Of everything that goes into a basement floor, the vapor barrier is the cheapest layer and the most expensive one to not have. Here's what it does.
The ground is always wet enough
Even soil that looks dry holds moisture, and that moisture moves as vapor — constantly, upward, into anything porous above it. Concrete is porous. Without a barrier, a basement slab wicks ground moisture up through itself for the life of the house.
You notice it as a musty basement, damp-feeling floors, flooring adhesives that let go, carpet pads that grow things, and efflorescence — that white mineral dust — blooming on the slab.
What the barrier is and where it goes
It's a heavy polyethylene sheet — we use a proper under-slab grade, not painter's plastic — laid over the compacted base, seams overlapped and taped, sealed around plumbing penetrations, run to the edges. The slab gets poured directly on it. Done right, it's a continuous break between ground moisture and your floor, forever.
Its cost is a rounding error on the job. Its absence is permanent: there's no practical way to get a barrier under an existing slab short of tearing the slab out.
Why it still gets skipped
Two reasons: it's invisible in the finished job, and it makes finishing slightly less convenient (the slab dries from the top only, so it holds bleed water longer). Neither is your problem — both are reasons a crew in a hurry leaves it out.
Every basement floor we pour gets a barrier, sealed and photographed before the pour. If you're getting bids, ask each one specifically. Then get ours — the on-site estimate is free.
Related service
Basement FloorsPoured over a vapor barrier on compacted ground — dry, flat, and quiet for decades.