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The focus of this lesson is on ways to evaluate the sustainability of buildings.
By the end of this lesson, you should be able to:
To Read | Lesson 5 Online Content | You're here! |
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If you have any general course questions, please post them to our HAVE A QUESTION discussion forum located under the Discussions tab in Canvas. I will check that discussion forum regularly to respond as appropriate. While you are there, feel free to post your own responses and comments if you are able to help out a classmate. If you have a question but would like to remain anonymous to the other students, email me through Canvas.
If you have something related to the material that you'd like to share, feel free to post to the Coffee Shop forum, also under the Discussions tab in Canvas.
I am going to start this lesson with a very simple question: What is the purpose of a building? The U.S. EPA says that [1]Americans spend on average around 90% of their lives indoors, so there must be a good reason to use them. Please think about an answer before moving on.
When you think about it, the use of buildings boils down to two primary objectives. In no particular order, these are 1) safety/securty and 2) to control the climate and to "keep nature out" of the building. There is no disputing the need for safety, from the earliest human habitations to modern buildings. Climate control is of course very important as well: Most of the things you do in buildings - eating, sleeping, gathering with family or friends, getting work done, relaxing, etc. - often require, or at least are much easier to accomplish with, some form of climate control. Have you ever tried to get a good night's sleep in a cold, pouring rain with no shelter, get some work done on your computer in the middle of an unbearably hot, humid, sunny day, or try to read a book on a cold, windy winter day? I can't say that I have, but I know it would not turn out well.
Over time, humans have gotten better and better at controlling the climate inside of buildings. While some ancient societies clearly understood the benefits of using natural means to make buildings more comfortable and minimize heating, cooling, and lighting needs (as you will see below), in the U.S. it was not until 1978 [2]that there was any federal requirement for states to enforce energy efficiency in new buildings. Two consistent feature of colonial-era buildings in the U.S. is that they were cold (they did not hold heat well in the winter) and drafty (aka leaky). This is not unique to the U.S. The United Kingdom, for example, has some of Europe's oldest building stock [3] (that is saying something, considering how old most European cities and towns are!). About 50% of residential buildings and 39% of non-residential buildings in the UK were built before energy standards were widely introduced in the 1970s (source: RICS [4]). There have been major movements in recent decades [5] to reduce energy use in UK buildings, with limited success.
That stated, buildings built after the 1970s have generally become more efficient, largely due to increasingly robust energy standards in the U.S. [6] required by the federal government (though some states, e.g. California, consistently have more stringent standards than required). Most of these codes are based on international energy standards such as the International Energy Conservation Code [7] (IECC). Technology related to things such as insulation, air sealing, and high performance windows has also improved.
Generally speaking, we are headed in the right direction in terms of energy efficiency of buildings. But anecdotal evidence and published research indicate that there are still an abundance of opportunities to improve energy efficiency. One important consideration ist that a lot of existing houses in the U.S. and elsewhere were built before energy standards were implemented. I can tell you from personal experience and from talking to folks that have been in the industry for decades that any building - commercial or residential - that was built around or before 1980 (unless it has been retrofitted) has negligible insulation and is very leaky. Modern buildings must be more efficient because of codes but are still usually built with the idea that we should just use technology such as heating and cooling equipment, insulation, air sealing, and lighting to reduce energy usage. The overall point is that very little consideration has been given to how the building can utilize the surrounding natural conditions to reduce energy usage and increase occupant comfort. Enter passive design principles.
Before moving on to learn about passive design, please read the following articles about the benefits of energy efficiency.
It is a widely agreed-up fact that the sun provides enough energy every hour to provide all of humanity's energy needs for one year. Thus, it stands to reason that there is likely enough sunlight to provide the average household's energy needs throughout the year. We will save that calculation for a later time, but some related research from the National Renewable Energy Laboratory (NREL) has shown that much of the electricity in the U.S. could be provided by solely installing PV arrays on suitable rooftops in the U.S. The images below show 1) the percent of buildings in each state that have suitable rooftop space and 2) the percent of the total electricity sales that could be provided by suitable rooftop space in each state. You can read the full report here [16]if interested.
