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Greenhouse Gases and Home Building: Manufacturing, Transportation, and Installation of Building Materials

Special Studies, September 9, 2008

By Warren Carnow, with special thanks to Gary Ehrlich, other staff in NAHB’s Construction Codes and Standards Group, and staff at Home Innovation Research Labs for help in specifying the components of a single family home.
 
Report available to the public as a courtesy of HousingEconomics.com
 

Introduction

Global warming has become a major topic in local, state, and federal politics. Politicians confronting the issue are looking for new ways to limit greenhouse gas emissions in all sectors of the economy, and that includes the home building industry. In fact, some local governments have already put in place building regulations requiring reviews of development proposals to determine possible adverse impacts on the environment. King County, in the Seattle Washington metro area, for example, requires developers to provide estimates of emissions from transportation (by the anticipated occupants), annual energy consumption (heating, cooling, etc…), and embodied emissions generated during construction of the home and provides a spreadsheet that helps them do so.[1]

 
Previous articles have looked at emissions generated by use of the home[2] and vehicle miles traveled[3]. This article focuses on the embodied emissions component. In general embodied emissions are the emissions generated from the production, transportation, installation, maintenance, and disposal of the materials and products used to build a house. However, our study focuses primarily on the “cradle to gate” embodied emissions created during production, transportation, and installation. To date there has been relatively little quantitative information about how the embodied emissions of residential homes are affected by choosing one material over another. The definition of a green building material is relatively loosely defined, and the advantages and disadvantages of the green products are often unclear. For example one product might claim to be a green product because it is constructed from recycled materials, but to what degree the emissions produced by recycling offsets the reduction in consumption of natural resources is not always transparent. As a result the real benefit of choosing one building material over another can be unclear. It is our hope that this study can provide a better picture of which materials reduce (increase) embodied emissions and how by how much.

Estimating Emissions for a Baseline Home
The estimation of emissions embodied in a product as complex as a house presents many challenges. Partly because of this, some analysts have chosen to concentrate more on emissions that result from energy consumed inside the home after it is built, or from vehicles operated by household members. Now enough data has become available that at least one private company, the Athena Sustainable Materials Institute (hereafter Athena), believes it can produce reasonable estimates of environmental impacts of many of the products and systems that go into a building. Athena produces computer software that estimates embodied greenhouse gas emissions for an entire building, provided the users specify some details of various systems (walls, roof, etc) that go into the structure. NAHB investigations uncovered no other company providing an equivalent product at this time. The King County spreadsheet uses a software product from Athena to estimate greenhouse gas emissions.
 
This article is based on results from a slightly different Athena software product, called the Impact Estimator. NAHB chose this product after consultation with Athena employees, who recommended the Impact Estimator as the most appropriate tool they had so far developed for analyzing residential structures. The article compares embodied emissions for a baseline home, defined primarily by the building materials and systems that are currently used most frequently in residential construction to various alternatives in order to quantify the impacts that these tradeoffs have on embodied emissions.
 
Specifications for the baseline home are derived largely from Home Innovation Research Labs' Builder’s Practices Survey 2006 to calculate the “baseline” or average dimensions, and pick the most frequently used building systems and materials. The baseline home is intended as a starting point to analyze the tradeoffs between materials and is not meant to be a precise representative average of homes built over a certain period. The calculations assume a 2,420 square foot, one story home with a basement built on concrete footers which support a concrete slab floor and concrete basement wall. Wood stud walls, a wood joist floor, and a light frame wood truss roof constitute the remaining structure of the house. For a detailed specification, see the Baseline House sidebar attached to this article as a separate file.
 
Of all the embodied emissions produced in building the baseline single family house, Athena’s Impact Estimator estimates that the overwhelming majority are produced during the manufacturing of materials, which accounts for roughly 50 metric tons of CO2 equivalent (MTCO2e). Construction and transportation have relatively little effect, as greenhouse gases generated during construction activity accounts for roughly 2.0 MTCO2e and transportation of building materials to the construction site accounts for only roughly 0.1 MTCO2e. The Impact Estimator also shows relatively little variation in MTCO2e from the transportation and installation of building products as the specification of house changes, unlike manufacturing, which changes total MTCO2e dramatically in some cases. Embodied emissions are often measured in MTCO2e, where the “e” for “equivalent” is used to include the impact of other greenhouse gasses produced during manufacturing. Other gasses besides CO2 have a different (usually much greater) impact on the environment per unit of weight, so in order to compare the overall emissions each greenhouse gas is calculated in terms of the equivalent amount of CO2 it would take to have an equivalent impact on climate change.
 
