How you insulate a wall depends on one question: is it open or closed? An open wall, in new construction, an addition or a gut renovation, takes batts, mineral wool or spray foam between the studs, and the energy codes in New Jersey, Connecticut and New York now ask for more than a 2x4 cavity can hold on its own. A finished wall has three routes: insulation blown or injected through holes, rigid board added outside when the siding comes off, or opening the wall from inside.
Find out what is in the wall first. Many older walls are empty or hold thin, slumped batts, and some hide knob-and-tube wiring, balloon framing or lath and plaster that decide which methods are safe at all.
How to tell if your walls are insulated
The Department of Energy’s outlet check:
- Turn off the power to an outlet or switch on an exterior wall.
- Remove the cover plate and shine a flashlight into the gap beside the electrical box. A crochet hook can pull out a few fibers, power still off.
- Repeat on every floor and in old and new parts of the house. Insulation in one wall doesn’t mean it is in every wall.
An infrared camera shows the rest. The Building America Solution Center uses thermal imaging to find empty bays, hidden blocking and voids, which show best when indoor and outdoor temperatures are far apart, so a winter energy audit sees the most.
Two findings stop the project. Pebble-like gray-brown or silver-gold fill may be vermiculite, which the EPA lists among asbestos products in wall insulation and says to leave undisturbed. Wires strung on porcelain knobs and tubes are knob-and-tube wiring, covered below.
Insulating open walls: new construction, additions and gut renovations
What the energy code requires for walls
The prescriptive table for wood-framed walls gives the same four options in every climate zone we work in: New Jersey’s zones 4 and 5 under its adopted 2024 IECC; all of Connecticut, zone 5, under the 2021 IECC in its 2022 State Building Code; and zones 4, 5 and 6 under New York’s 2025 Energy Conservation Construction Code, for permits applied for from 31 December 2025.
- R-30 in the cavity, or
- R-20 in the cavity plus R-5 continuous insulation, or
- R-13 plus R-10 continuous, or
- R-20 continuous alone.
Continuous insulation, the “ci” in the code, is an unbroken layer across the studs, usually rigid board over the sheathing. The code counts cavity and continuous insulation separately and gives no credit for sheathing or drywall (Section R402.1.4), and a batt squeezed into a shallower cavity counts only at the R-value it delivers there. U-factor calculations and whole-house energy modeling are other compliance routes.
Additions follow these numbers. Where a renovation exposes existing wall cavities, Connecticut’s 2021 code asks that they be filled with insulation (Section R503.1.1), and New York’s 2025 code asks for full-depth insulation of at least R-3 per inch (Section R503.1.1.3). New Jersey handles existing homes under its Rehabilitation Subcode, where installing insulation other than foam plastic against or within 1 1/2 inches of an interior finish is ordinary maintenance with no permit (N.J.A.C. 5:23-2.7).
What fits in a 2x4 or a 2x6 wall
- 2x4, 3 1/2 inches deep: R-13 or R-15 batts, or about R-24 to R-25 of closed cell spray foam filling the bay. None reaches R-30, so a new 2x4 wall needs R-10 of board over R-13 batts, or at least R-20 in the cavity plus R-5 outside; 3 inches of closed cell, about R-21, qualifies.
- 2x6, 5 1/2 inches deep: R-20, R-21 or R-23 batts, or a full bay of open cell foam at about R-21 to R-23, each with R-5 outside. About 4 1/4 to 4 1/2 inches of closed cell reaches R-30 with no board.
Skip R-19 in a 2x6: Johns Manville’s fact sheet says its 6 1/2-inch R-19 batt, compressed into the 5 1/2-inch bay, delivers R-17 to R-18.
Batts, mineral wool or spray foam
The Department of Energy calls fiber in open stud bays the least expensive way to insulate a wall, and spray foam much more expensive but better at air sealing and moisture resistance. How we choose:
- Fiberglass batts, about R-3.1 to R-4.3 per inch: the budget choice for dry, regular stud bays, after air sealing.
- Mineral wool batts, about R-4 per inch: denser, and described by ROCKWOOL as noncombustible, water resistant and good at sound absorption. Suits bathroom walls and bedrooms.
- Open cell spray foam, R-3.9 to R-4.2 per inch: seals and insulates in one pass, and fills the irregular framing of older houses that batts never fit.
