Batt insulation, for the spots loose fill can't stay put

Batts are the insulation most people picture: pink, yellow or gray blankets cut to the width of a stud bay. They are also the insulation most often installed badly, because a batt only performs when it fills its cavity completely. A batt stopped two inches short of the top plate, or stuffed behind a pipe, is a batt that is mostly decoration.

So batts are a fit-and-finish product. In the right spot, cut by someone patient, they are simple, clean and durable. In the wrong spot, blown-in insulation does the same job faster and with fewer gaps. This page is about telling the two apart.

Where batts make sense in a Bay Area house

Batts need open framing. You have to see the whole cavity and press the batt into it. In Bay Area houses, that usually means one of four places.

• Walls opened during a remodel. Kitchen and bath remodels expose exterior walls stud by stud. That is the cheapest moment the wall will ever be to insulate, and the code has a number for it (below).
• Knee walls. Story-and-a-half houses and finished attic rooms have short walls between the room and the low part of the attic. The knee wall gets batts, and the attic floor behind it gets insulated too, so the room isn't wrapped on one side only.
• Floors over a crawl space or a garage. Plenty of Bay Area houses sit on raised foundations, and plenty of bedrooms sit over garages. Batts go between the floor joists, tight against the subfloor, and get held there. See crawl space insulation and garage ceiling insulation.
• Attic floors with clean, even joist bays. Possible, and sometimes a good first layer. For the R-49 depth the Energy Code asks of many altered attics, a blown layer on top is usually faster and fills around the joists. That is on the blown-in insulation page.

Where batts fight you: attics crowded with ducts, wiring and bracing; low-pitched attics where nobody can kneel; and walls that are already closed. Closed walls get existing-wall insulation, blown in through small holes.

What the 2025 Energy Code asks for, spot by spot

California's 2025 Energy Code (Title 24, Part 6) took effect on January 1, 2026. Adding insulation to an existing house counts as an alteration, and each part of the house carries its own minimum.

The attic row is the tricky one, because it depends on your climate zone and on what is already up there; the zone-by-zone rules are on our Title 24 attic insulation page. For attics, the building department checks the work, not a third-party rater.

Fit is the whole job

The Department of Energy's Building America guide is blunt about batts. Each one has to fill its entire cavity, with no gaps, and it must not be folded, tucked in or pinched. Small gaps add up to a big loss in effective R-value, the way a few holes ruin a blanket. On a real job that comes down to a handful of habits:

• Batts are cut to length so they meet the top and bottom plates, not stopped short where the tape measure ran out.
• Where a wire or pipe crosses the bay, the batt is split around it, half behind and half in front, instead of being shoved behind it.
• Under floors, batts sit tight against the subfloor and are supported so they can't sag. A sagging batt leaves an air gap where air circulates and carries the heat away anyway.
• Nothing gets compressed. A batt squashed to half its thickness loses R-value.

Facings matter too. The Bay Area sits in DOE's mild climate zones, where the guide calls for unfaced batts in vented attics, and for no facing between an old layer and a new one. Kraft paper facing burns, so it shouldn't be left exposed in a garage or an unfinished room; batts with flame-resistant facings exist for those spots.

If you want to check an old batt job yourself, DOE's rule of thumb for fiberglass batts is thickness in inches times 3.2. A batt that has sagged, been pushed aside for a light can, or been flattened under storage boxes is worth less than its label.

Blown-in vs batt insulation

The labor hours come from a Bay Area contractor's published figures. Neither material is an air barrier: DOE notes that batts are not airtight, and air leaking through a ceiling or wall carries heat straight past them. Seal first, then insulate. That is the air sealing page.

Before a single batt goes in

• Knob-and-tube wiring. Live knob-and-tube can't be surrounded by insulation without an electrician's sign-off under the electrical code, so we stop and refer you to one; see the knob-and-tube guide.
• Rodents. If there are droppings or chewed batts, the entry points get closed first and the soiled material comes out. See rodent proofing and insulation removal.
• Recessed lights. Older cans not rated for insulation contact can't be buried. In many Bay Area attics the Energy Code wants them replaced or fitted with fire-rated covers before insulation goes over them.
• Suspected vermiculite. Loose, gray-brown, pebbly fill under the old batts? EPA says to assume it contains asbestos and leave it alone. We stop and refer you to an asbestos consultant; see the vermiculite guide.

Fiberglass, mineral wool, or something else

DOE's guide lists batts made of fiberglass, mineral wool, cotton and sheep's wool. Fiberglass is the inexpensive default. Mineral wool doesn't burn, and it is the usual upgrade when the same wall also needs to be quieter (see soundproofing). California requires insulation sold in the state to appear in the insulation directory kept by the Bureau of Household Goods and Services. The exact product and R-value go on your written quote, and the product carries its manufacturer warranty.

How a batt job gets quoted

Batt work is measured work: square feet of wall or floor, joist and stud depth, how many bays are interrupted by pipes and ducts, and what has to come out first. You can book a free in-home inspection, or send photos or a short video of the open walls, the knee wall or the crawl space and get an estimate from those.

The written quote lists the product, the R-value and every area covered. Nothing is due before you sign.