Passive design - including passive solar design - seeks to minimize heating, cooling, and lighting energy use in a building using passive strategies. Before we begin, please note the difference between passive and active solar design, as described by the California Energy Commission [17]:
"Homes constructed with passive solar design use the natural movement of heat and air to maintain comfortable temperatures, operating with little or no mechanical assistance. It's called passive solar because the design of the home maximizes the benefits it receives from the sun with standard construction features. Passive solar takes advantage of local breezes and landscape features such as shade trees and windbreaks, and uses a simple system to collect and store solar energy with no switches or controls.
On the other hand, active solar systems use mechanical devices such as pumps and fans to move heat from collectors to storage or from storage to use. Photovoltaic panels that collect solar energy, turning it into electricity, are also considered an active solar system."
It is important to distinguish between passive and active strategies. While active strategies such as solar PV and high-efficiency heating and cooling equipment are essential components of minimizing energy use and reliance upon unsustainable energy sources, passive design requires a different mentality and in some ways, design intent. For example, many homes in the U.S. have enough roof (and/or yard) space to accommodate enough solar PV modules to provide all of their electricity needs even if the houses were not originally designed with that intent. This is especially possible if the home upgrades to highly efficient heating and cooling, as well as improved insulation and air sealing. However, it would be difficult for most houses in the U.S. to become truly passive, because passive design principles are more specific than simply having enough properly-oriented roof or yard space.
Though you may see some minor disagreements on these depending on which source you use, the following are generally accepted to be essential passive solar design principles. It is important to note that you can minimize energy use by selectively implementing these, but in order for a building to be truly passive, all of them must be used. As you will see, they often complement each other, and some simply do not "work" if not combined with others. These principles are orientation/azimuth, overhangs/shading, windows, insulation/air sealing, and thermal mass. Please read through the following descriptions of passive solar design and some passive solar design principles. Note that I provide a link as well as a .pdf since the links are archived and don't always work. All of the readings are from the California Energy Commission.
Please read through the following descriptions of passive solar design and some passive solar design principles. Note that I provide a link as well as a .pdf since the links are archived and don't always work. All of the readings are from the California Energy Commission.
Please read the following for some additional insight into thermal mass, which is essential for passive design but is not often used.
The foolwing provides a brief summary of these principles:
Generally speaking, most of these principles are easy to understand. It's all but common sense that you need good insulation and air sealing for an efficient home, and understanding that good orientation is important is easy to understand if you know that the sun is usually in the south in the northern hemisphere. We all know that windows allow sunlight in. There is some subtlety and basic trigonometry (uh oh) to overhangs, since you must consider sun angle at your latitude for proper design, so that can be a little tricky. But the fact that deciduous trees lose their leaves once a year is common knowledge. Thermal mass is the least commonly understood, but the basics are not difficult to understand (things that heat up slowly and release heat slowly). It bears repeating that for a fully passive solar design, all of these elements must be present and work in conjunction. As noted above, this almost certainly must be considered in the design process, since the building's orientation cannot be changed after it is built and some overhangs are difficult to retrofit. Windows, air sealing, insulation, and possibly thremal mass can generally be added after it is built, though.
Here is an example of a modern home that uses some passive design principles. See if you can find examples of using orientation, windows, shading, insulation, and air sealing.
Passive solar design refers to taking advantage of ambient daily and seasonal sunlight (or lack thereof) to heat and cool a building. Passive design in a more general sense includes any measure that reduces energy use by taking advantage of local conditions. Natural ventilation is one way to do this, as the video below details.
Note the use of prevailing breezes, orientation, and windows to help passively cool a home.
One final note about passive design before we move on: In case you think that these principles represent some modern breakthrough in knowledge, well, you are off by a few thousand years! Many ancient and historic civilizations all over the world not only understood these principles, but deployed them on a wide scale.
Please read through the following descriptions of how passive design has been used for thousands of years.
There is a saying that "if you want to pay attention to something, you need to measure it." As you are hopefully aware, there are many ways that buildings and urban/suburban development impact sustainability, e.g. through energy use, natural resource use, waste generation, impact on ecosystems, impact on biodiversity, impacts on pollution, and more. If we want to minimize these impacts and to be more sustainable, we need to figure out a way to measure these impacts. Enter green building ratings systems, aka green building metrics. There are too many ratings sytems to fully cover here, but the following provides some insight into some of the more common ones, as well as some that are less common.
