My question on that (and I'm sure my contractor and subs question) will be, how do you fasten windows and siding securely through 2" of foam?

I assume you are strapping the foam to hold the siding - so we screw strips of Advantech around the windows and doors & fasten the windows to them. And/Or fasten them with "masonry clips" into the framing and do not rely on flange attachments. We've done both and both work well.

How ever you do it, consider glue & screw (or nail) instead of just screw.

OK for siding then you are basically putting furring strips over the foam which spaces the siding out a bit, but gives you something to fasten it to.

Some pics or detail drawings would be awesome here! Am I right?

What I am hearing/reading/learning, is that for a 2x4 wall in zone 5, you'd want drywall, dense packed cellulose, OSB, 1" of foam on the exterior (maybe 2 would be a lot safer), Tyvek (Necessary?), and a 1/4" to 3/4" rain screen before the siding.

For the basics on dealing with windows and rigid foam sheathing, read this.  (You may need to sign up for a trial membership to see it.)

And this.  (Doesn't require membership.)

In zone 5 with a 2x4 framing it takes at least R5 of continuous exterior insulation to meet IRC 2015 code minimum thermal performance, and that is sufficient for dew point control at the structural sheathing, which means the interior side doesn't need a vapor retarder tighter than standard latex paint.  More exterior R is clearly better.

To do it with 1" foam you'd need to use polyiso.  While the labeled R value of XPS is R5 at 1" , it's performance drops to R4.2 over a few decades.  Polyiso has a higher labeled R, but underperforms at cold temps, but you can pretty much count on R5-ish performance for the duration, boosted a bit by the presence of a foil facer and the rainscreen gap.  There is some graphite loaded EPS that get you there at 1"  too, but it's more expensive.

I use an inch of Thermax, which also makes house wrap redundant.

IMO, a little drying to the exterior is better than the none that you get with aluminum foil facer. "Doesn't usually condense because of the R value ratio" may not be the same as "no moisture problems" (as some SIP roofs have discovered). This is worth reading wrt skipping building wrap. Also think carefully about your interior and exterior air barriers.

Posted By Dana1 on 29 Jul 2016 05:46 PM
For the basics on dealing with windows and rigid foam sheathing, read this.  (You may need to sign up for a trial membership to see it.)

And this.  (Doesn't require membership.)

In zone 5 with a 2x4 framing it takes at least R5 of continuous exterior insulation to meet IRC 2015 code minimum thermal performance, and that is sufficient for dew point control at the structural sheathing, which means the interior side doesn't need a vapor retarder tighter than standard latex paint.  More exterior R is clearly better.

To do it with 1" foam you'd need to use polyiso.  While the labeled R value of XPS is R5 at 1" , it's performance drops to R4.2 over a few decades.  Polyiso has a higher labeled R, but underperforms at cold temps, but you can pretty much count on R5-ish performance for the duration, boosted a bit by the presence of a foil facer and the rainscreen gap.  There is some graphite loaded EPS that get you there at 1"  too, but it's more expensive.

Thanks I definitely have some reading to do. One benefit for me of going with 2" of foam and 2x4 is that the house plan was designed with 2x6 exterior framing. The 2x4 plus 2" of foam should be very close to the same overall thickness so dimensions wouldn't need to be adjusted much if at all.

What do you think of this information? Right now we're trying to figure out how to detail the house...

https://foursevenfive.com/an-interior-air-barrier-does-it-better/

there are different methods that are used successfully, including this one from 475. They have excellent, workable suggestions.

As it hints in the second sentence, air barriers on both sides is best.

Consider putting foam on the inside - this allows the walls to be highly breathable to the exterior (a well proven moisture safe design for cooler climates). Even 1" is a big plus.

Here is a more recent article from 475.

The more I read the more I struggle with all of this. Bottom line is there is a huge lack of knowledge in my area of anything above code min. While at first I sought to build a highly energy efficient home properly, it seems even among experts in the field there are major disagreements. If you read the comments on the link I posted, it's obvious some people that know a world more than me on the subject can't even agree to what is best. I've found lots of builders that will "build whatever I want", but if there are moisture issues etc, it's my problem not theirs. We've found a GC that will stand behind a home that he builds, but only if he builds it his way. I realize that this is likely the same way he's built homes for 20+ years (adjustments for code min), but you honestly can't blame someone for doing what they believe is safe and right.

