Air sealing closes the holes that let heated air out of a house and outside air in: the gaps between the living space and the attic, the basement, the garage and the outdoors. Insulation slows heat moving through a surface. Air sealing stops the air itself, and that air carries heat and moisture with it.
It comes before insulation because the leaks sit under the insulation, and fiberglass and cellulose don’t stop them; the Department of Energy notes that air passes right through fiber insulation. The energy code is blunt: air-permeable insulation shall not be used as a sealing material.
Why air sealing comes before insulation
Warm air rises. In winter it pushes out through every opening in the top-floor ceiling, and the air that replaces it is pulled in low, through the rim joists, the sill plate and the basement. The Department of Energy calls this the stack effect. ENERGY STAR adds that drafts are felt at windows and doors, but in most homes the most significant leaks are hidden in the attic and basement.
The leaks move water too. The Department of Energy notes that most water vapor movement in and through buildings is carried by air, and that insulating an attic without sealing it can make moisture problems worse, because the roof framing ends up colder. Order also matters for cost: sealing top plates under a foot of blown insulation means raking it all back first.
Is it worth it? EPA estimates that homeowners save an average of 15% on heating and cooling costs, or 11% on total energy costs, by air sealing and adding insulation in attics, floors over crawl spaces and basements. Weatherization and weatherproofing are the wider names for the same work; the Department of Energy defines home weatherization as sealing air leaks and adding insulation.
If a quote specifies a depth of insulation and never mentions air sealing, it is half a job.
Where a house leaks air, biggest first
ENERGY STAR’s rule is to plug the big holes first. Roughly in the order ENERGY STAR and the Department of Energy weight them:
- The attic floor: top plates, soffits and chases. ENERGY STAR says leakage is likely greatest where interior and exterior walls meet the attic floor, at dropped soffits over kitchen cabinets and bath vanities, and behind or under knee walls. Chases, stairwell walls and the open tops of balloon-framed walls also run straight into the attic.
- Rim joists and sill plates. ENERGY STAR names the top of the basement wall, where masonry meets wood framing, as a common leak, and says sealing it matters most at bump-outs such as bay windows.
- Recessed lights. ENERGY STAR calls cans that poke into the attic a big source of air leaks, and warns that in cold climates their heat can melt roof snow into ice dams.
- Bath fans. The gap around the housing leaks, and a fan ducted into the attic dumps wet air there. The residential code requires exhaust to go outdoors, never into an attic, soffit, ridge vent or crawl space (IRC Section M1501.1).
- Chimney and flue chases. ENERGY STAR calls the opening around a chimney or a furnace or water heater flue a major source of warm air moving into the attic.
- Attic hatches and pull-down stairs. The 2024 IECC wants attic access openings sealed with gasketing that allows repeated entry.
- Plumbing and wiring penetrations. Holes for wires, pipes and the vent stack pass through the attic floor and the basement ceiling. On exterior walls, the Department of Energy suggests foam gaskets behind outlet and switch plates.
- The garage-to-house connection. The Department of Energy’s Building America program says sealing an attached garage helps keep carbon monoxide and other contaminants out of the house. The leaks are the rim joist, the plates along the shared wall and floor cavities that run from over the garage to under living space; the door should be self-closing and weatherstripped.
- Fireplace dampers. A closed damper that doesn’t seal leaks all winter; more below.
Windows and doors leak too, but the hidden holes do more damage. Dark, dusty streaks in attic insulation show where house air gets through.
How air sealing is done
Match the material to the gap
The Department of Energy’s weatherization job aids size the material to the hole:
- Up to 1/4 inch: caulk, high-temperature rated near a heat source.
- 1/4 inch to 2 inches: one-component foam or mastic.
- 2 to 3 inches: two-component foam.
- 3 inches or larger: rigid material that won’t bend, sag or move, with support across spans wider than 24 inches.
Spray foam
Spray foam seals and insulates in one pass, the fast answer at rim joists, sill plates and wide, irregular gaps. Closed cell runs R-6.9 to R-7.1 per inch and also acts as a vapor retarder; open cell runs R-3.9 to R-4.2 per inch. Foam has fire rules about what covers it; see is spray foam safe and open cell vs closed cell.
Rigid board over soffits, chases and open walls
The Department of Energy’s air sealing checklist for contractors says uncovered soffits, openings under knee walls and the tops of balloon-framed gable walls are closed with rigid foam or OSB sealed at the edges, and that fibrous insulation is not an air barrier.
