Roof
Design and construction of the roof affects the maintenance, structural, and thermal performance of the entire house. In order to meet the goals of minimizing material resource impacts while maximizing energy efficiency, the performance and interaction of all components of the roof assembly should be evaluated together. The following comparative analysis identifies the relative economic, energy, and environmental implications of four different roof systems: wood trusses, I-joists, structural insulated panels (SIP), and structural engineered panels (SEP). The components of the roof system that are analyzed in this comparison are the following: insulation (fiberglass for truss and I-joist systems, rigid polystyrene for SIP and SEP systems), sheathing, waterproofing membrane, structural framing (trusses, I-joists, beams, and/or purlins), and fascia. Roofing products like asphalt shingles are discussed separately in the page on roofing materials.
General Recommendations
SIP construction has the least environmental impact, due to reduced material resource use and increased energy efficiency. Energy savings and smaller heating and cooling equipment can offset the higher first costs of SIPs. The new SEP construction system is also a high-performance alternative to traditional roof construction, although first costs are roughly similar to SIPs. In general, when planning exterior systems, the following should be considered to reduce environmental impacts:
- Use locally or regionally manufactured wood products to reduce transportation and increase community economic benefits. Wood framing used in the Midwest generally comes from the Western or Southeastern U.S., as well as Canada. Panel products are manufactured within 500 miles and insulation is manufactured within 1000 miles.
- Use materials that maximize rapidly renewable resources. Exterior oriented strand board (OSB) uses fast-growing aspen and waste products. Also use materials from FSC-certified producers to ensure sustainable growing and harvesting practices and prevent use of plantation-grown products.
- Use less toxic materials. When selecting composites such as OSB, use formaldehyde-free sheathing products to protect workers during installation. In panel construction, use expanded polystyrene or polyisocyanurate insulation, which do not use HCFC blowing agents during manufacture.
Environmental Context
Roof systems for residential construction rely heavily on wood
products, which may or may not include engineered wood products or
products provided from sustainable sources. Engineered wood products generally reduce
material resource impacts by reducing needs for virgin or prime
materials, but may increase negative impacts associated with binders,
resins, and finishing materials required for their production. Using
wood from certified sources can
reduce environmental impacts associated with growing and harvesting wood
fibers as well as improve social and community aspects related to local
harvesting, production, and manufacturing of finished products.
The insulation used in roof systems also has a significant impact on the environment. In general, cellulose insulation is the most environmentally benign type of insulation with the lowest global warming potential, the least embodied energy, and the least emissions to air and water. Insulation types with the greatest environmental impacts include mineral fiber batts (also known as rock wool), polyisocyanurate, and extruded polystyrene, none of which are frequently used to insulate residential roofs. (For more on insulation types see the page on insulation.) However, all insulation types will save many times more energy over their life than was used in their manufacture.
Options and Analysis
| alternatives | cost/sf of building footprint (materials + labor) | global warming potential (lbs of CO2/sf building footprint) | additional LCA measures (per sf of building footprint) |
IAQ | practice | |
| roof truss | $6.68 | 6.86 | energy consumption (Btu) | 67,603 | typical | standard |
| solid waste (lbs) | 1.75 | |||||
| air pollution index | 1.00 | |||||
| water pollution index | 0.002 | |||||
| I-Joist | $6.70 | 5.94 | energy consumption (Btu) | 60,814 | typical | standard |
| solid waste (lbs) | 1.71 | |||||
| air pollution index | 0.86 | |||||
| water pollution index | 0.005 | |||||
| SIP | $16.27 | 7.95 | energy consumption (Btu) | 90,723 | better | requires training |
| solid waste (lbs) | 0.53 | |||||
| air pollution index | 1.59 | |||||
| water pollution index | 0.007 | |||||
| SEP | $13.10 | 13.71 | energy consumption (Btu) | 157,908 | better | emerging technology |
| solid waste (lbs) | 1.34 | |||||
| air pollution index | 2.59 | |||||
| water pollution index | 0.006 | |||||
Costs based on Means CostWorks 2007, unless otherwise noted. LCA measures and global warming potential modeled using Athena EIE 3.03
Cost
Costs associated with roof framing are roughly 6% of the overall house budget. Roof truss and I-joist construction fit within this general trend. Material costs for I-joists are less, but may be absorbed by additional labor costs. SIP and SEP roofs generally cost more than twice as much as standard truss roofs. However, because framing costs are only a small portion of the overall house budget, construction with SIP or SEP systems typically increases total house cost by only 2%.Construction with roof trusses, SIP, and SEP require a crane for setting members, which can lend further economic benefit to the use of I-joists in some cases.
