From SE Portland garage to Net Zero ADU

May 03, 2017

Among the many good reasons to build an accessory dwelling unit (ADU)—Or, in this case, to convert a two-stall garage into an ADU—is the sense of community it brings. Whether you're looking to rent out and generate some passive income, minimize your own living space while renting out your main house, or accommodate an aging family member or returning college student; ADU’s can encourage a greater sense of community. They also provide a partial answer to our current housing crisis by providing affordable housing options for those looking to live in Portland’s most coveted neighborhoods.

Chief among the benefits of this soon-to-be Net Zero Ready ADU is a drastically reduced carbon footprint. Because of its small size—at 665 square foot—the ADU will be extremely more energy-efficient, water-efficient and materials-efficient than your average home. In order to be Net Zero Ready certified, the ADU must offset any energy it consumes with a renewable source such as solar PV (photovoltaic) arrays.

The best way to lower the energy demand of a building is through the application of Passive House strategies. Passive House (PH) is the world’s most energy efficient building certification. It is achieved through five key factors:

1. An optimal level of continuous thermal insulation

​2. High quality windows, usually triple glazed with insulated frames

3. “thermal bridge” free construction; “Thermal bridges are weaknesses in the building envelope’s thermal barrier that allow more heat to pass through than might be expected. Following the path of least resistance, heat travels from a warmer space toward a cooler one.”

4. An airtight building envelope

5. Mechanical ventilation with heat recovery

As a result, a PH building is more comfortable and takes very little energy to heat or cool. However, it's not necessarily in the homeowners best interest to aim for the PH plaque in every circumstance. One must pay attention to the building's size and keep in mind cost versus performance.

For instance, it may not always be cost effective to chase the final few BTU’s of energy to reach the PH standard by adding insulation or energy systems in small homes. Recently, I caught up with our Designer Ben Valentin to get his thoughts on incorporating PH principles into his design of this future Net Zero ADU. 

“After a few studies on the existing site and running a preliminary energy model,” says Ben, “we determined that PH was not the best route to take. Small buildings are much harder to meet this level of high performance due to their volume to surface ratios. It's not impossible, but the added cost of achieving PH for such a small building doesn't make much sense. Instead, we decided to design a high performance ADU using PH strategies that would still make for a very comfortable ADU while using very little energy to maintain that comfort.”

The ADU’s construction began in February and is expected to be finished as early as this summer. The building’s high performance features include: 

• a 665 square foot floor plan
• computer modeled to locate and size windows for maximum comfort and energy usage
• graphite impregnated EPS foam insulation with an R-value of R5 per inch
• extremely efficient electric heat pump hot water heater
• overhangs modeled to manage heat gain from the sun
• liquid applied prosoco air barrier
• one bedroom and one bath
• heated/cooled with a single head mini-split heat pump
• ventilated using a Lunos spot HRV
• highly insulated, triple pane windows and door
• a tightly detailed air-barrier (less than PH standard)
• 8.5 inches of wall insulation (R-39), Roof R-80, floors R-64

Tags: Net Zero ADU, Net Zero Ready, High Performance, Passive House, Design-Built

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