Last updated: April 7, 2026
The 10 Most Common Air Leaks in BC Homes
Key takeaway: Most blower door test failures come from the same 10 locations. If you know where to look and how to seal them, hitting 2.5 ACH50 (Step 3) is straightforward and 1.5 ACH50 (Step 4) is achievable. Below 1.5, aerosol sealing becomes the most reliable method because it finds gaps that manual methods miss.
Air leakage is not random. It follows patterns, and the same spots show up on blower door tests project after project. This guide covers the 10 most common leak locations in new Part 9 homes in BC, why they leak, and how to seal them using both traditional methods and aerosol sealing.
How Air Leaks Affect Performance
A quick reference for why this matters:
| ACH50 | Step Level | What It Means |
|---|---|---|
| 3.5+ | Below Step 3 | Significant air leakage, common in pre-2023 code builds |
| 2.5 | Step 3 | Baseline mandatory target since 2023 |
| 1.5 | Step 4 | Expected 2027 mandatory target |
| 1.0 | Step 5 | Net-zero-ready performance |
Every 0.5 ACH50 reduction in a 2,000 sq ft home translates to roughly $200 to $400 per year in energy savings and 3,000 to 5,000 BTU/h reduction in peak heating load.
1. Rim Joists (Band Joists)
Where: The horizontal framing member that sits on top of the foundation wall, between the subfloor and the sill plate. Also present at second-floor transitions.
Why it leaks: The rim joist is a transition zone where multiple materials meet: foundation, sill plate, subfloor, and wall framing. The gap between the sill plate and the foundation top is often the single largest leak in a new home. The rim joist cavity itself, between floor joists, connects the wall cavity to the floor system.
Traditional sealing: Two-part spray foam applied to the interior face of the rim joist, filling the full cavity between each floor joist. Minimum 2” thickness for air sealing. Sill plate sealed to foundation with acoustical sealant or gasket.
Aerosol sealing: Pressurized sealant fills the sill-to-foundation gap and any openings in the rim joist area from the inside. Particularly effective at sealing the small, irregular gaps between the sill plate and concrete that are difficult to reach with manual methods.
Impact: Sealing the rim joist area alone can reduce ACH50 by 0.5 to 1.0 in a typical home.
2. Electrical Penetrations
Where: Every electrical box, wire penetration through plates, and exterior wall outlet/switch location.
Why it leaks: Standard electrical boxes are not airtight. Wires pass through drilled holes in top and bottom plates. Exterior wall boxes have gaps between the box and the drywall cutout. In a typical 2,000 sq ft home, there are 80 to 120 electrical penetrations through the air barrier.
Traditional sealing: Airtight electrical boxes (vapor-sealed type). Acoustical sealant around wire penetrations through plates. Gaskets behind cover plates on exterior walls. Some builders wrap standard boxes in poly or use putty pads.
Aerosol sealing: Pressurized sealant fills the gaps around wires and boxes automatically. This is one of the areas where aerosol sealing excels because the number of penetrations is high and the gaps are small and irregular.
Impact: Individual boxes leak small amounts, but cumulatively they can add 0.3 to 0.8 ACH50.
3. Plumbing Stack and Vent Penetrations
Where: Drain/waste/vent (DWV) pipes that pass through top plates to exit through the roof. Supply lines that pass between floors.
Why it leaks: Plumbing penetrations through top plates are typically drilled oversized (3” to 4” holes for 1.5” to 3” pipes). The gap between the pipe and the framing is rarely sealed properly. DWV stacks create a direct chimney effect from the basement to the attic.
Traditional sealing: Fire-rated expanding foam or fire caulk around penetrations through top plates. Metal flashing collars at roof penetrations. Note that some penetrations require fire stopping, so the sealant must be rated appropriately.
Aerosol sealing: Fills the annular gap between pipe and framing. Works well because these gaps are often in hard-to-reach locations where manual application is awkward.
Impact: A single unsealed 3” plumbing penetration can contribute 0.1 to 0.3 ACH50 due to the stack effect.
4. Recessed Lights (Pot Lights)
Where: Any recessed light fixture installed in an insulated ceiling, particularly in cathedral ceilings and ceilings below attic space.
