Chris wrote:
I want to install wood grain finish panel on interior walls and on ceil
... panels are 4' x 8' sizes and thickness is 2.6 mm.
I would need some advice how to install them on the wall and on the
ceiling of the cathedral roof.
I want to avoid them to be undulated because of dilatation (expansion).
I did just add R-20 pink fiber glass insulation on the wall (studs is
2"x6" at 16" on center) and cover with 6 mil vapor barrier.
How should I put these panels in order to cover the walls and the ceil?

May I put them directly on studs in contact with vapor panel? Or is
better to use some horizontally furring strips?
Also I would not like to use a gypsum wallboard as a back support.

Don't know the specific and you don't say what the material is, but the
dimensions given indicate this is a very thin ( 1/8") panel. In order
to keep something that thin flat you definitely will need furring
strips or other stiffer material behind them, especially on a ceiling.
I'd think 2-ft spacing might be adequate. As always, for required
spacing between sheets to account for expansion and recommended use
including backer material, etc., check manufacturer's recommended
installation notes which are almost always available either in the
retailer's location or online. I'd venture a small gap just so they
don't butt hard up against each other would be adequate for interior
use, but that will obviously depend on what the panel is made of as
various composites have differing expansion characteristics (mostly
humidity-related).