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October 2009
guest feature
article
A French Green Roof
Research & Development Program
By Maxime Darnis and
Philippe Faucon
Photos Courtesy Maxime Darnis
Compared
to Germany, green roof total surface in France is small but the annual
market is getting more and more important. In 2008, more than half
a million square meters of rooftops were vegetated here. This shows the
increasing interest for this technique, but market expansion is still
slowed down by lack of knowledge: contactors are still questioning green
roof benefits and techniques. To give answers and fill the lack of
research, we launched a research program in 2007.
The CRITT
Horticole is a regional center of innovation and technology transfer
specialized in horticulture approved by the French Ministry of Research
and Universities. Our team of 10 engineers is located on the West Coast
of France (oceanic climate), and works on several subjects related to
the Horticulture Industry: greenhouse engineering, market analysis,
plant death research activity and technology transfer to industrial,
environmental and energy diagnostics for horticultural companies,
horticultural activity support… and more recently: extensive green
roofs.
We focused our research program on extensive vegetated rooftops with low
maintenance because we think this is the most sustainable way to “green”
buildings. Now, the main topics are:
- Widen the range of species used on green roofs (plant diversification)
- Understand and model green roof thermal behavior in order to quantify
the benefits
- Get references on commercialized systems (costs, environmental
performance…)
Here is a short description of our green roof research and development
program, partially financed by the European Community (EC):
Plant Diversification
In France, the majority of extensive green roof projects rely on a small
number of species, principally Sedum species. Selection of
species in French green roof companies is limited and mostly based on
German research carried out in the nineties. According to the
literature, 300 taxa could be used on green roofs, hence the CRITT
Horticole started the plant research program.
Widening the range of plant species used for green roofs could help to
improve green roof functional performance (e.g. thermal efficiency,
water management), improve aesthetics, reduce maintenance costs and
enhance conservation and biodiversity.
We are looking for physiologically and morphologically adapted species
to withstand rooftops' extreme environmental conditions. We try to
identify some native species to improve the green roof relationship with
the local environment, facilitate biodiversity development on the roof
and also promote local identity. Moreover, a review of plant
species associations is conducted to create new design patterns and more
sustainable vegetation covers. The use of special plant communities with
sustainable and self sufficient vegetation can reduce maintenance and
resource input on green roofs. For instance, Fabaceae species, by
fixing nitrogen, could enrich the soil locally and help other plants'
development.
A 200 square meter (2,153 sf) green roof platform was initially installed at
CRITT Horticole, and we have recently extended the trials for a total
area of 300 square meters. It reproduces a double layer extensive green roof
system with different substrate depths. Two hundred plant species
have been tested here since 2009. Two hundred more species or
cultivars are going to be planted.
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Figure 1:
A part of the 200 square meter green roof system for
plant resistance tests. |
Plant material comes from nurseries, botanical collections, seed
companies or natural ecosystems. They were planted as plugs or
cuttings. For each species, morphology, aesthetic characteristics,
flowering, survival rate, multiplication, development, and coverage are
evaluated. This year was particularly selective because of a six
week dry period this summer: around 20% of the species tested died.
Among the survival species, some are very promising and showed high
drought resistance level, good coverage abilities, nice visual
appearance and large flowering period.
A second step consists in studying production protocols of the most
interesting species in partnership with a horticultural young plant
company. Our green roof industry needs cheap plants, easy to grow
(from seedlings or cuttings) with a high survival rate after planting.
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Figure 2:
A very promising Sedum cultivar, colorful! |
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Figure 3:
Another Sedum variety, changing with seasons, very drought
tolerant. |
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Figure 4:
An Erigeron species blooming generously. |
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Figure 5:
A very drought tolerant and carpeting Artemisia species
growing into 8 centimeters substrate depth. |
Thermal Performance Studies
Concerning thermal benefits (in summer and winter), an international
review shows that many studies have been conducted worldwide (North
America, Asia, Greece, for example) and gives different results.
Nevertheless, no data are available under French climatic conditions.
