Northern Sask. Canada. 2x6 framing on 16", 2 ' XPS covering entire wall to foundation (all thermal bridging is eliminated) OSB on outside of XPS (Screwed to studs, supported by sill plate at foundation) Tyvek wrap exterior Hadiboard or vinyl Side building. All cavities filled with 3inch of spray foam directly onto XPS board. Rough opening of windows boxed in with OSB which bridges the 2inch XPS to the exterior OSB sheet. Where deck will occur bracing is used as described in the REMOTE wall system from the cold climate building center. Any thoughts or concerns??

With the OSB on the outside of the XPS, will wind bracing be sufficient?  Will inlet bracing be required to augment the continuous OSB sheathing (bracing)?  Or am I mis-understanding the stackup?

Good question! As the 2x6 wall will have the cavities filled with 3 inches of high density closed cell spray foam I would think the structure would be more than sufficient for wind bracing. The OSB should be continous from the foundation wall to the ceiling.

You may be right about the foam bracing the wall, but will the local code official accept it?

I am not sure. I saw the REMOTE system and nobody here builds in this manner. Many people here are clading the exterior with taped 1 inch XPS foam sheeting which in this climate can cause drying issues. So my thought was to combine the two! And use spray foam. All thermal bridging is defeated, the studs can breath to the inside if needed, and the OSB can still breath to the exterior if water penetrates the drainage plane. Build the wall as per typical steps. The significant change is laying the foam down prior to screwing on the OSB. Provide the appropriate overlaps on the wall in the corners and on the boottm etc to provide a continous piece of foam. Finsh the build and run your wires then spray foam the whole thing and your tight.

So what is going in the ~2.5" cavity between the spray foam and the drywall?

As previously mentioned, the OSB doesn't do much for strength if it is separated from the studs by 2" of foam.

If it was me, I would be spraying 5.5" of cellulose in those stud cavities instead of using spray foam and saving ~2/3 of the cost (5.5" of sprayed cellulose costs roughly the same as 1" of closed cell foam here). 5.5" cellulose is ~R19-20 in the cavity while 3" closed cell spray foam is ~R18 (aged value) in the cavity. Then attach the OSB directly to the studs for strength. Then apply 2 layers of 1" rigid unfaced EPS foam with off-set seams over the OSB. Then housewrap and furring strips for drainage plane under siding.

I hear you on the cost. My thought is that the spray foam is a better air barrier then the cllulose, prevents convection heat losses, and is a vapour barrier as well. This moves the condensation plane 1.5 inches from the drywall, which would not be anywhere cold enough for condensation. Any moisture in that cavity would ventilate into the interior. Having the OSB on the inside of the foam with an interior vapor barrier seems to me to cause a drying issue. In this case the foam is the drainiage plane. If water gets behind it it will not breath out very easily, poor drying through the foam, moisture traped between it and the OSB and rot. If moisture penetrates from the inside into the wall cavity it can not dry well in either direction. In my set up the drainage plane is on the outside of the foam. If any water penetrates in the OSB can still breath to the exterior, good drying capability. If we use a 2inch blocking at selected points, the corners of rough opening for example, we could create a mechanical coupe that could restore the lost strength you mention by having the OSB on the outside of the foam.

Just to add, I have talked with local framers and they laughed at the ideas presented as far as doing the exterior foam with firring strips. They thought it was not strong enough. In this region we would need at least 4 inchs of XPS foam to avoid the need for interior vapour barrier.

Are metal skin polyurethane SIPs available in your area?  If so, the wall could be thinner and yet contain a drainage plane between the SIPs and Hardie Board.  Should not be any worry about moisture or bracing with this system.

Unless the OSB is nailed to the 2x6 studs, the OSB is NOT providing any shear strength. From what I know, this would NOT pass code in most areas.

What is the purpose of this build style?

