Kerdi Board Thinset and Porcelain Tile for Basement Floor
Hi, I have an old home built in 1930 with no vapor barrier & no insulation under the slab, climate zone 4a, just north of NYC. I just bought the house 6 month ago and installed a perimeter interior drain + sump. So far, there has not been any moisture or water in the basement. I want to run this by the GBA community before I move ahead and make the investment.
Starting from the basement slab, building up: 3/4″ Kerdi Board thinset to the existing uninsulated slab (Tile substrate & R-3.4 insulation) + Schluter Ditra uncoupling membrane + 3/8″ porcelain tile.
Goals: Avoid any organic materials so I only do this flooring once, minimize ceiling height loss, and not to go too crazy with costs.
Questions:
1. Are there any concerns with the impermeability of the Kerdi Board and porcelain tile? My thought is the Kerdi board is essentially the waterproofing layer. In this assembly, there is no “drying to the interior” as in the case with other basement floor assemblies, right?
2. Is this overkill? A few contractors have suggested that it is, but I really don’t want to redo the flooring if a toilet overflows etc. in the future.
Thanks!
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Is Kerdiboard rigid enough as a substrate for tile? I wouldn't have thought so, at least not for larger tiles.
Yes, Kerdi board is primarily used as a substrate for tile. I've approved this with Schluter.
That substrate is usually on the wall. Was it clear when talking to Schluter that you were talking about a floor? The only mention of horizontal application in the manual is for countertops, and they specify a minimum thickness of 1.5" for that.
Kerdi board can be used on a floor. It is not made with regular grade EPS, as Andy lower down in the thread mentioned. Some high density grades of EPS are even used in foundations if I'm not mistaken. I don't think doba1400 will be able to acquire it at his local HD but he should be able to order it. Maybe he can even locate salvaged EPS, though that's doubtful. But I think it would still be way cheaper than Kerdi Board.
See further down in this thread. He just needs to bond Kerdi to it. He should try bonding some Kerdi to a small sample of EPS before committing to it to make sure the Kerdi is well bonded and that he's using the proper stuff, i.e. modified or unmodified to attach it. If he reassures himself about that then he can just lay out the high density EPS over polyethylene sheeting. Then bond Kerdi over the entire EPS sheets and let the EPS float over the concrete. Then tile over the Kerdi. It will be the simplest and cheapest way to do it. But it will require high density EPS.
He might also have to use a leveling compound like they use under linoleum for the bare concrete. It depends how even it is. If its already pretty smooth then the EPS will just float over it as is.
Why use kerdi boards instead of polyethylene plus regular foam?
AT 3/4", Kerdi board has R-3.4, plus it serves as an approved substrate for tile. If I went with XPS or EPS, I would need to use OSB or plywood as a substrate for the finish floor. In other words, Kerdi board serves double duty - tile substrate and insulation.
Impermeability is not a problem. Best practice now is to have a vapor barrier in the floor, which a 1930's floor probably doesn't have, so that's an upgrade.
Is it overkill? It is true that tile gets laid directly on concrete floors all the time. Porcelain tile has a permeability close to zero and probably is sufficient for a vapor barrier. The R 3.4 insulation is a benefit, in energy savings, comfort and in reducing the chance of condensation in the summer.
Thanks DCContrarian. Sounds like you don't have any reservations about this strategy? Thanks!
Thanks DC. Since Kerdi board has a low perm, I assume it's not considered to be trapped between the porcelain and the concrete slab? In other words, the Kerdi board itself is pretty much a vapor membrane and there's no concern for mold growing on the Kerdi board?
On the contrary, considering a different basement floor system - EPS + OSB + Vinyl, the low perm vinyl is a no-no because it traps moisture in the subfloor, which has organic material (OSB)?
I think what you're doing is fine. The most important part of the Schluter system is the Ditra uncoupling layer. Tile needs to move and if its laid directly on concrete it will crack either at the grout or in the tile itself or at both. Obviously they engineered kerdi board to work with Ditra with minimum fuss, so I say go with it Just follow Schluter's directions and you'll be fine and won't need to worry in the future. People in the trades are like people everywhere: they don't want to change from the way they used to do it even if progress has passed them by. They also don't mind that the old way saves them a buck because they won't have to live in the house.
Eric,
I agree with you, although it isn't tile movement that is the problem. De-coupling membranes are there because the slab underneath moves or cracks.
Thanks Eric and Malcom! Any suggestions for modifications or is this good to go? It's just a big expense and I want to make sure I'm not missing anything before proceeding.
