ASHRAE Journal - December 2014 - 57

©SKYSHOTS, PORTLAND, OR.

ABOVE At 31,597 ft2, this was the largest

contiguous ecoroof in Portland in 2011. The
roof is planted with drought-tolerant native
species, has PV panels on the south half and
solar water heaters on the north half.

Problem to be Solved
The project team set out to meet the Architecture 2030
Challenge for reducing energy use by 50% from other
existing, similar buildings (http://tinyurl.com/qaq66fy).
Besides the technical challenges, there were two complicating factors:
* In an apartment building of this type, tenants control most of the energy use. The design could not rely on
complicated systems or on central controls.
* The budget, already limited by the financing of
affordable housing, was further constrained by the difficulty of obtaining financing in 2009.
The design and construction process aimed to maximize
collaboration between team members who had worked
together on several projects and could build on the relationships and on the lessons learned. From the very beginning
of design, everyone was at the table and the major subcontractors were active participants. Before making design
decisions, the team analyzed multiple options, modeled the
energy savings, and tested the pricing. The team put a great
emphasis on an airtight, thermally efficient building envelope. This was considered the most cost-effective way to get
energy savings, the best way to reduce reliance on tenant
behavior, and a good strategy to avoid future maintenance
costs related to maintaining equipment.

Design Process and Decisions
Building Enclosure
The team's first step was to design an efficient building
and an efficient envelope. The team began by studying
eight to 10 massing models and assessing them for cost
and energy efficiency as well as for aesthetics and for
suitability for the site. The U-shaped design that was
selected provided a high ration of floor area to skin and,
therefore, provided the most energy-efficient shape.
The team developed and priced 12 different options for

LEFT The courtyard sits above an underground

parking garage, but is designed with a drainage system that filters all of the stormwater
before it drains to the municipal storm sewer.

framing and insulating the exterior walls. Each of the
12 wall assemblies was modeled, including calculation
of an overall R-value for the opaque walls and glazing. Three different window performance levels were
considered for the initial models (U-0.45, U-0.35, and
U-0.29); a total of 36 possible assemblies were analyzed. The energy model showed that the windows were
extremely important. The least performing opaque wall
with the best window had a better R-value than the best
performing opaque wall with the U-0.45 window.
Focus was put on finding energy efficient windows
and reducing the overall window to wall ratio. Smaller
windows were put in the bedrooms where they weren't
needed as much during the daytime; larger windows
were put in the living areas. Screens were added to
shade living room windows on the south and west elevations (if they weren't already shaded by a balcony). The
windows are vinyl casement with a high performance
U-value of 0.26, exceeding the standard for ASHRAE
Standard 90.1. The windows have low air infiltration as a
result of a design that includes three layers of gasketing
and cam locks that have three contact points. Balconies
have fiberglass doors with U-value of 0.26 and air infiltration rating of 0.03 cfm/ft2 (0.15 L/s·m2).
Exterior wall insulation is blown-in cellulose within
the stud cavity rated at R-23 nominal. The exterior cladding is brick veneer. The calculated effective overall
R-value of the wood-framed walls, accounting for framing and thermal bridging, is R-16. Additional exterior
insulation is installed outboard of sheathing in small
areas of steel stud framing. Full exterior insulation was
considered, budget constraints focused the team's attention elsewhere. The roof has two layers of rigid insulation under a two-ply SBS membrane over wood trusses
for an effective R-32. Continuous layers of insulation
minimize thermal breaks. The eco-roof (planted roof)
D ECEM BER 2014

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