Do I need a vapour barrier in my Calgary bathroom walls?

Yes, Calgary bathroom walls require a vapour barrier — specifically, a 6-mil polyethylene vapour barrier behind the drywall on exterior walls, as required by the Alberta Building Code. However, the vapour barrier requirements differ between general bathroom walls and the wet area inside a shower or tub surround, and understanding this distinction is critical to avoiding moisture problems.

On exterior bathroom walls (walls that face the outside of your home), the standard 6-mil poly vapour barrier is installed between the insulation and the drywall, just as it would be in any other room. This vapour barrier prevents warm, moist indoor air from reaching the cold exterior sheathing where it would condense and cause rot and mould. In Calgary's climate, where winter temperatures regularly drop to -25 to -35°C, the temperature differential between the warm bathroom interior and the cold exterior wall cavity is extreme, making a continuous, well-sealed vapour barrier essential. The Alberta Building Code requires this vapour barrier on the warm side of the insulation in all heated spaces.

On interior bathroom walls (walls between rooms), a vapour barrier is not typically required by code, but many Calgary contractors install one behind bathroom drywall as best practice, particularly if the bathroom serves a shower or tub. The reasoning is straightforward — bathrooms generate more moisture than any other room in the home, and a vapour barrier provides an extra layer of protection for the wall cavity.

Inside the shower or tub surround, the approach changes significantly. The waterproofing system in this wet area — whether Schluter Kerdi membrane, RedGard liquid membrane, or another approved system — serves as both the waterproof barrier and the vapour barrier. You should not install a poly vapour barrier directly behind cement backer board in a shower. The concern is creating a moisture trap — if water gets past the tile and waterproofing membrane (even in small amounts through vapour transmission), it needs to be able to dry. A poly sheet behind the backer board can trap moisture between two impermeable layers, promoting mould growth on the backer board and within the wall cavity. The waterproofing membrane applied to the face of the backer board is the correct vapour management strategy in the wet zone.

Calgary's extremely dry winter climate creates an interesting vapour dynamic in bathrooms. Indoor humidity can drop below 20% during cold snaps when the furnace runs continuously, then spike to 80% or higher during a hot shower. This extreme cycling means that vapour management must handle both directions — preventing warm moist air from reaching cold surfaces during and immediately after showers, while also allowing drying during the extended dry periods between uses. A properly functioning exhaust fan rated at 1 to 1.5 CFM per square foot is essential to remove shower moisture quickly and return the bathroom to ambient conditions.

For basement bathroom renovations — a common project in Calgary homes — vapour barrier considerations are particularly important. Below-grade exterior walls in Calgary basements should have an interior vapour barrier as part of the insulation assembly, and the bathroom must be ventilated to the exterior through proper ductwork. If you are adding a new bathroom in an unfinished basement, the framing, insulation, vapour barrier, and ventilation all require permits from the City of Calgary, with inspection by a Safety Codes Officer.

If your bathroom renovation involves opening walls, ensure your contractor installs or repairs the vapour barrier correctly — seal all seams, penetrations, and electrical box openings with acoustic sealant or vapour barrier tape. Verify that your contractor carries WCB Alberta coverage before work begins, and get multiple quotes to compare approaches. Find local bathroom renovation contractors through the Calgary Construction Network at calgaryconstructionnetwork.com.

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