Take a minute or two to think about the following (feel free to write down your answers): If you wanted to compare how sustainable one building is compared to another building, what would you look for? Think of specific things that 1) impact sustainability and 2) can be measured.
In the 1990s, the U.S. Green Building Council [31] (USGBC) undertook the same exercise. The result of this was Leadership in Energy and Environmental Design (LEED). LEED was the first widely-accepted "green" building rating system, and is now recognized worldwide. (Side note: the USGBC is a non-profit organization that provides an incredible amount of resources related to sustainable builiding design, many of them freely available. See their Courses and Events [32] site for details. There are also many local chapters [33] that host events.)
LEED is one of the most commonly recognized and used green building ratings systems in the U.S. and the world. You have almost certainly seen a LEED building, even if you did not know it was certified. Please read and watch the following summaries of how LEED "works."
As you can see, LEED is a points-based rating system. Remember all of those aspects of sustainable buildings you listed at the beginning of this page? LEED almost certainly provides "points" for each one of them. On a basic level, a building gets a specific amount of points for sustainable physical features such as water reduction, energy efficiency, renewable energy, use of native plants, use of local materials, use of non-toxic materials, and more subtle ones such as promoting public transportation, providing bike infrastructure, and designing the building to be used as an educational tool. Afte the building is designed and tested, the building achieves a certification level based on the points earned:
There are more complexities to actually getting certification, e.g. the the builiding owner works with a LEED professional and registers with USGBC from the outset, etc., but that goes beyond the scope of this course. There are also a number of different types of certifications, from single new buildings to interior design to operations and maintenance and more. More information on these is here [35]. If you are interested in reading more about the full LEED process, you can dig into the details here [36]. Note that there are job and training opportunities in this process. For more information see their Credentials [37]site.
Here is one example of a LEED building. Note the variety of sustainable features it has, from water to waste to energy and more. If you are so inclined, you can browse for LEED projects on the U.S. Green Building Council [38] website, including with a searchable database.
OPTIONAL - You can play around with the LEED for Homes checklist here [39], and access other checklists here [40].
One of the common critiques of LEED is that building owners/designers are not required to check all of the boxes (literally, in this case). In other words, it is possible to have a LEED building that overuses water, does not provide 100% of it's own energy, does not use entirely non-toxic or local resources, and so forth. You can pick and choose what you want to address, and the only penalty is that you might not get the certification level that you wanted. In addition, a building receives its LEED rating before the building is used. This can be a problem if the folks using the building do not operate it in such a way that the LEED goals are accomplished. For example, if the building has high levels of insulation and air sealing but the building occupants turn the thermostat up too high or low, or leave windows open, etc., then the building may use much more energy than the design claims.
The Living Building Challenge seeks to address these points, but more importantly it seeks to be the most robust and holistic sustainable building rating system available.
The Living Building Challenge is overseen by the Living Future Institute. Their goal is to "move beyond merely being less bad and to become truly regenerative." Based on personal experience and extensive research, Living Building Certification is indeed the most robust and holistically sustainable building rating system available today.
Please read and watch the following introductions from the International Living Future Institute. They provide a good overview of the intent and philosophy behind Living Buildings.
As you read [41]and saw, the core questions asked by the International Living Future Institute (ILFI) are:
It is important to take a minute to think about the implications of the first question in particular. The goal of ILFI goes beyond simply not making things worse in terms of sustainability to actually making things better by building buildings. The intent is summarized when they say: “Nothing less than a sea change in building, infrastructure and community design is required. Indeed, this focus needs to be the great work of our generation. We must remake our cities, towns, neighborhoods, homes and offices, and all the spaces and infrastructure in between. This is part of the necessary process of reinventing our relationship with the natural world and each other—reestablishing ourselves as not separate from, but part of nature, ‘because the living environment is what really sustains us.’” The Living Building Challenge is nothing if not ambitious!
The Living Building Challenge has seven (7) "petals" for certification. Each petal represents a different category of sustainability:
There are twenty (20) total "imperatives" within these petals. Each imperative has a specific, measurable standard that must be met in order to receive certification.
