Where to put concrete perimeter insulation on a double wall assembly to avoid thermal bridging
I am planning to build a slab on grade energy efficient home in NH, climate zone 6. I am aiming for a whole wall assembly R-Value of between R-40 to R-46. It seems that a double stud wall with dense pack cellulose is the most economical and practical way of achieving this.
In doing research on GBA and on other websites, almost every foundation design I saw for a slab involved either putting all of the insulation on the exterior perimeter of the foundation wall/slab or splitting it 50/50. There are several problems I saw with this design that I am trying to figure out a solution for.
1. It creates both a difficult flashing detail where the foam meets the sheathing (foam sticking out 2″ inches), unless you go with cantilevered LVL bottom plate, or a 2×6 exterior stud cantilevered, which is are both expensive options. Additionally you have to deal with applying some form of protection for the exterior foam.
2. If my goal is to have 4+ inches of EPS foam separating the foundation wall from the slab, wouldn’t it make more sense to have all of the foam on one side of the foundation wall? With only 2 inches of exterior foam against the foundation wall, it looks the two inches will allow cold to more easily spread into the foundation wall, which will then only have another 2″ of foam (with an ICF foundation) insulating the slab from the wall.
I was unsure what to do until I read the recent GBA article titled “A case for double-stud walls” https://www.greenbuildingadvisor.com/article/a-case-for-double-stud-walls.
In this article the author described putting the entire perimeter insulation of 4″ EPS foam on the interior side of the foundation and relied on the walls 12″ thickness to extend the interior wall beyond the foam. This would create a continuous layer of insulation from the foundation wall up into the wall cavity. Since the interior stud would be partially resting on foam, the author called for using a plywood layer under the bottom plates to provide rigidity for the interior stud. This would effectively create a cold foundation wall, but warm interior slab. I initially had a hard time figuring out how this was done, as there were no pictures of the slab/wall detail in the article and nobody in the comments seemed to mention doing this method, so I went ahead and created a detail that I thought represented what was described and attached it to this post. In my case, the wall is 13 inches, so its possible I may be able to have the plywood only under the interior stud. One detail I didn’t see mentioned was the creation of a foundation wall “Chair” where the top 4 or 6 inches of the foundation wall height are slimmed down to 6″ thick to allow for more interior perimeter EPS insulation.
In doing research on youtube, I also found a builder in Canada who seems to have done a similar approach. https://youtu.be/qO7M_nYQAII?t=475
The whole point of this post is that I am just wondering if anyone else has done a wall/foundation assembly such as this and also if anyone foresees any problems with building a foundation in such a manor. Does it seem that this assembly would actually perform better from a thermal and cost efficient standpoint? Or for example would having a cold exterior foundation wall create a thermal bridging problem up into the wall cavity (my initial thought is no).
I appreciate everyone’s help. Thanks!!