XPS and EPS have very different R values.

You need to first design the wall system and determine what R value you need then consider the insulation types that are appropriate.

Renovator8, it is mineral wool, a consultant suggested this as an option but the architect has not used it in this application before. I've linked an article describing the product that would be used on the exterior.

My notes on the wall system are sketchy but generally speaking on the exterior we will have an insulating layer, a housewrap and siding, my understanding is that the placement of the housewrap/barrier would depend on the type of insulation used.

(The consultant is for certification to get a building permit and CO in this location. New construction or major remodels require you to satisfy local green building requirements or get LEED certified.)

I like using XPS in our cold climate behind masonry, as in the pic below.

Photo: Heather Joy Investments Ltd.

Why at this point exterior mineral wool doesn't appeal to me:

1) Need for a sheathing inboard of the insulation. By contrast, the XPS can be used for both purposes as long as there is internal bracing (seismic and other local requirements may differ);

2) The attachment, as described in the linked Halliday article, looks awfully fiddly to me. As well, the carpenters attach the XPS to wall sections being completed on the building platform and then raise them. It looks like this may be more difficult with much thicker sections of wool.

3) The walls sheathed in XPS stand up well to the weather as the house awaits the final cladding. I can't imagine leaving mineral wall in the rain and exposed to insects etc. for two months.

4) XPS does not require a separate weather or air wrap.

Behind masonry, though, I let the bricklayers put on their usual felt.

I wouldn't even consider EPS as an exterior above grade insulation. In most climates you'd need a thick layer, creating concerns about attachment. As well, it's so flimsy, there's bound to be mechanical damage, especially at the edges.

Below is a schematic from Roxul on exterior installed mineral wool.

In past decades I have seen rigid rockwool and fiberglass boards used in the cavity of a brick veneer wall system, under membrane roofing and on the face of foundation walls below grade but in general, rigid foam insulations have taken over.

I have never seen rigid rockwool used behind siding but I guess it's possible. There are a lot of issues to consider and the advantages are unclear to me. That's all I know.

The issue I would be concerned about is rigid mineral wool's ability to absorb water regardless of how fast it dries out in the open air. Wood also dries fast in the open air but it rots when wet in a wall system. There is too much contradictory information about that aspect of the material and I need better information before putting it on the face of a building.

Renovator8, sorry I was not trying to be confusing, all of this terminology is new to me -- I am trying to learn as much as I can and "Roxul" was the name that the consultant and architect used. I saw something about the mineral wool being water repellent but I will certainly ask the consultant about that!

Think I have more reading to do, I very much appreciate the education.

Mineral wool is a fine insulation. And unlike XPS, for instance, aging doesn't reduce its R value. But for what I build in my climate there are simpler energy efficient alternatives when it comes to insulated sheathing.
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