Air sealing is the part of home insulation work that most contractors underprovide and most homeowners underestimate. A well-insulated home that isn’t air-sealed is like a well-insulated sleeping bag with all the zippers open — the insulation slows conductive heat loss, but the convective heat loss through air movement overwhelms it. In many local homes, air leakage accounts for 25–40% of heating and cooling costs. Addressing it is often more impactful than adding more insulation depth.
“Blower door testing changes everything about how you understand a house. You can see on a thermal camera exactly where the air is moving. The pot lights are usually obvious — they look like blowtorches on the camera. The top plates at the attic floor. The penetrations around pipes and wires. Once you’ve done enough blower door tests, you know exactly where to look. And you know that insulation without air sealing is doing maybe half the job.”
— Priya Sundaram, certified insulation contractor and building envelope specialist with 16 years of experience in residential insulation and energy efficiency upgrades in the local area
Table of Contents
Questions About the Air Sealing Scope
Will air sealing be performed as part of this insulation project?
Ask directly — and ask what’s specifically included. “We seal as we go” is not an adequate answer. A proper air sealing scope describes specific locations.
For an attic insulation project, air sealing should address at minimum:
Top plates: The gap between the interior wall framing and the attic floor (where the wall meets the ceiling) is one of the most significant air bypass locations in a typical local home. Spray foam or caulk seals this gap.
Penetrations: Every pipe, duct, wire, and chase that passes through the ceiling/attic floor. Utility penetrations, plumbing vents, and electrical wiring all create gaps that must be sealed.
Pot light boxes: Recessed lights that aren’t airtight require an airtight cover installed from the attic side. See the attic insulation article for more detail.
Attic hatch: Weather stripping and an insulated cover box.
Chimney chase: The gap between the attic floor and a chimney must be sealed with fire-rated materials (metal and fire-rated caulk, not foam).
Dropped ceilings (soffits above cabinets): A common but often overlooked air bypass — the space above kitchen cabinet soffits or bathroom soffit ceilings is often open to the attic.
What a good answer looks like: A written list of specific air sealing locations included in the scope.
Will blower door testing be used to assess and verify air sealing?
A blower door test uses a calibrated fan mounted in an exterior doorway to depressurize the house, then measures the air flow rate needed to maintain a specific pressure difference. This measures the overall air tightness of the building envelope (reported in air changes per hour at 50 Pascals — ACH50).
Before air sealing: Establishes the baseline leakage rate and, with thermal imaging, helps locate the major leakage sites.
After air sealing: Confirms the improvement achieved.
Blower door testing is required for many rebate programs (national energy-efficiency rebate programs) and gives objective measurement of the improvement. Ask whether it’s included in the scope.
What a good answer looks like: Blower door testing confirmed as part of the scope, or a clear referral to an energy advisor who will perform pre and post testing.
Questions About Air Sealing Materials
What materials will you use for air sealing?
Different air sealing applications require different materials:
Expanding spray foam: Appropriate for sealing around pipes, wires, and gaps in wood framing. One-component low-expansion foam for gaps up to approximately 3 cm; two-component or specialized foam for larger gaps.
Acoustical sealant / acoustic caulk: Flexible, remains pliable — appropriate for sealing around electrical boxes, at the base of walls, and at penetrations where movement is expected.
Rigid foam board + tape: For larger openings like dropped ceiling soffits and chimney chases.
Metal flashing + fire-rated caulk: Required at chimney chases and any penetration through a fire-rated assembly.
Weatherstripping: For attic hatches and access panels.
Ask what material is being used for each major air sealing location.
Questions About Air Sealing in the Rest of the House
Is attic air sealing the right priority, or are there other significant leakage sites?
An energy audit with blower door testing identifies where the air leakage is actually occurring — which is often not where homeowners assume. In some homes, the basement is a larger source of leakage than the attic. In others, leaky windows or sliding doors dominate.
Ask whether the contractor or their energy advisor partner can identify the major leakage sites before work begins — to ensure the most impactful air sealing is prioritized.
What about basement and crawlspace air sealing?
For homes with unconditioned basements, the rim joist — the framing member that sits on top of the foundation wall and supports the floor structure — is a major source of heat loss and air infiltration. Spray foam (typically closed-cell) at the rim joist is one of the most cost-effective insulation improvements in many your region homes.
Ask whether rim joist insulation and air sealing is included in the scope or can be added.
Questions About Air Quality After Air Sealing
Does improved air sealing affect indoor air quality?
This is an important question that many contractors don’t raise proactively. A home that’s made significantly tighter through air sealing has reduced natural ventilation. If the home already has mechanical ventilation (an HRV — heat recovery ventilator — or ERV), the unit should be commissioned and balanced after air sealing work to ensure adequate fresh air exchange.
If the home doesn’t have mechanical ventilation, ask whether an HRV should be considered alongside air sealing work. A tight home without controlled ventilation can accumulate moisture, CO2, and indoor air pollutants.
What a good answer looks like: The contractor proactively raises the ventilation question when significant air sealing is planned and recommends an HRV assessment if the home doesn’t already have one.
Related Questions
- Questions to Ask an Insulation Contractor Before You Hire One
- Questions to Ask an Insulation Contractor About Attic Insulation
- Questions to Ask an Insulation Contractor About Spray Foam
- Questions to Ask About Home Energy Audits and Rebates
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