Environmental econ roofing paper

Consciousness has been growing about the environmental consequences of our building technology. Significant innovative efforts have been put into improving roofs in particular. This paper is intended to serve as a starting point for a government representative trying to understand what roofing technology is out there, its environmental consequences, and the related economics.

Two major urban environmental concerns are urban heat islands and the management of stormwater, both of which construction affects. The United States Green Building Council, a coalition of leaders from the building industry, has developed standards for high performance, sustainable buildings, including standards for roofing. There is a tremendous variety of roofing types with very different ways of managing precipitation and solar radiation. These different approaches have very different environmental consequences.

This paper is divided into four sections. The first part of this paper will examine the environmental concerns of urban heat islands and how current roofing practices relate to them. The second part of this paper will outline a few representative types of roofing, their construction and what they do. The third part will examine the economics of the costs and benefits. The fourth part will provide a framework for examining the tradeoffs involved and propose a method for making decisions as to what roofing technology to encourage in particular areas, and methods for encouraging these decisions.

1. Environmental Concerns

In nature, precipitation and sunlight are critical parts of the local ecology, but in building design, these are the very elements from which roofs are used to protect building residents. In nature, soil rainwater flows into the soil, where it is absorbed by plants and used with carbon dioxide and solar radiation in photosynthesis, producing oxygen and biochemical energy.

Buildings and their foundations block off hydrological access to soil, and typically force the available land around the building to absorb the water. In dense urban environments with a large proportion of the land covered with impermeable surfaces, this has led to the creation of expensive systems for storm drainage. Conventional roofing sheds water directly and immediately to the street level where it adds to the water absorbtion burdens of the storm drainage system. Indeed, in “Roofing Handbook” by Robert Scharff and Terry Kennedy, sites lapping, the overlaying of lower ends of any roofing component on top of the next element, as “one of the earliest and still most important principles of roofing” due to its role in “draining water off the building”.

Meanwhile, conventional roofing systems absorb solar radiation directly. This is a design element that recieves less coverage. In conventional roofing, sunlight is considered mainly in terms of its indirect heat effects and damage to the roofing materials. The solar energy transforms into heat, similar to the what happens in unshaded parking lots on sunny days. Along with other heat producing activities, this leads to the urban heat island effect, in which more densely built up areas have higher tempeartures than less built up areas, which typically have more greenspace. This also results in wide variation in roof temperature throughout the day, and thus significant, regular expansions and contractions, which are a major source of roof damage, and a major limitation to the lifespan of any roofing system.

2. Roof Construction

a. Conventional Roofs

There have been a variety of roof constructions employed throughout history, and these roofing technologies have been under continuous development. Among the roofs that are currently common in the United States, in commercial applications, built up roofs and single ply roofs are commonly used while asphalt roofs (often in the form of shingles) dominate the residential roof market. The significant variations occur above the support structures and insulation (if any).

Built up roofing is built out of sandwiched layers of felt, saturated with and glued together by melted bitumen. Above this, an additional layer of water proofing bitumen is laid on. The surface may then be covered with gravel or protective mineral-surfaced sheets. This is one of the earliest roofing styles still in common use in commercial projects.

Single-ply roofing is another major category of conventional, commercial roofing technologies. It consists of flexible, waterproof membranes, which are laid out in lapping layers spanning the length of the roof. The membranes are manufactured off site, allowing for better control in the manufacturing process to reduce errors.

The final example from this category is asphalt roofing, surface coverings and underlayments produced from asphalt, which comes out of the petroleum refining process. These include the well known shingle, which dominates the residential roofing market and roll roofing.

These forms of roofing all operate by blocking stormwater and redirecting it off the roof, and by absorbing solar radiation and converting it into heat. The primary design concerns for these forms of roofing are that they be able to endure the heat and water flows without losing functionality. There are certainly significant differences in the installation, costruction, costs, and other notable properties of these materials, but functionally speaking, the primary considerations for such roofing are how long it will last before it needs to be replaced, how much maintenance it requires, and how costly it is to install.

