Help needed guys - been getting lots of good info here, but haven't seen this one - I am having trouble getting kerdi to stick to hardi backer - I had already bought the hardi backer when I learned about kerdi, so rather than rip it out I decided that overkill wouldn't hurt, but my kerdi seems to have air bubbles behind it 24 hours after installation - I figure I wasn't fast enough with my installation of each sheet after putting up the thinset, and even though it seemed to be well bonded the thinset was too dry - do I need to pull the whole sheet down and start over, or do I cut out the bubbles and overlap a new section? I had sponged water onto the hardibacker to try to prevent this, even had a slightly wet unmodified thinset (held the notch 3mmX3mm, but not by much) but my lack of trowelling skills might have left it up on the wall too long before trying to set the kerdi...I don't seem to have the problem in the smaller sections I did (bench, curb)... Did I use too little thinset, or is the thinset being too dry the most likely cause, or did I try too hard and by making it wet did I screw that up??? I have tried to take a picture, but I can't make it show the bubbles.

If I have to pull each sheet down (assuming this ruins the kerdi where it did stick) should I think about a liquid waterproofer instead of buying kerdi for the walls again? Looking at about 90 sq feet if I have to pull it all down, about 10-15 sq feet of areas with bubbles in them. Thanks in advance.


Keith

I am no pro and you might get better feedback but I had the exact same problem. I am doing a shower and I have 3 walls 3x8 foot and since the notch is so small on the trowel the thinset dries too quickly to get an entire sheet up and my kerdi literally fell off the wall when I tried to stick it on. Same issue for me, I was too slow with the trowel. I scraped all the mortar off and started over. This time I just cut my kerdi into 3 sheets overlapping them by a foot for each wall. That worked great. Then I also got bubbles. Some of them were 12X12. I didn't want to start over so I just cut out the bubble with a razor blade and patched in an overlapping piece. That worked out well. I even had an outside corner I had to redo because of a bubble, same solution worked fine. I believe I had to patch at least 4 spots but had enough kerdi to do it.

How big are the air bubbles? An small one, or even two, is not a big problem to fix. Many of them in a relatively small area can be a problem.

You've run into one of the problems that I don't like about Hardibacker: it sucks water like a shop vac. While Kerdi is relatively easy to install, it does take a little practice, and 3mm of thinset on hardi is still going to dry pretty quickly when you're covering an entire wall. The first part you put on is going to be too dry by the time you put the last part on, so by the time you're ready to hang a large sheet, it's too late. This is also why it's recommended to use regular drywall with Kerdi. It doesn't draw the moisture out of the thinset like CBU's.

It's possible to fix the air bubbles, but if there are a lot of them, it may be easier to replace that section. If you can cut out a section that has a lot of air bubbles, you could replace it with another piece or even a couple of scrap pieces, as long as you overlap it two inches.

Another tip when you're applying a sheet is to immediately start trowling from the center of the sheet out toward the edges in every directions. You're trying to push the membrane into the thinset and squeeze out the excess. I use a concrete finishing trowel that has rounded ends. Keeps me from sticking the corner of the trowel into an adjoining piece of Kerdi.

correction, overlap the walls by 4-6 inches, not an entire foot.

Kman/Todd -
Thanks for the info and the quick reply - I'll cut out the sections with big/lots of bubbles, then scrape the thinset and reapply a patch that overlaps by at least 2 inches - same thing for small bubbles on their own, or do I just slice them and use a silicone sealant to stick the kerdi down/make the repair for the cut?

Thanks again

Keith

I say cut them all out and patch unless you have dozens. Then it would be a huge pain to do. I think my smallest I cut out was only a couple of inches. But stick with using mortar behind the kerdi. This might get into trouble but some of the very small ones I never really worried about since I installed 12X12 tile on the wall and had plenty of good kerdi to stick them to. My thoughts are when I get all the tile up cured and grout in place there will be no way the can come back up. Again I am not pro, so maybe I should post back in a year or so to see how it worked out.

I agree, if it's a very small spot and you're using a large tile, it shouldn't be a problem. The idea is to make sure you have enough contact to keep the tile in place, probably 80-90% should be good.