Vapor Barrier Crawl Space Sealing & Insulation

An open crawl space with a dirt or partially exposed floor is the largest uncontrolled soil-gas opening most houses have. Sealing it with a durable vapour barrier does two useful things at once: it holds back ground moisture that drives mould and rot, and it creates the sealed plane that a radon mitigation system needs in order to draw from beneath rather than from the room.

That second point is worth being clear about, because it is frequently oversold. Sealing a crawl space is not by itself radon mitigation. A vapour barrier alone will usually reduce entry, sometimes substantially, but it is not a substitute for an active system where a radon test has shown elevated levels. Where both are needed, sub-membrane depressurization does both jobs at once - the barrier is sealed and the system draws from underneath it.

If your crawl space also takes water, the sequence matters. Getting drainage and a sump pump right before laying a barrier avoids trapping water under plastic, and it is far cheaper than doing the work twice. Crawl space conditions vary a lot across the region, which is why we look at the space before quoting - see the communities we cover.