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Bathroom floor build-up & Underfloor Heating: 15mm ply + 10mm insulated board — overkill or smart?

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Hi everyone,

I’m planning of redoing my bathroom floor and I’d appreciate some input from the experts here. The main joists are spaced 43 cm apart. Right now, there are a few bridging pieces (I guess you could call them noggins) running between the joists about every 2 meters, at lest this is what I managed to see.

I have 2.5cm to play with between the main joints and tiles. My plan is to use 15 mm plywood and then put a 10 mm insulated cement board on top to have the best thermal efficiency. To maximise structural rigidity, because of the fact I am using 15mm ply-wood compared to more commonly used 18 or 22 mm, I am thinking of adding few extra noggins in between the joints, say every 80-90 cm, to get more support for the plywood and avoid deflection.

Then I’ll lay the electric underfloor heating mat, apply tile adhesive, and install 60x60 cm tiles.

My questiona for you are:

1) does this setup sound good to you? Buy adding some extra noggins will I be able to use 15mm PlyWood instead of a thicker one?

2) Is it worth doing some extra work in order to usev10 mm insulated board instead of a 6 mm? Is the extra thermal benefit of the 10mm really worth the extra work of adding more noggins?

Thanks a lot for any advice!
 
I know this doesn’t answer your questions, but… why are you going to put down cold-under-foot tiles and then heat them up with expensive electricity, rather than just putting down a naturally warm-feeling floor?
 

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