Continuous air sealing and insulation hidden beneath new siding and trim
A well made case for a new $150,000 'house coat'
There are 120 million homes in America, and we need to figure out how to retrofit them for the new reality that we live in. We cannot all choose to move to the country and build a net-zero home and let someone else deal with the existing houses. To get down to 350 ppm of carbon dioxide in the atmosphere and avoid catastrophic climate change, the building sector is going to need to reduce its energy contributions which are estimated at something like 40% of all emissions. Scientists and politicians like to use the figure of 80% reduction in greenhouse gases by 2050 as a goal, and the work on my house fits into that target. We need to get away from talk about payback or return on investment and set goals (energy budgets) for our energy use and make plans to achieve them. Ed Mazria’s Architecture 2030, the Thousand Home Challenge, and Passive House help with the goal setting. Architects like Steve Baczek and HERS modelers like Mike Duclos help with the planning. Remodelers like Byggmeister can make the plans a reality.
It helps to get field confirmation and adapt
The idea of simply adding a complete new skin (exterior insulation, 2 by 2 light frame, new siding and windows) was a pretty radical approach I had been considering. But seeing it in a presentation by Steve Baczek about Peter Yost's Brattleboro project confirmed that I wasn't alone or insane; others were thinking "outside the box" too. We did a couple of our own neat twists on the concept:
1. Stacked scrap foam: instead of metal brackets, we stacked rigid insulation scraps to hold the screwed 2 by 2s the proper distance off the old siding.
2. Wall thickness set by depth of new windows: the new windows lined up exactly with the existing ones, we just tacked them in place and the spray foam locked them in. I really liked the appearance of the old sashes, so they stayed.
3. Soffit detail at bottom of wall: Steve's detail provided drainage, air flow for the new back-vented siding, and a clean look where the new exterior wall system terminates at the first floor level.
Steve Baczek, the architect, describes the project this way: "At the start, I was pretty skeptical about the double window, but I really have come to appreciate its effectiveness. It took some planning, it took some convincing, and it took some doing, but the simplicity of wrapping an airtight blanket around the building is the success of the project."
There were really three budget-busters or outstanding difficulties on this project:
1. Third-story bay window: Complex little areas up very high are so time-consuming. The bay on the front gable looks great inside and out, but it was inordinately expensive to achieve.
2. The shed dormers: On the east and west sidewalls of the south dormer where I kept the slate, I stopped the foam about 2" off the roofline. The alternative was to cut 5" off the slates and install new step flashing, which would have ended up with me re-slating practically the entire thing.
3. Slate roofs and PV systems: We kept the 95-year-old original slate roof because it is beautiful and in great shape. But I ended up having to strip the slate roof on both the slopes where the PV would be installed. I really tried to get an installer to install posts and boot flashings and patch the slate, but none would do it and I think they were right not to. Each row of panels would require six penetrations, so the whole system would need 42 patches plus the slates that broke by accident. And on top of that, the city required me to bring the rafters up to code, so I had to sister all the rafters with 2x10s. I got more depth for insulation in the process, but it meant taking out the plaster slope in the master bedroom. Overall, slate is great if you don't have to work on top of it. It's a lot of work and a significant barrier to putting rooftop solar systems on old houses.
One final eye-opener of the environmental type: at the NESEA 2010 Building Energy conference researcher Daniel Bergey of Building Science Corporation presented some sobering metrics on the relative global warming potential of the major types of insulation--cellulose, fiberglass, EPS, XPS, high density spray foam (HDSF) and Polyisocyanurate rigid insulation (PIR).
Global Warming Potential (grams C02 equivalent per square foot of R-value):
Cellulose - 2.3
Fiberglass - 25.1
EPS - 92
XPS - 2064
PIR - 115
HDSF - 1690
Given how poorly HDSF compares to PIR, if I were to do my house again I would strip all the siding and use rigid polyiso foam. I believe I am getting great performance with the spray foam, but we can't lose sight of why we are doing these deep energy retrofits in the first place.
General Specs and Team
Location: Somerville, MA
Living Space: 2966 sqf
The home is a duplex (upstairs/downstairs). The deep energy retrofit involved strictly energy upgrades (interior of walls in basement, exterior of walls above-grade, and ceiling plane of attic).
Design:Steve Baczek, AIA
HERS Modeler:Mike Duclos, EE Associates
Heating:Chris Ernst, Pipelines
Solar PV:Rich Herald, Nexamp
Foundation: 8” concrete block with 3” of closed-cell spray foam (R-18) on the inside and framed stud wall with mold-resistant drywall.
Walls:Existing walls are dense-packed with cellulose (R-13), 4” of closed-cell spray foam (R-24) on exterior.
Roof:R-60 loose-fill cellulose and 2-inch spray foam air seal at eaves
Windows:Jeld-Wen Premium Clad Siteline EX double-hung; basement windows are Harvey Industries double-glazed hopper windows
- Buderus GB 142/24 gas modulating condensing boiler, 22,700 to 75,200 Btus rated output, 95 AFUE
- 60 gallon Superstor indirect hot water tank
- HERS score - start: 119; finish: 37
- Photovoltaic: Packaged system by Nexamp 5.25 kW DC STC: 25 Kyocera panels (400 sq. ft.), Solectria inverter
- Basement Walls: R-18
- Above-Grade Walls: R-37
- Attic: R-60
- Windows: double-glazed, low-e, argon-filled, .32 u-value, .27 SHGC, .51 VT
- Basement windows: double-glazed, low-e, argon-filled, .32 u-value, .38 SHGC, .55 VT
- Indoor Air Quality
- Two HRVs (one for each unit)
- Green Materials and Resource Efficiency
- salvaged roof slates (for long-term repair)
Image Credits: Cador Price-Jones, Steve Baczek