www.diytileguy.com Tile installation blog for the Do It Yourselfer 
Do Uncoupling Membranes Really Prevent Cracked Tile Floors?
You want to tile your concrete floor. But there’s a problem. The concrete has cracks in it. You know that if you tile right over the crack that you soon will have floor tile that’s cracked in the exact same spot. So what can you do to keep the tile from cracking?
Why Schluter Ditra is so cool: Uncoupling membranes explained
Maybe you’ve heard of uncoupling membranes. You know, products like Ditra, Stratamat, and Redgard uncoupling membrane. These products are popular on DIY television and are found at big box home improvement stores. It’s possible that a store clerk or even a tile professional recommended uncoupling membranes as a way to prevent cracked tiles. Is this good advice?
Uncoupling membranes are designed for movement
It only makes sense that these membranes will help with movement. They have their geometric shapes. They are designed so the tile will move independently of the concrete below. Just look at this animation below.
https://youtu.be/QLlzaf7pqgA?t=1m7s
Surely these membranes will prevent floor tile cracks… or will they?
Crack isolation membranes
There’s a classification in the tile industry called crack isolation membranes (CIS membranes). These membranes have undergone particular tests and met certain criteria. They’ve been scientifically tested in what they can handle and how much they can handle. They will prevent some cracks from transferring through to your tile.
The membranes that have met these requirements are labeled as ANSI 118.10.
The Uncoupling Myth
Uncoupling membranes seem like they would prevent cracks also. But the fact is that they haven’t been (officially) tested as crack isolation membranes. The powers that be in the tile industry want uncoupling membranes to be their own classification- separate from the CIS membranes. So this means that they will have their own tests.
But certainly uncoupling membranes will stop cracks, right?
Not necessarily. The uncoupling membranes have undergone private unofficial tests. I’ve heard whispers of some that will pass the crack testing and others that won’t. But until these membranes are required to undergo industry testing we can’t be sure of how they will perform.
How to prevent cracked tile floors?
If you want to prevent cracks in the concrete from transferring through to your tile floor the best products are A118.10 crack isolation membranes. These have been tested and have the highest performance.
Although uncoupling membranes are easily available and it seems like they would prevent cracked tile floors we can’t and shouldn’t assume that they will. CIS membranes are still the top performers in the tile world.
Hi again! Do crack isolation membranes provide the same function as uncoupling membranes? I know uncoupling membranes can’t be assumed to have crack isolation properties, but what about vice versa? For instance, since rpm mats meet ansi 118-10, will they be just as “uncoupling” as ditra heat? Thank you!
This subject is a bit of a hornet’s nest because then you get into “what is uncoupling?” There have been many debates on internet forums and even the tile committees that have opined on this uncoupling feature and, more importantly, what the actual benefit is.
So RPM mats will provide a tileable surface. So will Ditra and Ditra-Heat and any of the other uncoupling mats. So will cement board or Hardibacker. Will they “uncouple?” First, we have to figure out what that is and define it.
So far the definition is a mat with a “geometric shape”.
I’m probably not answering your question but it would honestly depend on who you talked to. RPM mat people will tell you the benefits of their mats over the heat membranes. Uncoupling heat membrane people will tell you the benefits of their system over RPM mats.
What I can tell you is that I’ve used both and they both work. When I add up costs and the steps necessary to install I’ve found that I much prefer uncoupling heat membranes.
One thing: I saw the claim of crack isolation on the RPM website but it doesn’t give any details. I would want to know if it achieved that rating with the vinyl glue that the company recommends to install.
It used to be that to install RPM mats over concrete that a urethane adhesive was required. The urethane adhesive was a crack isolation membrane all by itself- whether you installed anything over it, or not.
So is the claim of crack isolation with that adhesive or the vinyl glue? That’s important to know and I don’t know the answer.
edit: upon closer examination, the instructions for going over concrete list the urethane adhesive (Hydroment Ultra-set Advanced). This is a good adhesive if installing over concrete. If you plan on using other adhesives and need a crack isolation membrane then I would try to find out more.
Thank you for all that info, really appreciate the time you take to answer. I’m installing over wood subfloor, so I guess it doesn’t seem like it could decouple as well as ditraheat if it is glued down to the subfloor.