When I was talking to our builder, he expressed a concern about using 2" foam board outside the sheathing.  We are using vinyl siding.  He said that in the past he has seen some houses that had the foam board under the siding, & when the siding is nailed in, it can "suck in" the siding since the foam board isn't as rigid as osb.  He said that you can potentially see "waves" when looking down the wall.

Has anyone heard of this?  If so, any solutions?

I don't know if this is legit/up to code, but would attaching the foam board to the outer side studs, then the OSB on the outside be an option?

Vinyl siding should be "hung" on by the nail rather than nailed down tight.    If they do it right, I wouldn't think it would be pulled in.  However, there seems to be more done wrong than right, especially when they use the nail gun to do it.   Also, vinyl siding should be able to move rather easily sideways on the nails.  This is to handle the expansion and contraction the siding experiences.

As to the other question about the insulation directly to the studs, the answer depends on how they accomplish the rest of the task.  If they use let in bracing, it might be possible.  They might be able to do it everywhere but the corners.  They may have to use solid sheathing on the corners only.  Your local codes would dictate.

Posted By jerkylips on 08 Feb 2010 09:01 AM

Posted By aardvarcus on 02/07/2010 4:03 PM
Bob is right, use a board over the foam to attach the siding to. I wouldn't recommend building any wall without using plywood sheathing, because without it you are inherently creating a weaker wall. Not only does plywood add diagonal bracing, but installed properly it captures the top and bottom plates to the studs, helping tie the wall together in case of high winds from a storm. Also, in high winds that can turn anything into a projectile, I would definitely want something more in between me and the outside than just vinyl, foam, fiberglass, and drywall. I would suggest going studs, plywood, foam, furring material, then vinyl.

Thanks for the advice, I can talk to him about this.  The only issue I see with your suggestion is that the wall is getting pretty thick at that point.  2x6 + 2" foam board + furring strips = 9" thick wall.

Reading through the responses, I want to clarify one point.  We are not considering using foam board in lieu of osb sheathing, it will be in addition.  My question really was, "should the foam board go under or over the sheathing.  Sorry if I didn't state that clearly..