Natural Ventilation and Green Roof Strategies
Atwater Commons, Middlebury College
Mark Gleason, BSE, MSCE, P.E. 1
Richard Maimon, B.A., M. Arch., AIA2
Jason Smith, B.S., B. Arch., RA 3
José Almiñana, D. Arch, MLA, RLA, ASLA, LEEDAP4
John Nystedt, BSLA, RLA5
1
Project Manager, Facilities Planning, Middlebury College, Phone +1 802 443-5424. Fax 802443-2076. E-mail mgleason@middlebury.edu
2
Associate, KieranTimberlake Associates LLP, Phone +1 215 922-6600. Fax +1 215 922-4680.
E-mail rmaimon@kierantimberlake.com
3
Project Manager, KieranTimberlake Associates LLP, Phone +1 215 922-6600. Fax +1 215 9224680. E-mail jsmith@kierantimberlake.com
4
Principal, Andropogon Associates, Ltd., Phone +1 215 487-0700. Fax +1 215 483-7520. Email alminanaj@andropogon.com
5
Project Manager, Adropogon Associates, Ltd. Phone +1 215 487-0700. Fax +1 215 483-7520.
E-mail nystedtj@andropogon.com
1.0 INTRODUCTION
Middlebury College is a small liberal arts institution located in Middlebury, Vermont. In 1995
the College committed to a ten-year expansion plan to ultimately increase academic year
enrollment to 2,350 students. In so doing, the College embarked on an aggressive campaign of
building projects, which included new academic, athletic, dining, and residential spaces. To
maintain environmental integrity throughout its expansion and to guide new construction,
College design decisions have focused on the relationship between the built and natural
environments. The ultimate goal is to attain the highest-quality built environment while
considering educational, community, economic, and environmental criteria.
Middlebury College‟s commitment to responsible and sustainable development is demonstrated
by recently completed and current building projects. Among the recently completed projects that
demonstrate these institutional ideals are the following:
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Bicentennial Hall (2000), where use of a local forestry cooperative resulted in
approximately 125,000 board feet of certified wood used for paneling, bookcases and
millwork on every floor of the building. Seventy percent of the wood came from forests
within thirty miles of the college.
Old Science Center (2001), which was “deconstructed” to make room for a new library,
and in so doing, the College was able to salvage for recycling and reuse over 97 percent
of the building.
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Ross Commons (2002), where over 60 percent of the finish wood used on the project
came from the college‟s own forest and where the college purchased design furnishings
from area woodworkers using local certified lumber.
Further testimony to the college‟s commitment is the design of Atwater Commons. Currently
under construction, this project consists of two new residence halls and a new dining hall. The
College‟s holistic approach has promoted, integrated, and applied its green design goals to all
aspects of the project from initial site planning, community impact, and design strategies, to site
demolition, the use of local and sustainable building materials, and construction protocols for
reuse of on-site material and waste handling. Two of the most prominent features of this project,
and the focus of this paper, are the design for natural ventilation in the residence halls, and the
implementation of ecologically-based site development and landscape strategies, including a
green roof on the dining hall, to enhance the built environment.
2. ATWATER COMMONS OVERVIEW
2.1 Site Development
Figure 1
Atwater Site Model – Plan View
Figure 2
Atwater Site Model – Northern View
The design for Atwater Commons, prepared by KieranTimberlake Associates, Philadelphia, PA
(KTA), supports and creatively fulfills Middlebury College‟s mission for residential life. The
three new buildings supplement three existing buildings of Atwater Commons: Coffrin Hall, Le
Chateau and Allen Hall. The existing architecturally diverse structures form an outdoor room
with two north-south ridges flanking a grass green. Viewing the panorama from the southwest,
the perception of this section of campus is of buildings set in the landscape - broad lawns,
sparsely populated by a variety of trees and Adirondack chairs - with landscape continuity into
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the distance. The new buildings, set around an intimate commons green, reinforce and
participate in this setting by following an irregular, nuanced landscape-to-building approach.
