Basement Tile

Hey guys hoping someone might have insight into tiling over xps rigid foam board? Part of my basement remodel i would like to tile most of the basement. Right now its just a slab below grade, so i was hoping to put xps down then plywood sleepers and radiant, then cbu, ditra the tile. Owens corning rates the f250 as having 250lbs psi. Any one have experience here?

Thanks

Thanks!

Thanks John, I read a PDF from the John Bridge tile forum that talks about using cork or foam and they seem somewhat close in their heat loss prevention properties.

You're right, there must be hundreds of opposing opinions on this subject.

A big part of your decision is if your slab is emitting water vapor or ever has liquid water present. If it's always been dry, that makes life easier. First thing I'd do is tape a piece of plastic (at least a foot square) to the slab and leave it for a few days. If there's any trapped moisture, you know you'll need to address this first. If it stays dry, then you can possibly go a different route. The channels in Ditra offer a limited path for some water vapor, but not liquids.

Thanks

the only moisture trouble I have experienced was two fold, a chimney that I ripped out: Problem Solved and my basement door/walk out...but that was during the 2008 '100 years rain' we had here in seattle, my back yard is clay so it doesn't drain well and the water just pooled to the point where it leaked in under the door sill, but not much at all.

otherwise my slab is pretty good. I do have another thread going regarding the actual renovation and making this place warmer, so trying to consolidate all the info, plus the info from JOhn Bridge forums.

Terry asked for a pic of the Aquia 2 when they installed it, not sure if he ever posted it, but here is a shot of my bathroom, before and after
BEFORE


Almost after (excuse the renovation materials on the floor):

K2 said:

Hey guys hoping someone might have insight into tiling over xps rigid foam board? Part of my basement remodel i would like to tile most of the basement. Right now its just a slab below grade, so i was hoping to put xps down then plywood sleepers and radiant, then cbu, ditra the tile. Owens corning rates the f250 as having 250lbs psi. Any one have experience here?

Thanks

Click to expand...

It sounds like a stackup that can work, but putting the Ditra above plywood sleepers traps moisture in the plywood sleepers. You may want to use composite PVC for the sleepers, since it's not susceptible to moisture. Whether the seams of the Ditra are sealed or not makes no difference in the drying capacity through that layer- with vapor diffusion it's all about cross sectional area and vapor-pressure. The seams are less then 1% of the total area, and the Ditra is built with poly vapor barrier. Even if the wood is saturated and heated to 85F by the radiant tubing for maximum H2O vapor pressure that water is staying-put under the poly.

To be stiff enough you'll need to use half-inch CBU rather than 1/4" and place the joints/seams where they are supported by the sleepers to limit flex cracking at the edges.

With the stiffness of the CBU and surface area of the sleepers you don't need the high density XPS underneath- the cheap stuff will do. But under radiant you'll want a minimum of R10 (R12 would probably still be cost-effective in the long term) to limit heat losses to the soil. R5-R7.5 would make financial sense even without the radiant floor, and an unheated floor would be on the order of 10F below the average temp of the radiant floor during the heating season.

The channels in Ditra will allow the stuff below to breath as long as the outter edges are not closed in. But, there is a limit on how much water vapor it can handle. You won't get any appreciable moisture to go up through it, but if you've got tile there, not much would get through in the first place. Between the fleece and the thinset holding it down the structure itself has about 1/8" high grid of interconnected channels throughout the material and is one approved material that can be installed on green concrete, or if there's a slight vapor issue. CBU generally isn't considered a structural material...not sure if your sleepers and foam are perfectly the same height it would make much of any difference on a floor with 1/4 or 1/2" stuff. The install instructions call for a spread of thinset underneath almost all cbu boards, and the whole purpose of that is not to anchor it, but to fill any minor voids to provide 100% support. I'd call the manufacturer to check out their thoughts. If you can stand the height and increased cost, go with 1/2" stuff, but I'm not sure you need it.

on my first floor, i pretty much cut 6" slats of plywood spaced them 3/4" apart and used a router to make arches in the plywood so they weren't really sleepers, it was essentially custom routed plywood as you can see here in the pic.

I did make sure each CBU board NEVER touched or ended over a gap.

Ground moisture alone would be an issue with Ditra on top of the wood. Putting poly under the foam doesn't change that much either- the wood HAS to be allowed to dry toward the room, since it has ground moisture drives that can only be mitigated, not 99.99% blocked. The channels need to be positively vented to the room air to change the long term moisture picture for the wood, since it won't pass moisture by any other means due to it's poly layer. Simply venting moisture into the channels does squat if it has no place to go.

Fasteners punching a bunch of holes doesn't change the vapor barrier aspects of poly. The fasteners aren't vapor permeable either, and with vapor diffusion it's all about the available cross sectional area anyway- what's the cross sectional area of all the fasteners as a fraction of the whole? What's the cross sectional area of all of those micro-tears in the poly surrounding the fasteners? (Answer: Not much!) Even if 1% of the area was holes it would have to be tiny holes on a very tight grid to allow much drying capacity. Perforated radiant barrier foil products are ~2% holes by area with tiny holes on a 1/4" grid and they still come in at under 10 perms. Any wood more than an inch away from the defect in the poly due to the fastener doesn't get any drying relief worth mentioning.

According to the spec, the permeance of Ditra is 0.006perms,which is as vapor-tight as it gets, a true vapor barrier. It's designed to protect the tile from moisture drives coming from below, not to protect the subfloor from spills on the tile or other moisture drives from above the floor. (Tile is usually porous even to liquid water, and usually relatively high permeance even compared to wood.) Ditra is really the right thing to provide the mechanical decoupling needed with a radiant floor, and it would be fine over a wood subfloor that had the capacity to dry downward, but good luck with downward drying on a basement slab. The only sure-fire option here is to limit yourself to materials that aren't susceptible to moisture under the Ditra.

Dana said:

According to the spec, the permeance of Ditra is 0.006perms,which is as vapor-tight as it gets, a true vapor barrier.

Click to expand...

If you seal the seams, Ditra IS essentially waterProof.

Depending on how you treated the edges of the room, the channels could allow water vapor to dissipate. There is a continous matrix of approximately 1/8x3/16" or so open channels throughout the material from edge to edge. DitraXL has significantly taller channels. This allows you to install Ditra on a green slab that is still outgassing moisture as soon as you can safely walk on it. Now, if you caulked the edges or didn't have a path for that potential moisture to go, it would hold any moisture in, and that would be bad for any wood sandwiched in there.

If there's no air-pressure difference to purge the air, the channels will still pretty much stagnate with only miniscule humidity transfer to the room air.

But if you slope the slab at a 2:12 pitch or higher, with the heat of the radiant it should convect pretty well!

This isn't much different than the venting problems of a flat roof, except that you have zero chance of wind helping you out.

Being able to install it on top of a green slab has no bearing on whether you can put it over wood on top of a slab- it's a different problem (and susceptibilty) entirely. Putting it over the green slab only means that even if it's outgassing it won't screw up the finish floor. Concrete tolerates high moisture content extremely well, wood, not so much.