Among the standards required are:
As you can see, there are a few things that separate Living Buildings from LEED (and other) sustainable building metrics. The list is long, but primary among them are the following "Principles that govern the standard":
As a result of this being such a robust standard (and the fact that it must be used for a year prior to certification), there are only 30 fully certified Living Buildings [47] in the world, as compared to thousands of LEED buildings.
The following provides a description of a Living Building. As of 2018, this was only the second one in existence.
LEED and Living Buildings are holistic rating systems. There are a number or energy-only rating systems in the U.S. The following is a sampling of a few of them.
Energy Use Index (EUI) and Benchmarking are very common metrics in the energy management industry and energy policy world. They focus solely on energy use, and are relatively simple ways to assess and influence the energy efficiency of buildings.
First, read the following short reading about Energy Use Index (sometimes called "Energy Use Intensity") and watch the video below.
Benchmarking is a common practice in the commercial energy efficiency industry. The most commonly used software for this is Portfolio Manager from the U.S. Environmental Protection Agency (EPA). Read the summary of both of these below
An energy use index (EUI) is relatively easy to determine. You just take a year's worth of energy bills, convert the total energy use to kBTU (thousands of BTUs), then divide by square footage. That's it! This gives you a snapshot of the general energy efficiency of a building, though not a comprehensive one. It is a helpful basis of comparison for other buildings, which is exactly what EPA Portfolio Manager does. Portfolio Manager compares your building's EUI with other similar buildings. What is meant by "similar?" Well, that depends on a few factors, but the most important aspects are 1) buildings in a similar climate zone and 2) buildings that are used for similar things. For example, the mean EUI of an education building in the U.S. is 51 kBTU/ft2, while the mean EUI of a hospital is about 188 kBTU/ft2 (source: Lawrence Berkley National Laboratory Buildings Performance Database [50]). Why the difference? Think about the difference in operating hours and equipment used in these buildings. Given this, it would be unfair to compare the "typical" energy use of a hospital with the "typcial" energy use of a college or middle school. Similarly, you would expect slightly different energy use based on the climate. The exact same building in a temperate climate such as, say, Coastal North Carolina would use less energy than one in a cold climate such as Minnesota or Maine or a hot, humid climate such as Florida.
EPA provides you with an "Energy Score" based on other buildings in their database. A score of 25 means you have a lower EUI than 25% of comparable buildings, a score of 50 means you have a lower EUI than 50% and higher than 50%; a 90 means you have a lower EUI than 90% of buildilngs, and so on. EPA provides the option of being Energy Star rated if you have a score of 75 or above. Some municipalities (such as Philadelphia [51]) require buildings to obtain their Energy Score every year so that they may constantly see how they compare to other buildings. The benefit of doing this every year is that you are forced to keep up with your peer buililngs. Let's say you get a score of 75, then think: "Cool, I'm very efficient. I don't need to make any upgrades." But then if other buildings become more efficient over time, your score will drop. In this way, the process of improving efficiency in theory never ends.
Energy Star is a program created and run by the U.S. Environmental Protection Agency (EPA). As the name implies, this is an energy efficiency rating system that has the following four components:
a high efficiency thermal enclosure system (thermal envelope!)
According to the EPA [52], Energy Star certified homes "are at least 10% more efficient than homes built to code and achieve a 20% improvement on average." Please read the following summary of the Energy Star Residential New Construction Program:
Here is a great video that introduces some of the benefits of Energy Star Homes. It is geared towards builders but is a helpful introduction for anyone.
OPTIONAL
Passive House Certification is effectively a more robust version of Energy Star Certification. There are actually two different certifying bodies in the U.S. - Passive House Institute (PHI) and Passive House Institute U.S. (PHIUS) - as detailed by ecohome here [55]. The distinction is not important for our purposes, but we will focus on PHIUS standards. The image below provides a sense of how robust Passive House standards are relative to Energy Star and other standards. See this [56] document for a fuller explanation.
Please read the overview below and click on the second link to look at some of the differences between PHIUS and other energy efficiency standards:
The following is a summary of the energy-only rating systems:
Benchmarking:
Energy Star Certification
Passive House Certification
Finally, let's go over a few other sustainable building types and techniques. Tiny homes have been gaining in popularity for over a decade now (there are multiple shows and documentaries dedicated strictly to tiny homes, for example), mostly as a solution to exponentially increasing housing prices. But there is an element of sustainabiliy and freedom (e.g. "van life") to a tiny house as well, and many people are simply tired of the overconsumptive American lifestyle. We will also briefly go over a few less common examples, including straw bale, cob, earth bags, and cooperative housing.