2.2 Residence Halls
The new residence halls face one another in a staggered manner defining the new Atwater
Common. Each is placed parallel to and engaged with the two ridges forming Atwater‟s
landscape. The halls bend slightly with the landscape contours, subtly breaking the long sides of
the halls into smaller increments. The halls are approximately 90' apart at their closest interval,
framing the view to and from the north. The residence halls vary in height relative to their
relationship to Atwater Common, a sloping green lawn. Their roof ridge lines stay constant. The
green falls away making Residence Hall A a four- to five-story building accommodating 84 beds
and Residence Hall B a three- to four-story building accommodating 70 beds. The buildings
contain various sized (3, 4, 5 bedroom) flexible suite arrangements and some single rooms. Both
halls are vertically accessed through entrance lobbies serving stairways. Each stair serves six-toten suites with a maximum number of students using any entry totaling 40. The suites are
oriented through the building (across its width) with exposures and views to the east and west.
Single rooms are grouped vertically about one stair and entry. At the lowest levels, each hall
contains shared programs including a library, seminar room and laundry.
Figure 3
View of New Common Green with Residence Hall B at Center
On the commons sides of the halls - which face one another - the windows are configured as a
regular pattern of three wood windows for each suite common room. This arrangement
maximizes light, view and ventilation. Local limestone in a warm gray tone, cut in linear pieces
with tight joints, is utilized. The windows are deeply set, for shading purposes, and the masonry
openings are „tied‟ with a granite lintel which provides additional shading.
Granite lintels and columns, with recessed wood and glass walls, provide larger openings at the
base for entries to lobbies, as well as the seminar room and library. The vertical pattern of
windows above these entries is staggered, reflecting the pattern of the entry stairs rising above.
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The eastern- and western-most sides, against the ridges, employ local granite in a rustic,
uncoursed pattern. Windows are set in a less regular pattern, suggesting the individual bedrooms
within. Lead-coated-copper roofs and trim finish the buildings with brick chimneys punctuating
the roof and eave line. These chimneys refer visually to the College‟s earliest buildings, but
serve here as ventilation stacks, assisting the natural ventilation of the buildings as discussed in
Section 3 below.
2.3 Dining Hall
Figure 4
Dining Hall from West
Figure 5
Dining Hall Interior
Atwater‟s new dining hall is positioned as a gateway on the path to the faculty residence to
conveniently serve all students of the Commons. This places the dining and social center of the
Commons in the geographical center of all buildings forming Atwater. It is oriented to capture
views to the east and southeast toward the Town of Middlebury and the Green Mountains
beyond. Engaging the dining hall into the western slope reduces the building‟s mass, and
provides for a view of a landscaped roof, extending the landscape of Atwater into, around and
over the building. Skylights, roof ventilation stacks, and a fireplace chimney provide a
sculptural landmark to the structure from the Common and College‟s main campus path. Eastern
(morning) and western (evening) light will illuminate the hall and serving areas.
Service spaces are located below in a plinth that is engaged into the slope. The ground floor level
of the dining hall will be faced in lead-coated-copper shingles and pierced by an exposed
pathway stair that leads down and through the building to the Commons Faculty Associate
residence. The main level of the dining hall will contain a student lounge, reception area,
seminar room, and other kitchen services, and will be enclosed by a curtainwall of glass in wood
frames with limited wood infill walls. The fireplace chimney is limestone and brick, sharing a
common element with the residence halls. Trims and other surfaces of the soffits and roof are
lead-coated copper.
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3. RESIDENCE HALL NATURAL VENTILATION
Middlebury College‟s residence halls are used not only during the academic year, but also in the
summer to house students attending its internationally renowned Summer Language School
program. The prospect of providing air conditioning to the new residence halls was reviewed by
the College and the design team at the outset of the Atwater planning process. Issues of climate,
geography, and program, in light of the College‟s environmental goals, suggested the
consideration of alternative approaches to building thermal comfort. KieranTimberlake
Associates consulted with the late Don Prowler, Philadelphia, PA, a noted environmental
architect, to assist in developing the natural ventilation approach. A formal study measuring
interior comfort levels was subsequently prepared by Arup, New York, NY. Design work was
completed by Lundquist Killeen Potvin & Bender, Inc., St. Paul, MN, who served as building
systems engineers for the project.
The following key issues led to the design for natural ventilation:
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Climate: With the exception of 7 to 10 days of the year, generally during July and/or
August, exterior temperature and humidity levels are well within the optimum thermal
comfort range for human beings.
Geography: The site is on a ridge line set between two major mountain ranges, with
north-south valleys providing a channeling effect which helps to flush stagnant air and
provide most days with air movement. The proposed siting and geometry of the two
buildings is intended to assist in flushing the common outdoor space.
