Shell construction input

[BigJim2185]

Hey guys -
I have been doing some work on my shell design and was hoping for some input.  Some of my concerns are listed below. 

1.  I am having a tough time finding consistent information about vapor barriers/retarders, but the consensus seems to be that if you are going to use air conditioning for part of the year, a warm side vapor barrier is problematic.  As such, I have not included one.  Am I on the right track here?

2.  With a perm rating of about 1.1, manufacturers state that XPS foam on the outside of the stud wall will not create a cold side vapor barrier, like polyiso would.  Will it allow my wall to dry the way I have used it? 

3.  Manufacturers also state that xps will serve as a drainage plane.  I am planning to mill thick clapboard siding, and I am having a hard time feeling like I could trust fasteners going through 2" of foam into stud walls to hold it without sagging.  I tried to solve this with a large sill that will partially support the strapping, but this has left me feeling uncomfortable with drainage at the sill, which right now consists of an additional drainage plane of #15 felt skirted over the sill.  There is some indication that xps will absorb some water and lose R value in doing so, so the redundancy of the felt isn't a problem so much as this designs long term capability to keep the sill dry.

Any input would be appreciated.  I don't know what I'm doing, so you won't insult me.  Should be a pic attached below, or view it in my gallery.   




 

[timberwrestler]

1.  No vapor barriers anymore.  Unless you live up near the Arctic Circle.

2.  The perm rating of the foam depends on the thickness.  The perm rating isn't really a factor into how much or what type of foam to use.  Read this:
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
(I think they're going to delete that link, so google "green building advisor minimum foam thickness martin"). 
Actually, read everything on Green Building Advisor.  And at Building Science Corporation.

3.  It can be a drainage plain, and I've done it, but I regret it.  It's not very expensive to add a layer over the foam.  I wouldn't worry about the strapping sagging if you use good screws.  I wouldn't worry about the XPS absorbing water unless it's in really wet area for a really long time.

[BigJim2185]

Thanks for the help.  I had referenced a different article with a similar chart to that on Green Building Advisor to determine the 2" of xps. It looks like the article you linked is in agreement that as long as I use a class III vapor retarder on the warm side, I should be good to go even if the foam doesn't allow the wall to dry to the outside. 

I am trying to talk myself into eliminating the sill lip at the bottom of the drainage plane.  It's encouraging to hear you say to use good screws and not worry about it. 

I guess the alternative is to think about flashing the sill with something more substantial than a fold in the #15 felt.  The sill is going to overhang my foundation wall by between 4" and 4.5" either way.  I am planning to pour a ledge around the foundation wall just below grade so I can eventually face it with fieldstone, in which case it might be nice to have the drainage plane flashed out over the fieldstone face, all behind the siding.  Is there a good flashing product for an application like this that is less expensive than lead?