The latest issue of Fine Homebuilding has a small article on R4.2/inch rigid mineral wool boards as a cheaper alternative to XPS with some perhaps better characteristics.

As I am thinking about building in a damp coastal place (with abundant ants) this would seem ideal for application over plywood sheathing and under rain screened siding - moisture resistant, hygrically permeable and free draining.

This could replace an outer course of XPS over a composite staggered stud wall designed to acheive R40 .   Using XPS for the outer wall  would introduce an undesirable outer vapour barrier that could trap moisture inside the wall assembly due to the low permeability of the XPS boards.

A composite wall design with an inner vapour barrier, cellulose in a thermally broken double stud wall cavity, sheathing and then Mineral Wool boards, house wrap, rain screen and then siding could dry outwards nicely. 

The catch is that it is not widely available - it is sold as drainboard (Roxul- www.roxul.com), Rainbarrier (www.Thermafibre.com) and FBX Drainage Board or Minwool.

Any experiences with using 2" of rigid mineral wool board in place of XPS?

Thanks, Graham