Bottom line in all of this is that I've delayed our build by nearly a year now trying to investigate all this and find the best, cost effective (while being efficient), and long lasting home we can. The wife is upset rightfully so, so I guess now we move forward. We're going to wind up with a 2x6 exterior framed home with dense packed cellulose, plywood or OSB wrapping the exterior, and house wrap. On the inside it'll be drywall/latex paint. I'm going to try to caulk/seal all the framing as much as possible during construction (we live only a walk away from our building site), but I really don't think there is much more we can do. Unfortunately this seems to still be a fringe science that's not even universally agreed to by the experts. I'll try my best to keep outlets and switches off of exterior walls as much as possible. Unless it's financially impossible, we plan to have radiant floor heat with ductwork for A/C in the ceiling (more comfortable in my opinion) as well as solar panels on the roof. If there are any additional things I can do without disrupting this plan I'd love to hear it I do thank everyone for some great discussions, but I'm 43 and I don't want to wait until I'm 50 to get this house built.

"even among experts in the field there are major disagreements"
I would disagree. Both Building Science Corp and Passive House have detailed the important factors - what you are seeing is mainly disagreements amount experts on the minute details of those factors. Whether the air barrier is on the inside or outside or in the middle does not affect the fact that an air barrier its critical. Whether you use foam, cellulose, roxul or something else, we all (mostly) agree on the amount of insulation (at least those of us who do this daily anyway), and that 6" is nowhere near enough. Exterior foam (enough so you don't get condensation inside the stud bay) is the best way to insulate and relatively foolproof, but is expensive. Double stud walls as we do is the least expensive, but in order for it to work the house must be airtight - meaning the builder should have experience building a very tight house. There are hundreds of builders across the country experimenting with different methods to achieve the same goal; in 10 or 25 years we'll know for sure. In the mean time, the most important factors are air tightness and added insulation. Do that really well and you'll thank yourself. And your wife will also.

All I can say is that done right, the house would be more comfortable than you ever thought possible. So building a minimum code house, at least you know what you get. Same old, same old.

Ok- So how do I go about determining the tried and true ways to add insulation and get the best possible air tightness? Exterior foam, like you said, is expensive. But you also said "relatively foolproof". When I start looking at the details for windows and doors, it makes me nervous that no one in the area will have any idea how to properly install and flash with 2" of exterior foam. A double stud wall also sounds great, but again, finding someone around here with the experience to do it right and not introduce other issues seems to be a major road block...

And I hope I'm not coming off as upset, I'm really not - I guess we're just not ready to jump on the experimentation wagon.

Given our current and predicted energy prices it is a hard sell for 2" of foam. I put two inches on the "inside" of my master suite when remodeling our 1978 2x4 walls. My carpenter had no problem flashing new windows, doors and cedar siding according to the manufacturers' specs and still making the inside trim fit.

Even if you went to a 1" Thermax indoors you would be eliminating the most taxing feature of a standard 2x4 or 2x6 wall with easier air-sealing where it really matters--at the source of potential vapor migration. No way for moisture to get trapped as long as the siding can breath.

"no one in the area will have any idea how to properly install and flash with 2" of exterior foam." Its actually pretty simple: we strap with 1x3 spruce or 1x4 rough pine 16"OC (screwed to the studs), and install strips of 5/8" Advantech around windows & doors and at corners. The wide width allows us to nail window trim. Above a window, we tape-flash to the foam. The are all kinds of discussions, articles & videos about these exact topics on BuidlingScienceAdvisor.com (I have a photo but can't seem to be able to upload it.)

Badger, why do you keep mentioning Thermax? Was that required for you, it's twice the cost of regular polyiso.

Anyways, to get guys (carpenters) to get you close to a great air barrier you need to find a contractor that gets it, not easy, or stress an exterior sheathing air barrier. Much easier concept to sell and execute, gets good blower door results, its a uniform flat plane from foundation to top plate. There will still be plenty of areas to screw up unless you're going to live in a cube, so k.i.s.s. concept should apply.

My take on interior air barrier method is it's garbage. Your stud cavities are still inside your envelope, why let exterior air mingle into your cavity insulation and degrade the r value?