Thermal bridging of the sill plate
I’m planning on building a SIP over timber frame home. The 1st attachment is the wall section detail that the frame company sent me to pass on to my drafting person.
Assuming this makes sense the drafter would reference this but do something slightly different than shown because we plan to use ICFs on the foundation wall. Also, because only 2 walls are buried and 2 are ground level “walkout” walls we want to have the outside of the SIP line up with an OSB sheathing placed over the outside of the ICF so that we can seamlessly carry the siding down the sides of the walkout walls to just above ground level.
One other consideration is I need to make sure that timer frame posts, which will stand on the main floor decking and will be blocked underneath in between the joists, are situated such that they have at least 6in of their 8in depth over the top of the foundation wall. After speaking with the ICF guys about this they suggested using a flared ICF at the top which expands the concrete thickness out to 11.5in. This is shown in the 2nd attachment.
The Timer Frame + SIP company’s detail (see 1st attachment) shows they use a shoe plate inside of the SIP and at its bottom. The shoe plate gets attached to the sill plate before the SIP (which has some of its EPS cut out so it fits over the shoe plate) is lowered over the shoe plate and fastened to it. They say they do it this way so that the exterior OSB sits over the sill plate.
The problem I see with this design is that it requires the sill plate to be carried all the way out to where it is in contact with the outside air, or at least the air in the gap where the siding is stood off from the outside of the building. The sill plate has the joist band and outside of the joists sitting on it. Those in turn are underneath the main deck. As a result this seems to me to be creating a direct thermal bridge from the main deck to the outside of about 2in in height going all the way around the outside of the building.
In addition to the above 2in of the shoe plate is in contact with both the inside and outside of the SIP’s OSB sheathing. The SIP’s interior sheathing is in contact with the joist band’s full height, the thickness of the subfloor on the main floor deck, and the interior wall finishing. So it seems there would also be additional thermal bridging going on there too.
My question is am I worrying too much in thinking that this is not the most energy effecient design?
Should I be looking for a different approach for securing the bottom of the SIPs which will reduce or eliminate this kind of thermal bridging?
My thinking is that the answers are no (I’m not worrying too much) and yes (I need a different approach). The 3rd attachment is one approach I thought of. This calls for trimming back both the sill plate and shoe plate so that the appropriately trimmed SIP can sit both on top of the shoe plate while still having 2 inches of EPS outside of both the shoe and sill plates and also allowing the exterior OSB sheathing to be fastened to those two plates by nails or screws which pass through the exterior OSB and EPS then are secured into the shoe and/or sill plates.
My thinking behind my approach (assuming it is structurally sound) is that it interrupts any direct thermal bridging, leaves only 1 gap (between the top of the ICF and bottom of the SIP) that has to be sealed, and also places about 2 inches of EPS (for about R-7) on the outside of any components that are in thermal contact with the interior (such as the sill plate) above and beyond the meager insulation values provided by the R-1 of the wood itself.
Assuming that I’m not worrying too much and should use a different approach from that shown in the Frame/SIP company detail does my approach make sense or is there a better way to do this?