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Community and Q&A

EPS or XPS for Exterior Wall Insulation

zox20 | Posted in Energy Efficiency and Durability on

I will be building a new house near Tucson. Lately, I’ve noticed some houses under construction using a synthetic WRB and Formular 250 instead of EPS and 15# felt. A typical exterior wall here in the southwest consists of 2×6, OSB sheathing, 15# felt, 1″ EPS foam board insulation, and one coat stucco system. Would the substitution of XPS and synthetic WRB for EPS and felt be worth the additional cost?

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  1. Expert Member
    Michael Maines | | #1

    XPS has significantly higher levels of up-front carbon emissions than EPS, so if you care about the environment and climate change, EPS is a better choice. XPS is not as vapor-permeable as EPS but at 1" thickness it's somewhat vapor-permeable.

    15# felt is pretty flimsy stuff; it used to be tougher but now it tears if you look at it. I sometimes use 30# felt but usually find that synthetic WRBs are tougher and easier to work with.

    Where is your air control layer? I usually tape the sheathing to create the air control layer, but sometimes WRBs are used for that purpose.

    1. charlie_sullivan | | #2

      As an addendum to this good answer, there are now improved version of XPS available, standard in some regions and by special order in others, that have much lower global warming impact. Most common are the gray version of Styrofoam called "beyond blue" and Owens Corning NGX.

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