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One flooring company said they would install an uncoupling membrane when installing tile over my 100-year-old basement laundry room's concrete floor. OIs there a good reason to do use a membrane?

It strikes me as overkill. There are no cracks in the concrete, and I believe the concrete has been there for a long time. Our floor is painted, and we have zero moisture escaping from below per the plastic bag test. There are control joints through the room and they are uneven height, though not cracked. They'll be using a paste (not a true leveling compound) to even out and flatten the area.

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    FYI, those are control joints, not expansion joints. The distinction is that expansion joints would have a compressible medium sandwiched between sections of concrete. That would change the nature of the project somewhat. – isherwood Jul 12 '17 at 17:16
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Yes. It isn't expensive, but it can also reduce the amount of efflorescence that comes up through the tile and/or grout. I have a basement bathroom where that wasn't done, and the natural stone tile is often covered with minerals that have worked there way up through the tile. The moisture coming up through the concrete basement floor is a key component of efflorescence.

To treat the question more generally beyond the original poster, I'd consider it in two parts:

  • can there be benefits? Of course. I mention efflorescence, but it can also prevent crack propagation.
  • are there always benefits, or is it always necessary? No, it sounds like there aren't benefits here for the original question.

Edited to add a picture of what I have on my tile floor, which I believe would be reduced had they used an isolation membrane, that would have also blocked moisture:

Efflorescence coming up through natural stone tile on basement slab

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    I'll add to my question pertinent information: our basement concrete floor is painted, and we have zero moisture escaping from below. I did the plastic bag test. – glenviewjeff Jul 12 '17 at 16:19
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    If you're able to rule out any benefit, then perhaps it isn't useful, and you could ask them to quote the work without it. But it still isn't expensive, and if that's the way the installer always does it, it might be easier to let them use the approach they have confidence in. – Tim B Jul 12 '17 at 16:21
  • The company I'm not planning to use twice mentioned that they use it, so I figured I'd investigate. I'm not going with them because their rates were more than double the cost of the one I'd planned to go with. – glenviewjeff Jul 12 '17 at 16:42
  • I added a little extra info; please take a look. – glenviewjeff Jul 12 '17 at 16:48
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This is a subjective question that depends on individual risk tolerance. I wouldn't bother. My previous home was a similar situation, with several cracks and no expansion joints. It was obvious that the basement was perpetually dry and the cracks stable. (They probably occurred as a result of initial settling 50 years ago.)

I applied fiberglass mesh to the cracks with tile mortar and did the project. 15 years later the floor looked just fine, with no sign of movement or cracking.

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Keep in mind that the substrate is only one side of the equation. The fact that your concrete has no cracks doesn't mean it doesn't contract and expand. It most certainly doesn't mean that it will expand and contract at the exact same rate as the tiles.

The magnitude of the movement may be small, while the ratio is huge. Concrete generally expands around ten times as much as ceramic, for example. If your laundry room floor expands an imperceptible 0.125" the tile field will have barely moved. Eventually the tiles will delaminate or the grout will crack.

I'll grant you that the opposite situation, tiles moving more than the substrate seems unlikely in your case. A basement laundry room seems a poor location to have problems with solar expansion, and an unlikely candidate for moisture absorbing tiles.

There are quite a few factors involved, and it is entirely possible you won't have any issues. I wouldn't immediately rule the membrane out as overkill though.

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    A basement is going to have a uniform temp. I would not waste the $ I have put tile over concrete pads outside with no tile problems. – Ed Beal Jul 12 '17 at 17:01

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