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What kind of exterior rigid insulation should I use?

[email protected] | Posted in Energy Efficiency and Durability on

I am working on a series of small 1950 vintage homes. I am planning on filling the walls with cellulose blown from the outside, adding exterior rigid insulation, and re-siding. The exterior finish is not yet known, but I would like to offer the Architect something that works regardless what is chosen (within reason).

I had been thinking about some kind of un-faced extruded foam (so that I do not add any vapor barriers to the system) but people keep giving me funny looks when I say this.

Any suggestions?

– Ron

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  1. GBA Editor
    Martin Holladay | | #1

    All three major types of rigid foam — expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate — have been successfully used as exterior sheathing.

    EPS's main advantages: it's inexpensive and relatively vapor-permeable. However, it tends to crumble at the edges and has a relatively low R-value per inch.

    XPS is chosen by many builders. It's denser, smoother, and stronger than EPS, and it has a higher R-value per inch.

    Poliso is usually sold with aluminum-foil facing, making it vapor impermeable. This may not be a problem, however, as long as the wall assembly's moisture performance has been modeled, and as long as there is no interior vapor barrier. Vapor-impermeable sheathing makes more sense in hot climates than in cold climates.

    Since you didn't mention where you are building, it's hard to recommend the best sheathing. Regardless of which sheathing you choose, remember that the performance of the wall will change depending on the thickness of the foam. In cold climates, for example, thin foam sheathing does not perform as well as thicker foam sheathing. The reason: the foam must be thick enough to keep the interior face of the sheathing warm enough to prevent condensation.

  2. [email protected] | | #2

    There already are some of what I call semi-vapor barriers of unknown permeability (such as somewhere just under 60 years of interior painting (at least 5 or six colors in spots) as well as the possibility of other unclear existing assemblies. Since I do not really know, it is simply safer not to introduce a barrier if I can avoid it. The project is in Boulder Colorado, and I try to never spec a vapor barrier without clear cause on my new construction projects. I would like to use XPS. What I am still wondering however is about the facing on the XPS. I was told a while back by Joe Lstiburek that "un-faced XPS" would be the way to go when you did not want to stop water vapor from going where it wants to go. Is this just outdated info? It was at least 10 years ago when he told me this.

    - Ron

  3. GBA Editor
    Martin Holladay | | #3

    I'm not sure why Joe referred to "unfaced" XPS. All of the XPS I've ever seen — whether blue or pink — is unfaced.

    As I recently noted in a response to another question, Dow Styrofoam XPS (1 inch thickness) has a permeance of 1.5 perm. That's 25 times more permeable than 6-mil polyethylene, which has a permeance of 0.06 perm, but it's still a fairly low number.

    Remember, though, that 1/2-inch OSB has a permeance that varies between 0.70 and 2 perms -- about the same at 1 inch of XPS -- so most U.S. homes already have a vapor-retarding layer on the exterior.

  4. homedesign | | #4

    Martin, Some of the xps available here (N.Texas)has a thin clear film on the surface of one side.

  5. [email protected] | | #5


    The homes that I am working on were built in the mid 1950's. They have no plywood or OSB sheathing. They have some kind of exterior sheathing that is a cellulose based product (we think). It looks a lot like Homosote. All said, if we can get good exterior insulation without stopping vapor flow, we would feel more comfortable.

    - Ron

  6. Riversong | | #6

    Ron, I agree with your concern. You have unknown vapor permeance on the inside (and likely unknown air tightness), an exterior skin that's already highly vapor permeable - so it makes no sense to add an impermeable sheathing to a structure that's been able to dry to the exterior for 60 years.

    If you must add a thermal break to the exterior, you'd be better off with EPS - perhaps 2" with strapping for a siding nailbase and "drainscreen" if you can manage the flashing details.

    Have you considered an interior thermal break, such as cross-hatched strapping or 2x3s, and extra cellulose - or are you not touching the interiors?

    [by the way, the XPS Martin refers to on this and another thread is Dow CladMate (low density) - normal XPS has a perm of 1.1 for 1"]

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