Passive House Level Retrofit of a Historic Stable and Carriage House

Project Description

To our knowledge this is the first Passive House level retrofit of a structure listed on the National Register of Historic Places. This DC Landmark Building, the Spencer Carriage House, was built in 1905 to serve one of the large mansions in the Dupont Circle area. It is one of the last of these carriage houses remaining. Originally stables and carriages were housed below, and servants’ quarters above.  Over the last hundred years the building has been used as a garage, as a car dealership, as a tony restaurant, and in its last incarnation, as a bar. Our clients turned it into a home for aging in place in a vibrant part of the District. They wanted their new home to exceed of the District of Columbia’s 50by30 goals.

Our role in the project was as consulting architects to the designer, Yoko Barsky, of Deco Design Studios. Yoko has done several homes with the clients and was the true designer of the project. Our job was to make the home as energy efficient and comfortable as possible.

The building envelope
When our clients purchased the building, it was abandoned, with nothing of the original interiors remaining. The exterior, however, with its 14” masonry walls, was in very good shape, though the slate roof had outlived its useful life. Because it is a historic structure, all of our energy retrofit work had to be done on the interior, and the exterior had to be restored to its original condition. Essentially we built a building within a building.

We removed the existing first floor concrete slab, which was badly cracked and sloped. We then laid down 4” EPS foam insulation over a gravel drainage bed, covered it with 20 mil vapor barrier, and poured a proper 4” reinforced concrete slab.  Because it is an existing structure, we could not insulate under the footings. Instead, we made the interior wall insulation continuous with the slab insulation, thereby thermally isolating the footings from the building.

We  cleaned the interior face of the exterior walls to pass the inspection of the Prosoco reps, and applied a continuous layer Prosoco  Cat 5 air/water barrier, returning into each window and door masonry opening. We embedded the perimeter edges of the under-slab vapor barrier into the Prosoco to assure one continuous air and watertight surface at first floor and at the interior face of the masonry walls. Because the Prosoco is vapor permeable, however, any water that penetrates the masonry walls will still be allowed to dry to both the interior and exterior.

We next applied continuous  2” rigid foam directly to the interior face of the masonry and erected a new double stud wall inside of that to carry the floor and roof systems.  The framing is filled with 9-1/4” of Manville’s Spider densepack fiberglass insulation. The final step before adding the gypsum wallboard was to install a secondary air infiltration barrier of Intello. This is a “smart” vapor barrier that inhibits vapor movement in winter into wall and ceiling assemblies, but allows those same assemblies to dry toward the interior in summer. We did this as an extra precaution for winter months against any vapor laden interior air getting to the masonry and condensing. Total R-value of the wall is R40.

We replaced all windows with wooden units by Zola, designed for historic retrofits. They each have a fixed top sash and an in-swinging tilt/turn bottom sash, which exactly approximates the look of a double-hung unit. These units have a u-value of .10.

At the roof we placed open-faced SIP panels above new Advantek roof decking; we filled the 9-1/2” TJI roof framing cavity with densepack cellulose; and we added layer of 1”EPS rigid foam at the underside of the framing. The purpose of the insulation board above and below the framing was threefold: to eliminate thermal bridging, to keep the bottom face of the roof decking warm enough not to have wintertime condensation, and to add to the overall R-Value (R52). As at the walls, we placed a continuous Intello barrier just above the ceiling gypsum wallboard. We matched the original slate shingles as closely as possible.

The only exterior change to the house was to cut out part of the roof that is hidden from view from the street to create a small roof deck for entertaining and for an adjacent hidden area for the heat pump condenser unit. The badly deteriorated wood and copper ventilation cupolas were rebuilt entirely to match the originals. One is used for ventilation ducts; the other has become the hoistway for the elevator.

The original masonry walls were repointed and patched as required using brick and mortars compatible with the original brick.

The owners wanted to preserve and reuse as much of the original wood as possible. Where joists and flooring had to be replaced, they were cleaned up and re-milled into new flooring and to make doors, tables, and kitchen and bath cabinets.

The entry foyer for stair and elevator, guest bedrooms, a small guest apartment, utility spaces and a two car garage occupy the first floor. The second floor houses the main living and dining spaces and the master bedroom suite. The attic floor, served by a small spiral stair and the elevator, is used for storage and to access the small roof deck mentioned above.

A Mitsubishi City-Multi HVAC  system serves the entire building. The selection was based upon the need to handle peak loads of very large gatherings in both summer and winter, and yet ramp down to handle much lower demands the rest of the year. The City-Multi does that, able to operate efficiently from 1.5 tons to 6 tons of demand. Hot water is provided by four solar thermal panels by AET Solar.

The house was occupied in January of 2019.

Project team:
Interior architecture and design:     Yoko Barsky, ASID, Washington, VA
Project manager:                              Izumi Kitajima, CPHC
Structural enginer:                            Rossetti Engineering, Fairfax, VA

General contractor:             Federalist Builders Inc., Washington, DC
Masonry:                              Vaughan Masonry Restoration, Alexandria, VA
Ornamental Metals:             Fred Crist Metalwork. Exterior grille work.
`                                            AK Metals, Alexandria, VA. Stairs, counters
Cabinetry and millwork:       Dovetail Millwork, Rixey, VA
Roofing:                               Clerkin Roofing and Sheet Metal, Washington, DC
Plumbing:                            H D Johnson and Sons, Washington, DC
HVAC:                                  Michael Bonsby HVAC, Gaithersburg, MD
Electrical:                             W&W Electric, Silver Spring, MD
Solar Thermal:                    Solar Energy Services, Inc, Millersville, MD

Project Details

  • Date February 5, 2019
  • Tags Aging in Place, Deep-Energy Retrofits, Remodeling and Additions
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