- Closed cell spray foam, R-6.9 to R-7.1 per inch: the most R-value per inch, a vapor retarder at about 1 1/2 inches, and the pick for shallow walls and the wall shared with a garage.
Foam in living space needs a thermal barrier such as 1/2-inch drywall (Section R316.4 of the 2021 IRC, R303.4 of the 2024 IRC). The Solution Center notes foam walls still leak at wood-to-wood joints, doubled studs, corners and headers, so those get sealed too. More in open cell vs closed cell spray foam.
How to insulate around electrical boxes and wiring
The Department of Energy’s rules for batts: fill the entire cavity, never fold, tuck or pinch, and slit the batt to fit around wiring and plumbing. The energy code adds (Table R402.5.1.1 of the 2024 IECC):
- Electrical boxes on exterior walls are caulked, taped or gasketed to the air barrier with their hidden openings sealed, or air-sealed boxes rated to NEMA OS 4 are used.
- Insulation fills the space behind and around each box and surrounds wiring, so batts are cut to fit, not stuffed. The Department of Energy notes blown-in fiber fills around boxes on its own.
- Corners and headers are filled with insulation of at least R-3 per inch.
- Tubs and showers on an exterior wall need the wall behind them insulated, with an air barrier between; the Department of Energy notes that wall may turn out to have no insulation or air barrier when a tub comes out.
Faced or unfaced, and which way the paper goes
Kraft facing is a Class II vapor retarder, and the paper faces the heated room. Whether a wall needs it depends on the climate zone (IRC Section R702.7, in the 2021 edition Connecticut uses and the 2024 edition New Jersey and New York adopted):
- Zone 4, most of New Jersey, Westchester and Long Island: unfaced batts behind painted drywall are allowed, and so is kraft. Polyethylene sheeting is not permitted.
- Zone 5, Connecticut, New Jersey’s Bergen, Hunterdon, Morris, Passaic, Somerset, Sussex and Warren counties, and the Hudson Valley counties beyond Westchester: faced batts or vapor retarder paint, unless the siding is vinyl, aluminum or brick veneer with an air space over plywood, OSB, fiberboard or gypsum sheathing, or the wall carries at least R-5 of board on a 2x4 or R-7.5 on a 2x6. Zone 6, Ulster and Sullivan counties, has fewer options.
Kraft facing burns and can’t be left exposed; it goes in contact with the drywall.
Thermal bridging and continuous insulation
Every stud is a strip of wall with no insulation. The Department of Energy’s Building America guide says heat loss through the framing can cut a wall assembly’s R-value by 25 to 40 percent, and foam between the studs doesn’t change that. Board across the studs does. The same guide lists extruded polystyrene at about R-5 per inch and expanded polystyrene at R-3 to R-4, with polyisocyanurate highest of the three.
Adding exterior insulation when you re-side
Re-siding is the best chance an existing wall gets. ENERGY STAR’s guidance for zones 4 through 8: if the wall is uninsulated, drill the sheathing and blow insulation into the empty cavities, then add R-5 to R-10 of insulating sheathing under the new siding; over an insulated 2x4 wall, add R-10. In New York, replacing siding and windows across a full elevation brings in the 2025 code’s continuous insulation requirement (Section R503.1.1.3). What the Solution Center and the Department of Energy call for:
- Repair the sheathing and framing once the siding is off.
- Install an air and water control layer, flash windows and doors into it, and extend jambs and sills for the thicker wall.
- Lay the board in two layers with seams offset, taped where the outer layer is the drainage plane.
- Screw vertical furring strips through the foam into the framing and hang the siding on them.
Plastic sheeting inside a wall wrapped in foil-faced board, a Class I vapor retarder on both sides, needs an approved design (Section R702.7). The siding contractor handles the siding; the board is insulation work.
Insulating existing walls without opening them
Standard spray foam needs an open bay, so closed walls are filled through holes. We insulate walls once they are open, from inside or out, and we fill closed walls through holes as well. Both through-the-hole methods below are specialized work, and what follows is how we do them.
Dense-pack cellulose or fiberglass through drilled holes
The Department of Energy calls drill and fill popular and time-proven:
- Holes of about 1 to 2 inches go into each stud bay, roughly 6 inches below the ceiling, through drywall or plaster inside, or through the sheathing after a strip of siding comes off.
