Inorganic basement subfloor
doba1400
| Posted in General Questions on
Is there a product or detail that doesn’t use an organic material in the subfloor (osb or plywood)? I have an uninsulated slab with no vapor barrier below the slab and I want to to finish in tile. I also want to add a layer of EPS insulation. So existing slab + EPS + (what inorganic substrate?) + uncoupling membrane + thinset and tile. I live just north of NYC in climate zone 5. Thank you!
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Replies
Doba1400,
It is a good idea to install a decoupling membrane to keep cracks from telegraphing from the slab to the tile. I've used the Schulter system, and it is easy to work with.
Thanks Steve, yes I will plan on using the Ditra system. I forgot to mention I want to insulate on top of the slab with EPS. What can I put on top of the EPS to act as a substrate for the tile that's incorganic?
Given this is a basement floor the risk vs. reward equation for insulating an existing floor does not work in my mind especially since you have to give up head room to fit in the insulation.
If the old slab is more than 10 years old and un cracked I say thinset the tile directly to the concrete. You will have a very durable floor that will survive an occasional flood unscathed.
The amount to heat loss from your 68° basement to the 55° ground below is small enough that you are unlikely you be alive when that insulation has saved enough energy to pay for itself.
Walta
Thanks Walter. If I tile directly to the concrete floor (I think I'd still do an uncoupling membrane - there are some cracks in the concrete), and put down a rug on top of the tile, my concern is mold and moisture between the tile and the rug, since the slab is uninsulated. Is this a valid concern? It's going to be a kid's playroom. Thanks!
Yup, the rug will be your insulation, so the tile under it will be cold, and you are quite likely to have mold growing on the back of the rug. The fact that the tile and the stuff under it don't support mold growth won't help much.
"Wedi board" is a tile underlayment made from insulating foam. Here's a link to way more than you need buy likely local building supply places will sell smaller quantities.
https://www.homedepot.com/p/wedi-Building-Panel-3-ft-x-5-ft-x-1-2-in-Waterproof-Tile-Backer-Board-50-Sheets-010717064/312556488
This is great, thank you Charlie. Have you used Wedi board as a floor tile substrate before? Will one layer suffice as the substrate? Also I don't want to anchor the Wedi board to the slab because the slab is pretty thin - can I use the Wedi board as a floating subfloor? Perhaps if I do this, it would require two layers of Wedi board, staggered seams, and glued together?
I am not sure if uncoupling membrane will support mold growth but my guess is it would not.
Walta
Mold generally requires three things to flourish: temperatures above about 50°F, a source of moisture and a source of food. It can find food pretty much anywhere, so ignore that. Unless the substrate is somehow toxic, perhaps with very high or very low pH, or with biocides, it will support mold growth. I deal with similar situations regularly and agree 100% with Charlie--you don't need a lot of insulation, but you need a little to greatly reduce the chances of mold growth, by keeping the condensing surface warm enough to prevent condensation.
What you're looking for sounds like cement backer board.
I believe cement board is too soft a substrate for tile, no?
It's made to be used under tile. What I don't know is whether it would be rigid enough over foam to keep the tile from cracking.
Not a lot of insulation, but you might consider the ditra heat duo membrane. It creates something of a thermal break with a small R-Value. Skip the heating cable without a lot more insulation though.
Hi Tom, thank you for your recommendation. I've considered Ditra Heat Duo membrane directly to my uninsulated thin slab and my concern is heat loss to the soil, since there's minimal R-value to the thermal break (Martin Holladay's comment #1 in the link below. The comment is for hydronic heat but I believe the principle is the same). Also when the heated floors are not turned on, won't I have the same issues that I would per Charlie's comment #8? I'd really love this route to work as it's the simplest solution, but I'm concerned with mold growth under any rugs etc. and that it's essentially still an uninsulated slab. Thanks!
https://www.greenbuildingadvisor.com/question/radiant-flooring-on-top-of-old-slab
Thanks!