Work on the Passive House continues in Derwood, Maryland. The rough plumbing that goes under the slab–or “groundworks”–is now in place. You can see stubs of white pipes sticking up where the bathroom, powder room, kitchen, and laundry are located. The interior of the foundation has been backfilled with gravel, and is now being capped with a layer of sand.
Sand in this case is a somewhat unusual application, but Erhan Tolu, our structural engineer, required it as a cushion for the next layer: an airtight membrane, and above that, 9 inches of high-density polystyrene, or “geo-foam.” Geo-foam is capable of carrying intense loads–in this house, it will support the interior bearing walls, as well as several posts that carry much of the weight of the roof. Erhan’s concern was if the foam were placed directly over gravel, it would settle over time into the space between the sharp points of the individual stones. This would also perforate the airtight membrane.
In many Passive Houses, geo-foam supports the entire house; however, the slope of this lot required the masonry foundation walls to lift the northern portion of the house above grade. To further protect the exposed portion of the foundation walls from cold, they are clad with 3 inches of geo-foam, and protected by cement board. The exposed cement board will ultimately be covered with stucco.
Great care is taken in this design where the porch and the screened porch supports intersect the main house foundation. Passive house design requires the elimination of thermal bridges. In conventional construction, supports for exterior structures would have been directly connected to the main foundation. In this case, these supports are separated by 3 inches of geo-foam.
Another essential Passive House feature is air-tightness. Under the slab we are using 15 mil polyethelene (6 mil is customary), with seams sealed with a rugged tape with an aggressive adhesive.