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Active Green Roofs by pablolaroche

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A smart green roof that can couple or decouple its thermal mass with the space to help with cooling in the summer and heating in the winter.



A living or green roof is a roof that is substantially covered with vegetation. These have positive effects on buildings by reducing the stress on the roof surface and increasing their life, improving thermal comfort and reducing noise transmission inside the building, reducing the urban heat island effect, reducing storm water runoff, re-oxygenating the air and removing airborne toxins, recycling nutrients, and providing habitat for living organisms, all of this while creating peaceful environments.

In our proposal the passive cooling potential and overall environmental impacts of green roofs are augmented using a system that selectively couples and decouples the green roof’s mass with the building interior to provide more comfortable indoor environments with reduced mechanical heating and cooling. Previous experiments with green roofs indicate that indoor comfort is enhanced in some climates when non-insulated green roofs are combined with night ventilation. However, insulation is needed to block heat when it is too hot outdoors or to keep heat inside when it is too cold outside. In our active green roof system the insulation is separated from the green roof, creating an air space that allows for convective coupling or uncoupling of its thermal mass using a fan activated by temperature based rules. When the system is ON the plenum is ventilated and the mass of the green roof is coupled with the space below and when it is OFF the ceiling’s air space acts as additional insulation. Cool air from the outside is provided with additional controls and logic rules.

Multiple combinations of these rules have been tested over several years at the Lyle Center for Regenerative Studies in Cal Poly Pomona University. A simplified version of the green roof was also tested in the Tijuana House Prototype at the Lyle Center. A full size version is currently integrated in the design of a classroom in a design for a new high school in Irvine, California by HMC Architects.

Category of the action

Building efficiency, Social Action

What actions do you propose?

Two major actions are proposed to achieve the implementation of this goal:

a)    Applied research and evaluation of the systems


b)    Implementation in an architecture project

We have done substantial progress in these two areas and the funding from CoLab would support both of these initiatives.


The green roofs are being tested at the Lyle Center for Regenerative Studies in California State Polytechnic University Pomona, located 30 miles east of Los Angeles in southern California, in a hot and dry climate with mild winters.

climate data

Climate in the Location of the Testing Facility

Test cells have been used to evaluate this concept. All have the same dimension, 1.2 x 1.2 x 1.2 meters and were built using 2 by 4 inch stud wall construction with drywall on the inside, plywood on the outside and batt insulation in between for a U value of 0.12 W /m2 K. The exterior walls are white with 0.61 m by 0.61 m (2’ x 2’) double glazed windows and 3.8 cm thick concrete pavers. Different types of vegetation and growth material have been tested over the years above 2.5 cm of gravel. There is a plastic liner underneath the gravel, above a metal plate supported by wooden joists. Drainage tubes in the gravel capture excess water and drain it outside. The roof is the only component that changes between the cells permitting to compare the performance of different types of roofs. The first cell is the control cell and has a code compliant insulated roof, with a U value of 0.055 W/m2 K, painted white. The three green roofs have different conditions, a non-insulated green roof, a green roof with insulation underneath, and the green roof with the insulated plenum referred to as the smart active green roof. The growth medium in the un-insulated green roof is thermally coupled with the interior via a metal plate, while the other green roof has10 cm of matt insulation underneath.

test cells

One of the Test Configurations

The data logging system controls the active green roof while it records data from all the cells. It consists of a microprocessor controller with thermistors that measure temperature in different locations, a laptop computer with the control program that collects and stores data, four test cells and an active ventilation system, which consists of a 4-inch inlet and on the outlet side a fan that provides air changes and operates by microcomputer or a timer. The next image shows the data monitoring system and fans in the active green test cell, which can be operated using different sets of rules. The use of the fan for structure cooling with the smart controller was tested in previous research (La Roche 2004, La Roche 2011) and the results were very positive. The controller can be programmed using different logical rules that turn the fan on and off as a function of thermal relationships between the thermistors and to cool down the building's interior mass so that it can 'coast' comfortably through the next day. 

