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I am installing engineered hardwood flooring over concrete slab below grade in the basement. The flooring installation instructions mention that if using 6-mil poly underlayment, it should be lapped up the wall, but not touching sheetrock. However, I'm using a 2-in-one underlayment (vapor barrier as well as a bit of foam cushioning), and the instructions for that don't mention lapping, they say to just cut to the size of the room. Some other random sources have mentioned lapping 6-mil up so far that it's hidden by the base molding (so it's definitely touching sheetrock).

What's the deal? From what I understand, all of the barriers are meant to block moisture coming from below. Lapping makes sense for creating a more complete "seal", but it makes sense that if it touches sheetrock, then condensation could travel from the barrier to the drywall. The instructions I've seen that say not to lap are implying that rather than blocking the moisture at the edges, this instead gives it some room to evaporate.

Does anybody have a definitive answer?

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DO not overlap as it will cause a noticeable uneven floor when walking over the areas. We cut to size and tape where the underlayment meets.

Overlap the wall areas and once flooring is complete prior to baseboard installation, simply cut along the edge (leaving a 1/4"-1/2" overcut) and tuck the underlayment under the sheetrock.

This allows an overlap of the underlayment edging and does not allow any moisture to touch the sheetrock, preventing an environment where mold can grow.

  • I agree the pad needs to go wall to wall if cut short a void will be created so they suggest to cut it slightly long so it will cover under the baseboards, but not long enough to be up on the sheetrock.+ – Ed Beal Feb 6 '18 at 14:33
  • This is what we did for our wood floor on concrete slab with a similar underlayment. No issues. Manufacturer actually provided similar instructions for concrete. Our underlayment had a section for overlapping which was just the plastic part without the sound proofing membrane. I've also read that regular duct tape is fine to use for joining the pieces. – Micah Montoya Sep 6 '18 at 14:23
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This will be only my opinion or educated guess as what would be best to do, or at least, what I would do in your case.

I would lap the plastic loosely up on the wall, cutting the corners so the folds do not build up there while installing the flooring. There is also a need to have the sheetrock off of the concrete floor about 1", cut it that high if it isn't already. This should be the norm for any drywall in a basement.

After the work is done laying the floor making sure you keep your gaps proper at the perimeter, and since the drywall is cut high enough, the gap of the flooring can be measured from the bottom plate, not the drywall face, better insuring the base covers the cut edges of the flooring. Most laminate flooring needs only a 1/4" to 3/8" gap at the perimeter, so that is why the plastic is applied loosely, so it can roll up under the drywall edge when the flooring is set. These notes are a little out of sequence but I hope it can be understood. Again, after all the flooring is laid, protect the floor using a piece of metal sheet and cut the plastic off even with the face of the drywall and tuck the small flap of plastic into the space under the drywall. That way the drywall is fully exposed to the ambient air in the room any rising moisture will not get to the ends of the flooring and supposedly not at the edge of the drywall since the plastic is covering the edge and the base covers the cut at the drywall and hides it all.

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