According to the results produced by Athena’s Impact Estimator, total embodied emissions (cradle to gate) of the baseline house is 51.4 MTCO2e. By comparison, NAHB estimates that a typical new home built in the Los Angeles metro area generates 7.0 MTCO2e per year from energy used inside the home, and the 14.5 MTCO2e per year from vehicles driven by the occupants of the home. In other words, the embodied emissions amount to about seven years of emissions generated by using the home or more than three times the annual transportation emissions.
 
Figure 1. GHG Emissions (in MTCO2e) for a Typical home Compared
 
 
Analyzing the Tradeoffs
When producing a commodity as complex as a single family house, builders inevitably face a number of tradeoffs. A number of these area shown in Table 1. For example, when choosing windows for a home low E alternatives increase costs up front, but reduce utility bills in the future, which results in fewer greenhouse gas emissions; however, that comparison doesn’t take into account the embodied emissions generated in manufacturing the windows. And in this instance, the additional embodied emissions of fitting a whole house with low E windows is less than 0.1 MTCO2e. However, other materials can embody greater differentials, such as the window framing, where wood fames have significantly lower embodied emissions than both aluminum and PVC, but have similar insulating properties once installed. In this instance it translates into a little under 2 MTCO2e saved by using wood frames over the more common PVC frames.
 
Figure 2. How Embodied GHG Emissions Vary with Window Materials
 
 
The ongoing CO2 emissions associated with maintenance (which tend to be relatively small according to results obtained from the Impact Estimator) are not included in this comparison.
 
The embodied emissions of siding materials vary significantly, particularly for certain types of brick. The most commonly used siding for new homes in the United States is vinyl, though it varies from region to region, and there are a number of other commonly used alternatives.[4] Their embodied emissions impacts range from wood, which is has the lowest embodied emissions, to brick (especially split face brick) which embodies a significantly larger amount of emissions. Among siding alternatives tested against the baseline, cedar embodied the lowest emissions, although pine and spruce weren’t that far behind and wood is typically 1.5 MTCO2e less than vinyl (+/- .3 tons), depending on the siding style and type of wood. Stucco has roughly the same impact as the vinyl, but the material with the greatest amount of embodied emissions is brick which add over 20 MTCO2e to the baseline home.
 
Figure 3. How Embodied GHG Emissions Vary with Siding Material
 
 
Again, ongoing maintenance and replacement, and the embodied emissions from those activities are not included in this comparison. It would be desirable to be able to analyze a mix of siding materials—for example, one for the front of the home, and a different one for the other three sides—but Athena does not offer this option in the current version of its Impact Estimator.
 
According to the 2006 NAHB Builder’s Practices Survey, asphalt shingles are by far the most common roofing material used in new home construction. Among the other materials used, concrete tile, clay tile, and steel all embody more emissions than asphalt. The concrete and steel alternatives each increase embodied emissions in the baseline house by roughly 1.5 MTCO2e, but the clay tiles stood out as particularly high causing an increase of about 7.0 MTCO2e. One roofing material that reduced the baseline home’s embodied emissions was the modified bitumen roofing system, which reduced emissions by 0.8 MTCO2e.
 
In new home construction the most frequently used material for sheathing and decking is OSB (Oriented Strand Board) [5]. Decking is found beneath the floor as well where it ranges from 1/2” to 3/4” in thickness.[6] An 1/8” increase in thickness of OSB in either case increases embodied emissions by 0.6 to 0.7 MTCO2e. The main alternative to OSB is plywood. The Athena Impact Estimators allows users to include the sheathing in the specification of exterior walls (which is done for the baseline home) and to specify whether the material is OSB or plywood, but not to specify the thickness. Compared to OSB, there are fewer emissions embodied in plywood of the same thickness.
 
The home assumes a wood stud frame for the first floor wall, but steel and concrete are sometimes used in home construction as well. If we replace 2x4 wood studs 16” on center with 2x6 studs 24” on center, embodied energy increases, but only by about 1/3 MTCO2e. If we replace the wood framing with the minimal steel framing option available in the Athena Impact Estimator, embodied emissions increase by roughly 1.0 MTCO2e. However, this doesn’t include any additional insulation or other changes. Building codes generally require additional changes—specifically, more insulation for a steel-framed home. The embodied emissions impact of insulation changes are shown separately in Table 2.
 