smart system

Smart Controller

There are many complex interactions in a green roof that affect its energy performance and each of its particular constituents has a significant role (Theodosiou T. 2009):  the leaves reflect and absorb solar radiation and reduce the amount of radiation that reaches the surface, there is long wave radiation exchange between the canopy elements, between the soil surface and the foliage and between the foliage and the environment, evapotranspiration on leaves, convective heat flux between the leaves and the ambient air, conductance within the soil which depends also on the amount of moisture, and there is some solar reflection and absorption on the soil surface. It was not the purpose of this research to study the specific impact of all of these complex interactions. Instead, the purpose was to determine the potential for cooling with green roofs when these were combined with ventilation to cool the thermal mass of the planting material from the inside, mostly at night when the thermal mass of the green roof is cooled by forced convection through the fans. This night venting process with green roofs has been discussed in other papers (La Roche (2006), La Roche (2009), La Roche et al (2012)) and has proven very effective. Nocturnal ventilative cooling is also a well known very simple strategy that has been used for many years, mostly in warm and dry climates (Allard F., Santamouris, M., (1998), Cook, J., (1989), Givoni B., (1994), Grondzik, Kwok, Stein, Reynolds, (2010), La Roche, P. Milne, M. (2004), La Roche, P., (2011). During the daytime, the cool mass of the green roof acts as a heat sink, reducing the rate of indoor temperature rise and the fan is turned off because the sensors indicate that it is not possible to cool the space with outdoor air.

summer operation of the proposed system

Summer Operation of the System

winter operation of the system

Winter Operation of the System

In 2014 we will be expanding the modest facilities in which we have conducted all our experimental work at the Lyle center. The new Laboratory for Advancements in Building Systems (LABS) will be a group of flexible buildings that will allow researchers and the industry to evaluate the performance of experimental systems and strategies for passive or active low-energy systems. This facility is discussed in the next section.

Research results are extremely promising. The rule developed by La Roche and Milne (2004) can also be used to control the exchange of air for cooling in active green roofs. Also, the best performance is achieved with the plenum fan working continuously at least during the selected periods.The tests have also demonstrated that all of the options with night ventilation and thermal mass keep the space under the active green roof cooler than in the control cell. Consistently the lowest maximum temperature is in the plenum of the smart roof. Maximum  temperatures are also lower in the non insulated green roof and the active green roof when the plenum fan is turned on. Overall active green roofs, when combined with night ventilation can lead to more comfortable conditions inside buildings, with increased energy efficiency.This type of system is not applicable in all climates and a simple way to determine when it is applicable would be to use givoni’s and milne’s climatic design guidelines and propose it when thermal mass with night ventilation is helpful.



Implementing a new idea in a project is always difficult because of the real or perceived level of risk that the client has to take adopting the new idea. I have evaluated this system for many years. The system works. The next step is to implement in a real building and demonstrate its potential. We are implementing this in “Irvine High School Number 5” a project under development at HMC Architects. Our project designer is part of the CoLab team and we are working with the project manager, principal in charge and client to implement the idea. Our mechanical engineer will also help to achieve full integration with the mechanical cooling and heating system.HMC Irvinbe High School #5

View of HMC's Irvine High School Proposal

The system consists of an insulated plenum and ducts and fans that move the air as needed to make the most of cooling or insulation requirements. Moving the insulation from its traditional position directly underneath the green roof, to a lower position in the ceiling, and changing the operation of the air flow systems, affects the plenum insulation level. Depending on the requirement, the classroom space can be directly coupled or decoupled with the green roof. Moving the insulation down and away from the green roof is critical to maintain the insulation when needed or eliminate it when appropriate also. We expect this system to work in in several modes. During the summer and mid-season these will be: outdoor air cooling, re-cooling, and mechanical cooling. During the winter it will be in insulation mode.

