Marmox board

[carlos21]

Hi. I have just put a new pvc door into old stone wall, i was hoping to insulate the inside revesls but there isnt enough room as id like, so i was thinking marmox board, my question is how ridgid is the board at small thickness ie 6 or 10mm? 

Because the stone work beneath is uneven iam wondering can i get away with not renderjng it flat behind the board?

Thanks carl

[Russell griffiths]

Bond it on with spray foam, then when it has gone off squirt more foam in the voids, I would not go thinner than 12mm too floppy. 

[Super_Paulie]

When I was stuck for space I used 6mm, foamed on and then mechanically fixed so I could get it plumb and level.