Concrete is the other material sometimes used by builders in above-grade walls. The impact of concrete use on the emissions embodied in the home depends on both the type of concrete wall and the fly ash content (or other recycled waste product) used. If we replace the exterior walls with cast-in-place concrete walls, emissions increase by over 10 MTCO2e, not accounting for changes in insulation, or maintenance, or durability. Use of insulated concrete form walls or concrete block walls increase embodied emissions further than this. Increasing the fly ash content in concrete reduces embodied emissions by a small amount.
 
The insulation thickness varies for a number of reasons; the type of framing material, the climate and location of the home, or the type of insulation being used. For this reason deciding on a baseline level of insulation is somewhat misleading, so instead our study assumes a home insulated for a region roughly around Kansas City, Kansas. In order to account for climates that require more insulation, we increased the thickness 50 percent to account for the impact of changes in colder climates. Of all the insulation types, cellulose has the lowest embodied emissions. The other insulation types all have higher embodied emissions compared to the fiberglass option specified in the baseline home.
 
The baseline home has a basement, but the incidence of basements varies greatly across Census Divisions. In New England, around 89 percent of new homes are built with basements. New homes in the Mid Atlantic and the Northern Central US are also very likely to have full basements. On the other hand, along the Pacific, in the Mountains and in the South basements are considerably less common. Only 6 percent of homes built on the West Coast have basements, for example[7]. Not surprisingly, the impact of a structural feature as large as a full basement on embodied emissions is considerable. The most common material used for basements is cement, and whether it is cast-in-place, concrete block, or insulated concrete form, all cement contains relatively high embodied emissions. Again, this accounting only for embodied emissions, and not any other possible benefits of concrete. The Portland Cement Association advertises concrete homes as being quiet and comfortable, low-maintenance, and built to last.
 
The baseline home is 2,420 sq. ft, based on the average reported in Home Innovation Research Labs' Builders Practices Survey 2006. Given the same house, increasing the square footage 500 to 2,920 sq. ft. produces an estimated increase of 7.4 MTCO2e in embodied emissions. The baseline home does not assume a garage. Although the majority of new single family homes have garages, it is more convenient to analyze the garage as a totally separate structure in the Athena Impact Estimator. Adding a two-car garage to the baseline house, increases the embodied emissions by 4.1 MTCO2e.

Other Green Alternatives

The Athena Impact Estimator does not include all building products; there are some other energy and resource efficient materials worth considering. Table 3 lists building material alternatives that can help lower embodied energy of home construction and/or reduce demand on natural resources. This list includes primarily recycled materials and alternative renewable resources that may lessen the production of emissions during construction. In particular, there are a wide variety of wood flooring alternatives, as well as carpeting and tile alternatives that produce fewer emissions. There are recycled and rapidly renewing hardwoods (such as bamboo and cork), linoleum (which is more environmentally friendly than vinyl), and recycled carpet, as well as more novel approaches such as compressed earth. Other lumber products can be substituted as well to replace wood with higher embodied energy. One disadvantage with some alternative building products like cork flooring is lack of durability, which requires that the product be replaced and means that total embodied emissions may be higher over the life of the house than they first appear. PATH (A Public-Private Partnership for Advancing Housing Technology) provides more information on these products as well as other environmental alternatives [8].
 
Conclusion
There has been a tendency for discussions of the impacts of residential development on greenhouse gasses to be governed by sound bytes and labels such as green, recycled or energy efficient that are not always clearly defined. Often, the role of embodied emissions is overlooked in these discussions. This article has sought to provide home builders with information about the magnitude of embodied emission effects, that they can take into consideration when making decisions about building products. The intent is to allow a more comprehensive analysis of which building products and practices are environmentally friendly by taking greenhouse gasses generated during the manufacturing phase of the process into account.
 
For more information about this item, please contact: Paul Emrath at 800-368-5242 x8449  (pemrath@nahb.org)
 
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Footnotes:
 
 
 
 
[4] NAHB Builder’s Practices Survey 2006
 
[5] NAHB Builders Practices Survey 2006
 
[6] HUD Residential Structural Design Guide: 2000
 
[7] NAHB Builder’s Practices Survey 2006
 

For more information about this item, please contact Paul Emrath at 800-368-5242 x8449 or via email at pemrath@nahb.org.


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