The last piece of the building enclosure that we are working through is the foundation and slab. We thought this would be simple, but turns out there are a lot of options here.
Basically we need a thermally broken slab and a minimum of R15 of vertical insulation down the entire concrete wall. We have an attached porch and garage that will need to have a thermal break between the unheated slab and the heated slab. While the insulation on the outside is not our preference for long term durability (insects and weed whacker concerns), it is required by code.
We have been drawing various forms of stem walls, thinking about ICF, and other options. We have meet with multiple builders that are not comfortable with the slab being supported solely by foam. Rachel confirmed this fear is valid. Even though we see lots of examples of passive homes that are built this way, apparently we have to find another solution. Most builders connect the slab directly to the concrete, but this creates a significant thermal bridge.
Today we met with Pete from Trana Masonry to talk through all of our options. We landed at a monolithic slab. Many passive houses opt for this approach. It has the following benefits:
- Air sealing from the slab to the wall air barrier is very simple - flashing tape connects the wall air barrier to the concrete which is already an air barrier.
- We can manage the foam ourselves and make sure all penetrations are properly sealed.
- It will bridge over minor soil issues - we have marl soils on the site - although they are more than 10 feet down, this has caused some stability concerns with other foundation types.
- It is about 1/3 less cost than some of the other options we were looking at.
So, here's the foundation assembly that Rachel Wagner presented for monolithic slabs. We are heading this direction -
Some resources that have been helpful:
Robert Riversong's article on a Passive Solar Slab
http://foundationhandbook.ornl.gov/handbook/section4-1.shtml
Slab Happy
Basically we need a thermally broken slab and a minimum of R15 of vertical insulation down the entire concrete wall. We have an attached porch and garage that will need to have a thermal break between the unheated slab and the heated slab. While the insulation on the outside is not our preference for long term durability (insects and weed whacker concerns), it is required by code.
We have been drawing various forms of stem walls, thinking about ICF, and other options. We have meet with multiple builders that are not comfortable with the slab being supported solely by foam. Rachel confirmed this fear is valid. Even though we see lots of examples of passive homes that are built this way, apparently we have to find another solution. Most builders connect the slab directly to the concrete, but this creates a significant thermal bridge.
Today we met with Pete from Trana Masonry to talk through all of our options. We landed at a monolithic slab. Many passive houses opt for this approach. It has the following benefits:
- Air sealing from the slab to the wall air barrier is very simple - flashing tape connects the wall air barrier to the concrete which is already an air barrier.
- We can manage the foam ourselves and make sure all penetrations are properly sealed.
- It will bridge over minor soil issues - we have marl soils on the site - although they are more than 10 feet down, this has caused some stability concerns with other foundation types.
- It is about 1/3 less cost than some of the other options we were looking at.
So, here's the foundation assembly that Rachel Wagner presented for monolithic slabs. We are heading this direction -
Some resources that have been helpful:
Robert Riversong's article on a Passive Solar Slab
http://foundationhandbook.ornl.gov/handbook/section4-1.shtml
Slab Happy