During outdoor air cooling mode the system will provide free cool air to the interior of the space. This air will flow from the outside through the plenum space, where it will also cool the thermal mass of the green roof from below. This could happen whenever outdoor air is cool enough, to reduce the indoor air temperature, probably in the night and early morning. The temperature mode can be adjusted by the user, depending on building characteristics and operating parameters, but would usually be close to the balance point of the building, around 65 F.

outdoor air cooling mode


Outdoor Air Cooling Mode

When the outdoor air becomes too warm to directly cool the interior space -in this case the classroom- the system goes into re-cooling mode and the air is moved by fans that circulate the air through the plenum, moving heat from the classroom and depositing in the lower portion of the green roof, where it is stored in its thermal mass, which acts as a heat sink. If additional outdoor air is required to maintain IAQ then a sensor activates the exterior fan and brings fresh air inside the classroom.


re cooling

Re Cooling Mode

When the green roof can’t work any longer as a heat sink and the plenum air temperature and the air temperature of the classroom have reached a certain point, in which the plenum can no longer cool and the space is beginning to reach the upper portion of the comfort zone (eg around 74-76  F), the AC turns on and the mechanical system is activated to supply conventional mechanical cooling.

AC cooling

Mechanical Cooling Mode

During the winter or during cold days when heating is required, the plenum system of the active green roof is shut off so that the air inside the plenum is relatively stable except for internal convection currents. Thus the plenum will work as an air space in the winter, increasing the resistance to heat loss of the insulated ceiling, and keeping more of the heat inside the space. This will reduce energy used for heating the space.

heating mode


Heating Mode

The first of the following figures shows real data for the active green roof tested in September of 2012, while the next figure shows hypothetical performance of the active green roof system during a typical September day in Irvine California, where the school will be located.

The whole system can also be installed without the first ceiling, leaving the ductwork exposed. This would also provide more daylight and views and a stronger educational opportunity leaving the systems underneath exposed.

no ceiling

System without the lower ceiling and with exposed ductwork

The following figure shows real data from the test cells collected at the Lyle Center in Septermber 2012.

september data

Recorded data in the Active Green Roof Cell

The next figure shows estimated performance in a classroom using Irvine Climate data for September. The operation of the system is indicated in the diagram: first outdoor air cooling, continues with re-cooling, then mechanical cooling when the "coolth" of the green roof is depleted and cooling again when outdoor conditions are favorable.

predicted data

Predicted performance in Irvine Classroom



In addition to the two main initiatives mentioned above, we are interested in the education of students of all ages, education of the clients and training of practitioners. Changes such as this one will only occur if there is an educated public that is willing to implement them and an educated team of professionals with the knowhow to do this. We will continue publishing in scientific journals and technical conferences and will also continue developing and implementing our children’s workshops to include these concepts. I also have a book on carbon neutral architectural design in which I have included some of the initial results. CRC Press Taylor Francis has asked me for a second edition which I would finish by the end of 2014. Some of this work already included, but I will include even more in the second edition of this book. We are also working on a children’s book to illustrate these ideas and would like to do a similar one explaining the benefits of green roofs.


Who will take these actions?


The proponents for the applied research and evaluation of the systems will be Higher Education Institutions, National Laboratories and consulting firms. In 2014 we will be developing the Laboratory for Advancement in Building Systems (LABS) at the Lyle Center for Regenerative Studies to evaluate the performance of experimental passive or active low-energy systems. The LABS Facility will consist of three identical building envelopes, with an overall length, width, and height of 8 feet. More precise testing of the active green roof system will be executed in these facilities. In addition we are collaborating with North Carolina State University to develop a network of testing facilities in different climates in the United States.

LABS Image

The internal steel structure carries all downward, shear, and moment forces. This gives greater flexibility to the walls and roof, allowing them to adapt to a variety of testing needs and simplifies the experimentation process because walls can quickly be disassembled and interchanged. The default walls and roof will be typical Type IV construction (wood studs, plywood, and insulation) with the possibility to have  “super-insulated” cells to reflect future standards and systems. The concrete foundation contains ThermaPEX tubing for experiments utilizing radiant heating and cooling.

labs exploded axonometric

Exploded Axonometric of one of the Buildings in the LABs facility.

green roof in LABS

View of the LABS Facility at the Lyle Center


The proponents for the implementation in design projects will be architecture firms and city, regional and state governments, and home owners. In this first phase HMC Architects and a forward thinking client are taking the lead through the high school project mentioned in the previous section. We expect that when results demonstrate the benefits of the system we will be able to expand the implementation in other projects. To demonstrate the benefits to the client we have also done a detailed cost analysis.

