To achieve the carbon reductions needed to prevent a global ecological catastrophe, almost every house in North America will need a deep-energy retrofit. If the projecting elements on a home’s exterior — especially the eave and rake overhangs — can be stripped away, the best retrofit option is to wrap the exterior of the house with an airtight membrane and a deep layer of insulation, followed by new siding, roofing, and windows.
Because the purest version of this insulation technique requires a building’s roof overhangs to be lopped off, the method is referred to as a “chainsaw retrofit.”
It started In Saskatoon
The chainsaw retrofit technique has a precise point of origin. The method was pioneered by two Canadian energy researchers, Rob Dumont and Harold Orr, who cut off the eaves and rakes of a modest ranch house at 31 Deborah Crescent in Saskatoon in the summer of 1982. The work was funded by the Institute for Research in Construction, a branch of the National Research Council Canada.
Dumont and Orr are both retired. As the first generation of building scientists and residential energy researchers gets old enough to pass the baton to younger practitioners, it’s important to remind today’s young architects and builders of their ground-breaking work.
Dumont and Orr were key members of the research team that designed the Saskatchewan Conservation house in 1977. Five years later, the two engineers set about to develop a technique for superinsulating existing homes.
In retrospect, it’s clear that Dumont and Orr were the first to combine a number of elements that have become part of the standard procedure for deep-energy retrofits. The work at 31 Deborah Crescent included the following steps:
Did they really use a chainsaw?
In August 1982, the retrofit work began on the 1,200-square-foot house at 31 Deborah Crescent, described by the researchers as “a bungalow built in 1968.” As it turns out, the remodelers never used a chainsaw. “We used a circular saw to cut the framing — the cut was about 2 1/2 inches deep,” Orr told me recently. “We finished the cuts with a handsaw. When I started giving presentations about the house, numerous people said, ‘You should have used a chainsaw.’ So I started to call it the ‘chainsaw retrofit’ job.”
In their research report, Dumont and Orr wrote, “In order to allow a continuous air-vapor barrier at the junction between the wall and roof, and to avoid having to wrap the existing eaves and overhangs, it was decided to remove the eaves and overhangs. To accomplish this, the plywood soffits were removed, and the shingles were removed from the eaves and overhangs. A power saw was then used to cut through the roof sheathing and part way through the roof truss eave projection and roof ladder in line with the outside of the existing wall of the house. … Finally, the saw cut in the trusses was completed with a hand saw.”
Fiberglass or foam?
These days, most builders specify rigid foam or spray polyurethane foam for a deep-energy retrofit. Back in the 1980s, however, Dumont and Orr used fiberglass batts, in part because the researchers were under strong pressure to keep expenses to a minimum. “The problem with polystyrene is that it is four times the price of fiberglass,” explained Orr.
At 31 Deborah Crescent:
- Workers removed the stucco, eave overhangs, rake overhangs, and asphalt shingles. At the roof perimeter, gaps between the roof trusses were covered with a strip of 3/8-in. plywood.
- The roof sheathing and wall sheathing were covered with 6-mil poly, held in place with plywood strips. All of the seams in the poly were sealed with Tremco acoustical sealant.
- A frame of 2×8 purlins (installed on edge) was built on top of the plywood strips.
- 2×4 rafters were installed over the purlins. The rafters were connected at the ridge with galvanized strapping. The new 2×4 roof framing included new overhangs at the rakes and eaves.
- Two layers of fiberglass batts were installed on the roof: 8-in. batts were threaded under the rafters, and 4-in. batts were dropped between the rafters.
- New plywood roof sheathing and asphalt shingles were installed over the rafters.
- New 2×4 wall studs were hung from the projecting rafters and toe-nailed into a wide bottom plate that was secured to the house with galvanized strapping. The 2×4 studs were positioned to create room for 12 inches of fiberglass insulation.
- Window rough openings were lined with plywood, and the new window sills were covered with painted metal flashing.
- The space between the old wall sheathing and the back of the new studs was filled with horizontal 8-in. fiberglass batts. Then 4-in. batts were installed vertically between the studs.