Program: The residence halls are generally unoccupied during much of the peak summer
cooling period between 9:00am to 5:00pm. Students are in classrooms, libraries and
dining halls, many of which are air conditioned in the summer months. By the time
students are returning to their rooms the building is beginning a potential flushing cycle,
thereby negating the need for mechanically generated cooling.
The natural ventilation design strategies developed for Atwater include both natural and
mechanical tools that are integrated to promote summer-time cooling without the need for air
conditioning. These strategies extend to all aspects of the design, and include the following:
1.
2.
3.
4.
5.
The buildings are oriented north-south with their long facades facing east and
west, taking advantage of spring, summer and fall prevailing winds.
Exterior masonry walls, consistent with other campus buildings, add mass and a
thermal sink for cooler air.
Student suites are designed as floor-through “flats” with vertical entryways,
allowing true cross-ventilation.
Transoms above doorways allow air flow between bedrooms and common spaces.
All spaces are equipped with ceiling fans to assist air flow and all windows have
operable shades.
A generous floor-to-floor dimension of 11‟-4” helps to stratify warmer air from
cooler air, further aiding comfort.
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6.
Figure 6
The maximization of thermal comfort comes through mechanical assistance in the
form of vertical ventilation stacks. Air can be removed from each suite by an
individually controlled attic fan to exhaust suite air through the roof-top chimneys
and draw in cooler night-time air through the windows.
Section through Residence Hall
Figure 7
Typical Suite Plan
The natural ventilation components can be deployed at the occupants‟ discretion in a number of
combinations depending on prevailing weather conditions. In the peak of summer the intent is to
use mechanically assisted natural ventilation with ceiling fans and shades. Typical bedrooms
and common rooms were analyzed for a peak external summer day condition, which is the most
critical case for the proposed system. The models were simulated with various air change rates.
The models indicate that ventilation rates of 2 air changes/hour during the day and 10 air
changes/hour during the night for night flushing are optimal to meet the comfort range described
by the ASHRAE Standard 55, Thermal environmental conditions for human occupancy.
The attributes of these natural ventilation strategies described above are principles of good
architectural practice which many of the earlier campus buildings embraced. For Atwater
Commons, these principles are reinterpreted for the 21st century.
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4. LANDSCAPE STRATEGIES
4.1 Site Development
The site and landscape design promotes the principals of maintaining a “balanced” site, natural
drainage, and using native, low-maintenance species, highlighted by the design for the
landscaped roof for the dining hall.
A significant portion of an existing parking lot has been replaced with a large landscaped green,
helping the project obtain a net decrease in impervious coverage. Reinforced lawn fire lanes also
limit paved areas. The reduced runoff is directed along vegetated swales which naturally process
the stormwater and encourage stormwater recharge. Although wet basins are not required by
stormwater regulations (i.e., calculated post-development runoff is less than that calculated for
pre-development conditions), they are included in the design to manage this stormwater, thereby
further slowing and filtering the runoff.
Figure 8
Site Plan for Atwater Commons
Figure 9
Planting Plan for Atwater Commons
Plant species native to the Middlebury area will be utilized throughout the site in order to
minimize maintenance and encourage native fauna. Plants that are common in Vermont‟s red
maple swamps have been utilized in the swales and basins, creating lush layers of sedges,
grasses, ferns, and flowering forbs along with occasional shrubs and trees.
The woodland edges will be planted with native trees and shrubs that naturally grow on the
fringes of Vermont‟s upland hardwood forests, and the ground layer will be covered with
locally-obtained leaf mulch. Higher up the slopes, the “red pine ridge” plant community that
was in decline will be restored with pines and shrubs common in that plant community. Local
pine needle mulch is utilized at the ground layer to aid in the establishment and restoration of the
pine ridge plant community.
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A major portion of the lawn area is planted in “no-mow” lawn which utilizes low-growing
species that require minimal maintenance. It is anticipated that this “no-mow” lawn will be
utilized in more areas of the campus in order to significantly decrease the need for mowing. For
areas with higher foot traffic and increased passive recreation, lawn species that thrive in the
local climate and existing soils without fertilization have been utilized.
The buildings are set into the existing slopes to minimize the visual impact on the neighboring
community. This building placement has created excavated rock that has been used in its blasted
form for small retaining walls, or has been crushed and re-used throughout the project as
construction aggregate for fill placed beneath and around the buildings.