Therefore, we are developing an energy balance model of a vegetated roof
and integrating it in a building energy simulation program (TRNSYS)
in partnership with the
LEPTIAB
(Université de La Rochelle). At the same time, in order to
understand precisely the phenomenon, we study and quantify thermal and
mass fluxes. The roles of growing media, vegetation and drainage
layer characteristics on thermal behavior are evaluated experimentally
on a green roof modular platform. The experimental results will be
usable for model validation.
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Figure 6:
Green Roof Modules
Platform. |
The platform consists of 30 green roof modules (1 square meter
or just under 11 sf each).
These are made in stainless steel covered with root-proof membrane and
filled with different green roof systems (single layer with 2-substrate
depth, multilayer with different drainage layer types). Vegetation
structure and density may also influence thermal efficiency of green
roof. Therefore, two plant types were planted: grassy forms (Festuca
glauca) and carpeting plants (Sedum acre and Sedum album).
The green roof modules are in a greenhouse in order to control climatic
conditions but will be transported outside for other experiments.
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Figure 7:
A Green Roof Module |
A data logging
system has been installed and records: temperatures (air and substrate
at different points under the surface), solar radiations (incoming and
reflected), relative humidity, wind velocity, and weight of the module
(the green roof modules are suspended from load cells).
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Figure 8:
Green roof modules suspended from load cells. |
Energy conduction through the vegetated roof varies in response to
thermal growing medium properties (thermal conductivity, specific heat
capacity, albedo). They vary in response to the moisture content
of the soil. Soil moisture depends on precipitation, soil water
storage capacity and evapotranspiration. Therefore, a greenhouse
experiment was conducted to determine evapotranspiration on a green roof
and adapt the theoretical formula of Penman-Monteith. Our weighing
lysimeters will be used for other experiments (comparison of growing
medium water content evolutions between green roof systems…).
We have also explored variation of thermal conductivity with soil
moisture for different green roof substrates. The data will be
used in the green roof energy balance model.
Model validation will also use data from an in situ data recording.
The site is a three-building area next to the city of Poitiers.
Rooftops are divided and different green roof systems will be installed.
This site is exceptional for green roof thermal benefits studies and
will be equipped next year with data logging systems.
Get References on Commercialized Systems (Costs,
Environmental
Performance…)
Our 30-module platform is also used to study the performance of the most
common vegetation systems. So we conduct measurements and
observations on coverage rate, vegetation health and development,
drought tolerance, weed invasion, and maintenance needs.
This platform is the only one in France and could be used as a reference
to explore many subjects, such as water management (quality or
quantity), fertilizer management, acoustics benefits, new substrates
compositions…
Summary and Perspective
Our R & D program is unique in France. We installed our
experimental platforms last year, with financial support from the EC,
but it is only a middle or long term program. We are now looking
for collaborations in France or within the European Community with other
institutes, universities, technology transfer centers, and greenroof
suppliers involved in the building industry.
Maxime Darnis and Philippe Faucon
Publisher's Note: See the ARRDHOR CRITT Horticole Green
Roof Research profile in The Greenroof Projects Database
here. For complete information, visit the ARRDHOR CRITT
Horticole website.
Maxime
Darnis:
"I have
studied horticulture and landscape engineering
at the INH High School in Angers, France. I have worked in tree
management in cities and now in the green roof research program at the
CRITT Horticole.
I am
interested in the different ways to green our towns with both scientific
and commercial approaches. I am open to discussions and
partnerships worldwide to contribute to “city greening” development."
Contact
Maxime Darnis, ingénieur horticole, chargé
d'étude toitures végétalisées at:
m.darnis@arrdhor.com
Philippe
Faucon:
"I am a horticultural engineer, with 20 years of
experience in greenhouse engineering, technical, economic and
organization study. I am very interested in innovation in
horticulture, and I have already worked on new containers (avoiding root
spiraling), new substrates (made from pine barks), and new organic
fertilizers (made from wool).
Greenroofs are an aesthetically pleasing and elegant way to put nature
in the city. From my point of view, greenroofs have to be really
“environnemental friendly,” and research programs may help, by testing,
measuring, and innovating the greenroof industry to progress in this
way."
ARRDHOR CRITT Horticole
18 rue de l'arsenal
17300 Rochefort/Mer
tel +33 05 46 99 17 01
fax +33 05 46 87 28 63
www.arrdhor.com
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