I wish to install a large scale porcelain tile in roughly 600 square feet of living / dining area in a 1920's home. Currently the home is heated by underfloor radiant heat - Wirsbo tubing in an aluminum thermal plate "Joist Trak" system. I have removed the existing finished floor and am left with the original diagonal 3/4"+ wood subfloor. Floor framing consists of 2x10's @ 16" O.C. with the longest unsupported span being 13'-0". My question revolves around the hope of creating the most thermally efficient base for our radiant installation.

Is it possible and advisable to create a mud bed over the existing conditions without adding another plywood layer? It seems as if this would be the most efficient method in regards to heating capabilities. By adding the additional ply, I think it would eliminate the benefits of the mud bed.

If the above is not advisable is there another product that I could use in place of 1/2" ACX that would strengthen the floor while at the same time increase the thermal capabilities beyond what plywood would achieve. I was thinking that a Hardie Panel, structural siding panel might be an option.

In either scenario, would you suggest using Ditra as the final underlayment.

Thanks in advance.

What thickness is the existing subfloor thats over the diagonal plank?

Hello-

The existing 3/4"+ plank is the existing subfloor / sheathing. No additional subfloor exists.

Thanks,
Ryan

Sorry for the confusing language, the existing plank is strong of 3/4". (3/4"+) Closer to 7/8".

Thanks.

If all the diagonal planks are in good condition then a min. 1/2" plywood underlayment with Ditra on top of that should be fine.

A mud bed would be ideal, but the increased height transitions may present there own problems. You would still need the additional plywood layer plus the mud bed. You might as well just use the plywood/Ditra and save all that labour building a mud bed.

Thank you Randall.

Any feedback on the use of another structural panel other than plywood, that may help to increase the thermal quality of the floor?

Also, when applying a self leveling compound, would you put it down before or after the layer of ply?

Thank you.

There's nothing I know of that would work for your type of installed heating system. The best method obviously is to have the hydronic tubing embedded in concrete, gypcrete or mud bed.

I think you'll find that the extra plywood layer and Ditra combination will work fine.

On one job I did, the homeowener did not want infloor heating under the tile initially. Needless to say, once it got cooler they wanted a warm floor. The solution was to do a modfied hydronic heat system under the subfloor. This system ran off the existing hot water tank. Let me tell you, that room where the hydronic is, was plenty warm.
Your system would be pretty much the same. I wouldn't worry about it not being warm enough or retaining the heat.

Thank you for your input Randall.

Any advice on the application of the self leveling compound addressed above?
Nothing major out of level, just a few areas that need adjustment.

If this is addressed in another thread, let me know and I can go there.

Regards,
Ryan

Does this area need to be level or are you just trying to make it flat?
How much is the adjustment, 1/8", 1/4",1/2"or more ?

The plywood needs to be installed over the diagonal plank first before any leveling or thinset patching.

I would avoid an SLC application if possible.

Can you post any photos of the area your working on?

Randall-

I would be happy to post an image or 2 when I can. I am not overly concerned with level, as the floor is probably only out 1/4 - 1/2" over 25-30'. There are a few low spots (1/4" - 3/8") over maybe a 10sf area do to the original framing being crowned. I would like to correct these. I would probably correct them under the floor from below, however access is "touchy" due to the radiant tubing.

Maybe it would be best to compensate for it during Ditra and tile install? Your thoughts are appreciated.

Thanks