Energy
SIP and SEP construction offer superior control of air infiltration, which can account for 20% - 30% of home heating costs.1 Homes built with these construction systems consistently outperform stick construction, in terms of energy efficiency and comfort. (For a discussion on envelope construction systems and energy efficiency please refer to the wall page.) If using more typical truss-based roofing systems, raised heel trusses are required to ensure the full depth of attic insulation to the outer edge of the roof.Convection Currents in Fiberglass Insulation
Historically, roof systems with ventilated attics and low-density, blown-in fiberglass insulation have been at risk for reduced R-values due to air convection currents that accelerate heat loss through the insulation (Convection currents are not an issue with higher-density fiberglass batts.) The impact of the convection currents depends on numerous factors, including the temperature of the attic air, the density of the blown-in fiberglass insulation, and the size of the fiberglass fibers. With some types of low-density fiberglass, convection currents can begin to significantly decrease the R-value of the insulation, beginning around 30° F or less. However, Minnesota energy code now requires that, despite a potential drop in performance, the insulation must still meet the energy code minimum R-value at design conditions (typically R-38 at -13° F). The installer can do this in a variety of ways, including blowing the fiberglass at higher densities, using different fiberglass formulations with thicker fibers, or simply installing more insulation.Settling in Cellulose Insulation
While blown-in cellulose insulation does not suffer from convection currents, it is at risk from significant settling, which can reduce the R-value of the installation over time. However, like fiberglass, cellulose insulation is required to meet the same energy code minimum R-values, despite the potential drop in performance. To account for settling, cellulose insulation is now installed to greater thicknesses. This way, the R-value of the installation will still meet energy code requirements even after settling has occurred.Durability
Protection against both water penetration from the exterior and water vapor from the interior improves the life and performance of a roof system. Systems and products that effectively reduce moisture penetration can help eliminate mold growth, ensuring longer-lasting construction and healthier indoor environments. Because of their resistance to moisture intrusion and condensation problems, SEP roofing systems perform very well, increasing durability and improving indoor air quality.Also be sure to select an appropriate roofing material for the type of roof. Many types of asphalt shingles are rated only for "cold" roofs. "Cold" roofs have a ventilated attic space that is not conditioned or a ventilated air space between the insulation and roofing. Generally, truss and I-joist roofs are considered cold roofs because the insulation is placed in the ceiling while the attic above is ventilated with cold outside air. SIP and SEP roofs are generally considered "hot" roofs because there is no ventilated air space between the roofing and the insulation.
Life Cycle Analysis (measures are given per square foot of building footprint)
Global Warming Potential
Typical roofs insulated with fiberglass and constructed with trusses or I-joists result in similar global warming potentials. The use of rigid expanded polystyrene insulation (EPS) in SIP panels increases the global warming potential of a SIP roof, despite a more efficient use of wood. SEP roofs result in substantially higher global warming potentials than other alternatives primarily from the use of two products: 1) a thick petroleum-based water-proofing and air-tight membrane that is applied over the sheathing and 2) rigid extruded polystyrene insulation (XPS) which has significantly higher global warming potential than EPS and fiberglass. (See the insulation page for more information on insulation types.) However, the increased global warming potential of SIP and SEP roofs is recovered over time by the superior energy performance of homes built using these systems.Energy Consumption
Engineered wood products require significant amounts of energy for processing and drying. Rigid insulation products, including EPS and XPS, require significantly more energy to manufacture than batt products. The use of rigid insulation in both SEP and SIP roofing drives up the embodied energy of these systems. However, the increased embodied energy is quickly recovered by the superior energy performance of homes built using SIP or SEP.Solid Waste
The use of pre-fab roof panels in SIP and SEP construction reduces the amount of waste during home construction.Indoor Air Quality
Mold development related to moisture condensation within roof cavities is the primary concern in terms of indoor air quality. Systems that eliminate roof cavities are at a lower risk. These systems include SIP and SEP construction. However, when properly air sealed, conventional systems can also reduce the risk of mold development. Off-gassing from roof structure materials does not significantly impact IAQ because the materials are either outside the air and vapor barrier or encased by finishes.Materials
SIP construction has the minimum overall material use, combining structure and insulation in one panel. SEP roofs use slightly more lumber than truss roofs, due to the fact that both use trusses for structure, but SEP incorporates extra thicknesses of sheathing on top. I-joist roofs use the least lumber of any roofing options. SEP roofs could also be modified to use I-joists for support, rather than trusses, which would reduce lumber consumption. Raft truss roofs use more lumber than any of the options listed above. Raft trusses may prove to be advantageous for roof construction in 1-½ story homes if the bottom chord of the truss can be designed to act as the floor of the upper story. This can reduce costs and minimize material use by eliminating the need for separate floor joists. All the options reviewed use less material than traditional rafter construction utilizing solid 2x8 framing.Future Recycling
Plywood sheathing used for roof deck construction has the most direct reuse potential, primarily because it can be downcycled one more step before being landfilled or used for fuel. OSB sheathing is harder to recycle because there are greater amounts of chemical binders, resins, and glues used to hold the wood chips together. Use of adhered moisture barriers, such as those used in SEP construction, makes recycling or material reuse difficult. Recycling SIP panels poses similar problems because the rigid insulation is adhered to the panels' OSB skins.Practice
Alternative roof systems such as SEP and SIP roofs require training for subcontractors and traders and may include the use of larger equipment (cranes for lifting the panels into place, or additional crew members to handle the panels if cranes are not employed) It is important to train all crew members when using SIP and SEP to ensure quality of construction. Large developments using alternative roof system construction methods will benefit from added efficiencies, as crews carry experience from one unit to the next.
1 1United States. Environmental Protection Agency. "Air Sealing, Building Envelope Improvement." December 2000. EPA 430-F-97-028. Accessed October 2009. www.energystar.gov/ia/new_homes/features/AirSealing1-17-01.pdf
