To poly or not to poly - that is the question

By Dave Henderson

July 6, 2026

There has been an ongoing debate for longer than I care to remember about the use - or rather need to use - polyethylene under concrete basement floor slabs as a means to control soil gas entry into the home.

There are two camps: one says the concrete in and of itself is an effective barrier; the other argues poly is absolutely necessary. Which side is right? Where is the proof?

I may not be correct, but I believe the debate started in the U.S. decades ago when poly was required to address dampproofing. In the U.S., they generally pour their concrete basement slabs directly on the ground, so they don’t get the benefit of the capillary break provided by the gravel layer. Because they were using poly for dampproofing, it naturally morphed into protection against soil gas entry as well. And because they did it, we adopted it as well. I don’t know that this was the sequence of events, so I admit it might have happened differently.

Sometimes we adopt things because we think they make sense. In the absence of facts, we make judgement calls based purely on speculation, and that’s never good.

Of course, everyone in Ontario is now acutely aware of the change to the Ontario Building Code which requires polyethylene under the basement floor slab. However, what most people don’t realize is it landed there not necessarily to prevent radon entry into the house, even though that was part of the rationale. The main reason it is there is to facilitate radon mitigation in the event radon accumulation in the house becomes a problem in the future. Traditionally, we have relied on the concrete basement floor slab to be an effective air barrier.

What changed? Why is the concrete no longer acceptable as an air, and consequently, soil gas barrier?

The short answer is, the National Building Code of Canada changed. Plain and simple. The logic is concrete cracks so therefore it can not be considered an effective air barrier. The odd flaw in their logic is that they are quite happy to recognize normalized leakage rate (NLR) when measuring building air tightness. NLR fundamentally acknowledges there are cracks and resulting air leakage in buildings, and that’s okay provided the air tightness tests produce acceptable NLR results.

So yes, concrete cracks but we have literally thousands of houses built in Ontario that easily meet or exceed the stringent air-tightness requirements for high-performance housing programs - all of them built without poly under the basement floor slab.

The proponents for the use of poly under the slab also maintain that because concrete cracks, it is not possible to connect a sub-slab radon mitigation fan to create depressurization in the gravel layer because the cracks in the concrete mean the fan will simply draw air from within the house, potentially making the radon concentration worse.

When you ask radon mitigation contractors if they are able to extract radon from below a basement slab without poly, they will say, “Of course.” They do it all the time.

What changed between those older houses and houses built today? Some argue increasingly greater air tightness of new construction, but the answer is perhaps speculation won out over facts.

RESCON is undertaking a (potentially) groundbreaking study to finally answer the question. In true Mythbusters fashion, we are going to test the efficacy of a code-built concrete basement floor slab to act as an effective air barrier. Conversely, we can use the test data to determine what difference the addition of poly makes in terms of being able to depressurize the gravel layer under the slab.

We have a unique opportunity while the market is slow to use existing inventory housing to do some field testing and air-tightness testing of basement floor slabs constructed to both the old OBC requirements without the poly as well as new OBC homes that have it. By using two sets of houses of similar size and parameters built by builders using the same trades and crews, workmanship becomes more consistent and less of a potential contributing factor.

We will also be able to measure air distribution throughout the gravel layer by placing sensors in all four corners of the house.

The testing will take place over the summer, so stay tuned for an update on the results of the study in an upcoming issue. Hopefully this will finally settle the debate one way or the other.