Chimneys and flues: fire-rated materials and clearances
Ordinary spray foam never goes against a chimney or a hot flue; ENERGY STAR says not to use it there, and the Department of Energy calls for fire-resistant materials such as sheet metal and furnace cement caulk. The rules:
- An interior masonry chimney needs 2 inches of airspace to combustibles, and that space may not be filled except for fireblocking (IRC Section R1003.18, in both the 2021 and 2024 editions).
- Where a chimney passes through a floor or ceiling, the gap is fireblocked with noncombustible material, fastened in place and self-supporting or laid on metal strips or lath (Section R1003.19).
- For metal flues, ENERGY STAR cites the 1-inch clearance building codes usually call for; that gap is closed with sheet metal or aluminum flashing bedded in high-temperature caulk.
- The Department of Energy’s job aid adds a durable fixed dam at least 2 inches taller than the final insulation, keeping combustibles at least 3 inches from the flue or chimney.
Hatches, stairs and doors
A hatch gets weatherstripping on its stops, a latch that compresses it and insulation on its back. For pull-down stairs, ENERGY STAR says to treat the attic door like a door to the outside: weatherstrip the edges and add rigid foam or an insulated cover. Exterior doors get a continuous gasket.
Recessed lights and bath fans
In new work the code wants recessed lights IC-rated, labeled at no more than 2.0 cfm of air leakage and gasketed or caulked to the ceiling (2024 IECC Section R402.5.4; 2021 IECC R402.4.5). Old cans that aren’t IC-rated need 3 inches of clearance from insulation, so the options are an airtight IC-rated replacement from an electrician or a sealed cover over the can that keeps that space. Fan housings get sealed to the ceiling.
What a blower door test measures
A blower door is a fan set in a panel that fills an exterior doorway. The Department of Energy describes the test: the fan pulls air out, outdoor air flows in through every gap, and a gauge called a manometer reads the pressure difference. Insist on a calibrated blower door; an uncalibrated one finds leaks but can’t measure them.
The reading is taken at 50 pascals of pressure. The airflow needed to hold that pressure is CFM50; CFM50 times 60, divided by the house volume in cubic feet, is ACH50, air changes per hour at 50 pascals. Lower is tighter. While the fan runs, a smoke pencil or infrared camera shows where air comes in. In older houses, Building America guidance warns that depressurizing can pull mold or asbestos into the house; pressurizing may be safer.
What the code requires for new homes
- New Jersey (2024 IECC): every new home is tested and must reach 3.0 ACH50 on the prescriptive path, or 4.0 on the performance and Energy Rating Index paths (Sections R402.5.1.2 and R402.5.1.3).
- Connecticut (2021 IECC as amended): 3.0 ACH50 on the prescriptive path, and never worse than 5.0 on any path (Sections R402.4.1.2 and R402.4.1.3).
- New York (2025 energy code): 3.0 ACH50 in climate zones 4 and 5, which cover Westchester, Long Island, Rockland, Putnam, Dutchess and Orange, and 2.5 in zone 6, which includes Ulster County.
Existing homes don’t have to test
No state we work in makes an existing house pass a blower door test when it is insulated or air sealed. New York’s 2025 code says testing is not required for alterations (Section R503.1.1.6), Connecticut’s code leaves it out of its alteration rules (R503.1.1 of the 2021 IECC), and New Jersey’s Rehabilitation Subcode doesn’t require one when an attic is insulated. For scale, NYSERDA’s Comfort Home program targets 10 ACH50 or lower when it seals a one- to four-family home. Connecticut’s Home Energy Solutions visit and PSEG Long Island’s assessment include a test; see what is a home energy audit.
Combustion safety and ventilation after tightening
The Department of Energy warns that negative pressure in a house can make fireplaces and fuel-burning appliances that draw room air, such as a natural-draft furnace or water heater, backdraft: exhaust spills into the house instead of going up the flue, carrying carbon monoxide. Its guidance is to test chimneys and combustion appliances before air sealing and again afterward, with a worst-case depressurization test using blower door equipment.
- Appliances. ENERGY STAR recommends having a heating technician check gas- or oil-fired furnaces, water heaters and dryers for proper venting after tightening. The Department of Energy favors sealed-combustion, induced-draft or power-vented replacements.
