A builder’s net zero envelope course by Xerox Builders · October 2026
The air barrier is the first envelope decision on a high-performance home, and it has to be made before framing starts. This Detail shows how to choose the airtight layer, how to check the plan on paper with the pencil test, and how to make sure one person owns it on site.
Why it comes first
- Air leaks drive heat loss and draughts.
- Leaking air carries moisture into walls and roofs, where it can condense.
- Ventilation systems and heat pumps are designed for a tight house, so they only perform as intended when it is tight.
- Once drywall is up, most of the air barrier is hidden.
The choice changes the sheathing, the way windows are installed, the order of the trades and the testing plan.
The pencil test
Take a section through the house and trace the air barrier all the way around in one colour without lifting your pencil: from the base of the wall, up the wall, across the ceiling and back down. Every place you have to lift the pencil is a detail you must solve. Typical ones are the base of the wall, the rim joist, the top plate under the attic and every window. Do it for the plans, the sections and every penetration.
Four ways to build the airtight layer
| Approach | How it works |
|---|---|
| Sealed interior poly | Polyethylene behind the drywall, with every lap and edge sealed with acoustical sealant |
| Airtight drywall | The drywall itself, sealed to the framing with gaskets and sealant |
| Exterior sheathing membrane | A taped, airtight membrane over the sheathing, often also the water-resistive barrier |
| Sealed sheathing | Plywood or OSB with every joint taped or sealed |
Whichever you choose, the materials must be airtight. The BC Building Code limit for air barrier materials is 0.02 L/(s·m²) at 75 Pa.
Interior or exterior?
| Interior | Exterior | |
|---|---|---|
| Floor lines and interior walls | Must be sealed at each one | Passes by unbroken |
| Penetrations | Many (boxes, pipes, ducts) | Few |
| When you can test it | Poly: before drywall. Airtight drywall: only after | Before insulation |
| Weather exposure | Protected | Needs care while the building is open |
BC Housing’s airtightness guide reports that exterior approaches consistently do better, especially on larger buildings.
Primary outside, backup inside
Many high-performance builders use both: a primary air barrier outside (membrane or taped sheathing) and careful sealing inside, which often doubles as the vapour control layer. Two lines of defence make a low blower door result much easier to reach.
Make one person own it
- Mark the air barrier on every drawing in one colour.
- Name one owner, usually the site supervisor.
- Brief every trade at the pre-construction meeting.
- Protect it, and repair any damage the same day.
- Test it with a blower door before insulation and drywall.
What good looks like
| Benchmark | ACH50 |
|---|---|
| Typical new home (rough) | about 3 to 4 |
| Vancouver houses, building by-law maximum | 2.5 |
| High-performance home (rough) | about 1.0 |
| Passive House limit | 0.6 |
ACH50 is air changes per hour at 50 Pa. Targets change over time and differ between cities, so confirm the current requirement with your city and energy advisor.
Site checklist
- Air barrier approach chosen and agreed before framing
- Air barrier drawn in one colour on every plan and section
- Pencil test done; every break has a drawn detail
- One owner named on site
- All trades briefed at the pre-construction meeting
- Damage repaired the same day
- Blower door test booked before insulation and drywall
Test yourself
Try to answer each question, then click it to check.
What is the pencil test?
Tracing the air barrier around every section without lifting your pencil. Every place you lift it is a detail you must solve.
Name one advantage of an exterior air barrier.
It passes floor lines and interior walls unbroken, has few penetrations, and can be tested before insulation and drywall.
Who should own the air barrier on site?
One named person, usually the site supervisor, who briefs the trades, protects it and arranges the test.
Next: Detail 02, Air Barrier Materials & Compatibility. New to the physics? Start with our Building Science Deep Dive, lesson 1.3.
Planning a high-performance home in Vancouver, Burnaby or Surrey? Talk to our team about getting the envelope right from day one.