Why it leaks: Standard recessed light housings have ventilation openings to prevent overheating. These openings connect the conditioned space directly to the attic. Even IC-rated (insulation contact) fixtures are not airtight unless specifically rated as IC/AT (airtight).
Traditional sealing: Use only IC/AT rated fixtures in insulated ceilings. Build airtight enclosures over non-AT fixtures using rigid insulation or manufactured covers sealed to the ceiling drywall. In new construction, specify IC/AT fixtures from the start.
Aerosol sealing: Less effective for recessed lights because the leak path is through the fixture itself, not around it. Aerosol sealant can help seal gaps between the fixture housing and the drywall, but the fixture must still be AT rated.
Impact: Each non-airtight recessed light can add 0.05 to 0.15 ACH50. A home with 20 pot lights using non-AT housings could lose 1.0+ ACH50 through the ceiling alone.
5. Window and Door Rough Openings
Where: The gap between the window/door frame and the rough framing. Every window and door has this gap.
Why it leaks: Windows and doors are shimmed into rough openings that are 1/2” to 1” larger than the unit. The gap is traditionally filled with low-expansion foam, but the foam does not always form a continuous seal. Settlement, wood movement, and foam shrinkage can create air paths over time.
Traditional sealing: Low-expansion foam applied in a continuous bead on all four sides. Some builders use backer rod and caulk. Interior flashing tape (like Siga Fentrim) provides a more reliable air seal by bridging from the window frame to the interior air barrier.
Aerosol sealing: Fills the rough opening gap under pressure, ensuring complete contact. Works particularly well on windows where the gap is irregular or where foam has not fully expanded to fill the cavity.
Impact: Poorly sealed windows can contribute 0.1 to 0.2 ACH50 per window. A home with 15 windows and 3 doors could see 1.5+ ACH50 from rough openings alone.
6. Attic Hatches and Scuttle Holes
Where: Access panels to attic space, typically in hallways or closets.
Why it leaks: Attic hatches are one of the most overlooked leak points. A standard plywood hatch sitting in a framed opening has no weatherstripping, no compression seal, and a gap on all four sides. Warm air rises directly into the attic.
Traditional sealing: Weatherstrip the hatch frame with adhesive foam tape. Add rigid insulation to the top of the hatch panel (R-30+ to match ceiling insulation). Install latches that compress the hatch against the weatherstripping. Some builders use pre-manufactured airtight attic access panels.
Aerosol sealing: Can seal the frame-to-ceiling junction but cannot replace weatherstripping on the hatch itself. This is one area where manual attention is always needed.
Impact: An unsealed attic hatch can add 0.2 to 0.5 ACH50, especially in winter when the stack effect pulls warm air upward.
7. Cantilevers and Bay Windows
Where: Floor or wall sections that project beyond the foundation wall: bay windows, bump-outs, cantilevered floors, and second-story overhangs.
Why it leaks: Cantilevers create complex junctions where the floor system transitions from interior to exterior. The underside of the cantilever is exposed to outside air, and the cavity between floor joists is often open to the wall cavity above. Insulation is stuffed in but the air barrier is frequently incomplete.
Traditional sealing: Rigid blocking between joists at the exterior wall plane. Spray foam on the underside of the cantilevered floor. Air barrier continuity from the wall to the underside of the cantilever. This is one of the most difficult details to execute manually because access is limited once the structure is assembled.
Aerosol sealing: Highly effective for cantilevers. The pressurized sealant fills gaps in the joist blocking and at transitions that are essentially impossible to reach manually once the structure is assembled.
Impact: A poorly sealed cantilever can add 0.3 to 0.8 ACH50 depending on size and complexity.
8. Bathtub and Shower Drain Penetrations
Where: The drain penetration through the subfloor, particularly for bathtubs on exterior walls or above unheated spaces.
Why it leaks: Bathtub drains require a 2” to 3” hole through the subfloor. The gap between the drain pipe and the subfloor is rarely sealed because it is hidden behind the tub. On second floors, this connects the joist cavity to the floor below. On ground floors over crawl spaces, it connects directly to outside air.
Traditional sealing: Acoustical sealant or fire caulk around the drain penetration through the subfloor, applied before the tub is set. This must be done during rough-in because it is inaccessible after the tub is installed. Many builders miss this.
Aerosol sealing: Can reach and seal drain penetrations even after the tub is installed, which is a significant advantage. The pressurized sealant finds the gap from the accessible side of the subfloor.
