How to Achieve 1.5 ACH50 in Wood Frame Construction in the Okanagan
Key takeaway: Hitting 1.5 ACH50 on a typical Okanagan wood-frame build requires a deliberate air barrier strategy across five construction stages: design, framing, sheathing, rough-in, and pre-drywall sealing. Manual sealing alone gets most builders into the 2.0 to 2.5 ACH50 range. Reliably crossing the 1.5 threshold (Step Code 4, expected 2027) usually requires aerosol sealing as a finishing layer. This guide walks the full sequence.
What 1.5 ACH50 actually means
ACH50 stands for “air changes per hour at 50 Pascals of pressure.” A blower door fan depressurizes the house to 50 Pa (roughly the equivalent of a 20 mph wind on every wall) and measures how much air leaks in. Divide that by the interior volume of the house and you get ACH50. A result of 1.5 ACH50 means the house exchanges its full air volume 1.5 times per hour under that test pressure.
For context across BC Step Code levels:
| Step Code Level | ACH50 Target | What it represents |
|---|---|---|
| Step 3 (current minimum) | 2.5 | Achievable with disciplined manual sealing |
| Step 4 (expected 2027) | 1.5 | Demands deliberate air barrier strategy |
| Step 5 (net-zero ready) | 1.0 | Belt-and-suspenders required |
| Passive House | 0.6 | Specialized envelope detailing |
A 1.5 ACH50 result represents a 40% reduction in air leakage from the current 2.5 ACH50 minimum. That’s not a small step. It changes how you frame, sheath, and detail the air barrier, and it usually changes which trades touch the building envelope.
The five stages of a 1.5 ACH50 build
Reaching 1.5 ACH50 reliably is a sequence of decisions made across the entire build, not a single intervention at the end. Treat it as five connected stages.
Stage 1: Design (before permit)
Before framing starts, the air barrier strategy needs to be drawn, not just described. Specifically:
- Identify the air barrier plane. On most Okanagan wood-frame builds this is the exterior side of the sheathing (using sheathing tape and a fluid-applied or self-adhered membrane). Some builds run the air barrier on the interior side (smart vapor retarder doubling as air barrier). Pick one and detail it consistently.
- Trace the air barrier on every section drawing. Highlight the line on every elevation, section, and detail. Where it crosses framing transitions (rim joist, wall-to-roof, slab-to-wall), draw the connection.
- Engage your energy advisor early. At Step 4, the energy advisor isn’t an afterthought. Their HOT2000 model needs to reflect the actual assembly you’re going to build, not a generic Step 3 spec.
- Confirm window and door specs. Most Okanagan Step 4 builds use windows with U-value 1.4 W/m²K or better. Specify them in the contract documents, not on the fly.
See the energy advisor guide for selecting and engaging an EA.
Stage 2: Framing
Framing is where most ACH50 failures start. The framers don’t have to be air sealing experts, but they do have to understand which choices affect the air barrier:
- Glue-and-screw the bottom plate to the subfloor. Or use a sill gasket. The plate-to-subfloor joint is the single highest-frequency air leakage point in BC homes (see common air leaks).
- Foam-fill the rim joist before sheathing closes it in. Closed-cell spray foam or pre-cut rigid insulation with sealed edges. Don’t trust fiberglass batts to provide the air seal at the rim.
- Block off interior partition tops. Where interior walls meet the ceiling plane, gaps to the attic create direct air paths. Block and seal.
- Tape sheathing seams as the wall goes up. Don’t wait for “after framing.” Tape goes on while the walls are vertical and accessible.
Stage 3: Sheathing and air barrier installation
This is where the air barrier becomes physically continuous. The discipline here determines whether the final result lands at 1.5 or at 3.0.
- Tape every sheathing seam. Use a flashing tape rated for exterior exposure (e.g., 3M 8067, Siga Wigluv, or equivalent). Roller-press every seam.
- Detail every penetration as you make it. Service penetrations (electrical, plumbing, mechanical) get their own air seal at the moment of installation, not retroactively. Use airtight grommets or sealant.
- Fluid-applied or self-adhered membrane at corners and openings. Prefab corners and pre-applied window flashing membranes outperform field-cut detailing.
- Window rough openings: backer rod plus low-expansion foam plus interior air seal. The interior side of the window-to-frame joint is often skipped; don’t.
Stage 4: Rough-in coordination
Plumbers and electricians cut through the air barrier dozens of times per house. If they don’t know there’s an air barrier strategy, they’ll defeat it.
- Walk every trade through the air barrier on day one. A 15-minute conversation per trade, with the section drawings open, prevents most penetration failures.
- Spec airtight electrical boxes on exterior walls. Or use poly box wraps with sealed edges. Standard plastic boxes leak.
- Seal plumbing stack penetrations. Where DWV plumbing penetrates ceiling planes or top plates, fire-rated sealant at every penetration.
- Mechanical ducts in unconditioned space need to be inside the air barrier. Ductwork in vented attics is a Step 4 killer; relocate to conditioned space or insulate to R-12 minimum.
Stage 5: Pre-drywall blower door test (the moment of truth)
Run a blower door test before drywall closes everything in. This is the single highest-leverage decision a Step 4 builder makes.
- Schedule the test after rough-in is complete and the air barrier is fully detailed, but before drywall. That’s typically a 1 to 2 day window.
- Target a pre-drywall reading of 2.5 ACH50 or below. If you’re above that, find and fix the leaks now. After drywall the cost to fix triples.
- Expect to find leaks. Even the best framing crews leave 3 to 5 air barrier discontinuities per house at this stage. Finding them in pre-drywall is normal and cheap.
