Diagonal siding...Which way to slant??

[Erin]

Yeah, one of these days I need to start a build thread, since I'm finished with the basement and nearly done framing the first floor!  lol

We're building a post and beam house similar to a First Day Cottage http://www.firstdaycottage.com/pictures/building.html (pictures in the link to see what I'm talking about)

Posts/beams, then floor decking, then walls:  1x shiplap nailed to posts, then housewrap (or tar paper), then polyiso insulation, finally strapping and exterior siding. 
First Day puts their interior siding on horizontally, but a diagonal is stronger IIRC. 
I can go either way.  My boards are long enough to trim 8" off each piece (got a steal at an auction this summer sot they're not to order) if I just go horizontally. 
Or, particularly because they're long, I can put them on the diagonal for some extra strength.

What are some thoughts? 
And if I were to put them up diagonally which way do they slant?  Intuitively I'm thinking against the prevailing winds (that is, sloping down to the east and south) but I know intuition isn't always right when it comes to engineering principles... 

[lobster]

slant in the same direction the sun beams (on average)

[Erin]

 

[firefox]

This is just a guess on my part. What I would  do is make a small section on some 2x4's maybe 4' x4'. then get a hose and a fan and experiment to see which way lets the least amount of water through. You will need to put some tarps around so you don't accidently get water in the back way. Hopefully this may help figure it out.
Bruce

[MountainDon]

Posts/beams, then floor decking, then walls:  1x shiplap nailed to posts, then housewrap (or tar paper), then polyiso insulation, finally strapping and exterior siding. 

Is this "1x shiplap" on the outside? Or is this on the inside? If that's on the outside then it is what should be applied at a diagonal. Interior wood is most often nailed with finish nails, close to the edge, and does not offer a lot of structural strength. Also, IMO, the bracing should be built into the exterior sheathing so that would mean the interior could be covered in anything any one desires.

[Erin]

Yep, it's on the outside of the frame, but the interior side of the wall. 

Think stress-skin panels-- this is the side of the "panel" that is up against the posts.  So it'll be nailed on with 12d nails, then house wrapped with either Tyvek or tarpaper (I still haven't decided this one!), then polyiso, then strapping, and finally the last layer of the "panel," the exterior siding.

[MountainDon]

The polyisocyanurate board is sometimes used as the weather resistant barrier when foil faced and seams caulked and/or taped well.

[Erin]

Which was my first plan, actually.   Skip the vapor barrier entirely and just make sure I tape/Great Stuff my seams well.

But vapor will migrate OUT of the house (since we live in a semi-arid area), which means it will collect between the shiplap and the polyiso, right?  Consequently, I was thinking I want to have house wrap, or tar paper, to control that vapor.  Rather than letting it build up between the polyiso and wood, it'll build up between/within the tarpaper and polyiso... 

[MountainDon]

Hmmm.  I'm not sure I see any need for tarpaper or house wrap on the interior facing side of the polyiso.    If water vapor travels from the interior to the polyiso, the polyiso should be warm enough on the inside face to prevent any condensation from happening. That's assuming you'll have enough thickness of polyiso board to provide proper insulation; I'm sure you will. Inserting a layer of housewrap in there seems redundant and if, by some fluke or accident of some kind, liquid water gets inbetween the house wrap and the foam board it will not have a easy way to exit. Tarpaper would be my choice if I felt I needed something there between the inside polyiso face and the interior. At least tar paper can absorb liquid water and allow it to evaporate off to the interior. But I don't think I'd want to use either in that position. If anything I might consider applying a layer of tarpaper to the outside surface of the polyiso.  But then I'd also want to determine if there is any potential interaction between the tarpaper and the foam... but foil facing on the polyiso should prevent any direct contact and thus head off that issue.

[Erin]

Well that's why I was originally thinking about skipping a vapor barrier in the first place...

But then I probably just don't know enough about vapor migration.  Will it want to go "out" in the first place? 

Like I said, we're semi-arid.  Even in summer, 30% humidity is pretty high for our area...  So, interior humidity will almost always be higher than exterior.
OTOH, if I'm insulated enough, both sides of that shiplap should be fairly close in temperature, and the vapor shouldn't be trying to wick outward anyway, should it? 
I guess my biggest concern is the fact that the polyiso is going to be directly up against the shiplap and won't have a drainage plane.  So if water DOES get between the two, it has nowhere to go except into mold production.

[MountainDon]

we're semi-arid.  Even in summer, 30% humidity is pretty high for our area...  So, interior humidity will almost always be higher than exterior.

We're in high desert NM, also arid, very dry air most of the year. I had similar concerns when contemplating adding insulation to our home. It's 2x4's from the mid 80's, with R11 fiberglass batts. I was concerned that adding foam to the exterior would trap moisture in the fiberglass infill between the studs. My research indicated that when adding foam to the exterior there is a certain foam thickness that when used or exceeded, will make the inside face of the foam warm enough to prevent any condensation between the inner foam and the interior. The colder the climate the more exterior foam is required to make the inner face of the foam warm enough to alleviate any condensation issues. If the wall cavities have no other type of insulation at all then the inner face of the foam will be guaranteed to be warm enough to eliminate the condensation issue. It's the condensation that is the major potential issue in most cases. If the walls and insulation faces are warm, then you only have the airborne moisture to deal with. In our case any excess interior moisture is removed with the operation of the A/C unit.

[hpinson]

This was on Fine Homebuilder this morning. I was intrigued-- wish they had explained their approach a little more:

http://www.finehomebuilding.com/item/25035/douglas-fir-stained-siding-how-to-details-for-enduring-beauty?&lookup=auto&V27=&V28=&V29=&V30=&V31=&V32=&V33=&V34=&V35=&V55=&V56=&Taun_Per_Flag=True&utm_source=email&utm_medium=eletter&utm_content=201208020-old-sink&utm_campaign=fine-homebuilding-eletter