Impact: 0.05 to 0.15 ACH50 per penetration. Not huge individually, but combined with other plumbing penetrations, it adds up.
9. HVAC Penetrations and Ductwork
Where: Supply and return duct penetrations through floors, walls, and ceilings. Fresh air intake and exhaust for HRV. Range hood and dryer exhaust.
Why it leaks: Duct boots are screwed to subfloor or drywall with gaps around the perimeter. HRV ducting passes through exterior walls for intake and exhaust. Every duct penetration through the air barrier is a potential leak.
Traditional sealing: Mastic sealant at all duct connections and boot-to-surface junctions. Caulk around exterior wall penetrations. Sealed backdraft dampers on exhaust terminations.
Aerosol sealing: Seals duct boot gaps and penetrations under pressure. For duct leakage inside the envelope, some builders use aerosol duct sealing (a related but separate technology) to seal the duct system itself.
Impact: 0.2 to 0.5 ACH50 from combined HVAC penetrations. HRV penetrations through exterior walls are particularly important because they are large diameter (6”) and pass directly through the air barrier.
10. Foundation Sill Plate and Mudsill
Where: The junction between the concrete foundation and the bottom of the wood frame wall assembly.
Why it leaks: Concrete is not flat. Even with a sill gasket, the irregular surface of the foundation top creates gaps under the sill plate. Foundation anchor bolts create additional penetration points. The sill-to-foundation junction runs the entire perimeter of the building.
Traditional sealing: Closed-cell foam sill gasket installed before framing. Acoustical sealant applied to the interior side of the sill plate after framing. Spray foam applied at the sill-to-foundation junction from inside.
Aerosol sealing: Fills the sill-to-foundation gap comprehensively. The sealant follows the irregular concrete surface and fills voids that gaskets bridge over. This is one of the most impactful applications of aerosol sealing because the gap is long (entire perimeter) and irregular (concrete surface).
Impact: 0.3 to 0.8 ACH50 from the sill plate perimeter. On a 2,000 sq ft single-story home with 180 linear feet of sill plate, even a small average gap creates significant cumulative leakage.
The Cumulative Effect
Here is why these 10 locations matter: add them up.
| Leak Location | Potential ACH50 Contribution |
|---|---|
| Rim joists | 0.5 - 1.0 |
| Electrical penetrations | 0.3 - 0.8 |
| Plumbing penetrations | 0.1 - 0.3 |
| Recessed lights | 0.2 - 1.0 |
| Window/door rough openings | 0.5 - 1.5 |
| Attic hatches | 0.2 - 0.5 |
| Cantilevers | 0.3 - 0.8 |
| Bathtub drains | 0.05 - 0.15 |
| HVAC penetrations | 0.2 - 0.5 |
| Foundation sill plate | 0.3 - 0.8 |
| Total potential | 2.65 - 7.35 |
A home where none of these are sealed could easily test above 5.0 ACH50. A home where all are sealed with traditional methods by an experienced crew will typically hit 1.5 to 2.5 ACH50. A home sealed with aerosol sealing after traditional methods will consistently hit 0.5 to 1.5 ACH50.
The difference between passing Step 3 (2.5 ACH50) and passing Step 4 (1.5 ACH50) often comes down to how thoroughly these 10 locations are addressed.
The Approach for Step 4
For builders targeting 1.5 ACH50, the recommended approach:
- Design stage: Plan the air barrier continuity. Identify complex details (cantilevers, plumbing stacks) and plan sealing access.
- Framing: Use airtight electrical boxes, seal plates during assembly, install sill gaskets.
- Rough-in: Seal all penetrations through the air barrier (plumbing, electrical, HVAC) before insulation.
- Pre-drywall air sealing: Comprehensive manual sealing pass at all 10 locations.
- Pre-drywall blower door test: Test before drywall to find and fix remaining leaks.
- Aerosol sealing: If the pre-drywall test is above target, aerosol sealing can bring it down. Even if the pre-drywall test is on target, aerosol sealing provides margin for any leakage introduced during drywall installation.
This layered approach gives you the best chance of passing the final blower door test without schedule delays or costly rework. For a step-by-step breakdown with results data by method, see our guide on how to achieve Step Code 4.