Work underway on Landmark DC Carriage House Deep Energy Retrofit
Construction is underway on our first deep energy retrofit of a historic structure. The Spencer Carriage House, listed as a DC Landmark and on the National Register of Historic Places, 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, only a 3 minute walk to Dupont Circle. In its original configuration stables and carriages were housed below, with 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 as a bar. Our clients are turning it into a home, and what they hope to be the first Passive House certified historic retrofit in the country.
When they purchased the building, it was abandoned and essentially nothing of the original interiors remained. 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 retrofit work must be done on the interior. Essentially we are constructing a building within a building.
Here is a rundown of the key features:
SIZE: 5,700 SF ~ 4 Bedrooms; 4 Baths and 2 Half baths
SLAB: 4” concrete with 4” EPS below (R20)
WALLS: Existing 14” masonry with liquid air water barrier applied to inside face + 2” EPS + 9-1/4”
double stud wall with densepack fiberglass + Intello at interior face (R40)
ROOF: 9 1/2” TJI’s with densepack fiberglass + 2” EPS above and 1” EPS below (R52)
WINDOWS: Zola wood triple glazed (R6)
AIRTIGHT LAYER: Inside face of masonry
MECHANCAL: Mitsubishi City-Multi with ducted and cassette air handlers, Zehnder ERV
HOT WATER: Solar with electric back-up
The first step was demolition of the accretions of 100 years of various occupants.
With regard to the existing floor slab and footings, we were able to remove the first but hardly the second! Our approach is to take up the existing concrete, which was badly cracked, settled and sloped, and replace it with a new insulated floor slab. The insulation we are providing at the interior face of the historic masonry walls will continue down and tie into the new under-slab insulation, thus thermally isolating the existing footings. The only un-isolated footings will be the interior footings under existing posts and columns.
The existing concrete slabs have now been removed, ground insulation installed, and rough slabs poured. Here are some images of that work:
With all retrofits, the biggest challenge is dealing with thermal bridges. In this case the issues were the existing floor joists’ connection to the exterior masonry and the places where the walls and columns meet the ground.
We originally planned to cut off all existing floor joists at the wall, fill the joist cavities, and build a new exterior bearing wall that was connected to the interior face of the exterior masonry wall but completely isolated thermally. The extra thousands of dollars required for temporary shoring of the existing masonry walls made this approach prohibitive. Instead, we have amended this approach, leaving most of the existing joists in place, then encapsulating them in foam at the first 12” away from the wall. The foam encapsulation will prevent any potential condensation on those surfaces in winter.
The interior face of exterior walls have been cleaned and scraped, and Prosoco’s vapor permeable CAT5 air water barrier system applied. This wall surface will be the air barrier for the building and the CAT5 will lap over the under-slab vapor barrier. Here are some photos of that work.
Because of the poor condition of the existing roof and under-designed existing roof rafters, we are replacing the existing roof entirely and taking that opportunity to rebuild a passive house level roof assembly. The existing roof lumber will be reused for cabinetry and furniture.
New TJI roof framing and roof decking were installed. Prosoco Joint and Seam Filler were applied at all joints for continuous air/water membrane.
Next steps are the first blower door test, then cavity insulation installation.