Homeowners tend to think about insulation and air sealing as the same category of upgrade. They're related but distinct, and understanding the difference changes how you prioritize your retrofit dollars.
What air sealing actually does
Insulation slows heat conduction — the movement of heat through solid material. Air sealing stops air infiltration — the movement of conditioned air out of the house (and cold air in) through gaps, cracks, and penetrations in the building envelope.
A house with good insulation but significant air leakage can still lose a large portion of its heating energy through air movement. The stack effect in a two-storey house pulls warm air out through gaps in the attic and upper floors while drawing cold air in through gaps in the basement and lower walls. Insulation has no effect on this exchange — only air sealing does.
What the blower door test tells you
A blower door test depressurizes the house and measures how quickly air escapes, expressed as air changes per hour at 50 pascals (ACH50). An ACH50 of 3 or lower is considered tight. Many pre-1990 Ontario homes test at ACH50 8–15 or higher.
For a home at ACH50 12, air leakage may account for 30–40% of total heating energy loss. In that case, comprehensive air sealing will often deliver more energy reduction per dollar spent than adding more insulation to an already-adequate attic.
A common pattern: Homes with adequate attic insulation (R-30 or higher) but high blower door results (ACH50 8+) are better candidates for air sealing as the priority upgrade — not more insulation. Without a blower door test, this is invisible.
Where air leaks typically occur in Ontario homes
- Attic bypass paths — gaps around light fixtures, plumbing stacks, partition walls, and electrical boxes that penetrate the attic floor
- Rim joists — the perimeter of the first floor where floor framing meets the foundation, a common and often overlooked source of significant leakage
- Attic hatch — often has no weatherstripping and no insulation on the back
- Service penetrations — where gas lines, electrical conduit, and HVAC ducts enter the conditioned space
- Window and door frames — particularly in older homes where caulking has shrunk and cracked
Air sealing and insulation together
Air sealing and insulation are most effective when done together or in sequence. Sealing attic bypasses before blowing in insulation makes the insulation more effective, since those bypasses allow warm air to bypass the insulation layer entirely. Most contractors who do attic insulation should also seal the common bypass locations as part of the same job — but this isn't always done unless specified.
We confirm that air sealing is included in the scope of work before connecting clients with attic insulation contractors, because the two measures are only partly effective when done in isolation.
Rebate implications
The HRS Program includes an air sealing rebate of up to $1,000, typically requiring that insulation work is completed alongside it. The post-retrofit blower door test verifies the improvement achieved. Homes that can demonstrate a significant reduction in ACH50 between pre- and post-retrofit audits receive the air sealing rebate in addition to insulation rebates.