Attic Insulation on Long Island. What R-Value You Need and What It Costs in 2026.

That insulation has been sitting there for 50 years. Blown-in fiberglass settles and loses R-value over time — the R-19 you started with is likely performing at R-12 or R-13 today after settling. Meanwhile, current code has moved significantly: New York adopted IECC 2021, which requires R-49 for attic assemblies in Climate Zone 4 (Nassau and Suffolk counties both fall in Zone 4).

The gap between R-13 and R-49 is not a technicality. It is the difference between losing 25% of your heating energy through the ceiling and losing 8%. PSEG Long Island customers pay electricity rates well above the national average. That gap shows up every month on your bill.

Two other factors compound the problem in Long Island homes specifically:

• Blown-in fiberglass loses R-value. Fiberglass is not as stable as cellulose. It settles more, gaps more, and its R-value per inch can drop meaningfully over two decades. If your attic has old blown fiberglass from the 1990s or early 2000s, it is probably performing below its original spec.
• Knob-and-tube wiring. Many pre-1950 Long Island homes still have knob-and-tube wiring in the attic. Code prohibits covering knob-and-tube with blown-in cellulose without an electrician's sign-off — the wiring needs air to dissipate heat. This is a legitimate constraint that affects what insulation type you can use and what it will cost.

IECC 2021 (adopted in NY) requires R-49 for attic assemblies in Climate Zone 4. This is the current code for new construction and significant remodels. It is also the minimum required to qualify for most rebate programs.

NYSERDA Comfort Home (the main rebate program available in Nassau and Suffolk) requires R-49 minimum for attic upgrades to qualify. Going to R-60 adds marginal cost — about 3 extra inches of blown cellulose — but does not change the rebate amount significantly. Most of our jobs land at R-49 to R-60.

What you probably have: If your home was built before 1980, the original attic insulation was likely R-11 (3.5 inches of fiberglass batt) or R-19 (6 inches of batt or early blown-in). Anything under R-30 is a meaningful improvement candidate. Anything under R-20 is urgent.

Blown-in fiberglass is the most common choice for standard open attic floors. It is relatively inexpensive, installs fast, and does not have the knob-and-tube issue that cellulose does. Downside: it settles more than cellulose and its R-value per inch (R-2.5 to R-4.2) is lower than cellulose at its best.

Blown-in cellulose is made from recycled newsprint treated with borate (fire-retardant and pest-resistant). It runs R-3.7 per inch, fills gaps and voids that batts miss, and is more dimensionally stable than fiberglass over time. It is the best choice for open attic floors where there is no knob-and-tube concern. Cannot be used over active knob-and-tube without electrician clearance.

Spray foam (open-cell) is the right call when you are converting to a conditioned attic assembly — insulating the rafters instead of the floor, bringing the attic space inside the building envelope. This is common when the attic has HVAC equipment or ducts you want to keep inside the conditioned space. Open-cell is also used for targeted air sealing at the ceiling plane before adding blown insulation on top.

Rigid foam board is occasionally used in very low-headroom attics where blown-in depth cannot be achieved, and in knee wall assemblies where dimensional control matters.

• Blown-in cellulose: R-3.7/inch, best for open attic floor upgrades
• Blown-in fiberglass: R-2.5–R-4.2/inch, works where knob-and-tube is present
• Open-cell spray foam: R-3.8/inch, best for conditioned rafter assembly
• Closed-cell spray foam: R-6.5–R-7/inch, highest density — used for unvented assemblies and tight spaces

Blown-in to R-49 in a 1,200 sq ft LI Cape attic: $1,800 to $3,200 installed. The range reflects duct penetrations, access difficulty, and whether old deteriorated batt material needs removal first. Add $400 to $700 for old batt removal if the existing insulation is matted, contaminated, or covering knob-and-tube.

Blown-in to R-49 in a 1,800–2,200 sq ft colonial attic: $2,400 to $4,200. Colonials with two-story attic access and clear floor space are faster to install. Older colonials with low-pitch roofs or limited hatch access add cost.

Spray foam for unvented attic assembly (rafter insulation): $4,500 to $9,000 for a typical 800–1,200 sq ft attic. This is a conditioned attic conversion — the roof deck becomes the insulated assembly. Substantially more expensive than floor-plane upgrades, but the right call when HVAC equipment lives in the attic.

