Posted By jdebree on 09/03/2009 4:13 PM
You would lose almost all of the structural strength of a SIP. The strength comes from the two OSB panels being held rigidly X inches apart- it's almost as rigid as a solid piece. Putting a single layer of OSB between foam would be very flexible- you would have to have framing to support it. The idea of a structural SIP is that it is self-supporting- you don't need any framework (or very little).

Posted By toddm on 09/03/2009 6:11 PM
If you are talking about standard stud wall construction with foam sheathing on the outside and spray foam or foam boards stuffed in the cavities, yes, it is eminently doable, and a fine answer for DIYers who have more time than money. A stick-built version of SIPs was my Plan B, in fact, buying reclaimed foam from these folks: http://www.insulationdepot.com/ You'd get a R30+, 2x6 wall for a very small premium. You'd use spray poIyurethane to fill in any gaps between foam pieces and studs.

I'd address water vapor issues by treating the wall as if the cavity insulation was all spray foam, which is done all the time. If you aer were pretty religious about fitting the foam pieces and sealing them, you'd get 80 to 90 percent of spray foam's tightness.

But none of its stuctural, adhesive properties, so if you have more money than time, spray foam is better. [/quote]
Yes I am talking about standard stud wall construction, you are understanding that correct good sir. However, I'm not sure you caught what my conundrum is, if I used xps foam 2" or greater on the exterior of the sheathing, and then I did as you understood and described in the stud bays, that was the setting for my question. The osb will be in the middle of all of that sealed up tight foam, and that is precisely what I'm worried about.

Posted By toddm on 09/04/2009 7:44 AM
Here is the spray foam industry's manual on vapor barriers http://www.sprayfoam.org/downloads/pdf/AY%20118.pdf

and Building Sciences take on vapor control in walls http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design/view?topic=/resources/vapor_barrier_code_changes

The Cliff's Notes: Spray polyurethane and EPS are not vapor barriers, and XPS is considered semi impermeable only at an inch or more. You're a step ahead with the wall I described because there would be little or no air movement through it. How you deal with vapor pressure depends on where you are building.

Finally, you may not need OSB sheathing at all, except for the odd corner, depending on your building code.

**ok so apparently if you try to edit a post that has a quote, it's totally going to trap you in that box forever**

[quote]Posted By toddm on 09/04/2009 11:40 AM
Cudda been clearer I guess. No problemo in heating or cooling climates as long as you have the vapor barrier on the proper side and choose the right foam. In general you want the wall drying to the side with the least humidity, which translates to vapor barrier inside up north and outside down south. In the north, you'd want an inch or less of XPS as sheathing or any amount of EPS. In the south, more than an inch of XPS sheathing would sub for vapor barrier, but you'd need to add one over EPS sheathing (or use foil-faced boards.)
In the north XPS in the cavities could sub for a vapor barrier. Or add a barrier to EPS or polyurethane. Again, an XPS or foil backed EPS vapor barrier would be tighter than the kraft paper that does this work in walls with fiberglass batts. In the south you'd want EPS or polyurethane as cavity insulation.
Needless to say, we're talking about unfaced foam boards unless otherwise stated.
[/quote]

Perhaps I should be drinking more coffee... Btw, I live in Flint Mi, which is zone 5. I think it's between something like 6 to 8 thousand heating degree days.

Anyway, while I know you were trying to be clearer, actually I got a bit more confused. Regardless the effort is still appreciated of course.

It sounded like you were trying to say, that the exterior sheathing should be vapor permeable, and I should use only 1" of xps, or any depth of eps (because it is permeable) & I assume any non foil faced ISO would work too...

This is in conflict with what I have read on building sciences though?
http://www.airtightinsulation.com/UserFiles/File/building-science.pdf
(unless I'm reading that wrong too)

This is why I'm getting so confused, I read things on building science, I read things here and there and everywhere and they are all different, and I can't find but a scant few that point out how something was actually done, but not why, or specifically how things were done...

All of the assemblies I can find specific info on at building science and such use at least a 2" airspace, but I've heard many talk of no airspace at all, Dana on this forum for example said that airspace did little and was largely useless. I believe it was that person also who said you should at least observe a 1/3 2/3 rule of rigid R value on the exterior to the cavity R value on the interior. Now in the building science document there, they have more than one assembly similar to that, however, I don't really understand "potential for condensation" definitively other than taking it literally, but a roof that has that potential when it's 20 degrees outside and 70 inside @ 45% humidity is going to happen at some point and worse I'd wager...


I forgot to add, that I saw something on solar vapor drive, say the roof is wet, and then the sun comes out, wants to drive vapor inward into the roof assembly, key is to make sure it can't get in. I'm thinking about green roof design here, and as far as I know, they don't have any ventilation, just throw down a waterproof membrane and a bunch of other layers over foam and apparently that works? But do they fill in rafter bays with something I wonder...