density

The Building America page says cellulose "does a good job of slowing airflow, even at loose-fill densities," and lists fiberglass's weaker resistance to airflow as a drawback. Air washing through the top of a loose blanket carries heat with it, so that's a real point in cellulose's favor.

It's a smaller point than the gaps underneath. Wiring holes, plumbing chases, the attic hatch and old recessed lights leak far more air than any loose fill can stop, which is why ENERGY STAR recommends air sealing the attic before adding insulation. Seal first, and the gap between the two materials on airflow shrinks to a footnote. Skip sealing, and neither one fixes the draft.

Moisture: the fog side and inland

Climate is where the Bay Area adds a twist the national guides don't.

On the fog side (climate zone 3: San Francisco, Daly City, the Peninsula, Oakland, Berkeley, Alameda and the East Bay flats down to Fremont), summers are cool and marine air is damp. That alone doesn't wet an attic's insulation. What does is a roof leak, a bathroom or dryer vent dumping into the attic instead of outdoors, or condensation on the underside of the roof. Those are exactly the conditions ENERGY STAR lists as reasons to bring in a professional before insulating.

DOE's guide is direct about cellulose and water: accidental wetting and high humidity can make it "absorb large quantities of water, reducing thermal resistance, draining fire retardant, and creating risk of mold." It also says to address mold issues before installing cellulose. Fiberglass, by the Building America page's account, "will not be permanently damaged if it is saturated with water," though wet insulation of any kind stops insulating until it dries.

So on the fog side the material question comes after a moisture question: has this attic ever been wet, and is the cause fixed? If the answer is "yes, and not yet," fix it first. If the attic has a history of leaks you can't rule out recurring, fiberglass is the more forgiving fill.

Inland (zone 4 in the Santa Clara Valley, zone 12 from Concord and Walnut Creek to Livermore, and zone 2 in Napa and Santa Rosa), the attic problem is summer heat. Moisture rarely tips the choice there. Depth, weight and how much room there is at the eaves matter more.

Fire: two different ideas of safe

Fiberglass starts from glass, which doesn't burn; DOE notes the resin binders and paper facings used with it do. Cellulose starts from paper, which does, and it's the chemical treatment that makes it fire-resistant. That treatment is regulated. Cellulose sold as insulation has to meet the Consumer Product Safety Commission's interim safety standard, and every container must say: "This product meets the amended CPSC standard for flame resistance and corrosiveness of cellulose insulation."

A second federal rule requires a caution label on cellulose, including the line "Keep cellulose insulation at least three inches away from the sides of recessed light fixtures," and a warning to keep it away from furnace, water heater and space heater flues. The label asks installers to hand it to the homeowner at the end of the job. DOE adds that cellulose may smolder if temperatures get high enough. None of this makes cellulose a fire hazard in a properly built attic, but it does mean the dams around flues and old light fixtures aren't optional.

Knob-and-tube: a fact, not a ranking

If your house still has live knob-and-tube wiring in the attic, the choice is made for you. California's electrical code allows insulation around it only through an exception with several conditions. Berkeley's Form 146, which sets them out, includes this one: "the insulation shall be noncombustible as defined in the California Building Code." Treated paper isn't a noncombustible material, so cellulose can't meet it. A fiberglass or mineral wool product might, and that has to be confirmed from the product's documentation and with the building department.

The other conditions start with an electrician's survey of the wiring. We don't do that part ourselves; we stop and refer you to an electrician. The knob-and-tube guide explains the order of work.

Rodents: what the treatment does and doesn't do

Cellulose is often sold with the hint that its borate treatment discourages pests. DOE's guide does describe cellulose as treated to resist "fire, corrosion, fungal growth, and vermin." But the federal standard cellulose must meet tests flame resistance and corrosiveness, and we found no independent study showing that borate-treated cellulose keeps rats or mice out of an attic. Rodents nest in both fills. What stops them is metal over the holes they use, which is rodent proofing, done before any new insulation goes down.

Weight, old ceilings and topping up

At R-38, cellulose can weigh roughly twice what fiberglass does per square foot, by the Building America figures. A sound drywall or plaster ceiling carries either. A ceiling that already sags, or old plaster that's letting go of its lath, is a reason to look before adding weight, and on those ceilings fiberglass is the lighter load.

Most Bay Area attics already have something in them, so the real question is usually what goes on top. Either fill can go over dry, clean insulation. Matching the existing material keeps the job simple, but it isn't a rule.

Cost and dust

There's no meaningful price gap to chase. DOE's guide rates both loose fills in its lowest relative-cost tier, and the Bay Area sources behind our cost page don't separate them, so it shows one blown-in range. Both are dusty to install and both need masks, eye protection and covered skin on install day; is blown-in insulation safe covers what the health sources say about each.

How it usually shakes out

• Live knob-and-tube staying in the attic: not cellulose, and the electrician comes first.
• A history of leaks or attic moisture: fix the cause; fiberglass if you can't rule out more water.
• Tight eaves on a low-pitched roof: cellulose's fewer inches help.
• A sagging or fragile ceiling: fiberglass's lighter load helps.
• None of the above: either one, to the coverage chart.

At a free inspection we'll recommend one for your attic and write the product, R-value and depth on the quote; the product carries its manufacturer warranty. Book an inspection, read more about blown-in insulation, or go back to the full comparison of insulation types.