Accessory dwelling units have always been pretty sustainable – they’re small, located in friendly neighborhoods, generally in walking distance of parks, schools, shops and means of transit – but if you also make them “green’, then you have really created something special.

ADUs, also known as Backyard Cottages or Granny Flats, are secondary living units built on residential lots. They can be fully self-contained attic or basement conversions, attached additions or detached structures. Whatever the case, they have to be under a certain size to be categorized as accessory dwelling units (in Portland they have to be under 800 square feet. Because they were an affordable housing option for people who could not afford a bigger living space, and offered a lot of flexibility, ADUs were very popular in US cities, before World War 2, and although their popularity waned after society shifted toward low-density development, accessory dwelling is now experiencing a renaissance.

The Alameda Ridge ADU, one of Hammer & Hand’s latest sustainable projects has a series of features that aim to convince that accessory dwelling units can be a viable, eco-friendly alternative to conventional housing. Here are its technical highlights:

1. Thermally-broken insulated slab.  The ADU’s foundation was set on top of 4 inches of EPS geofoam insulation, creating a “break” that will keep thermal energy from leaking out of the house into the ground.
2. Liquid-applied air barrier.  This liquid-applied system provides a continuous air barrier on the exterior sheathing of the building and through into all window and door openings.  It’s a seamless, vapor permeable, weather resistant barrier, and it gained the project an “innovation point” in Earth Advantage Institute’s certification calculations.
3. High performance insulation.  For the walls, we blew in high-density cellulose insulation.  For the roof, they applied low-density spray foam and installed a continuous layer of polyiso foam over the rafters to reduce thermal bridging through said rafters.
4. Ductless mini-split.  The ADU uses a high-efficiency Mitsubishi inverter heat pump system.
5. Extensive testing.  In addition to the requisite Earth Advantage blower door test and ENERGY STAR insulation inspection, Hammer & Hand conducted an independent blower door test to verify performance of the liquid-applied air barrier system.  They filled the ADU with theatrical fog and simulated a 50 mph wind inside the structure to detect and fix any leaks or flaws in the air barrier.

For superior energy efficiency, water conservation, indoor air quality and resource efficiency, the Alameda Ridge ADU is scheduled to receive the “Platinum” certification from the Earth Efficiency Institute. On November 16, Hammer & Hand is holding an open-house and plans to offer visitors insights on the advantages of building a sustainable housing project.

Hammer & Hand