The Mt Airy Passive House has been an enourmously rewarding project. We’ve taken a tricky site and designed a site-specific, modern house that will be one of the most sustainable residences in the region.
Since the site is a steep hill, we found the perfect spot to orient the tallest roof due south. The foundation is a three-sided retaining wall, creating a walk-out basement that is still 80′ above the street below. Designed to Passive House standards, the entire shell is insulated far beyond the typical building code minimums, including continuous exterior insulation that wraps from below the basement slab up over the entire roof. Let’s look at the foundation construction in detail.
Out of the ground, the foundation is essentially just three concrete walls and a floor. The footprint of the house is a basic rectangle, and this is important architecturally. The simpler the footprint, the easier it is to insulate and seal. (Also, the foundation is literally the starting point for the rest of the structure, so the simpler that this can be, the easier it is for the construction team to build it square, plumb, and true.)
Below grade, things aren’t quite as simple. This steep site required the walls and footings to be designed as structural retaining walls, and we had to excavate down to ensure the base was sitting on solid bedrock (not always easy in hilly Cincinnati).
The footings on the side of the house step down with the hill slope, and there is a stem wall below the south “open wall” that ended up being much deeper than originally planned due to the geotechnical results helping us determine the bearing capacity of the soil. Ihe inside of the walls themselves is a grid of engineer-designed reinforcing, sized and placed in order to turn our basic foundation into an effective retaining wall to hold up the hillside and the water pressure inevitable from the wet soil for years to come.
The design of the house also has a bearing wall through the center of the structure. This allows for the big, long living spaces that face down the hill, and helps organize the smaller rooms on the uphill side of the house.
But below grade, this bearing wall also gets a foundation. A bit of coordination was required to get the plumbing (all nicely consolidated to one small portion of the plan) to drain through our network of foundation elements and run out of the house.
The gap with the floating steel in the photo to the left is where a column footing will be poured, integrated with the basement floor.
A key element of this design is the continuous insulation, and it is the reason why this level of sustainable construction requires proper planning. After the footings and foundation walls were set and poured, the below-grade plumbing was installed, gravel set, and the 6″ of rigid insulation was installed, continuous inside the foundation walls.
This was all prepped and organized in order to pour the concrete slab on top and ensure the correct final elevation.
In order for the insulation to be continuous, it also has to turn up just the few inches around the edges of the slab. The contractors did this with all of the cutoff and extra pieces. The plan was to cut them flush with the top of the slab afterwards, so the top edge didn’t need to be clean now (and actually having them up high prevented the slab pour from damaging them.)
Inside our raft of insulation, the vapor barrier gets installed, and then the wire mesh reinforcing is set in place to strengthen the slab.
In this photo, you can see the freshly poured basement slab next to the perimeter insulation not yet cut down. The south wall is right up front with an anchor bolt that will accept the wood sill plate.
After the slab pour, the concrete cures for a minimum of 7 days to reach an appropriate strength to begin framing. The yellow on the outside is a waterproofing/drainage mat applied everywhere that will remain below grade. Later, all of the yellow area will be backfilled with earth to build the final landscaping grade. We chose to keep the uphill side of the wall open until the interior framing was complete, to help strengthen the retaining wall.
After all of the underground work, the elements that remain above ground look simple and clean. This footprint is a 50′ x 28′ rectangle.