- Carbon monoxide alarms. EPA relays the Consumer Product Safety Commission’s advice: CO alarms on each level of every home, outside each sleeping area. New homes with a fuel-fired appliance or an attached garage must have them (Section R315 of the 2021 IRC, R311 of the 2024 IRC; New York’s code sends the rule to its fire code).
- Fresh air. The Department of Energy says a mechanical ventilation system is often needed once a home is air sealed, and the energy code requires mechanical ventilation in new homes built to its air barrier rules (Section R403.6).
- Radon. Sealing a basement or crawl space can raise indoor radon. Test for it.
We don’t do HVAC work. Ventilation systems, appliance venting and combustion repairs belong to a licensed heating contractor, and a combustion problem gets fixed before the sealing starts.
Drafts, fireplaces and backdraft dampers
A drafty house is usually the stack effect at work: cold air pulled in low because warm air escapes high. Caulking windows alone leaves the attic pulling just as hard. Seal the top and bottom of the house and most of the draft goes with it.
Backdraft dampers on fans and dryer vents
Every exhaust duct is a hole to the outside closed only by a flap. The code requires a backdraft damper at a dryer exhaust termination (IRC Section M1502.3) and in a range hood duct (M1503.3), and the energy code wants ventilation intakes and exhausts fitted with dampers that close when the fan is off (Section R403.6). A new damper or vent cap is work for an HVAC contractor or handyman. Our part is sealing the fan housing and duct penetration.
Fireplace and chimney drafts
The Department of Energy says to check that fireplace dampers close properly, and explains why they fail: metal flues warp or break after years of heating and cooling. For a fireplace that isn’t in use it suggests an inflatable chimney balloon, which deflates within seconds if a fire is lit under it. New wood-burning fireplaces must have tight-fitting flue dampers or doors and outdoor combustion air (Section R402.5.2 of the 2024 IECC, R402.4.2 in 2021). Top-sealing dampers and glass doors are for a chimney professional. Our part is the chimney chase in the attic, sealed with metal and high-temperature caulk to the clearances above.
One cold room
One cold room usually means a bedroom over the garage, a cape bedroom behind knee walls, a bump-out, or a weak supply duct. The first three are air sealing and insulation work; see garage insulation. The duct is an HVAC question.
Ice dams: an air leak showing on the roof
Ice dams need air below freezing and a roof deck above it. Snow melts where the roof is warm and refreezes at the cold eave, and meltwater pooling behind that ice can work back under the shingles. The Department of Energy counts warm air leaking up from the living space among the causes, with icicles as the telltale sign. ENERGY STAR calls a history of ice dams an indication of serious air leaks.
Building America’s prevention steps start with an airtight ceiling: limit the holes and seal every one, insulate well over the top plates, and keep a vent channel under the roof deck. The ice barrier membrane the residential code requires under shingles in colder areas (Section R905.1.2 of the 2024 IRC) limits the damage but doesn’t stop the dam.
Do ice dams melt on their own? Eventually, but water can back up meanwhile, and the Department of Energy warns that heavy ice can collapse a roof. Steaming off a dam is work for a roofer or an ice dam service. Stopping the next one is air sealing and attic insulation.
What air sealing costs
Most of our projects run between $3,500 and $12,500, and air sealing is usually priced with the insulation that goes over it. Square footage says little. What moves the number:
- Access. A walkable attic is quick; a low roof pitch, deep old insulation to move or a floored attic takes longer.
- Big openings. Soffits, chases, knee walls, balloon-framed walls and a chimney each add time and material.
- Recessed lights and rim joists. Old non-IC cans need covers or an electrician; long, crowded rim joists take longer.
- Material. Caulk and one-part foam cost less than spray foam, which seals and insulates in the same pass.
- Work by others first. Building America guidance holds off attic sealing and insulating while the roof leaks, knob-and-tube wiring is live, a fan exhausts into the attic, or vermiculite is present. Those fixes are priced by the trades that do them.
Rebates exist through state and utility programs; see New York and New Jersey insulation rebates and Connecticut insulation rebates. Program money is paid only on work by each program’s participating contractors, so ask any contractor, including us, whether they participate before you sign. There is no federal tax credit for air sealing on current work: the 25C credit, which covered it, ended for anything placed in service after 31 December 2025.
