Boston, Massachusetts has just been through one of its roughest winters in history. But the builders here have seen snow before — and spring is here now. Before the snow had melted, Nick Falkoff, Mike Dutra, and the crew of Auburndale Builders were breaking through frozen ground to start the foundation for a custom Passive House in the Boston suburb of Wayland. By mid-March, Auburndale had the project well underway.
Passive House details add cost to any building, and designers often compensate for the added cost factors by simplifying the building's form, sticking to simple compact boxes. But this project is different. Designed by Framingham architect Donald Grose, this house will have a complex footprint, a 14-pitch slate roof, brick cladding, copper gutters, and luxury interior amenities like an island cooktop range and — wait for it — a steam shower.
Details like the steam shower are a challenge for the project's all-star cast of consultants, which includes Marc Rosenbaum of South Mountain Company (on mechanicals) and Deap Energy Group's Mike Duclos (on envelope details). The Passive House consultant for the project is Maryland-based Michael Hindle, who contributed the foundation insulation detail shown here (see Slideshow). Hindle developed this detail in collaboration with his wife, architect Carri Beer of Brennan and Company Architects in Ellicot City, Maryland; Hindle says the pair have used the same system successfully on an earlier custom Passive House project in Maryland.
Passive House builders tend to avoid foam insulation when possible, but many projects still involve large quantities of expanded polystyrene (EPS) insulation below grade, where acceptable substitutes are hard to find. Foundation insulation can be controversial; while some critics will argue that insulating between the footing and the relatively temperate ground is not cost-effective, others will make the point that foam insulation comes with its own environmental penalties. So the Passive House subculture is actively exploring alternatives — such as the product Hindle specified for insulation under the house footings: Foamglas from Pittsburgh Corning (foamglas.com/building). Above, carpenter Tim cuts a piece of Foamglas with a steel trowel and fits it into place in a pad footing form.
Foamglas is inert and watertight, but it's somewhat brittle and friable, and it is rumored to be vulnerable to surface degradation if it goes through repeated freeze-thaw cycles while wet, according to Mike Duclos. With the weather warming, freeze-thaw was a relatively minor concern on this project. But to give full support and protect the material from damage, Hindle's detail calls for multiple layers beneath the Foamglas: first, a sub-base of strong two-inch to three-inch "trap rock"; next, a course of carefully compacted "crusher run" containing a graded mix of fine gravel and stone dust, which forms a stable, compact surface; and finally, a thin layer of sand to provide firm, uniform support for the Foamglas. For a step-by-step tour of the process, view the slideshow.