Old timer told me to skip the vapor barrier under my slab. That was a mistake.

Back when I was pouring a patio for a house in Boise, a guy who'd been finishing for 30 years told me vapor barriers were a waste on small residential jobs. Said the ground there is dry enough, just pour right on the compacted base. I was green and figured he knew better than the spec sheet. Three months later the homeowner called me complaining about moisture spots and a musty smell in their basement next to the slab. Pulled up a corner and the ground underneath was damp, no barrier at all. Cost me $400 to rip it out and redo with proper 6 mil plastic underneath. Who else has been burned by taking advice from someone who swore it was fine?

2 comments

2 Comments

lane.joel22d ago

lol honestly I gotta say I think you might be overreacting a little bit here. I've done slabs without vapor barriers on dry ground and never had a problem, it really depends on the soil and drainage situation. Not saying the old timer was right for sure, but $400 and a whole redo sounds like you might have jumped the gun. Moisture spots can come from a million things like a leaky pipe or high humidity, not necessarily the barrier. Plus that 6 mil plastic is cheap and easy to throw down anyway, but I've seen plenty of guys skip it on small pads and it's fine for years. Maybe it was just bad luck with that specific spot or the weather that season. I'd say take the advice with a grain of salt but don't think every job without a barrier is doomed.

noahhall22d ago

Three months later" tells me that moisture had time to wick up through the slab, which a proper capillary break would have stopped before it started.