- The walls were covered with new plywood wall sheathing and stucco.
- The existing basement slab was covered with 3 inches of polystyrene.
- The basement walls were insulated on the interior with 12 inches of fiberglass behind and between new wall studs.
- Storm windows were installed to provide a third layer of glazing.
- An HRV programmed to provide 0.5 air changes per hour was installed.
Although builders in warmer climates might hesitate to wrap a house with polyethylene, the technique works very well in Saskatoon. “Wrapping the exterior of the entire building with a polyethylene air-vapor barrier proved to be a very effective method of reducing air leakage in the building,” the researchers reported. “This particular house, after retrofitting, proved to be the tightest house in Saskatchewan measured to date by the National Research Council. … The air leakage of the house as measured by pressure tests was reduced from 2.95 air changes per hour at 50 pascals to 0.29 at 50 pascals, a reduction of 90.1%. Before and after measurements were taken of the space heating requirements of the house. The design heat loss of the house was reduced from 13.1 kW at -34°C to 5.45 kW by the retrofit.”
Twenty-seven years later, the house at 31 Deborah Crescent remains in excellent condition, and Orr reports that the fiberglass used to insulate the basement walls has stayed dry. “I talked with the homeowner just last year,” Orr told me. “He said the house is very comfortable.”
How much did it cost?
The experiment at 31 Deborah Crescent was controversial at the time. “The people in Ottawa thought this was way too radical and costly,” Dumont told me recently.
According to the research report, “As the retrofit procedure involved major alterations to the entire envelope of the structure, costs for the total retrofit were high. The total cost for the project, which included upgrading the shingles and the stucco on the house, was $23,700 in 1984 dollars.” In 1984 U.S. dollars, the cost of the work was $18,230; in 2009 U.S. dollars, the cost is equivalent to $37,510.
To modern readers, the investment sounds like a bargain. These days, most deep-energy retrofit jobs are far more expensive — in the $50,000 to $100,000 range.
Ripples far and wide
Dumont and Orr’s research findings were influential. When a Massachusetts engineer, J. Ned Nisson, learned about the Saskatchewan Conservation House, he was galvanized into action. “I invited Harold Orr and Rob Dumont to Massachusetts, and we started running workshops,” Nisson recalled recently. “The workshops were wildly popular.”
Nisson went on to found a monthly newsletter, Energy Design Update, in 1982. With co-author Gautam Dutt, Nisson wrote a landmark book, The Superinsulated Home Book, published in 1985. The book became the bible of the shrinking band of residential builders who carried the energy-efficiency torch through the dark years from 1986 to 2004.
The global climate crisis now compels our country to face a Herculean task — performing deep-energy retrofits on most existing buildings. “In construction, making decisions is not like solving a mathematical equation,” Dumont told me. “The economics are changing all the time: labor, materials, and energy costs always change. We have nine million existing dwellings in Canada, and over the next three decades I can see virtually all of them being retrofit. Many of the older buildings have wood siding or something that needs renewal periodically, so it makes sense to combine the retrofit work with the renewal of the exterior siding.”
Not all existing buildings are good candidates for chainsaw retrofits. If a historic building is decorated with elaborate, irreplaceable exterior trim, retrofit workers will probably leave their chainsaws at home. But the technique works well on simple ranch houses with plain trim. If you drive around your town with a “chainsaw retrofit” eye, as I do now, you’re likely to spot entire neighborhoods ripe for a skilled crew equipped with gas-powered Husqvarnas.
For more information on the first chainsaw retrofit, see the research report by Orr and Dumont,
Matt Wasse of Herrison Architects in Seattle has created a video animation of a chainsaw retrofit using Larsen trusses filled with dense-packed cellulose insulation. To see the video, .
A GBA video of a recent chainsaw retrofit job — including footage of workers cutting off roof overhangs — can be found here: Video Series: Exterior Insulation Retrofit.
Last week’s blog: “Getting Insulation Out of Your Walls and Ceilings.”