Many have been waiting for us to start posting on specifics of our Passive Project which contains two homes – one of which is seeking the stringent Passive House certification for extreme energy efficiency. We have been waiting to post until we have completed each section of the build. This ensures that we post once on exactly what we ended up building rather than what we hope to build.
Now that we have finished pouring our foundation and have begun putting up our first SIPs panels, we can dig into the nitty gritty of our Passiv Haus construction details. Let’s start with the foundation wall/slab construction and insulation details.
Our Passive House consultants from PHIUS (Passive House Institute of the US) gave us the following basic design rules for our foundation and slab insulation:
- Under slab insulation of R-50 (10″ of XPS Rigid insulation)
- Outer foundation wall insulation of R-10 (2″ of XPS Rigid insulation)
- Separation of the floating slab from all foundation walls with a minimum of 1″ (R-5) of rigid XPS Insulation
I’ll spare you the details, but we did not get to this design in one day. Once these reqs were finalized, we came up with our own version of how we would build the foundation and had it approved by PHIUS. Below are the three main types of foundation details we needed to cover. The standard foundation wall is how most of the perimeter of the foundation is built. The new party wall is the foundation wall in between the two new homes we are building. Finally, the existing party wall is the detail for the party wall next to the existing home on the site next to our Passive House.
Standard Passive House Foundation Wall Detail
Passive House New Party Wall Foundation Detail
Passive House Existing Party Wall Foundation Detail
There are some air sealing details in the above diagrams that we will talk about in more detail in another post coming up. The basic main issue that was difficult to pull off in our foundation, was ensuring that the slab was completely floating and isolated from the foundation walls with at least 1″ of rigid. Typically in Philly, the slabs will rest right on top of the foundation walls for extra support. In our case that would cause too much thermal bridging from the slab to the foundation.
Below are a few more pics of the actual construction.