Kicking ideas around in my head - I am planning an ICF basement with a SIP first story.  I'm concerned with the 2.5" thick EPS on the ICF and how to match the SIP panel into it, siding, water, etc.  Obviously can't cantilever the mudsill/sip/etc that far out - at least I wouldn't do it! 

I'd really like a superinsulated (i.e. near or greater than R-40) wall system.

So, has anyone considered or tried placing rigid XPS board over the outside of the SIP panel to better match the ICF block?  I see benefits:

1) better match to ICF block
2) more R value for wall
3) Water protection for OSB SIP panel (I would tyvek over the XPS board in addition - no tape on the XPS joints)

I'm concerned with potential moisture problems - but I guess I'm having a hard time seeing any??  The SIP panel itself is acting (if I'm not mistaken) as a 8 1/4 thick air/vapor barrier.  Does installing XPS over the top of this cause a potential moisture/rot issue??

Thanks for any insight!

Consider using a "taper top" ICF block at the top of your basement wall which would generally result in only ~ 1/2" of foam on the outside top edge of the wall.

I would use a doubled bottm plate and have the exterior face of the panel align with the exterior face of the ICF. The doubled plate in most one story conditions would take the cantilever load. This would be the simplest configuration to build

You could cantilever the mud sill over the ICF insulation, make it a 2x12 and offset it until it is flush with the ICF insulation, then set a 2x8 sill plate on top of that to receive the wall panels and offset that 1/2" off of flush, so the panel skins line up with the 2x12 & ICFs. As JeffD says above, this double bottom plate combination should handle the small cantilever and line up the exterior wall surfaces and simplify your exterior wall finish.

By the way, a OSB skinned SIP does not act as a 'vapor barrier', however, a metal skinned SIP does and is my preference as a designer. Your main task will be to protect the outer OSB SIP skin from moisture damage. A vapor barrier like Tyvek or building felt should be placed directly to the outer OSB SIP skin and then apply a siding or additional foam sheeting over that. Some would even recomend an airspace before adding the foam sheeting or siding, but that depends on the climate in your region.

I would use caution in trying to match the exterior edge of the foam of an ICF foundation by cantelevering the panel past the concrete. A double bottom plate extended over the foam may work fine, on the other hand it may not. I have not seen an engineer approve this detail nor have I seen this detail in a SIP manufacturer's installation manual.

Again, it may work ok. But, we don't know completely what kind of house you are building or what loads will be put on only one skin of the panel.

The conservative approach would be to place 3/4" XPS (R-7.5) around the exposed exterior of the foundation, build out the wall 3/4 to match. The benefits would be venting, more nailing and no bump on the exterior finish.

Jay Shultz
The Murus Company

The whole reason I'm asking the question is I have no desire to cantilever a structural element (the outer OSB panel) based on a doubled up plate.  Keep in mind, that plate is sitting in it's weakest possible orientation, with loads applied transverse to the tubular/grain orientation of the wood.  Also, this would increase thermal transfer.  I know light residential isn't exactly dealing in monstrous loadings - but I can't wrap my brain around doing that one.  I live in big snow country - heavy rood loads that eventually make their way to the foundation through the wall system.  Steel SIPs, would like to investigate them, but not available in my area.   But thank you for the suggestions, they are much appreciated.

Jay - thank you for your reply - this is probably my most likely option, a combo of 3/4 - 1" XPS followed by furring for an air gap/ nice drainage plane.  I'm building this thing for longevity, and to my mind, getting that OSB as protected as I can is a HUGE priority.

 

Putting insulation to the exterior side of a vented airspace will not add any insulation value to the structure. You would be better served to either use no insulation and save the money, furring the wall out with something else, or putting the vented airspace between the xps and the siding, in my opinion.

Sorry if that wasn't clear - but I would certainly put the insulation underneath the furring. From inside out I would go SIP-XPS-Tyvek-Furring-siding

On page 202 of Joseph Lstiburek's book "Structural insulated panels (SIPS)" he shows the cantilever design that JeffD mentions. If its ok with Joe its got to be ok when done properly.

it is OK, the foam also has some value for bearing capacity, its not like a true cantilever

If a floor system is being placed within the envelop of the SIP walls and is also bearing on the mudsill, it can assist in the support of the cantilevered SIP walls by attaching the rim board to the SIP wall in some substantial way and then of course attaching the rim board to the floor joists/trusses.

It depends on your design priorities, if you really want a super insulated wall, go with the XPS, but if you are just adding it to coordinate the thicknesses of your wall assemb inly, it's not necessary, since there is a design solution as mentioned above. Actually, I would cantilever my SIP so it's flush with the ICFs and then add the XPS/VB/Siding, so it overhung the foundation and shed water properly.

You could also use a piece of engineered wood for the sill, so the plate wouldn't shear along the grain

Here's a diagram...

 I would also consider SIP-WRB(Tyvek)-XPS (Taped joints)-Furring-Siding.  That would keep the WRB on the warm side of the insulation, and protected.  You would end up with 4 drainage planes for bulk water movement away from the panels, i.e. Siding, back of siding,XPS, and WRB.  I think you would be pretty well covered.

Didn't know Joe had a specific SIP book, just picked that one up, thanks for the tip Brawler.

From what I know, The skins on a SIP never take a load equally in a real world application and I am pretty sure that when they axial test SIP panels, they offset the load onto one skin. So it ends up that one skin is mostly load bearing and the other is sort of bracing the other laterally. I'm pretty sure my detail does not voilate any structural principles.

Thanks for mentioning that in Joe's SIP book - I had read that in there, forgot about it. I'm an engineer, and something with that detail just doesn't sit right with me - it's probably fine, you have to remember we engineers LOVE safety factors!

Simon - interesting about the axial testing of SIP panels - I'd love to tour a plant and see a structural test done.

Thanks all.

I am a SIP distibutor in Edmonton and my partners run a manufacturing plant here called Ecopan Edmonton.
In all cases with the ICF as a foundation, the customer places the exterior face directly to the outside of the ICF insulation.
Even if they do not run the ICF to the top of the floor system to wrap the floor cavity with 2" of rigid, it would be the same idea.
If you are still unsure about it, have your SIP manufacturer cut the panels an extra 1 1/2" and place a second 6 1/2" bottom plate under the whole panel as previously mentioned. You will have a bit more thermal transfer down there, but sometimes piece of mind is worth it.

This may be a little off topic, but here is a pic of a standard axial load test done in the horizontal on a metal sip, which as a designer I am more experienced with. If you look closely, you will see the end of a metal rod placed between the 2x4 plate and the testing machine in line with the upper skin. This puts all the axial load on the upper skin and then they test to failure. Richntiff, as an engineer I thought you would find this interesting. 

The concept of this test applies to my assembly detail in that if the inner skin of the SIP ends up taking most of the axial load of the wall and roof, that is how the axial load figures a company posts for there panel were determined in the first place. Just food for thought. Personally, I think the tests they do on panels are too conservative and do no reflect the 'real' strength of SIPs.