Insulation for walls that are already closed

California's first statewide energy standards arrived in 1978. A house built before then went up with no state rule about what went inside its exterior walls. Some got insulation anyway, some got it later, and some still have nothing in there but air, wiring and the occasional lost tennis ball. You can feel the difference on a January morning: the outside walls are cold to the touch, the heater runs, and the edges of the room never quite warm up.

Opening every wall to fix that would mean new drywall or plaster throughout. Drill-and-fill skips that. One hole per stud bay, a hose, and a lot of fiber packed in tight.

First: is there anything in your walls?

Don't assume either way. DOE's homeowner guide describes a simple check. Switch off the breaker for an outlet on an exterior wall, take off the cover plate, and shine a flashlight into the gap beside the electrical box. Insulation, if there is any, shows up right there. Check outlets on each floor, and in the old and new parts of the house, because one insulated wall says nothing about the others. An addition from 1995 can sit next to a 1938 living room with empty walls.

A thermal camera on a cold morning tells the same story for the whole wall at once: empty bays show as stripes between the studs.

How drill-and-fill works

DOE's Building America wall guide calls the one-hole dense-pack method the best practice for closed walls, and it runs like this.

• Pick a side. Filling from outside is usually preferred for occupied homes with wood, fiber-cement or vinyl siding, because it doesn't disturb the rooms. Filling from inside is preferred for stucco, brick and stone, where it is easier and cheaper than drilling the exterior. In the Bay Area that splits neatly: shingled and lap-sided houses usually get filled from outside, stucco houses from inside.
• Find the blocking. Many walls have a horizontal block partway up each bay. Every bay is checked, and blocked bays get a second hole so both halves fill.
• Pack it to density. A fill tube goes in to the top of the bay, fills it, and is reinserted to fill the bottom, withdrawn slowly as the cavity fills. DOE gives target densities of 3.25 to 4.0 pounds per cubic foot for cellulose and about 1.6 for blown fiberglass. Too loose and the fill settles, leaving a cold band at the top of the wall.
• Close it up. Outside, the holes get tapered wooden plugs and the siding goes back. Inside, the plugs are spackled and left ready for paint.

When the weather gives enough temperature difference across the wall, a thermal camera can show afterwards whether every bay filled.

What can stop a wall job

Most walls can be filled. These are the ones that need something else first.

• Knob-and-tube wiring. The electrical code bars insulation around concealed knob-and-tube except under a strict sign-off route, and DOE's advice is to insulate the walls when the wiring is upgraded. We refer you to an electrician; the conditions are in our knob-and-tube guide.
• Moisture. DOE says not to proceed if the siding, sheathing, window framing or interior finish show moisture problems. Find and fix the leak first, or the new insulation just holds the water against the framing.
• Weak plaster or cracked finishes. Dense-packing puts pressure on the wall from inside. Old plaster with cracks or loose keys gets repaired before filling, not after.
• Bays that don't hold fill. Bays open to the attic or crawl space, the open framing behind a tub, and bays used as return-air ducts. The return-air ones must be sealed off from the duct system before any insulation goes in; fill in a duct is not insulation.
• Painted surfaces in a pre-1978 house. DOE's guide says to assume lead paint in homes built before 1978, and federal lead-safe work rules apply when work disturbs more than 6 square feet of interior or 20 square feet of exterior painted surface.
• Cement-asbestos shingles. Older siding shingles can contain asbestos. Don't drill or pry them; fill from the inside. If there is any doubt about a material, we stop and refer you to an asbestos consultant.

Which fill for a closed wall

Both work in a closed wall, and both give the wall a little extra air tightness once they are packed in, though DOE is clear that this is no substitute for real air sealing. DOE's materials table puts cellulose at about R-2.9 to R-3.4 per inch. Blown fiberglass, packed to about 1.8 pounds per cubic foot, can reach R-15 in a 2x4 cavity and R-21 in a 2x6, according to DOE.

Those numbers line up with the code. When a remodel opens a wall, California's 2025 Energy Code expects R-15 in 2x4 framing and R-21 in 2x6. If the walls are coming open anyway, fitted batts are often simpler; see batt insulation. If the wall is staying closed, the cavity depth is fixed, and the material and density decide how close you get. Where knob-and-tube stays in the wall under the electrician sign-off route, the noncombustible requirement narrows the choice further. The product, target density and side of the wall all go on the written quote, with the product's manufacturer warranty.

Do the attic first, usually

Walls are a big surface, but the attic is usually the bigger leak and the cheaper fix. If the attic hasn't been done, start there: air sealing and blown-in insulation above the ceiling, then walls. Doing them in that order also means the wall quote isn't paying to fix a problem that was really coming through the ceiling.

Walls facing a busy street or freeway are another reason to fill them. Dense fill in an exterior wall won't stop traffic noise on its own, but it helps, and it is the same job. The soundproofing page has the straight version of that story.

Getting it quoted

Walls are easier to quote than people expect. Useful things to send, or to have on hand at the inspection:

• The year the house was built, and the years of any additions.
• A photo of each side of the house, so the siding or stucco is clear.
• What the inside walls are: drywall or plaster.
• What you found behind an outlet cover, if you checked.

You can book a free in-home inspection or send photos and a short video for a photo estimate. The written quote lists which walls get filled, from which side, with what material and at what density, plus exactly what patching is included. Nothing is due before you sign.