House made of sip panels. Walls gypsum fiberboard sheet 10mm in one layer on the metal frame.
Step 400 mm. Above it is the coating waterproofing. Ceramic tiles 500 by 300. Glue – K77.
Should I add another layer of fiberglass sheet, how critical is it that I have one layer? Grout is also supposed to be elastic, as well as the glue? Do I need an epoxy grout or can cement grout be elastic as well?
The glue you have chosen is good. The grout does not necessarily have to be epoxy, you can add a latex plasticizer to the cement grout instead of water.
The gypsum fiber sheet is better to lay in two layers. But judging by the fact that you have made a framework of the profile, it means that the main wall is not framed, so perhaps one layer may be enough, but not 10 mm, at least 12 mm.
And I would still recommend 2 layers.