Is a vapour barrier required behind bathroom walls in BC according to building code?

Yes, the BC Building Code requires a vapour barrier on the warm side of exterior walls in all residential construction, including bathroom walls. However, there is an important distinction between a vapour barrier (which controls moisture diffusion through wall assemblies) and a waterproofing membrane (which prevents liquid water penetration in wet areas like showers). Bathrooms in Metro Vancouver typically need both, serving different functions in different locations.

The vapour barrier — typically 6-mil polyethylene sheeting — is installed on the interior (warm) side of exterior wall insulation, behind the drywall or backer board. Its purpose is to prevent warm, moist indoor air from migrating through the wall assembly and condensing on cold surfaces within the wall cavity. The BC Building Code (Part 9, Section 9.25) specifies vapour barrier requirements as part of the building envelope assembly. In Metro Vancouver's climate zone, the vapour barrier goes on the interior side of the wall because the heating season dominates — you are keeping warm, humid indoor air from reaching the cold exterior sheathing.

In bathroom wet areas (showers, tub surrounds), you need a waterproofing membrane in addition to — or in some cases instead of — the standard vapour barrier. This is where many homeowners and even some contractors get confused. The waterproofing membrane (Schluter Kerdi sheet membrane at $4-$8 per square foot, or liquid-applied RedGard/Hydroban at $2-$5 per square foot) is installed over the cement backer board, directly behind the tile. Its job is to prevent liquid water from showers and baths from penetrating into the wall cavity. BC Building Code Section 9.29 mandates waterproofing in shower and tub enclosures.

Here is the practical breakdown for a Metro Vancouver bathroom renovation:

Exterior bathroom walls need a vapour barrier (6-mil poly) behind the insulation, with cement backer board and waterproofing membrane in wet areas. Some building science experts and Vancouver-area building inspectors now recommend against using polyethylene vapour barrier directly behind cement backer board in shower areas, because the poly can trap moisture between the backer board and the poly, creating a double vapour barrier situation where moisture cannot dry in either direction. The preferred modern approach is to use the waterproofing membrane (Kerdi or liquid-applied) as both the vapour retarder and the waterproofing layer in wet areas on exterior walls, with proper detailing at transitions.

Interior bathroom walls (walls that do not face the exterior) do not require a vapour barrier under the BC Building Code, but wet areas on interior walls still require a waterproofing membrane behind the tile. This is code-mandated and absolutely critical in Vancouver's humid climate — water that penetrates through tile and grout on an interior wall will saturate the wall cavity and create mould conditions just as readily as on an exterior wall.

For bathroom floors, a waterproofing membrane is required under tile in shower areas and strongly recommended across the entire bathroom floor, particularly in upper-storey bathrooms or condo units where a water leak would damage the unit below. In strata buildings, many strata corporations require full bathroom floor waterproofing as a condition of renovation approval.

Practical advice for homeowners planning a renovation: When you discuss your project with a bathroom contractor, ask specifically about their waterproofing approach for both wet areas and wall assemblies. A qualified contractor in Metro Vancouver will be familiar with the vapour barrier versus waterproofing membrane distinction and will know how to detail the transitions properly. If a contractor suggests using greenboard (moisture-resistant drywall) or standard drywall behind shower tile with no waterproofing membrane, that is a serious red flag — this does not meet BC Building Code requirements and will fail in Vancouver's climate.

Permit fees for bathroom renovations in Metro Vancouver are typically $150-$600 depending on scope, and a building inspector will check for proper vapour barrier and waterproofing installation during framing and pre-tile inspections. All plumbing and electrical work requires separate permits and must be performed by licensed tradespeople, with electrical inspections through Technical Safety BC.