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Drainage plane above or below exterior XPS?

user-669103 | Posted in Green Building Techniques on

Over at they show various combinations of laying of sheathing, house wrap, and XPS. polyisocyanurate

The preferred (most durable) on page 13 of 30 is XPS, wrap, sheathing.

However, it seems to me that wrap, XPS, sheathing may have an advantage:

1) The joints in the XPS are likely the weak points, whether mastic or tape is used. There are most XPS joints than wrap joints. Thus having the wrap on the outside to protect the XPS joints seems advantageous.

2) If water penetrates the exterior insulation (via joints) then wrap in the middle will work as a barrier to infiltration. However, if the wrap is on the outside the water should not infiltrate that far in the first place.

The rational that gives for XPS, wrap, sheathing order is that the wrap is protected. Which leads me to the question of maybe using stronger wrap is wise if on the outside. In this case I could use GAF Deck Armor (16 perms) instead of Tyvek (20 to 56 perms depending on type of Tyvek).

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  1. homedesign | | #1

    maybe this will help confuse things...a discussion with no clear conclusions....
    Riversong makes some good points

  2. user-669103 | | #2

    Don't know how I missed that thread. Good points made all round. In the end I'm not sure that I am convinced that any house wrap is required if there are two lapped layers of XPS. It is not like XPS will soak up bulk water, especially if the outer siding is mounted on 1" furrings thus creating an air gap and natural drainage plane.

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