These are the baseline from which we will judge other roofing technologies. Urban stormwater management is a concern which society has been dealing with for generations. Conventional roofing experiences much more dramatic swings in temperature, mostly on the high side, this has long been the case, and heat is a long time factor in roofing. Whether heating tar to make built up roofs, or selecting a waterproofing that won’t melt off the roof on a hot day. But current concerns over urban heat islands are a new consideration to which this technology is starting to adapt.

b. Reflective & Photovoltaic Roofs

To control the heat problems with conventional roofing systems, many are changing the way roofs deal with sunlight. This can be accomplished by increasing the incident solar energy reflected by the roof, increasing the heat energy radiated by the roof, and absorbing the solar energy for more productive uses, such as generating electricity with photovoltaics. All of these decrease roof temperatures, and help to some degree with the urban heat island effect, providing the private benefit of extending the life of the roof. Part of the appeal of these roofing strategies is that they constitute only small variations on the roofing techniques that are already well known and commonly practiced.

There are a multitude of different ways to build cool roofs. Membranes with high reflectivity can be purchased; some roofs can be painted or coated to increase reflectivity and emmittance. Increasingly, roofing materials are being made for their reflective and emmitive properties. Metal roofs are another highly reflective roofing option, and have been used in construction for centuries. Photovoltaics are newer and come in multiple forms. Some are mounted units, angled to catch a large quantity of solar radiation, and thus not an integral part of the roof surface. There are also solar shingles, which serve the same purpose, and work the same way as regular shingles, with the added advantage of generating electricity, though generally not as effectively as mounted units. All of these various options provide effects that are generally similar.

The benefits of cool roofs accrue both to the building residents and to the surrounding area. Cool roofs’ reduction of absorbed heat makes cooling the building easier. It also reduces the ambient temperature which produces the heat island effect, and thus air pollution problems. Cool roofs also suffer less thermal-related wear and tear, as there are fewer expansions and contractions. These represent significant gains to building residents and maintainers.

There are complications to all of these plans. In order for cool roofing to remain effective, the surface has to be kept clean, which results in an ongoing maintenance expense. Seasonal variation is another important factor in evaluating cool roofs. It is only relevant if costs due to excessive heat from the sun are not offset by the loss of the heat absorbtion in cooler months. Due to the reduced sunlight incidence in colder parts of the world, at colder times, this concern is not as significant as it may seem at first. A major problem with photovoltaics, particularly in the more effective mounted applications, is that they are highly expensive additions to a roof. Thus cool roofing is not as obvious a choice as it might seem.

c. Green Roofs

The third and final category of roofing this paper considers is green roofing. Green roofs are topped with plants and soil. The soil provides a medium for plant growth and allows the roof to retain water, filtering it slowly to the ground, allowing it to evaporate in place and absorb heat in the process, or aiding in photosynthesis. The soil also provides a thermal buffer to absorb and release energy slowly. The plants absorb solar energy in photosynthesis, store carbon, produce oxygen, and hold the soil together.

Green roofs are elaborate, layered constructions. On top of the normal waterproofing for a roof, a layer of drainage material is placed to allow water beyond the soils holding capacity to flow off the roof. Above that is a root proof membrane to protect the roof from the roots of the plants. The growth medium goes on top of the root barrier, and contains the plants. This allows the roof to support plants, absorbing water, and allowing the excess to run off without eroding the growth medium for the plants. All of these layers have important design considerations, the soil and plants most particularly.

Green roofs vary from extensive designs, which have shallow soil and a narrow range of acceptable plants, typically small in size and weight, to intensive designs, which have deeper soil and a larger range of acceptable plant types. In general, the benefits increase with the intensiveness of the roof, while the costs decrease with the extensivity of the roof, both with diminishing returns. This explains the constrained range of typical green roof soil depths.