Straw bale, Cob, and earth bags are much less common than most of the examples on previous pages, but are perhaps the most sustainable in terms of materials use since most of the structure is made from natural and local materials - mud, straw, gravel, and/or soil.Straw bale also provides excellent insulating value, and as you will see in the video below, cob is a good thermal mass and can be made into almost any shape.
Please read the following for an overview of each of these, then watch the video that provides details of a cob/straw bale hybrid house. in the video, note also all of the passive design principles that are used!
There is no single definition of cooperative housing, but at a fundamental level refers to housing that is co-owned and co-managed by a group of people. There are many different kinds of cooperative housing, and it has been practiced for thousands of years. In a Western context, cooperative housing usually refers to intentional communities that cooperatively own and manage a building and share resources and spaces. The following provides a good intro to some of the key ideals behind most cooperative housing.
Finally, tiny homes! Tiny homes were a niche application but have become much more mainstream now. They are small (usually less than 500 ft2), but relatively inexpensive ($10,000 - $100,000, depending on how fancy), and most of them are on wheels and so can be moved seasonally and avoid some permitting issues due to not having a foundation.
Here is a nice introductory video to tiny homes.
Here is one interesting application for tiny homes that helps address homelessness and substance abuse issues in Colorado.
I have visited and toured a number of cooperative buildings in Switzerland. Feel free to take a look at the pictures below for some insight into what they look like and how they operate. Note that there is no single set of rules for cooperative housing. The examples below only describe the ones that I saw in Switzerland. That stated, the types of rules at these Swiss examples are fairly typical for cooperative housing in that they are meant to foster low-impact, community-oriented living. One aspect that all cooperative housing shares in common is that the buildings are owned and managed cooperatively. All of the buildings below are owned by all occupants, including any businesses. All photos are my own.
Here are some more site visits! Again, this is not required reading. I suggest browsing through them if/when you have time. This may help inspire your final project proposals!
By now you should be able to do all of the following:
You have reached the end of Lesson 9! Double-check the to-do list on the Lesson Overview page [63] to make sure you have completed all of the activities listed there before you begin Lesson 6.
Links
[1] https://www.epa.gov/report-environment/indoor-air-quality
[2] https://www.wbdg.org/resources/energy-codes-and-standards
[3] https://www.bre.co.uk/filelibrary/Briefing%20papers/92993_BRE_Poor-Housing_in_-Europe.pdf
[4] https://www.rics.org/uk/news-insight/latest-news/news-opinion/time-to-retrofit-decarbonising-uk-buildings-and-economic-recovery/
[5] https://www.theguardian.com/commentisfree/2021/sep/28/britain-homes-energy-crisis-governments-insulation-low-carbon-heating
[6] https://www.wbdg.org/resources/energy-codes-and-standards#fn1
[7] https://www.iccsafe.org/products-and-services/i-codes/code-development/iecc-2024-and-beyond/
[8] https://aceee.org/sites/default/files/cost-of-ee.pdf
[9] https://www.energy.gov/eere/analysis/downloads/state-level-electric-energy-efficiency-potential-estimates-0
[10] https://www.energy.gov/sites/default/files/2017/05/f34/epri_state_level_electric_energy_efficiency_potential_estimates_0.pdf
[11] https://www.energy.gov/eere/analysis/downloads/doe-webinar-energy-efficiency-potential-states
[12] https://www.energy.gov/sites/default/files/2017/07/f35/EEpotential%20webinar_7-13-2017.pdf
[13] https://www.energy.gov/sites/default/files/2017/06/f35/energy_efficiency_resources_fact_sheet_june2017.pdf
[14] https://www.energy.gov/eere/slsc/energy-efficiency-savings-opportunities-and-benefits