- Each bay is packed to the manufacturer’s density; the Solution Center gives 3.5 to 4.5 pounds per cubic foot for cellulose and 1.8 to 2.6 for fiberglass, dense enough to resist settling and slow air.
- Bays over headers, under windows and behind cabinets get holes too, with a second hole below any blocking.
- An infrared scan confirms every bay is full before the holes are plugged and patched, or the siding and its drainage layer go back.
At cellulose’s R-3.2 to R-3.8 per inch, a full 2x4 bay reaches about R-11 to R-13. The Department of Energy says only a trained professional should do it, and the walls must be sealed and strong enough for the pressure.
Injection foam
Injection foam is pumped into closed bays through similar holes. The Department of Energy describes slow-expanding foams made for existing walls, which expand slowly to reduce the chance of damaging the wall, and slow-curing foams that flow around obstructions before setting. Its foamed-in-place types include cementitious and phenolic foams, and it notes phenolic foam can shrink up to 2 percent after curing. What we settle before the first hole is drilled:
- Which product we use, and its data sheet: R-value per inch, shrinkage, cure time.
- How we find blocking, and the infrared scan that confirms each bay is full.
- What happens to old batts in the wall; we pull out what blocks the fill.
- Whether a permit is needed; New Jersey’s no-permit maintenance rule excludes foam plastic.
What decides whether an old wall can be filled
- Knob-and-tube wiring. The Department of Energy says not to drill and fill where it is in the wall, because insulation can push the wires against the studs, and to replace active knob-and-tube before insulating; the Solution Center notes the National Electrical Code doesn’t allow insulation around it. An electrician checks what is live and replaces it first.
- Blocking. The residential code now wants fireblocking in stud walls at every floor and ceiling and at least every 10 feet (Section R302.11). Old blocks and diagonal bracing leave voids unless each gets a hole below it.
- Balloon framing. Older houses often run stud bays unbroken from basement to attic. The Solution Center says those walls need blocking at top and bottom before they can be dense-packed.
- Lath and plaster. The Department of Energy warns that drilling from outside can break the plaster keys, and drilling inside damages plaster, raises dust and can disturb lead paint. For plaster worth keeping, it suggests insulating outside under new siding, or air sealing and leaving the walls empty.
- Lead and asbestos. Paid work that disturbs paint in a home built before 1978 requires an EPA- or state-certified firm using lead-safe practices. The EPA lists siding shingles among asbestos products, so old cement shingles get tested before anyone drills them.
- Water. A filled wall dries more slowly. The Solution Center says to fix leaks and flashing first, calls stucco over wood framing a high-risk cladding, and rates vinyl, aluminum and vented brick veneer relatively safe.
Brick, stone and block walls
Brick veneer over framing has an air space behind it on purpose: the code requires a nominal 1-inch gap and weepholes so water drains (Table R703.8.4(1), Section R703.8.6). The insulation cavity is the stud bay behind the sheathing, reached from inside.
Solid brick, stone and block are harder. The Department of Energy warns that insulating masonry from inside leaves it colder and wetter, and freeze-thaw cycles can damage it permanently; its researchers don’t put fiber against masonry. What has worked: rigid foam outside under new cladding, or 2 to 4 inches of spray foam on the inside face behind a new stud wall, once rain is kept off the wall. “Cavity wall insulation” is mostly a British term for masonry houses with a gap between two layers of brick or block; in most houses here, the cavity is the stud bay.
What wall insulation costs
Most of our projects run between $3,500 and $12,500. On wall work, open cavities, closed walls filled through holes and exterior board, the number moves with:
- Area and height: square footage of open bays, and upper-story or gable walls that need staging.
- Material: batts cost least, open cell foam more, closed cell the most per inch.
- Exterior board: thickness, layers, taped seams and openings to detail.
- Prep: pulling old batts and sealing corners, headers and wiring holes.
Drill-and-fill and injection foam are ours, priced by bay count and access. Electrical, siding, drywall and lead-safe work are priced by those who do them. Rebates exist through state and utility programs; see New York and New Jersey insulation rebates and Connecticut insulation rebates. The federal 25C tax credit ended for anything placed in service after 31 December 2025.
Can you insulate walls yourself?