Green roofs have many of the same benefits as cool roofs, plus the ability to manage stormwater. They keep the roof cool and regulate roof temperature quite effectively. The absorbtion of direct radiation, particularly ultraviolet radiation by a non-structural layer helps to preserve the structural membranes. In addition to absorbing the sunlight for photosynthesis or evaporation, the soil layer serves as a thermal reservoir, gradually absorbing heat during the day and slowly releasing it at night. This can extend roof lifetime by as much as a factor of three. They also have major public benefits. Green roofs directly improve air quality, absorbing and storing carbon dioxide while producing oxygen through photosynthesis. Through their retention and filtration of precipitation, they effectively manage stormwater, reducing the peak flow. Some places, notably Singapore, are working on the use of extensive green roofs for agricultural purposes, but this is not a typical case. Many of the benefits increase with the intensiveness (soil depth) of the roof.

The two major problems with green roofs relate to their weight and the installation cost. While the dry weight of a green roof is heavier than a normal roof, the additional weight when the soil is saturated represents an extraordinary architectural burden, varying with the amount of soil involved. If one’s support is adequate, the installation of a green roof can still be directly costly, and this too varies with the extensive or intensive nature of the roof. Also, intensive roofs often need care, particularly watering.

3. Characterizing Costs and Benefits of Roofing Technologies

This section of the paper will start to examine the cost and benefit tradeoffs for the various roofing technologies, and how they are influenced by environmental considerations. Each of these categories of roofing alternatives involves cost and benefit tradeoffs, and contains construction options to consider. Considerations of the tradeoffs between specific varieties within the broader categories receive cursory examination.

Built up roofing, will be the baseline in our considerations. It has low reflectivity, meaning it reflects little light, absorbing most of it and converting it to heat. It has high emissivity, which means that it emits large quantities of heat as radiation. And finally, it is water-proof, shedding water from the roof level onto the street level.
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There are several relevant factors to consider in picking a roof. A major factor is the ambient temperature. In areas where heating expenses are high, and cooling expenses are comparatively minor, a cool roof may actually represent a liability. Other weather factors include the angle of incident sunlight, the amount and degree of cloud cover, and peak daily or hourly rainfall. Other factors include the water drainage and holding capacities of the soil, the price of electricity, roofing replacement costs, and the frequency with which replacement is needed for the roof.

The private benefits relate largely to the cooling of the building and the preservation of roof integrity. Cooling buildings can be quite expensive. This is particularly true in areas at lower latitudes with sunlight angles of incidence that approach the perpendicular and longer hot seasons. Less cloud cover increases the daytime heat load for the roof, increasing cooling costs. High daily temperature variations produce greater wear and tear on the roof. This is most common in environments which lack the temperature buffering effects of cloud cover.

A final element in the category of private benefits would be the productive use of the roof, whether that be the farming of a green roof, as proposed in Singapore or the inclusion of solar in the roofing design. These depend in large part on the local value of the resource. In places with heavy demand for food or electricity and a limited ability to supply such, these can be profitable industries, but cannot supply the entire local demand.

Public benefits related to stormwater concerns are twofold: flood control and water contamination. In terms of flood control, the primary concern is how much precipitation is produced, relative to the drainage capacity for a given area. Green roofs add a buffer capacity to this, reducing the area, and thus the water flows that storm sewers need to handle. Thus the use of green roofs reduces the needed quantity of stormwater drainage, often provided as a public good. Cool roofs, and particularly green roofs, aid with the contamination concerns by reducing the heat of the water proof membranes, and thus their tendency to leech complex synthetic substances into rainwater. In this fashion cool roofs improve the quality of the local water supply. Thus alternative roofing strategies do provide benefits to the public that are not fully incorporated into private decision making.

Public benefits related to the urban heat island effect are simpler. Roofs shed heat in two ways, by radiating the heat away through emissivity, or by direct convection with the local atmosphere. It is the convection that creates the urban heat island effect. Reflective and emissive roofing seeks to maximize the amount of the incident energy that sheds as outgoing radiation, thereby reducing urban temperatures. Green roofs prevent the heat issue from arising by absorbing the electro-magnetic solar energy to produce chemical energy through photosynthesis, and by evaporating stored water.