[15] https://www.energy.gov/eere/analysis/downloads/infographic-united-states-has-potential-cost-effectively-reduce-its
[16] https://www.nrel.gov/docs/fy16osti/65298.pdf
[17] https://web.archive.org/web/20161206200352/http://www.consumerenergycenter.org/residential/construction/solar_design/
[18] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/Home%20Construction%20-%20Passive%20Solar%20Design.pdf
[19] https://web.archive.org/web/20160417105132/http://www.consumerenergycenter.org/residential/construction/solar_design/orientation.html
[20] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/Passive%20Solar%20Design%20-%20Proper%20Orientation.pdf
[21] https://web.archive.org/web/20150906071238/http://www.consumerenergycenter.org/residential/construction/solar_design/overhangs.html
[22] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/Passive%20Solar%20Design%20-%20Overhangs%20and%20Shading.pdf
[23] https://web.archive.org/web/20150906055333/http://www.consumerenergycenter.org/residential/construction/solar_design/windows.html
[24] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/Passive%20Solar%20Design%20-%20Windows.pdf
[25] https://web.archive.org/web/20160401163645/http://www.consumerenergycenter.org/residential/construction/solar_design/thermal.html
[26] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/Passive%20Solar%20Design%20-%20Thermal%20Mass.pdf
[27] https://web.archive.org/web/20150906053957/http://www.consumerenergycenter.org/residential/construction/solar_design/balance.html
[28] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/Passive%20Solar%20Design%20-%20Everything%20in%20Balance.pdf
[29] https://www.smarterhomes.org.nz/smart-guides/design/thermal-mass-for-heating-and-cooling/
[30] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/7%20Ancient%20Wonders%20of%20Green%20Design%20%26%20Technology%20_%20WebEcoist.pdf
[31] https://www.usgbc.org/
[32] https://www.usgbc.org/courses-and-events
[33] https://www.usgbc.org/membership/individual
[34] https://www.usgbc.org/leed
[35] https://www.usgbc.org/education/leed-v41
[36] https://www.usgbc.org/leed/v41#bdc
[37] https://www.usgbc.org/credentials
[38] https://www.usgbc.org/projects
[39] https://www.e-education.psu.edu/emsc297/sites/www.e-education.psu.edu.emsc297/files/LEED%20For%20Homes%20Project%20Checklist%2010-01-14_0.xls
[40] https://www.usgbc.org/resources?LEED+Resources=%5B%22Checklists%22%5D
[41] https://living-future.org/lbc/basics4-0/
[42] https://www2.living-future.org/LBC4.0?RD_Scheduler=LBC4
[43] https://living-future.org/lbc/water-petal/
[44] https://living-future.org/lbc/energy-petal/
[45] https://living-future.org/lbc/place-petal/#03-habitat-exchange
[46] https://living-future.org/declare/declare-about/red-list/
[47] https://living-future.org/lbc/case-studies/?certs=living
[48] https://www.energystar.gov/buildings/facility-owners-and-managers/existing-buildings/use-portfolio-manager/understand-metrics/what-energy
[49] https://www.energystar.gov/buildings/facility-owners-and-managers/existing-buildings/use-portfolio-manager/learn-how-portfolio-manager
[50] https://bpd.lbl.gov/
[51] https://www.phillybuildingbenchmarking.com/
[52] https://www.energystar.gov/partner_resources/residential_new/about
[53] https://www.energystar.gov/partner_resources/residential_new/homes_prog_reqs/national_page
[54] https://www.energystar.gov/sites/default/files/National%20Program%20Requirements%20Version%203.1_Rev%2011.pdf
[55] https://www.ecohome.net/guides/2191/everything-you-need-to-know-about-passive-house/
[56] https://www.energy.gov/sites/default/files/2017/02/f34/PHIUS%2B2015 Passive Building Standards.pdf
[57] https://www.phius.org/phius-certification-for-buildings-products/project-certification/overview
[58] http://energy.gov/eere/buildings/downloads/phius-information
[59] https://www.motherearthnews.com/green-homes/earthbag-cob-strawbale-zbcz1605
[60] https://discovergeos.com/
[61] https://greenbuildingnews.com/2012/08/08/new-exhibit-the-denver-zoo-leeds-animals-and-guests-unique-experience/
[62] https://www.craftbrewingbusiness.com/featured/new-belgium-brewing-shares-highly-detailed-carbon-neutral-toolkit-for-breweries-aiming-for-net-zero-carbon-emissions/
[63] https://www.e-education.psu.edu/emsc297/828