What is ACH50 and how is it measured?

ACH50 (Air Changes per Hour at 50 Pascals) measures how many times the entire volume of air in a home is replaced in one hour when the building is pressurized to 50 Pa using a blower door. Lower numbers mean a tighter building.

What ACH50 target is required for Step Code 4?

Step Code 4 requires 1.5 ACH50 or better. This is a significant jump from Step 3's 2.5 ACH50 target and typically requires a deliberate air sealing strategy beyond manual methods alone.

When should I do a mid-construction blower door test?

The mid-construction test should happen after framing, insulation, and air barrier installation but before drywall. At this stage, every air barrier connection is visible and accessible, making leaks easy to find and fix.

ACH50 Airtightness Targets by Step Level

Key takeaway: Airtightness is the one Step Code metric you cannot fake with modelling. Step 4 requires 1.5 ACH50. The jump from Step 3 (2.5) is where most builders struggle.

Targets by Step Level

Step	ACH50 Target	Context
Step 1	None	Base code confirmation only
Step 2	3.0	Standard construction practices
Step 3	2.5	Current BC minimum. Requires air barrier attention
Step 4	1.5	Expected 2027. Deliberate air sealing strategy
Step 5	1.0	Net-zero ready. Passive House-adjacent

ACH50 targets do not vary by climate zone or house size. BC also allows NLA and NLR metrics that account for building surface area. Talk to your energy advisor.

How Blower Door Testing Works

A calibrated fan depressurizes the building to 50 Pascals. The fan measures how much air flows through the building. Airflow divided by home volume = ACH50.

The official test happens after construction, before occupancy. A registered energy advisor conducts it and produces the compliance report. For the full process, see our blower door testing guide.

Why Mid-Construction Testing Matters

At Step 3, some builders skip the mid-construction test. At Step 4, that is a dangerous gamble. Every air barrier connection is visible and fixable before drywall. After drywall, the same leaks cost 5 to 10 times more to fix.

Kelowna offers a $325 building permit rebate for mid-construction testing. See our pre-drywall guide.

Common Failure Points at Step 4

Ranked by typical contribution to total leakage:

Bottom plate to subfloor. Largest source of leakage in BC homes. Sealant or gasket is essential.
Window and door rough openings. Gap between frame and rough opening. Low-expansion foam plus air barrier tape.
Electrical boxes on exterior walls. Airtight boxes or sealed poly boots required at Step 4.
Plumbing and mechanical penetrations. Fire-rated sealant at all air barrier penetrations.
Attic-to-wall transitions. Interior partitions meeting ceiling plane often create direct air paths to the attic.
Rim joist areas. Spray foam or rigid insulation with sealed edges.

Air Sealing Approaches by Target

2.5 ACH50 (Step 3): Careful manual sealing. Poly with taped seams, acoustical sealant at plates, foam at openings.

1.5 ACH50 (Step 4): Manual methods become inconsistent. Many builders add aerosol-based sealing to close the gap from 2.0 to 1.5. The computer-controlled process targets a specific ACH number.

1.0 ACH50 (Step 5): Advanced air barrier systems plus aerosol sealing. Every imperfection adds up.

What Affects Your Result

Building size and shape. Complex geometry = more transitions = more leak points.
Number of penetrations. Simpler mechanical layouts test tighter.
Air barrier continuity. Fewer material transitions = easier to seal.

Use our rebate calculator to estimate how airtightness connects to your bottom line.