Floor heating in an old house with 800cc. How do you do it?

[Grudd]

I am going to install underfloor heating in an old house that has 800mm center-to-center between the floor joists.
How should I proceed to ensure it doesn't flex between the joists?
I have 22mm boards with grooves for pipes, which are 600x1800.

 

[Kakelfog]

You can either beam up so you get cc40 or you can set cross-beams to brace it, depending on how it looks under the floor. If beaming up is possible, it is the best option. Otherwise, the second alternative is to lay another chipboard first, but then you raise the entire floor and it can create issues with adjoining rooms due to the height difference and problems with doors.