I've been thinking about your question. I installed the Schluter system in my bathroom and it worked perfectly but it was a different situation than yours. The substrate was plywood. That makes a difference from concrete. I think it would be good for you to just call up a Schluter representative so you don't make a decision you'll regret. If I'm just thinking out loud it almost would make more logical sense to put down the Ditra on the concrete first to decouple and seal the concrete from any water intrusion. THEN put down the Kerdi board. Tile is meant to be bonded directly to Kerdi board. That way you decouple the substrate (Kerdi board) from any cracks occurring in the concrete.
But you should call Schluter as this is a special kind of question.
Thanks Eric and good question. I asked my Schluter rep and he confirmed, Kerdi first, then Ditra on top.
Kerdi board is extremely expensive, on the order of $80 or more per sheet, is it not? Another VERY common Schluter installation is tile over high density EPS foam that's 2" or more thick, with only Kerdi membrane in between. I've done numerous shower floors this way. Could you find the specs on Shluter's proprietary EPS, and then just use big sheets of that, secured with thinset? Seems like this assembly would serve as a decoupler and then skip the Ditra (also crazy expensive!) Shluter may have a tech paper on this sort of thing...
I didn't know you could bond Kerdi membrane directly to EPS foam. That's good to know. I don't think he needs to worry about damage if a toilet overflows even if the Kerdi membrane is sealed to the walls at the edges because you can't hurt the tile. The only issue would be if water comes up to the level of furniture. Then install a drain if you're worried about that, just like in a shower.
I still think he needs to decouple from any cracked concrete that moves, like Malcolm said. Maybe just lay 1' EPS foam floating, like some types of manufactured wood flooring. Bond Kerdi membrane to each EPS piece so they all act as one unit and float above any concrete cracks. Then just bond the tiles to the Kerdi. Seems like a plan and fairly reasonable in price.
One thing more. If he went with idea of a floating tile floor then he could lay polyethylene sheets over the concrete. That would allow the EPS to move freely just like happens in faux wood floating floors. and would also keep water seeping up through the concrete from degrading the EPS or otherwise causing problems.
Andy, I'm not familiar with Schluter's proprietary EPS and I'm not finding it on their website. Can you send a link? Anyway, I don't have the ceiling height to do EPS under the Kerdi board unfortunately. Which is why I like Kerdi - it has a good r-value per inch and serves as a substrate for the tile.
https://www.schluter.com/schluter-us/en_US/faq/kerdiboard-rvalue
Hi doba,
I don't think Andy was suggesting adding high density EPS board "in addition" to Kerdi board. He was just pointing out that Kerdi board is extremely expensive and you can make your own by bonding standard Kerdi material to high density EPS board hat you ordered separately. It would be a lot cheaper that way but would require you to find out how Andy bonded Kerdi to high density EPS board and what kind of thinset (if it was thinset) he used to do it. I myself have never attempted to do that.
Shluter makes an array of prefab shower pans and curbs out of, according to their website, "expanded polystyrene (EPS) foam with a high compressive strength", called Kerdi-Shower. They're sold at all the big box stores, and described on the Shluter site here: https://www.schluter.com/schluter-us/en_US/faq/shower-system-kerdishower-prefabricated-substrates-support
One sets the pan in a thinset bed, then waterproofs with Kerdi membrane on top, followed by tile. The point is that an EPS underlayment of sufficient compressive strength can serve as the ONLY underlayment for tile, other than a waterproofing membrane. How effective this assembly is for crack isolation from a concrete substrate, I don't know. And you obviously won't cover your basement with sloped shower pans. Finding suitable high-density EPS sheets will be a chore, and may end up costing more than Kerdi-Board, so this is has become just an academic pursuit.
As for the Kerdi-Board, here is how Shluter describes its composition, on the above linked page: "KERDI-BOARD prefabricated substrates are made of Schluter®-KERDI-BOARD." (It's sure not EPS--looks more like fiber-reinforced poyiso.) I don't use this product on walls. It doesn't save enough labor to justify its absurdly high cost of replacing regular drywall faced with Kerdi membrane. This is the first I've seen it suggested for use on floors, and upon looking I see indeed Shluter advocates that use. Again, it strikes me as far too expensive for the benefits it accrues to a floor job.
Doba, I know you're constrained with headroom in this application and want to keep your floor overlay to a minimum, and you "don't want to go crazy with costs." You never did mention how big of a floor area you're talking here... is it a huge basement?
Thanks Andy. Not too big of an area, about 350 SF.
Hello doba1400,
I was wondering If your plan to finish your basement slab the way you wanted (Kerdi 3/8” + Ditra +Tile) has been completed ?
I was also wondering if you tried to find a XPS or EPS replacement to Kerdi.
Thank you!