Built Right Detail 16: Pipes, Ducts & Wires Through the Envelope

A builder’s net zero envelope course by Xerox Builders · October 2026

Every trade that follows the air barrier makes holes in it. This Detail shows how to reduce penetrations with a service cavity, map the ones you need before framing, and seal ducts, HRV lines, electrical boxes and plumbing vents so they stay airtight and dry.

Why penetrations matter

A well-planned air barrier can be undone by the trades that follow it. Electricians, plumbers, gas fitters and heating and ventilation installers all need to get services through the envelope, and many of their holes are made after the air barrier is finished. Each gap is small, but together they can push a house over its airtightness target. Ducts that carry outdoor air through the house add a second risk: condensation on cold duct surfaces that drips into ceilings and walls.

The approach in this Detail comes down to three rules: fewer holes, planned holes, and every hole sealed.

Fewer holes: the service cavity

A service cavity is a layer of strapping, or a thin framed wall, built on the room side of the airtight layer. Wiring, boxes and supply pipes run in that space and never touch the air barrier, so the airtight layer (often taped sheathing, a membrane or sealed poly on the inside face of the studs) can stay unbroken. It also makes later changes easier.

• Make it deep enough for the electrical boxes you plan to use.
• It can stay empty or be insulated. If you insulate it, keep most of the wall’s insulation outside the airtight layer so that layer stays warm, and have your designer confirm the wall.
• A service cavity costs some floor area and adds a step, and it does not remove every penetration: vents, gas lines and outdoor fixtures still pass through.

Planned holes: map them before framing

Walk the drawings and list every service that crosses the envelope. A typical house has penetrations in three zones:

Group wall penetrations where you can, keep them away from corners and window openings, and mark on the drawings who makes each hole and who seals it.

Sealing through an exterior membrane

Make the hole before the wall membrane is finished, so the seal goes onto clean, accessible surfaces. Seal a membrane collar or a ready-made boot to both the duct or pipe and the wall membrane, lapped shingle-style: the membrane above over the collar, the collar over the membrane below. Run a sleeve through any exterior insulation, slope ducts slightly down to the outside, and flash over the hood. For outdoor lights and outlets, install blocking before the insulation and a mounting block through the exterior insulation, then seal the wire or conduit where it passes the airtight layer. Fill conduits once the wires are pulled.

HRV ducts: the cold side

An HRV has a warm side (supply and exhaust ducts to the rooms) and a cold side (the fresh-air intake and the exhaust to outdoors). The cold-side ducts carry air close to outdoor temperature through heated space, so they must be insulated with a vapour jacket that is sealed and taped at every seam; otherwise indoor air reaches the cold duct surface and condenses. Seal the jacket to the air barrier where the duct leaves the house. The BC Building Code sets a floor for the outdoor-air supply duct: where it runs through conditioned space it must be insulated to at least RSI 0.75 and have an effective vapour barrier. Treat the exhaust-to-outdoors duct the same way, following the maker’s instructions. Warm-side ducts inside the heated envelope generally need no insulation.

The BC Building Code requires outdoor inlets and exhaust outlets to be shielded from weather, birds and rodents. Keep the intake well away from the exhaust outlet, the dryer vent, gas appliance vents and anything else that could contaminate fresh air; take the distances from the equipment maker and your mechanical designer.

Exhaust vents, dampers and make-up air

• Dampers: range hoods, dryers and bath fans need a damper that closes when the fan is off (good practice and part of most fan and hood kits, rather than a specific Part 9 rule). A missing or stuck damper is an open hole to outdoors.
• Dryers: the BC Building Code requires dryer exhaust to discharge directly outdoors, in its own duct of smooth, corrosion-resistant material that can be cleaned.
• Duct joints: the code calls for joints in exhaust ducts to be sealed, for example with mastic or foil tape, and exhaust ducts running through or beside unconditioned space to be insulated (never less than RSI 0.75).
• Range hoods: a large hood can depressurize a tight house and pull air back down flues. Where a home has a naturally aspirating vented fuel-fired appliance, the code requires interlocked, tempered make-up air for exhaust equipment above 0.5 air changes per hour. Choose a sensible hood size and confirm with your mechanical designer.

Electrical boxes

With an interior air barrier, every box on an exterior wall or ceiling is a hole. Airtight boxes have a gasket where the cable enters and a flange that seals to the poly or the drywall. Seal wire holes through top plates and sheathing too; they connect wall cavities straight to the attic.

Plumbing vents and stacks

Vent stacks rise through partition top plates and the ceiling air barrier into the attic. Stub the pipe up first, slide a flexible gasket down over it, bed it in sealant on the plate or air barrier, and tape its edges. Flexible parts matter, because wood framing shrinks and pipes expand and contract. Do it before the attic insulation is blown in.

Site checklist

• Every envelope penetration shown on the drawings, with who makes it and who seals it
• Service cavity or other plan for keeping wiring out of the air barrier
• Blocking and sleeves installed before insulation
• Collars or boots sealed and shingle-lapped at every wall penetration
• HRV cold-side ducts insulated with a sealed, taped vapour jacket
• HRV intake located away from exhausts, dryer and gas vents
• Dampers fitted and working on range hood, dryer and bath fans
• Airtight electrical boxes on the air barrier; top plate holes sealed
• Plumbing vents gasketed and sealed before attic insulation
• Penetrations photographed and smoke-checked at the blower door test

Test yourself

Try to answer each question, then click it to check.

Next: Detail 17, Sequencing & Trade Coordination. For the theory behind ventilation and airtightness, see our Building Science Deep Dive.