I am getting my knickers in a twist over what appears to be conflicting information so hope those with greater experience will be able to clarify for me. Here is the background:-
New build, timber frame house that I wish to install a contract grade, one piece vinyl in the bathroom. The build-up is joists, 18mm OSB subfloor, 30mm expanded polystyrene which has the wet UFH running through, and 18mm floor grade, t&g Caber Board more or less floating on the styrene. (More of less because where is did not did not sit flat, I have put a few screws through to the subfloor.) Around the perimeter, is some special blocks provided by the UFH company to support the edge board and 10mm of foam to reduce cold bridging and provide an expansion buffer. The Caber Board is smooth and flat with minimal gaps at the joints.
The vinyl installer wants to overlay this with 6mm WBP. He says it will bridge the Caber joints (but surely there will be joints in a different direction?) He also proposes to take it to the edge, leaving no expansion gap. He says he cannot stick the vinyl to the edge(10mm) foam. But I will be having a skirting board so I don't see that as an issue. He also wants to fully adhere the vinyl.
I have spoken with the CaberBoard techs, and they recommend 6mm WBP generally, but over wet UFH, not having it would be OK. They say the expansion of their product is minimal and would happen at the edges so leave a gap (I have.)
I have spoken with the vinyl techs (Forbo) and their advice is, fully gluing only necessary if the are is over 18 sq.m; it is only 8 sq.m. Additionally, an additional 6mm will mean altering the bath panel height and affect the threshold height which I hope to keep (more of less) level.
Do I really need this 6mm WBP? Could the vinyl be edge glued to at threshold and around the shower area and left floating over the rest of the floor to allow for expansion and contraction, which would happen under the skirting board and bath panel?
It's not a question of saving money (only £150) or heat output, it just seems unnecessary to put flat another surface over a solid, already flat surface.