New heated tile floor substrate

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Old 10-16-16, 10:34 AM
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New heated tile floor substrate

I'm working on a bathroom renovation (complete tear-out) and am doing a hot water heated floor. I've sistered the ~100yr old somwhat damaged 2x10 joists with new 2x8's which I've made perfectly level. I then installed 3/4" x 1.5" nailers to the sides of the joists 1.0625" below the tops of the joists and spanned the between-joist space with 1/2" osb. I then spaced out my 3/8" pex heating pipe runs and filled in all of the spaces with strips of 1/2" osb. I pressed aluminum transfer plates into the groves and am about to install the 3/8" pex. The resulting assembly will be a perfectly flat and level floor with all the heating installed level with the tops of the joists.

Right now I have 1-1/8" to 1-3/16" from the tops of my new joists to the finished hardwood floor outside of the bathroom. That means if my subfloor, cement board, and tile could be ~1-3/16" thick I'd have a nice even transition.

I always prefer to use 3/4" ply/osb, then 1/2" cement board, then tile, but that would make my assembly around 1-3/4" thick. I'm wondering if, given my heated floor assembly between the joists, which makes things a lot stronger, I should be thinking thinner. Maybe 5/8" osb plus 1/4" cement board plus tile. Any assembly that brings the tiled floor close enough to the hallway level and avoids having a transition piece would raise my eyebrows.

There are fewer 5/8" sheet options out there and I would normally only use high quality plywood for something as important as a bathroom.

BTW, we're thinking about 2" octagonal tile sheets.

Ideas?
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The work is already done as described, but looking back, I'm curious what you guys would thinking of me putting a more solid 3/4" ply between the joists to level, then running my pipes/plate and strips of 1/2" osb perpendicular to the joists then putting cement board and tile straight on top of the heating assembly. This gives better heat output and simplifies the construction a bit, at the cost of not having a nice thick continuous subfloor. I was also worried/unsure about the aluminum plates reacting with cement board.
 
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Old 10-16-16, 12:19 PM
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Seems like another option might be 3/4" plywood subfloor plus 1/8" ditra (a product which I've seen a lot but never used myself).
 
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Old 10-16-16, 08:23 PM
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1/4" backer board is fine for a floor. I would not use less than 3/4" plywood under it.
 
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Old 10-16-16, 09:27 PM
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Today I pressed in the 3/8" pex and ran some ~140deg water through it. Everything looks good except the pipe makes some noise as it expands within the aluminum plates. It kind of pops for about a minute while heating up. I wonder if I should put something in the omega shape of the plates, either adhesive or lubricant. I also wonder how the plywood pressing down against the pipe will affect things.

At the moment I'm leaning toward 3/4" ply plus 1/8" ditra which will make my finished tile flush with the hardwood floor in the hall.
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Old 10-17-16, 10:10 AM
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After thinking about it for a while I decided the best thing to put in the grooves of the plates would be a caulk that stays gummy. Best thing I had on hand is acoustical caulk made by Green Glue so I put a 1/4" bead of that down a couple runs and pushed the pex back in place. The caulk fully squished up around the pex and I had to wipe the excess off the top. Since this stuff takes forever to dry, it'll be a while to test, and I still don't know how the plywood subfloor will affect noise, but I'm thinking of running a bead down the top of the pex before subfloor.

Then a few hours after I did that, I found this: http://media.wattswater.com/WR_Radia...XPlates-EN.pdf
Instructions from Watts to put a bead of silicone caulk in their aluminum plates exactly like I did. Watts specifies that ONLY silicone caulk should be used, but doesn't explain why. I would guess they're afraid of anything that could harden or chemically damage the pex?
 
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