Open walls, yes: the Department of Energy counts batts in open, standard stud bays as do-it-yourself work. Air seal first, cut batts to the cavity, split them around wires and keep kraft facing toward the room. Sealing outlets on exterior walls is a good homeowner job: the Solution Center’s method is breaker off and verified, caulk or low-expanding foam in and around the box, and a foam gasket under the plate.
Closed walls, rarely. The Department of Energy says drill and fill takes a trained professional, and the Solution Center says dense-pack density takes the right machine and a feel that only comes from experience. Spray foam kits carry the hazards in is spray foam safe. Stop for knob-and-tube wiring and anything that looks like vermiculite.
Common questions about wall insulation
How do you insulate existing walls without removing drywall?
Through holes. Dense-pack cellulose or fiberglass is blown into each stud bay through holes of about 1 to 2 inches, from inside or from outside behind the siding, then patched. Injection foams go in the same way. Neither suits walls with live knob-and-tube wiring, and balloon-framed walls need blocking at top and bottom first.
What is the best insulation for exterior walls?
In an open wall, closed cell spray foam gives the most R-value per inch, open cell fills irregular framing for less, and batts cost least when fitted carefully. Board outside stops heat loss through the studs. In a closed wall: dense-pack through holes, or board under new siding.
What R-value do walls need in New Jersey, Connecticut and New York?
For new wood-framed walls, the same options in all three states: R-30 in the cavity, R-20 plus R-5 continuous insulation, R-13 plus R-10 continuous, or R-20 continuous alone, under the 2024 IECC in New Jersey, the 2021 IECC in Connecticut and New York’s 2025 energy code.
Should I use R-13 or R-15 in a 2x4 wall, and R-19 or R-21 in a 2x6?
R-15 in a 2x4: same thickness as R-13, more R-value. R-21 or R-23 in a 2x6. An R-19 batt is thicker than the bay, and Johns Manville says it delivers R-17 to R-18 compressed into a 2x6 wall. For a new wall, the code options add continuous insulation either way.
Should wall insulation be faced or unfaced?
In zone 4, most of New Jersey, Westchester and Long Island, either works behind painted drywall, and plastic sheeting isn’t permitted. In zone 5, including Connecticut, use faced batts or vapor retarder paint unless the siding is vinyl, aluminum or vented brick veneer, or the wall has enough exterior foam. Facing goes toward the room.
Is injection foam or dense-pack cellulose better for existing walls?
Both fill closed bays through holes, and both depend on finding blocking and confirming every bay is full. Dense-pack is the drill-and-fill method the Department of Energy calls time-proven. We do both, and for injection foam we give you the data sheet, including R-value per inch and shrinkage, and scan to check the fill.
Is it better to insulate walls from the outside or inside?
Outside, when the siding is due anyway, the plaster is worth keeping or the walls are masonry: you stay in the house and board stops thermal bridging. Inside, during a gut renovation, when wiring and air sealing can be fixed too. Through holes when both finishes stay and the wiring and framing allow it.
How do I know if my walls are insulated?
Turn off the power to an outlet on an exterior wall, remove the cover plate and look into the gap beside the box with a flashlight. Check each floor and older and newer parts of the house. An infrared camera during an energy audit shows empty bays and voids.
Can you insulate walls that have knob-and-tube wiring?
Not until the wiring is dealt with. The Department of Energy says active knob-and-tube should be replaced before insulating, and drill and fill should not be used where it is in the wall. Have an electrician check which circuits are live and replace them first.
How much does wall insulation cost?
Most of our projects run between $3,500 and $12,500. For open walls, drill-and-fill and exterior board, the price follows wall area and height, batts versus open or closed cell foam or dense-pack, board layers and openings, and prep. Electrical, siding and drywall are priced by those trades.
Wall insulation in New Jersey, Connecticut, the Hudson Valley and Long Island
Before we quote a wall, we look at it: what the outlets and an infrared scan show, the framing, the wiring, the siding and signs of water. Then we tell you which route fits, what has to happen first and who does it, and what our part costs. The energy audit is free.
Call 914-825-6824 or use the contact form, 8am–8pm, seven days. The Home Insulators is family-owned, based in New Rochelle, NY since 2015, with a 100-plus-person in-house crew that never subcontracts. We work across all of New Jersey, all of Connecticut, New York’s Hudson Valley including Westchester, and Long Island; see our service area.