How do I know if my Ottawa slab has a vapour barrier underneath it

Determining Whether Your Ottawa Concrete Slab Has a Vapour Barrier

Unfortunately, there is no way to confirm a sub-slab vapour barrier without invasive investigation — you cannot see it from above. However, understanding Ottawa's building history and performing some simple tests can give you a strong indication of what lies beneath your concrete floor.

The Ontario Building Code has required polyethylene vapour barriers beneath concrete slabs on grade since the 1975 edition, with increasingly specific requirements in subsequent updates. If your Ottawa home was built after 1975, there is a good chance a 6-mil polyethylene sheet was placed under the slab. Homes built after 1990 almost certainly have one, as enforcement and building inspection standards tightened. However, homes built before 1975 — which includes much of the Glebe, Old Ottawa South, Sandy Hill, Centretown, Hintonburg, and Westboro — likely have no vapour barrier at all. Many wartime-era homes in Ottawa's west end and older Nepean neighbourhoods also lack sub-slab barriers.

Even when a vapour barrier was installed, it may have been damaged during construction, punctured by plumbing penetrations, or degraded over decades. A barrier that has holes or gaps may be worse than no barrier at all, because it can channel moisture toward the openings and concentrate it in specific areas of the slab.

You can perform a simple plastic sheet test at home to get a preliminary read on moisture transmission. Tape a 2-foot-by-2-foot piece of clear polyethylene sheeting to the concrete floor, sealing all edges with duct tape. Leave it for 48 to 72 hours, then check for condensation on the underside of the plastic or darkening of the concrete beneath it. If you see moisture, your slab is transmitting vapour — whether from a missing barrier, a damaged one, or simply high groundwater pressure overwhelming an intact barrier.

For a more precise measurement before an epoxy project, a contractor will perform a calcium chloride test (ASTM F1869) that measures the actual moisture vapour emission rate in pounds per 1,000 square feet per 24 hours. Standard epoxy systems require an MVER below 3 pounds. In Ottawa, spring and early summer readings tend to be the highest due to snowmelt saturating the ground, so testing during this period gives a worst-case reading that is most useful for planning.

If you have access to your home's original building permit records, the City of Ottawa Building Code Services (call 3-1-1) may have the original plans on file, which would show whether a vapour barrier was specified. For newer homes, the builder may have documentation. If you are planning an epoxy floor coating and suspect your slab lacks a vapour barrier, budget for a moisture-mitigating primer at minimum, adding roughly $2 to $4 per square foot to the project. For severe cases, a cementitious barrier system costing $3 to $5 per square foot may be necessary. Flooring contractors listed in the Ottawa Construction Network directory at justynrookcontracting.com can perform proper moisture testing and recommend the right mitigation approach for your specific slab.

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