Passive house details

The 100K House project has started building a passive house in Philadelphia, and they are doing a bang-up job of sharing the construction details. Their first article on their passive house goes into detail on the laying of the foundation, with thermal bridging eliminated by encapsulating the concrete slab with rigid XPS insulation:

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:

  1. Under slab insulation of R-50 (10″ of XPS Rigid insulation)
  2. Outer foundation wall insulation of R-10 (2″ of XPS Rigid insulation)
  3. Separation of the floating slab from all foundation walls with a minimum of 1″ (R-5) of rigid XPS Insulation

Passive House Foundation Detail

In their second article on passive house construction they discuss under slab air-sealing:

We get asked a lot what the biggest difference is between the 100K House project and the Passive Project. The answer is always the same. Air Sealing.

On a Passive House, we have to hit a very strict target on our blower door test of 0.6 Air Changes per Hour at 50 Pascals of pressure. This is no easy task. Every single seam must be caulked, or preferably taped, to hit this target. That all starts with the foundation. We’ll review a couple of the key details for the foundation air sealing below.

Air-sealing under the foundation seems a bit extreme to me, but maybe they have some concerns about cracking.

Passive House Foundation Detail