Insulation for bath floor outside curbless on old concrete slab

Nick M · Feb 5, 2021

I have a uninsulated slab in a 1910 carriage house for my bath remodel. I have plans to do a curbless entry to the shower. I was planning to use a foam pre-slope and have used the oatey shower pan liner method in the past with good success. Regardless of the system for the shower pan, I will need to raise my floor outside of the shower about 1-2 inches to allow for proper slope of the 5' shower pan and build up. Since the slab is uninsulated, I was wondering if you've seen any good floating products for raising and insulating floors. I know there's a Wedi foam board used for this, but it is crazy expensive. I'd ideally like to use something that has an R value of 5 or more, but is fairly quick and easy even if I need to lay backer board over the top.

I'd like to avoid doing the whole sleeper system with 3/4 OSB flooring and backerboard which only leaves me with about 3/4 inch of foam, plus little R value benefit where I have sleepers. I'd also end up with a good bit of time and expense for that build up.

I was reading a bit about the Schluter Kerdi system and thought that might be interesting to try, but their foam boards are not allowed as a flooring backer. This seems a little odd given that the foam boards are used in the shower, but maybe there is some difference there?

Any advice is appreciated. I am just about done with the waste piping (in the pic) so was thinking it would be good to know this soon before I backfill with concrete in case consensus is that I have to keep digging. For the sake of my sanity though, please don't tell me to remove more slab. ;-)

Nick M · Feb 27, 2021

I talked to Schluter and they do not approve Kerdi board to be used under the tile. So I bought 1" WEDI board and plan to lay tile down on it this week. If I run into issues I'll post here. Otherwise, assume the product worked as intended!

Sponsor · 2022-01-29T12:20:58-0800

Jadnashua · Feb 27, 2021

Wediboard is approved for use on a floor, and should work fine. One potential issue is if you’re planning small tile which may require an epoxy thin set, which will add even more expense. A larger tile more common on a floor, regular modified thin set will work fine.

The harder thing with any larger panel on a floor is getting full coverage...any pressure you can apply still means a very small psi pressure you can apply and the suction will make it nearly impossible to slide it back and forth to flatten the notched thin set. When setting large format tile, people have had good luck using a vibrating hand-held sander, obviously without any sandpaper installed on it. The vibrations help to flatten the notches left from troweling out the thin set. Another thing that will help is the use of a slant-notched trowel...after combing the thin set out, the taller, narrow notches fall over on themselves, leaving a much flatter surface, making it less likely to leave voids to the panel (or a tile) beneath it.

Nick M · Feb 27, 2021

Brilliant advice, Jim. Thank you! Funny you mention the smaller format tile: Just a few minutes after I brought home the selected 2 inch hex tile I thought to check the minimum dimensions from Wedi, which were 4 inch x 4 inch for Wediboard. I then proceeded to get a great workout returning ~700 lbs of tile and then purchasing a larger 6 inch hex.

Using a vibrating hand-held sander and a V notch trowel is brilliant. I definitely planned to use a single direction trowel application (parallel rows of thinset) with the boards laid down end to end to minimize air trapping underneath. I also wonder if back buttering the wediboard will help ensure I don't trap any airbubbles?

I wanted to make sure I didn't have any high points or recessed areas from my newly laid concrete drying and shrinking in the trench, so today I prepped for self-leveling compound to be laid out tomorrow. Instead of proving the benefit of a "level" surface which I think I already did a good job of, I plan to just make sure the thinset under the wediboard is being applied to a surface which is near perfectly flat so the boards aren't forced over high spots.

I already have the schluter kerdi membrane and foam shower basin so I am getting excited to try using the kedi hidden drain with a 6 inch hex tile over the top. There's decent risk to this, but I enjoy these projects and will keep posting updates in case these scenarios are helpful to others.

Nick M · Mar 14, 2021

Progress update here: Last night I finished laying the tile over the insulation boards and shower. All has gone well so far.

The idea of using a vibration sander to push out air bubbles under the foam boards was tried. I only saw one air bubble come out, so I either had very little air pockets or it wasn't really all that effective. I did stand on them and do a version of PeeWee Herman's Tequila dance which was very successful. I put boxes of tile and bags of thinset over the edges and that also seemed to help.

Wedi board is much harder on the surface but softer in the center foam area than the Kerdi board. Wedi is marked as "Made in America" though and I got great pricing and quick shipping on it to my local tileshop. I did crunch through the wedi board when I was kneeling on it to lay down the over-priced seem tape at the joints, but was easily able to patch with thinset.

I'm kind stuck with needing a 1.5 threshold at the moment, and this is a temporary issue as I'll be laying flooring elsewhere in the Fall, but wondering if there are good suggestions for building up. I purchased a 1" x 4" PVC trim board to lay under a honed stone threshold, but not sure if the adhesion will work well.

I abandoned the hidden drain idea. Instead, I placed the 4" square drain at the center of one 6" hex tile. It looks ab0ut as good as I could hope for.

I'll post pics a bit later.