Air sealing + blown-in combined: $2,500 to $4,500. This is the most common scope for Long Island home performance upgrades. Air sealing comes first — foam and caulk at all ceiling penetrations (recessed cans, plumbing, electrical, pull-down stairs) — then blown-in on top. Without the air sealing step, you are putting insulation over a leaky ceiling and the energy savings are significantly reduced.

Knee wall attics in Cape-style homes: Cape Cod dormers have a low-slope main attic floor plus knee wall bays on each side. The knee wall bays require separate treatment — either dense-pack cellulose in the stud bays or rigid foam on the knee wall face. Budget $600 to $1,400 extra for knee wall treatment in a typical Cape.

PSEG Long Island Home Performance with ENERGY STAR (administered through NYSERDA): rebates up to $400 for an air sealing and insulation combo on qualifying homes. The rebate is based on estimated energy savings, not a flat amount per job — actual rebate depends on your home's baseline and the scope of work. Requires a NYSERDA-approved contractor and a post-install inspection. We handle the paperwork in-house.

Federal IRA 25C tax credit: 30% of the cost of insulation and air sealing improvements, up to $1,200 per year. This is a dollar-for-dollar tax credit, not a deduction. On a $3,500 air seal and blown-in job, that is a $1,050 credit on your federal return. The credit resets each year — if you do attic insulation this year and basement insulation next year, you can claim $1,200 each year.

Combined, a $3,000 attic upgrade on Long Island can realistically net out to $1,600 to $1,900 after a PSEG rebate and the federal credit. Not every job qualifies — the IRS requires the insulation to meet applicable standards, and the PSEG rebate requires a NYSERDA-approved contractor.

Income-eligible households may qualify for EmPower+ through NYSERDA, which covers insulation and air sealing at no cost for households at or below 80% of area median income. Worth checking if your situation qualifies.

Knee wall attics in Cape homes. The classic Long Island Cape has a short-wall attic on each side of the dormers — the "knee walls." These are separate insulation problems from the main attic floor. If you only address the flat attic floor and ignore the knee walls, you leave a significant thermal bypass. We see this on roughly 60% of the Cape jobs we take over from prior contractors.

Ice dams. Ice dams — ridges of ice that form at the eave, back up under shingles, and cause water damage — are almost always a sign of inadequate attic insulation. Heat escapes through the under-insulated attic floor, warms the roof deck, melts snow at the ridge, and refreezes at the cold eaves. Proper air sealing and R-49 insulation on the attic floor eliminates the heat source. If you have had ice dam damage in the last two winters, your attic insulation is the first thing to fix — before roofing work.

Salt air and coastal homes. South Shore homes within a mile or two of the water see more moisture intrusion and HVAC equipment condensation than inland homes. If your attic has any signs of moisture — staining, rust on fasteners, soft sheathing — that needs to be resolved before insulation goes in, not after.

How much does attic insulation cost on Long Island in 2026?

A standard attic upgrade — air sealing plus R-49 blown-in on an average Nassau or Suffolk home — runs $2,500 to $4,500 before rebates. After a PSEG rebate and the federal 25C credit, net cost is often $1,500 to $3,000 depending on your tax situation.

What R-value do Long Island attics need to meet code?

R-49 under IECC 2021, which New York has adopted. That is the minimum for new construction and the threshold for most rebate programs. Best practice for new installs is R-49 to R-60.

Can I add insulation over my existing insulation?

Usually yes, if the existing material is dry, not moldy, and not covering knob-and-tube wiring. We run a moisture check and inspect for wiring issues on the audit visit before recommending an add-on approach. If the existing insulation is matted, contaminated, or too degraded to be useful, removal first is the better call.

Does attic insulation help with ice dams?

Yes. Ice dams are caused by heat escaping through an under-insulated attic floor, warming the roof deck, and melting snow at the ridge while the eaves stay cold. Getting the attic floor to R-49 with proper air sealing removes the heat source. Combined with adequate soffit and ridge ventilation, this is the most reliable long-term fix for ice dams.

How long does attic insulation take to install?

A one-day job for most homes. A standard Cape or colonial attic — air seal plus blown-in to R-60 — takes one day with a 4-person crew. Knee wall access, low-pitch attics, or old batt removal can add a half day.