Changing from a footing to a monolithic slab makes us re-think the wall assembly. Because slab would be wrapped in foam, it makes more sense to go back to the traditional larsen truss assembly with the larsen on the outside.
Ideally the foundation would be made from geofoam EPS - which is pre-shaped to be the form for the concrete. The amount of concrete needed would be greatly reduced. However, the house then is built on a layer of foam (yes, they use this for highways and airports, but it's only been around since the 70's so will it be a long-term foundation like concrete?)
The foam foundations are very common in Europe - Viking shows a variety of approaches.
What if we could put the larsen on the outside, use mineral wool batts, eliminate the exterior sheathing and have the outside rail on the larsen serve as both the rain screen and the mechanism to hold in the rock wool? Seems simple....
Roxul Comfortbatt is at Home Depot. 12 bags cost $512. They are R23 and are cut to fill a 2x6 cavity. One bag covers 39.8 sq ft. So, that's a product cost of $1.07/sqf - but we can install it ourselves and that's a huge savings. It would be about $1296 to fill all the exterior cavities. There's also a 2x8/R30 product if we wanted to downsize the interior wall to a 2x4 dimension and put more insulation on the outside.
Building thick walls causes a lot of consideration when it comes to window install. There's a great article from Building Science on this topic. It explains that the plywood sheating in the center of the wall would be the location of all the control layers - air, vapor, and WRB.
Here's a good blog on things to know about mineral wool from GBA.
This assembly puts us closer to Joe Lstiburek's Perfect Double Wall Assembly. And it avoids all the issues mentioned in this article on managing dew points. According to Martin Holladay, "The minimum R-value rules for rigid foam insulation in this location don't apply to mineral wool insulation, because mineral wool is vapor-permeable. That means that your mineral wool can be as thin or as thick as you want."
Instead of using the Larsen Truss approach - you can fasten the Roxul comfort board to the outside of the house. To us, this seems complicated - although you can read more about it on GBA. And we did see it done at Olson's Lake Elmo house. They did a double wall and then put the roxul on the outside - seems like a lot of additional work and materials for not enough gain.
There is some concerns about wind washing through Roxul bats - and that's why the boards are used in external applications. We will need to contact Roxul directly regarding that concern.
Ideally the foundation would be made from geofoam EPS - which is pre-shaped to be the form for the concrete. The amount of concrete needed would be greatly reduced. However, the house then is built on a layer of foam (yes, they use this for highways and airports, but it's only been around since the 70's so will it be a long-term foundation like concrete?)
The foam foundations are very common in Europe - Viking shows a variety of approaches.
What if we could put the larsen on the outside, use mineral wool batts, eliminate the exterior sheathing and have the outside rail on the larsen serve as both the rain screen and the mechanism to hold in the rock wool? Seems simple....
Roxul Comfortbatt is at Home Depot. 12 bags cost $512. They are R23 and are cut to fill a 2x6 cavity. One bag covers 39.8 sq ft. So, that's a product cost of $1.07/sqf - but we can install it ourselves and that's a huge savings. It would be about $1296 to fill all the exterior cavities. There's also a 2x8/R30 product if we wanted to downsize the interior wall to a 2x4 dimension and put more insulation on the outside.
Building thick walls causes a lot of consideration when it comes to window install. There's a great article from Building Science on this topic. It explains that the plywood sheating in the center of the wall would be the location of all the control layers - air, vapor, and WRB.
Here's a good blog on things to know about mineral wool from GBA.
This assembly puts us closer to Joe Lstiburek's Perfect Double Wall Assembly. And it avoids all the issues mentioned in this article on managing dew points. According to Martin Holladay, "The minimum R-value rules for rigid foam insulation in this location don't apply to mineral wool insulation, because mineral wool is vapor-permeable. That means that your mineral wool can be as thin or as thick as you want."
Instead of using the Larsen Truss approach - you can fasten the Roxul comfort board to the outside of the house. To us, this seems complicated - although you can read more about it on GBA. And we did see it done at Olson's Lake Elmo house. They did a double wall and then put the roxul on the outside - seems like a lot of additional work and materials for not enough gain.
There is some concerns about wind washing through Roxul bats - and that's why the boards are used in external applications. We will need to contact Roxul directly regarding that concern.