4. Framing the decisions

This section seeks to frame the decisions made by government actors and agencies with regard to roofing construction. First, it is important to determine locally appropriate strategies, taking into account climatic as well as economic factors. And secondly, a strategy to encourage and promote the desired technologies should be chosen. Taken together, these form the critical elements of a government role in the management of roofing technologies for public benefit.

Determining appropriate roof constructions for an area is an important and complicated task. Each region is unique, and has unique considerations, this is intended as a guide to some of the broader brush factors, not a comprehensive listing of all concerns. Among the more significant elements for a government agency to consider when deciding how to encourage particular construction technologies are both climatic and economic factors.

Different local climates call for different roofing technologies. In an area like Seattle, WA, a coastal, high latitude, temperate rain forest, with frequent cloud cover and heavy rain, relatively constant annual temperature, and a fairly cool climate, a green roof would be a better choice than a solar roof. However, in an arid, low latitude desert, with scant rain fall, high daily temperature variation, high temperature in general, low cloud cover, and plenty of sunlight, such as Phoenix, AZ a photovoltaic roof may provide greater benefits than a green roof.

Various factors suggest different appraches. High temperatures and high temperature variation are indicators in favor of alternative roofing. Green roofs, reflective roofs, and photovoltaic roofs all reduce the temperature of the roof, which is most desireable in the warmest climates. High sun exposure, through low latitudes and low cloud cover are a strong indication for alternative roofing, to extend roof lifetimes, and in particular photovoltaic roofing, to generate additional energy. Heavy rainfall is a positive indicator for green roofs. A major category of benefit for green roofs are their ability to manage stormwater. Additionally, with adequate rain water, the roof will not need to be watered for maintenance of the plant cover.

Another class of considerations in roofing technologies are the raw economic factors. The initial expense of photovoltaic and green roofs are a factor. The extension of roof life provided by cool roofing technologies, and the maintenance required to retain the benefits, provide long term calculations. The value of roof-producable goods like plant foods and electricity are other factors. The discount rate is another factor which is crucial to consider. In the case of high discount rates, any additional cost in roof construction needs to pay for itself quickly or dramatically. With more patient capital the long term benefits of novel strategies begin to outweigh the short term costs.

Having decided upon a technology to pursue, an agency must then consider how it will encourage the new technology, and there are several options available. The first step is to provide a regulatory streamlining for the desired technology, educating the relevant groups, including government agencies and private producers, as to the desired technology and its advantages, as well as its costs.

A second point is to examine the value of the improvement to the owners and manufacturers of the building. In the case of manufactured, reflective roofing, the advantages in reduced roof wear and cooling expenses may already outweigh the costs. This is not likely the case in terms of green roofs, where the benefits are mostly public, and the costs mostly private.

In cases such as green roofing, where the market provides an inadequate incentive, subsidy, taxation, and mandates are options to consider. Subsidies to parties constructing green roofs help to incorporate the public goods into the construction decisions of private parties. This may be more politically feasible in places with heavy corporate influence. Taxation of those who do not use the desired technologies would operate according to the “polluter pays” principle, defining the current market operation not as the baseline, but as a state of high externalization of costs. This is being implemented in many areas with stormwater managment fees. Mandates are more direct, requiring all construction to comply with the specifications under threat of criminal penalty.

Roofing plays an important and often neglected role in the quality of the environment. Agencies working to improve environmental quality will wish to carefully consider the tradeoffs to each technology, and their appropriateness to particular areas and building sites. Options exist for managing the market to produce a high quality environment, but should be pursued with caution and care.

References:

http://www.usgbc.org/LEED/

Roofing Handbook 2nd ed, Robert Scharff and Terry Kennedy c 2001

http://www.epa.gov/heatisland/index.html