Another question. If ISO is attached on the outside, do you even use OSB? Buildingscience site seems to show you don't and that the siding can be directly attached to the ISO through the studs. Sorry, I'm a newb at this wall structure stuff, but appreciate the help I'm getting.

I know of a house that only used OSB in the corners and put foam directly on the studs elsewhere. Seems to work fine.

Not to change the subject, we're considering a full wooden entry door, like doug fir or black locust, but after spending all kinds of money on good wall insulation, are we making a mistake getting a wooden door that presumably has little r-value. My wife I like the look of the wooden doors, but am having second thoughts about it.

Why not an insulated fiberglass door with a stainable wood grain?

I worked on a house built in the '80's with 1" of polyiso on the outside of the studs, followed by clapboards. The nail holes thru the foam had expanded from the movement of the wall over the years, rendering the foam pretty useless. The OSB should be nailed to the studs, with the foam outboard of that. Will it work backwards? Probably, at least for a while until you rebuild. Why take the chance? It gains you nothing except more issues to worry about.

I love nice wood doors and installed a 100 year old four panel door on my house, along with two old half glass doors, but rarely use them on other's houses because few clients understand that they warp, the expand and contract, and in general exhibit all the characteristics of wood. In short they are a PIA, but if you like real wood, nothing comes close. You'll need either a roof over the door or a good storm door to keep weather off the wood. Oh; they're also pretty bad energy wise, but the "standard" doors out there like Therma Tru are pretty bad also, so it's not that much of a hit if weatherstripped well. My last house was very tight overall, but the most significant infiltration was from all four TT doors. I'm trying TT doors with multi point locksets on the current house.

Unless you are an artist, get the door prestained by the factory - its very hard to do well so that it doesn't look like stained plastic.

I guess all the prow glass on the front of the house will negate any or most of our energy efficiency. And here I am worrying about a wooden door. Lol.

Posted By jzinckgra on 05 Sep 2012 12:54 PM
Another question. If ISO is attached on the outside, do you even use OSB? Buildingscience site seems to show you don't and that the siding can be directly attached to the ISO through the studs. Sorry, I'm a newb at this wall structure stuff, but appreciate the help I'm getting.

You can do that, but only if you cut in some other racking-prevention.  A common method is cross-bracing with T-steel cut into the studs, another is to build in a stout panels on the corners-only (using thinner foam over).  Not all methods work well enough to meet loca code in coastal wind zones or hurricane zones though.

You can't nail on siding directly through 2" of foam even from a structural point of view, let alone the severe thermal-bridging issues. The moment-arm of 2" of nail is too long- it'll bend or pull over time (even ring-shank).  To nail it on you have to provide something solidly structural, such as furring through-screwed to the studs on a narrow enough pattern to handle the load, but wide enough to minimize the thermal bridging of the fasteners.  For clapboard (or just about any siding with only 2" of foam) you can use 24" o.c. spacing for the furring screws.  The screws need to penetrate something structrural  (like sheathing + stud) by 1.5" min, and the heads need to be fat enough to not rip-through or split the furring over time.  Pancake-head  timber screws (eg FastenMaster HeadLok or similar).  Some of the home-center type box store stock a few lengths of HeadLok, but you can find them online in just about any length.

If you use 1x furring it won't always deliver the flattest exterior wall appearance with clapboard siding, but with 2x furring that's rarely an issue.  Also, some siding vendors have issues with only 3/4" (1x) wood- whereas 1.5"(2x) furring has plenty of bite for the siding nails.  Venting the bottom of the rainscreen gap with a combination of bug-screen and Cor-A-Vent or similar is advisable to keep it from becoming a critter-condo.

There are a lot of details on how it's done on this blog bit (including the detail-articles linked to in that blog.)

Where is the air barrier? I like the idea of the osb sheathing being the air barrier. You can tape the seams, use a spray applied wrb, caulk etc. When combined with dense pack cellulose or dense pack fiberglass you find a further reduction in air leaks. Top that off with rigid exterior insulation that has taped seams and you should have a tight house.

Posted By Bob I on 05 Sep 2012 03:49 PM
I worked on a house built in the '80's with 1" of polyiso on the outside of the studs, followed by clapboards. The nail holes thru the foam had expanded from the movement of the wall over the years, rendering the foam pretty useless.

Speaking of that, does XPS shrink? We just pulled siding at a 2nd level band joist of a mid 80's home with 1" foam t&g and there was probably 1/4" gaps at every joint.

I've never seen evidence of that, but that's not to say it can't happen. I have seen it with EPS. Another excellent reason to use a high quality stretchable tape like SIGA at the joints.

Posted By Roberth on 07 Sep 2012 12:17 AM
Where is the air barrier? I like the idea of the osb sheathing being the air barrier. You can tape the seams, use a spray applied wrb, caulk etc. When combined with dense pack cellulose or dense pack fiberglass you find a further reduction in air leaks. Top that off with rigid exterior insulation that has taped seams and you should have a tight house.

The final wall structure from the inside to outside will be: drywall, cellulose, OSB, 1" polyiso. All seams inside and out will be either taped and/or spray foamed including the sill plate.

You may want to consider using Sips. Check out Thermocore of Missouri.

Posted By Bob I on 07 Sep 2012 08:13 AM
I've never seen evidence of that, but that's not to say it can't happen. I have seen it with EPS. Another excellent reason to use a high quality stretchable tape like SIGA at the joints.

I've seen gaps like that on XPS from the 1980s that have been in service for well over a decade.

IIRC the manufacturers claim to have rectified the shrinkage problems with XPS, but wouldn't recommend counting on it as a primary air barrier no matter how good the tape is.