Simply trying to get a well insulated wall with no thermal bridging similar to the REMOTE system.  Around here no one will build 2x4 on 16", or 2x6 on 24". The stucco guys are getting really pissy building off the foam, and the inspectors will not pass a system without an internal vapour barrier. In my opinion a 2x6 wall with a internal vapour barrier and external XPS clading leaves me with a very poor wall for drying in our climate where we have significant days with high condensation risk. If I build the REMOTE system with 2x6 and 4 inch of XPS I will have an inappropriate ratio of insulation inside the condensation plane. If I use 6 inches to improve my ratio I have a 12 inch thick wall. So I am stuck. The wall I presented was an attempt to eliminate these problemsI saw Mike Holmes build a wall with no OSB, just XPS sheathing and srayed it, he thought it would be as strong a wall as one with OSB. WOuld the wall I presented as a system not be above code? The wall would be solid as a rock with the spray in foam. If I glued the foam to the studs and to the OSB it would be quite strong to shear forces. Or so I would think.

Posted By FTDA on 02 Feb 2012 11:22 PM
to eliminate these problemsI saw Mike Holmes build a wall with no OSB, just XPS sheathing and srayed it, he thought it would be as strong a wall as one with OSB. WOuld the wall I presented as a system not be above code? The wall would be solid as a rock with the spray in foam. If I glued the foam to the studs and to the OSB it would be quite strong to shear forces. Or so I would think.

Spray foam, per the ICC Building Code,  must be covered by a 15-minute thermal barrier due to fire/smoke concerns. Spray foam does increase shear & rack strength, it is estimated around 2x more strength vs. open framing (no OSB). So it can be said that open framing plus spray foam, would equal the strength of an OSB shear wall. The question is if code acknowledges this?

The other question is cost, what are you looking at cost wise to spray?

If done right this wall could create a mechanical couple between the XPS foam, interior foam, and outer OSB sheathing. Everything around here is crazy expensive. We pay 150.00 a foot for a typical spec home build. Spray foam is roughly 3-4$ a square foot. Framing a 2x6 structure is $10 a square foot plus material.

Posted By FTDA on 03 Feb 2012 08:25 AM
If done right this wall could create a mechanical couple between the XPS foam, interior foam, and outer OSB sheathing. Everything around here is crazy expensive. We pay 150.00 a foot for a typical spec home build. Spray foam is roughly 3-4$ a square foot. Framing a 2x6 structure is $10 a square foot plus material.

Where do you live?

I was quoted $10 per wall square foot area for an ICF wall, that was the block, re-bar and concrete.

Posted By FTDA on 03 Feb 2012 08:25 AM
If done right this wall could create a mechanical couple between the XPS foam, interior foam, and outer OSB sheathing.

How?

good question. The XPS is glued to the studs, the OSB is glued to the XPS. The OSB is screwed through to the studs. The XPS and studs are both held together by adhesion of the sray foam.

Saskatchewan. Here I was quoted $25-30 a linear foot for ICF construction (wall only) plus all materials and footing. Framing to dry in was actually $8-12 a foot for labour. With the plans I was looking at the number was $10. This is just labour.

FTDA, if you were quoted $30 a linear foot for ICF labor that's pretty good unless your home is five feet tall. Regarding your other wall system - your closed cell 2x section of wall is still thermally bridged by the studs. Since cranking up the cavity r-value does little I wouldn't bother with the closed cell. E.g. An extra inch continuous will outweigh closed cell vs open cell in the cavity. Regards.

$30 is for a 9 foot basement wall. In my described wall the entire stick build is covered by 2 inches of XPS from foundation to roof. There is no thermal bridging. The point of the spray foam is to create the required rigidity.

Posted By Lbear on 03 Feb 2012 03:54 AM

Spray foam, per the ICC Building Code,  must be covered by a 15-minute thermal barrier due to fire/smoke concerns. Spray foam does increase shear & rack strength, it is estimated around 2x more strength vs. open framing (no OSB). So it can be said that open framing plus spray foam, would equal the strength of an OSB shear wall.

That last statement would not be accurate in my opinion.  While spray foam may impart some increase in strength, it is nowhere near as strong as structural sheathing applied directly to the stud wall.  Adding the structural sheathing (plywood or OSB) to the stud wall increases the racking strength of the wall by at least a factor of 10.  See the following link as an example:

http://mgacon0.tripod.com/plywood.htm

More info here:

http://bct.eco.umass.edu/publicatio...hed-walls/