- In the City of Kelowna, the $325 mid-construction blower door test rebate offsets most of the test cost. See the Kelowna municipality guide.
For the full pre-drywall workflow, see pre-drywall air sealing.
Where manual sealing alone hits its ceiling
After the five stages above, well-executed manual sealing on a typical Okanagan wood-frame build lands somewhere between 2.0 and 2.5 ACH50. Some crews routinely deliver 1.8 to 2.0. Very few consistently deliver below 1.8 with manual methods alone.
The reason isn’t that builders aren’t trying. It’s that manual sealing depends on finding every leak by eye. Beyond about 2.0 ACH50, the remaining leaks are distributed micro-leaks at framing intersections, behind backer rod, around shifted gaskets, and through tiny gaps that aren’t visible during walk-through inspection. Fixing them by hand requires unrealistic levels of attention across every trade.
The 1.5 ACH50 target sits below where manual sealing reliably lands. To cross that threshold consistently, most builders add aerosol sealing as a finishing layer.
Adding aerosol sealing to cross 1.5
Aerosol sealing (most commonly AeroBarrier in BC) pressurizes the house to 50 Pa during a single mid-construction visit and atomizes a water-based acrylic sealant inside. The pressure differential drives sealant particles toward every gap in the envelope, sealing leaks as small as a human hair. The on-board blower door displays the live ACH50 reading, so the operator stops when the target is hit.
For an Okanagan wood-frame build:
- Typical pre-aerosol reading: 4 to 6 ACH50 (after framing crew’s manual seal but before aerosol pass)
- Typical post-aerosol result: 0.5 to 1.5 ACH50
- Typical seal-day duration: 4 to 6 hours on site
- Step 4 (1.5 ACH50) hit rate in published Okanagan project data: 92% (see case studies)
Aerosol sealing isn’t a replacement for the five build stages above. It’s a finishing layer that closes the gap between “well-sealed manual build” (2.0 ACH50) and “Step 4 compliance” (1.5 ACH50 or below). See AeroBarrier vs traditional air sealing for the full comparison.
Cost and timing
For a typical 2,500 sq ft Okanagan wood-frame Step 4 build, the air barrier strategy adds roughly:
| Stage | Typical incremental cost (vs Step 3) |
|---|---|
| Design (energy advisor extended scope) | $1,000 to $2,000 |
| Framing (sealants, gaskets, foam) | $500 to $1,500 |
| Sheathing (tape, membrane, flashing) | $1,500 to $3,000 |
| Rough-in coordination time | $500 to $1,000 |
| Pre-drywall blower door test | $400 to $800 |
| Aerosol sealing (if used) | $4,500 to $5,500 |
| Total Step 4 envelope premium | $8,400 to $13,800 |
Against this, the FortisBC New Home Program at Step 4 returns $9,000 to $15,000 per dwelling unit, which typically covers the full premium. See the Step Code rebates guide.
Common reasons builds miss 1.5 ACH50
When a build comes in at 1.7 to 2.2 ACH50 instead of 1.5, the post-mortem usually identifies one of these:
- Pre-drywall test was skipped or done too late. The 1 to 2 day window between rough-in completion and drywall is the only time leaks are cheaply visible
- Air barrier strategy wasn’t drawn. The framers built what they always build, and the air barrier became an afterthought
- Trades weren’t briefed. Plumbers and electricians cut through the envelope without resealing
- Window flashing was field-cut at intersections. Pre-formed corners and pre-applied flashings outperform field detailing
- Manual sealing was assumed sufficient at 1.5 ACH50. It rarely is, on Okanagan custom geometries
Frequently asked questions
Can I hit 1.5 ACH50 with manual sealing only?
Sometimes, on simple geometries with disciplined crews. In published Okanagan project data, manual-only Step 4 hit rates run roughly 30 to 40%. Aerosol-supplemented hit rates run 92%. If your build has cathedral ceilings, walkout basements, complex rooflines, or attached garages, manual-only is not the recommended path.
What’s the cheapest way to hit Step 4?
The cheapest way to attempt Step 4 is manual sealing alone. The cheapest way to reliably hit Step 4 is manual sealing as the baseline plus aerosol sealing as the finishing layer. Counted with FortisBC rebates, the aerosol path usually costs less than failed manual attempts that require post-drywall remediation.
When is the right time in the build to seal?
Aerosol sealing happens after framing, sheathing, windows, and rough-in are complete, but before drywall. That’s typically a 1 to 2 day window. Manual air sealing happens continuously through framing and sheathing. The pre-drywall blower door test happens at the same window as the aerosol seal.
Does Climate Zone 5 vs Climate Zone 6 change the airtightness target?
No. The 1.5 ACH50 Step 4 target is climate-independent. Climate zone affects insulation R-values, window U-values, and TEDI/MEUI targets, but the airtightness number is the same statewide.
What if my project is in Whistler at Step 4 already?
Whistler has been at Step 4 (1.5 ACH50) since January 2024 in Climate Zone 6. The same five-stage process applies. The harder part in Whistler is meeting TEDI in CZ6, which usually requires R-15+ exterior insulation rather than R-7.5. See the Whistler municipality guide.
Do I need an energy advisor for a Step 4 build?
Yes. Step Code compliance requires HOT2000 modeling and as-built compliance reports from a registered energy advisor. At Step 4, engage them at design stage, not after framing. See the energy advisor guide.
Next steps
- Run the rebate calculator for your specific project
- Review Step Code 4 requirements for the full compliance picture
- See aerobarrier case studies for real ACH50 results
- Compare air sealing methods before pricing your envelope