Where will these actions be taken?

Research will be done at Universities and implementation will be done at AEC firms. Collaboration between the research and implementation teams is crucial and is part of our team. This must be combined with collaboration with the roof contractor and the manufacturer of the green roof system. Finally, the client must be on board with the idea, and willing to be a leader in the field.

For the first phase Cal Poly Pomona University and HMC Architects will be working together. We expect other firms and universities to collaborate and we are working at these. The work will first be implemented in non residential buildings. 

The proponents for the applied research and evaluation of the systems will be Higher Education Institutions, National Laboratories and consulting firms specialized in research. As mentioned in the previous section, in 2014 we will be developing the Laboratory for Advancements in Building Systems (LABS) to evaluate the performance of experimental systems and strategies for passive or active low-energy systems and compare them in real-time with conventional systems through a computer control center with data logging instrumentation located adjacent to the envelopes. 

I am also working with a fellow scientist in a research proposal to further develop algorithms topredict performance, and permit adoption in energy codes. An example is the adoption of Night Ventilation – Whole house fan minimum; and Smart Vents as alternatives in C Zones 8‐14 in the 2013 Residential building Energy Efficiency Standards Measures in California. 

Implementation will initially be in non residential buildings in which a plenum is more common. It will probably not provide benefit in high rise multi story buildings and implementation in residential buildings will require additional R&D. Implementation in HEED an energy modeling program developed at UCLA in which I have been part of the development team is also a long term goal.

How much will emissions be reduced or sequestered vs. business as usual levels?

Emissions are produced as a result of interactions between the building and the environment around it. Initially  the active green roof system will be installed over one of the classrooms and will reduce GHG emissions by:  

a)    Energy operation.  Reducing annual electrical energy use for cooling from 46891 kWh to 23400 kWh and natural gas from 300 therms of gas to 280 therms will achieve a reduction of 16,974 lbs of CO2 for cooling and 240 lbs of for heating. If it was implemented in all the school it would reduce its carbon footprint by 2.8 million pounds of CO2/year.           

b) Fabrication. Because the roof will now last for the entire life of the building, the emissions from fabrication and substitution will be eliminated. We have not yet calculated this impact.

c) Water consumption in the building. This also generates GHG emissions by pumping and treatment for consumption. About 50lbs of CO2/year will be saved by using water condensed from the mechanical system.

What are other key benefits?

a) Reducing the stress on the roof surface and extending the life of the roof.

extended roof life

b) Improving thermal comfort inside the building, by reducing the heat gain during the summer.

b) Reducing noise transmission into the building,

c) Reducing the urban heat island effect by reducing “hot” surfaces facing the sky and reducing the air temperature above it.

d) Managing storm water runoff and improving the auqlity of water run-off. It also enhances urban hidrology and provides for a good use of rainwater.

e) Re-oxygenating the air and removing airborne toxins.

f) Recycling nutrients and providing habitat for living organisms,

g) Creating peaceful environments and establishing stronger connections with nature. It is one of the few environments that all agree on as having improved aesthetical features.

h) Reducing maintenance costs

low maintenance

i) It increases biodiversity and reduces the amount of habitat lost.

The key beneifts of our proposed active green roof are explained in the following image

benefits of green roof

What are the proposal’s costs?

The initial cost of the green roof system for the high school will be $40,200 instead of $24,200 for a typical roof. However we estimate an energy savings of about $980 for a simple payback time of about 18 years. This does not include addtional savings in the maintenance cost of the system. We plan to use $5000 of the CoLab award towards paying for the additional first cost of the system and reducing the simple payback period.

active green roof cost

In the costs of the proposal we should include the energy savings:

energy savings


Time line

Test Beds (Phase I). Ongoing. Summer series are continously evaluated with the current test beds. More data has to be collected during the cold season and will be collected in the winter of 2013-14 with the current test beds.

Test Beds (Phase II). Development of second phase of the test beds will begin with the new LABS facilities in the summer of 2014. These are full size prototypes to evaluate proof of concepts: 6 months. The following images show how these green roofs could be tested in these facilities.

insulated green roof


Test Cell with Insulated Green Roof

Non Insulated Green Roof

Test Cell with un-insulated Green Roof

Active Green Roof

Test Cell with active Green Roof

Development of Computer Model using existing data will begin in the Fall of 2013. This computer model will permit to determine the performance (energy savings etc) of the green roof system in different climates.