4.2 Dining Hall Green Roof
Numerous site strategies were investigated for the Atwater Dining Hall, with each evaluated for
campus planning, student life, architectural and environmental opportunities. The wooded site
east of the new residence halls emerged as the preferred location: its location is central to the
five residential buildings, it provides wonderful views and natural light, and the sloping
topography allows service and vehicular access to be separated from the pedestrian campus.
The location also suggested the potential for a green roof, which will provide the following
benefits:
1.
2.
3.
4.
5.
6.
Storm water management. Green roofs retain 70 to 100% of rainwater in summer
and 40 to 50% in winter. Excess water that flows to the storm system is cleansed
by filtration through the plants and soil.
Improved thermal insulation. European and Canadian research indicates that in
summer, the temperature of a conventional gravel-ballasted roof is 70 to 80
degrees F higher than the temperature of a green roof. This excess heat is
absorbed and re-radiated, substantially affecting air conditioning costs. In winter,
due to the added insulative layer of soil and plantings, heat loss and the resulting
heating costs are reduced. Based on these reductions in cooling, heating and
HVAC equipment costs, typical payback time for European green roofs has been
between two and five years.
Extended durability of waterproofing. The insulative value of the green roof
system also serves to protect the waterproofing layer from damaging ultraviolet
radiation. This prolongs the lifespan of the waterproofing system. Many
European green roofs installed in the mid-1970s retain their original
waterproofing today.
Improved acoustical insulation. Green roof systems can reduce airborne sound
levels by 40 to 50 decibels.
Absorbs heat. Eliminates the “heat island” and warming consequences of
conventional roofing materials.
Minimizes the development impact. The vegetation on the green room provides
moderate replacement to lost habitat and is a dramatic aesthetic improvement
compared to conventional roofing materials.
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Figure 10
Green Roof Detail Options
Figure 11
Roof Planting Plan
Green roofs are characterized in two categories: “Intensive” systems support large specimens
such as trees and shrubs through a deep soil layer (12” to 3‟); “extensive” systems support
smaller specimens with thin layers of engineered soil mixes. Intensive systems are capable of
supporting pavers and pedestrian traffic; extensive systems are generally not accessible. The
main dining hall roof is an extensive system with 7” of engineered soil; the lower roof is a
thinner extensive system with 3” of engineered soil.
The planting design for the green roof incorporates exclusively native species, and draws its
inspiration from flowering upland meadows and rock outcrop plant communities. Roof gardens
are potentially difficult places for growing plants – very sunny, hot, and dry with shallow soils –
so it was necessary to find plant species that would thrive under these conditions. Many green
roofs follow German tradition, and emphasize sedum species (low growing succulent plants
often used in rock-gardens), however, commercially grown sedum plants are invariably nonnative. The reference for this project was to natural areas where the growing conditions might be
similar, such as dry upland meadows, open rock outcrops, and cliffs. From the preliminary list
of possible plants, species that rely on deep roots systems for drought tolerance were eliminated.
Species with bulbs, tubers or shallow roots are utilized because they are more adaptable to the
relatively thin soils on the roof.
The pattern for the herbaceous plants and groundcover shrubs is based on the visual patterns and
drifts of species within a flowering meadow. Plant species naturally group into banded patterns
that have varying colors and prominence from season to season. To encourage these banded
patterns, the substrate undulates on the sloped roof in order to create bands of varying moisture
regimes. The planted bands are expected to shift slowly over time as different species find their
favored habitats; this is seen as part of the natural evolution of the plant community.
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Roof planting maintenance will include occasional selective weeding of non-native invasive
species, and supplemental watering during plant establishment period.
5. CONCLUSION
The majority of systems and products selected for the residence and dining halls - inside and out,
for construction and finish - reflect the College‟s commitment to selecting materials local to
Vermont and the region, durable and of long-life, and generally „green‟ or sustainable to the
environment. Sustainable design, construction and operation are evolving processes of planning,
implementation, evaluation and revision throughout design and construction. Middlebury
College recognizes the integral role environmental principles play in all of its activities and does
not apply a veneer of green strategy at the end its projects. The Atwater Commons project is a
testimony to the unique path the College, its consultants and its contractors are taking to raise
their own standards of what constitutes sustainable design and operation at Middlebury College.
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