Air sealing a house yourself
ENERGY STAR’s do-it-yourself guide says a homeowner can take on attic air sealing if the attic is accessible and not hard to move around in, and that its projects usually take a day or two. A realistic list:
- Pull insulation back, cover soffits and open wall tops with rigid foam sealed at the edges, and plug open joist cavities under knee walls.
- Foam wire and pipe holes, caulk small cracks, and weatherstrip the hatch.
- In the basement, caulk or foam the sill plate and the top and bottom of each rim joist bay.
- Weatherstrip doors, caulk window trim, and add foam gaskets behind outlet covers on exterior walls.
Wear a respirator or dust mask and knee pads, and step only on joists. Use metal and high-temperature caulk at flues, never spray foam, and keep insulation 3 inches from recessed lights that aren’t IC-rated; ENERGY STAR says to consult a professional before sealing old cans. ENERGY STAR’s list of attics to hand to a contractor includes roof leaks, moldy framing, vents blowing into the attic, knob-and-tube wiring and many unsealed cans. Vermiculite stays undisturbed until a lab tests it. After real tightening, have the fuel-burning appliances checked.
Common questions about air sealing
What is air sealing?
Air sealing closes the holes between the living space and the attic, basement, garage and outdoors: top plates, chases, rim joists, recessed lights, fan housings, hatches and penetrations, using caulk, foam, rigid board and weatherstripping. Insulation slows heat through a surface; air sealing stops the air that carries heat and moisture through the gaps.
Should air sealing be done before insulation?
Yes. The leaks sit under the insulation, and fiberglass and cellulose don’t stop air; the energy code says air-permeable insulation shall not be used as a sealing material. Sealing first is cheaper while the top plates and chases are uncovered, and the Department of Energy warns that insulating without sealing can worsen attic moisture problems.
Is air sealing worth it?
Usually. EPA estimates an average saving of 15% on heating and cooling costs, or 11% on total energy costs, from air sealing plus insulation in attics and floors over crawl spaces and basements. The house also gets fewer drafts, less dust and pollen, better humidity control and a lower chance of ice dams.
How much does it cost to air seal a house?
Most of our projects run between $3,500 and $12,500, with air sealing usually priced together with the insulation over it. The number depends on attic access, the soffits, chases, knee walls and recessed lights, the rim joists, and whether spray foam is used. Repairs such as roof leaks come first, priced separately.
Can a house be too airtight?
It can be tight enough to need mechanical ventilation, which the Department of Energy says is often the case after air sealing. The real risk is combustion: natural-draft furnaces, water heaters and fireplaces can backdraft in a tighter house, so they should be tested before and after the work, with carbon monoxide alarms on every level.
What is a good blower door test result?
Lower ACH50 is tighter. New homes must test at 3.0 ACH50 or lower on the prescriptive path in New Jersey and Connecticut, and in most of our New York counties; 2.5 in New York’s zone 6. Existing houses don’t have to test; NYSERDA’s Comfort Home program aims for 10 ACH50 or lower after sealing a one- to four-family home.
How do you air seal recessed lights?
Replace old cans with IC-rated, airtight fixtures, the kind the energy code requires in new work, gasketed or caulked to the ceiling. Cans that aren’t IC-rated need 3 inches of clearance from insulation, so they get a sealed cover built over them in the attic that keeps that space. Don’t stuff or foam an old can.
What is the difference between air sealing and a vapor barrier?
Air sealing stops air moving through holes; a vapor retarder slows water vapor diffusing through a material. Air sealing matters more for moisture, because the Department of Energy notes that most water vapor movement in buildings is carried by air. Closed cell spray foam does both jobs; open cell foam seals air but is not a vapor retarder.
Do ice dams mean my attic leaks air?
Often. Ice dams form when the roof deck stays above freezing in freezing air, and warm house air leaking into the attic is one cause. ENERGY STAR calls a history of ice dams an indication of serious air leaks. Sun and leaky attic ducts can warm a roof too, so look in the attic.
Air sealing in New Jersey, Connecticut, Westchester, the Hudson Valley and Long Island
We start in the attic and basement: top plates, chases, soffits, knee walls, lights, fans, hatch and chimney above; rim joist, sill and penetrations below. We note what has to come first, such as a roof leak, knob-and-tube wiring or a natural-draft water heater. You get it in writing: the sealing, the insulation over it, and the price of our part. 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.