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Exterior XPS sheathing over exterior 2x2 stud walls - good idea or not?

Hi Everyone,

New to this website, wonderful discussions and information!

I live in Michigan Zone 5a. I have an old Sears Craftsman house that has 2x2 stud walls with 1/4" plywood on the exterior for sheathing and on interior of the walls as the finish surface. The interior plywood has been painted over the years. It has steel frame windows with single glazing. Talk about cold and drafty. We are getting ready to install new windows and re-side with vinyl siding and would like to tighten the house and add a little R-value on the exterior when we do. We were going to install Tyvek over the exterior 1/4" plywood sheathing and then 3/4" XPS foam, then new windows and vinyl siding. We would love to do thicker foam but due to the roof details, we can only add the 3/4". I am not yet sure what, if any insulation is inside the wall, but if there is none, we could consider blowing them with cellulose.

With this thin wall assembly. are we setting ourselves up for moisture issues by adding the foam? Or is this little bit of R-value and thermal break a good idea?

Thank you, Joe

Asked by Joe Antis
Posted Jan 30, 2013 9:28 AM ET


6 Answers

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3/4-inch XPS has a low R-value -- about R-3.7. That's not much.

In your climate zone, the standard recommendation would be that any exterior rigid foam needs to have a minimum R-value of R-5. This recommendation would be for a 2x4 wall, however, not a 2x2 wall. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Your unusual wall assembly affects these calculations. You didn't mention whether there is any insulation between your 2x2 studs.

Here's the way the calculation works: the less insulation you have between your studs, the less it matters whether your exterior rigid foam is thick enough (according to the table in the blog I linked to).

Lots of insulation between your studs makes your sheathing cold. That's risky. Less insulation between your studs makes your sheathing warm. That's less risky.

So you'll probably be fine. If you really want to do some more math, you can perform a dew-point calculation. Here is an article that explains how to do it: Are Dew-Point Calculations Really Necessary?.

Needless to say, however, you're still going to end up with a pretty crummy wall. If I were you, I'd investigate options (perhaps extending your roof line with flashing) that would allow you to install thicker foam on your walls.

Answered by Martin Holladay
Posted Jan 30, 2013 10:28 AM ET
Edited Jan 30, 2013 10:31 AM ET.


Thank you, Martin. You are correct, it is a pretty crummy wall assembly. I find it amazing they made these "kit" houses with 2x2 walls with 1/4" plywood both sides. We have not opened a wall yet to see if there is insulation, but obviously there can't be much if there is since there is only 1-1/2" of space. I read through your reply and the links you sent. I have tried to figure a way to make thicker foam work, but I think we end up with a bad roof detail without re-working the roofing itself. I expect the tighter shell, new windows, and the little bit of foam will help with the comfort and efficiency nonetheless. Obviously, my big concern is avoiding any "condensation" issues. Thank you for easing my mind by taking the time to reply.

Enjoy your day!

Answered by Joe Antis
Posted Jan 30, 2013 12:00 PM ET


Joe: for how long will you be living in this house ?
removing exterior siding is not a small job, if you are there for 10years +
please consider extra exterior insulation. something in the 2" of XPS would be sufficient and
will pay itself in no time.
I am sure that the roof extra labor costs will be more than marginalized by the energy savings from the extra insulation.

Your 1.5" cavity probably has nothing or at maximum R4.5.. but you should realy consider
drilling a poke hole somewhere ( maybe inside a closet ) to verify before investing in insulation.
As Martin pointed out, even with 0.75"of XPS, if you've got insulation within the walls,
you might be running toward moisture problems.

I am pretty sure that some expert here could point out a way to calculate the payback of the additional insulation ( let's say 2" XPS or polyiso ) and you could add the cost of the roof modifications there.

Martin has published alot of very good articles ( that i am still currently reading and learning from )
about exterior insulation and what it implies, look at the blogs or use the search engine.

Have fun planning your investment!

Answered by Jin Kazama
Posted Jan 30, 2013 1:19 PM ET


You wrote, "As Martin pointed out, even with 0.75"of XPS, if you've got insulation within the walls, you might be running toward moisture problems."

I disagree. You are misrepresenting what I wrote. Even if there is 2-inch-thick insulation in the stud cavities of Joe's home, adding 3/4 inch of rigid foam on the home's exterior will not lead to moisture problems.

Answered by Martin Holladay
Posted Jan 30, 2013 1:31 PM ET


Well Martin it is easy to get away with it after EDITING ur POST!! ( ahha teasing you ! )

I was reffering that you pointed out to the article discussing the dew points situation with IN/EXT insulation placement, but my sentence syntax is probably wrong ..

J'aurais probablement du mettre cela dans 2 phrases separees!!
J'espere que je ne derange pas trop en essayant d'aider dans les QA comme ici Martin ??
Laissez moi savoir si vous croyez que je ne devrais pas commenter!

Answered by Jin Kazama
Posted Jan 30, 2013 11:09 PM ET


I didn't edit my comments for their technical content -- only for punctuation... (And I know you were joking.)

Si vous voulez écrire des commentaires, soyez le bienvenu. Tout le monde est invité. Plus de commentaires améliorent l'échange d'idées. Pas de problème!

Answered by Martin Holladay
Posted Jan 31, 2013 5:25 AM ET

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