Development of Computer Model using new data. Begins in the Fall of 2014 with summer data collected at the LABS facility.

Implementation of Active Green Roof System in non residential projects at HMC Architects. Ongoing. We expect to implement this system in several projects over the next two years.

We have the unique opportunity to develop this project though a collaboration between academia and practice. Establishing this connection has proven to provide excellent results. In the research facilities we develop and test proof of concepts while we implement and refine the ideas in practice so that they are safe, efficient, attractive and low cost. We are also working closely with the roof installer and manufacturer to ensure that the proposal is durable, buildable and affordable.

green roof idea

Related proposals

Passive cooling techniques are the only way to achieve average temperatures inside a building lower than outdoors, unless mechanical or active techniques are used. A passive cooling system is capable of transferring heat from a building to various natural heat sinks. Passive cooling systems provide cooling through the use of passive processes, which often use heat flow paths that do not exist in conventional or bioclimatic buildings. Smart Green Roofs utilize these heat flow paths to naturally cool and building when appropriate.

Not all passive cooling or heating systems are applicable in all types of climates and it is important to understand the climate variables which are most favorable. The implementation of each of the systems depends on specific climate conditions such as relative humidity, temperature, and radiant conditions. These conditions depend on the nature of the process, some systems will work better in under some conditions. 


Allard F., Santamouris, M., (1998). Natural Ventilation in Buildings a Design Handbook, James and James Science Publishers, London, UK

Cook, J., (1989). Passive Cooling, Massachusetts Institute of Technology. MIT Press.

Cook, J., 1989. Passive Cooling, Massachusetts Institute of Technology. MIT Press.

Eumorfopoulou E, Aravantinos D. (1998) The contribution of a planted roof to the thermal protection of buildings in Greece.  Energy and Buildings: 27, 29-36.

Givoni, B. (1976), Man Climate and Architecture, 2nd ed. Applied Science Publishers, London. 364 p.

Givoni, B. (1992): Comfort, Climate analysis and building design guidelines, Energy and Buildings. Vol. 18, N 1, 11-23.

Givoni B., (1994). Passive and Low Energy Cooling of Buildings, Van Nostrand Reinhold. 262 p.

Givoni, B. (1995), Passive and Low Energy Cooling of Buildings, Van Nostrand Reinhold, New York. 263 p

Grondzik, Kwok, Stein, Reynolds, (2010) Mechanical and Electrical Equipment for Buildings, 2010.

La Roche, P. Milne, M. (2003), Effects of Window Size and Mass on Thermal Comfort using an Intelligent Ventilation Controller. American Solar Energy National Conference, Solar 2003, Austin, Texas, June 21-26.

La Roche, P. Milne, M. (2004), Effects of Window Size and Mass on Thermal Comfort using an Intelligent Ventilation Controller. Solar Energy: Number 77  p 421-434.

La Roche, P. (2006). Green Cooling: Combining Vegetated Roofs with Night Ventilation, American Solar Energy Society ASES 2006 Conference, Denver, USA.

La Roche P. (2009) Low Cost Green Roofs for Cooling: Experimental Series in a Hot and Dry Climate. Passive Low Energy Conference, PLEA 2009, Quebec Canada.

La Roche, P., (2011), Carbon Neutral Architectural Design, Taylor Francis

La Roche, Pablo, Eric Carbonnier, and Christina Halstead (2012). "Smart Green Roofs: Cooling with variable insulation." PLEA2012 - 28th Conference on Passive and Low Energy Architecture, November 7-9. Lima, Peru: PLEA, 2012.

Santamouris, D.N. Asimakopoulos (Eds), (1996). Passive Cooling of Buildings, James and James Science Publishers, London UK.

Theodosiou T. (2009) Green Roofs in Buildings: Thermal and Environmental Behaviour. Advances in building energy research;3:271– and James Science Publishers, London UK.

The author's green roof research has been already peer reviewed and published in several conferences:

green roof research map