What's a vapour barrier, and do I need one in my bathroom walls in addition to waterproofing?

A vapour barrier (or vapour retarder) is a material that slows the movement of moisture vapour through wall assemblies, and yes — in most GTA bathroom renovations, you need to consider vapour control in addition to the waterproof membrane in your shower or tub surround. However, the vapour barrier and the waterproof membrane serve different purposes, and understanding the distinction prevents costly mistakes.

The waterproof membrane — products like Schluter Kerdi, RedGard, or Laticrete Hydro Ban — goes behind the tile in wet areas (inside the shower, behind the tub surround) to prevent liquid water from reaching the wall cavity. This is code-required in Ontario for all shower and tub surrounds. The vapour barrier (typically 6-mil polyethylene sheeting) goes on the warm side of the insulation in exterior walls to prevent warm, moist indoor air from condensing inside the wall cavity when it hits the cold exterior sheathing. In Toronto's climate, with winter temperatures regularly dropping to -10 to -20 degrees Celsius, this condensation risk is very real.

Where Each One Goes

Inside the shower or tub surround, the waterproof membrane replaces the need for a separate vapour barrier. In fact, installing both a poly vapour barrier and a waterproof membrane in the same wall assembly can create a moisture trap — water that gets between the two layers has nowhere to dry. The Schluter system, for example, is designed so the Kerdi membrane serves as both the waterproofing and the vapour retarder in wet areas. Follow the membrane manufacturer's installation guidelines rather than adding extra layers.

On exterior bathroom walls outside the shower area, standard Ontario building practice calls for a 6-mil poly vapour barrier on the warm side of the insulation, just as you would in any other room. If your bathroom renovation involves opening up exterior walls — for a window replacement, recessed medicine cabinet, or plumbing relocation — ensure the poly vapour barrier is properly reinstated with all seams and penetrations sealed with acoustic sealant or red tuck tape.

GTA-Specific Considerations

Toronto's freeze-thaw cycles (40–60 per year) make proper vapour control especially important on exterior walls. Without a vapour barrier, warm moist bathroom air migrates into the wall cavity, condenses on the cold sheathing, and causes rot, mould, and insulation damage. This is a common hidden problem in pre-war Toronto homes and post-war bungalows across Scarborough, North York, and Etobicoke where original bathroom renovations may have been done without modern vapour barrier practices.

In condo bathrooms, vapour barrier considerations are different. Interior condo walls (between units or between a bathroom and a hallway) generally don't need a vapour barrier because there's no cold side — both sides are conditioned space. Exterior condo walls should have vapour control as part of the building envelope, but this is typically handled by the building's original construction, not by individual unit renovations.

For interior bathroom walls in a house — walls that separate the bathroom from other heated rooms — a vapour barrier isn't structurally necessary since there's no temperature differential to drive condensation. However, using moisture-resistant drywall (green board or purple board) on all bathroom walls is good practice and standard in GTA bathroom renovations. Budget $2–$4 more per sheet compared to regular drywall.

If you're planning a bathroom renovation that involves opening walls, a qualified contractor will know exactly where vapour barriers and waterproof membranes are needed. Find experienced bathroom renovation professionals through the Toronto Construction Network directory.