I have a fiberglass freestanding tub, and the backside of the tub base wasn't caulked. Out of suspicion, I stuck a paper towel in the gap on the back and it came out fairly soaked with water (pic below). I thought it might be a leak in the plumbing fittings under the tub, but after soaking up the water, it doesn't immediately return. Also, filling the tub and draining it doesn't cause large pools of water under there, so I've ruled out a drain leak.

However, if I stick a paper towel under the tub and leave it for a few hours, the paper towel will come out lightly damp, but not soaked with water. What is going on if this is not a plumbing leak?

Is this on a slab? Or a house on pilings? Sounds like it could be condensation.

I've seen this before where hot and humid air was coming up around the drain connection under the tub. This house was on pilings, and the subfloor underneath wasn't sealed properly. There was no plumbing leak, but lots of condensation on hot and humid days.

Yes, this is slab on grade. The builder had to do some work under the tub to move the drain location in order to place the tub properly, and I suspect they didn't seal the vapor barrier properly which is causing excess moisture under the tub. How much of an issue is this long term? Enough to get them to chip a hole in the slab to patch the vapor barrier? Do I risk damage to the foundation if the moisture transmission continues?

Check the tub for leaks. Fill it up and mark a line. Check back in two hours and see if it is holding.

I've ruled out leaks already.

Of the drain or the tub itself?

Both

If it is vapor transmission through the slab, are there any other options to solve this other than chipping a hole in the slab to patch the vapor barrier?

Could a vapor barrier paint be put on top of the exposed slab on top? I guess this would cause the vapor to disperse through the slab in other locations and may cause moisture elsewhere?

And you tested the overflow?

It has been tested, but I haven't filled it to that level in 5 months.

The next thing would be the hot or cold line having some type of leak just very small. If not, then it's probably something to do with moisture coming up to the slab

Where that moisture is coming from is another issue. If there's any toilets or waterlines or drain lines or vent pipes in the area, it's possible that something could be compromised and water is coming underneath your tile flooring over there not likely but it is a possibility I've seen second-story stacks have a drywall screw that got into the pipe and water would come out of it

This is a one story house. The ground under the slab just naturally has moisture in it, so that will rise up as water vapor through the concrete if there is damage to the vapor barrier.

It sounds like the moisture coming up from the ground underneath your slab is very likely if they damaged the vapor barrier. Your concrete will need a way to dry out if it's getting wet, and right now it's drying out under your tub. If you put a moisture barrier on top of that area, then the moisture will travel elsewhere in the slab. This could cause issues with your flooring - warp wood floors or cause tile floors to pop up.

I'd talk with your builder about options. Do you know if your slab is rebar or post tension? I'd be very hesitant to start chipping into a post tension slab. If one of the cables is touched, then you risk a lot more damage than it would be worth.

You can chip around post tension cables and expose them. Just do not sawcut through them.

Post tension slab, but luckily, I have a picture of what the area under the tub looks like before they set the tub and filled the hole. There are no tension cables under the tub. I'm fairly certain that they didn't properly patch the vapor barrier under the tub. They just loosely placed a vapor barrier sheet in there and filled it with concrete. No taping to the existing vapor barrier, nor taping around the drain was done. Here's a before pic. This was later filled in with concrete and the tiles were repaired around the edges.



Edit: My main concern with them chipping away the concrete in that hole is the drain pipe getting cracked and then requiring digging under the slab to fix.