An old guy at the hardware store told me to always use a 6 mil vapor barrier on concrete, no matter what

I was picking up supplies for a laminate job in a basement in Tacoma about two years ago. This older guy, maybe in his seventies, was in line behind me and saw the vapor barrier roll in my cart. He leaned in and said, very seriously, 'You always use a 6 mil. Don't listen to the 2 mil stuff, it's junk.' I figured he knew his stuff, so I swapped it out. The job went fine, but a few months later the homeowner called me back. There was a musty smell. Turns out, the concrete slab had a lot of moisture and the 6 mil barrier just trapped it against the floor, creating a perfect spot for mold. I had to pull up the whole floor. A more experienced installer I know later told me that for that specific slab, a 2 mil barrier that breathes a little would have been the better call. I learned that blanket advice can get you in trouble. Has anyone else run into problems from following a 'one size fits all' rule too strictly?

3 comments

3 Comments

coleman.avery2mo ago

That's a brutal way to learn a lesson, but it's a good one. Blanket rules in construction are almost always wrong because every house and slab is different. That old guy probably had one specific situation where 6 mil saved him, so he thought it was a universal fix. Moisture needs a plan, not just a thicker piece of plastic. Sorry you had to deal with that mess.

ellis.victor2mo ago

Did you ever do a moisture test first?

william_miller1mo ago

Oh man, exactly what coleman.avery said about moisture needing a plan is spot on. You can't just wing it with plastic and hope for the best. I've seen guys throw down 6 mil on a slab that's still curing and wonder why they get bubbles and delamination later. You gotta test it first with a calcium chloride test or a relative humidity probe. Then based on that number you pick your vapor barrier, not just grab what's on the shelf. 6 mil might work for a dry slab in a climate controlled space but it's useless against a wet slab in a basement. It's like putting a raincoat over a sponge that's already soaked.