Greening Your Roof
Trying to green or sustain any building component these days is like trying to watch a moving target, and roofs are a prime example. In this column, I’ll take a look at where we are (at least at this particular moment) on the path toward more environmentally friendly alternatives to design and roofing materials.
The first design consideration that relates to the greening of roofs is water runoff. We want to keep as much water on the site as possible to improve water levels in the aquifer, but we also need to keep water on site in such a way as to prevent damage to the residential structure itself. I have troubleshot many roofs whose design permitted several hundred ft2 of roof to carry rainwater or snow melt to only 2–4 ft of gutter.This often creates serious ice backup problems in cold climates and rain backup and/or spillage in most climates when 1 or more inches of rain come down in just a brief time. Some architects and builders have designed homes with no gutter system, but with an effective drainage system around the foundation, which eliminates most ice and rain backup problems. If gutters are not kept clear of dead leaves and twigs in a wooded area, the water that pools in them can be a breeding ground for mosquitoes.
A further design consideration is the overhang on the roof. Overhangs are important to help reduce ice backup into the living spaces in snowy climates, and to reduce heat gain on southern exposures from the sun in the summer. Here in Ohio, roofs with no overhangs are notorious for ice backup problems. Finally, the orientation of a roof is an important design consideration if the home is to have solar panels, either to heat water or to produce electricity. If at least one-third to one-half of the roof surface can have a southern exposure, it makes it much easier to install solar panels. Much headway has been made in the last few years in defining the critical roof issues from a green building perspective, and in finding new materials and systems to address them.
What is a green surface material for roofs?Well, the product most commonly used on residential roofs these days—asphalt shingles—is not very green! Asphalt shingles are produced from a nonrenewable resource (oil), and given the diminishing supply of that resource, it is fair to say that there are many questions about the future of asphalt shingles. While old asphalt shingles can sometimes be recycled for use in hot-mix asphalt paving, most shingles end up in landfills because of the debris attached to them. Furthermore, virgin natural resources are required for the production of new shingles. Heavier asphalt shingles have a longer useful life, but this is still only a short-term solution.We must find an alternative that is more environmentally friendly and affordable.
Clay tile (used largely in the West), slate, and metal roofing are the obvious alternatives, but each is considerably more expensive than asphalt. Metal roofing is fully recyclable and requires considerably less energy to recycle than it did to make it in the first place. All three have long life cycles, which is a plus, and all but metal present no significant environmental debits. (CO2 and other pollutants are produced during the manufacture of metal materials.) A variety of new roofing materials made of postconsumer recycled content, such as rubber, plastic, and wood fiber, are now being produced.These are about half as costly as tile, slate, or metal and have a good, though short, track record to date. One plus is the ease of working with these recycled roofing materials, and another is the reduced damage incurred when installing or walking on them. They are clearly a step in the right direction because they can be recycled at the end of their useful life.
There is a lot of interest in roof gardens these days, though most roof gardens are still found on commercial buildings. Sloped roofs, common in residential structures, don’t lend themselves easily to this treatment. Adding flat roofs, which can be notoriously leaky, to homes may not be a fair tradeoff for the relatively small benefits gained by installing a roof garden.Also, roof gardens require considerable maintenance, which not every homeowner will wish to provide.
Using lighter-colored roof materials, such as asphalt shingles coated with white granules, especially in cooling climates, has clearly reduced cooling costs in residential structures.White roof coatings (elastomeric polymers) have also proven effective in reducing heat gain in row houses with flat roofs in inner cities. A lighter color also helps to extend the life of the roof, since some of the sun’s degrading rays are reflected away.Also of interest is a new generation of colored cool-roof surface (see “Cool-Colored Roofs,” HE March/April ’06, p. 12), which many homeowners prefer. Researchers and manufacturers have developed dark-colored pigments that reflect sunlight instead of absorbing it. These coatings are being created for metal and other surfaces as well as asphalt.
How is a green roof framed? More than 70% of residential roofs are framed with trusses these days, and that is a green plus. Large trees are required to produce 2 x 10s and 2 x 12s, while much smaller stock is required to produce trusses. In most cases trusses can be installed 2 ft on center, which also helps to reduce the amount of wood required. Roof sheathing today is mostly made of oriented strand board (OSB), an engineered wood made from chopped wood fiber.This means that no large trees are required to produce it.
Structural panels that are strong enough to span the distance from the sidewalls to the ridge of the roof are being used more and more often.This means that there is less of a need to timber frame the roof, which translates to a significant reduction in largedimension lumber. Structural insulated panels (SIPs) are one example. In my last column (Jan/Feb ’06, p. 12), I mentioned that steel stud/foam panels are being used more often in homes, though this poses environmental problems: The foam is produced from oil, a nonrenewable resource, and HCFCs are used as blowing agents. The good news is that steel is 100% recyclable; also, more and more renewable resources, including soybeans, are being substituted for oil to make the foam.
How do we insulate a green roof? Green building advocates want a roof that achieves the optimal level of insulation for the local climate, so that there is minimal heat loss in winter and heat gain in the summer. In the past we have insulated attics.With the emergence of panelized roofs, spray foam insulation, and foam panel insulation, it has become more common to insulate the roof surface itself (from either above the sheathing or below it). In cooling climates, the discovery that radiant barriers applied just inside the roof sheathing can reduce radiant heat gain into the attic space has dramatically changed the way we think; reducing heat gain in attic spaces can also reduce cooling costs for the living space. As we find new ways to insulate our roofs, we are also finding new ways to ventilate them.
Do we need to ventilate a roof to be green? For years, we would have answered with a definite yes, but according to studies conducted by the Building Science Corporation, ventilating a roof is no longer always necessary. We have learned, even in retrofit situations, to seal the thermal bypasses between the living space and the attic space. In so doing,we have nearly eliminated one of the primary reasons for venting an attic or roof space: moisture entering from the interior. I have investigated many homes over the years where mold was growing only on the sheathing in the rafter cavities where soffit vents were installed. In other words, vents were pulling in moist exterior air that was condensing on the sheathing (very likely when nightsky radiation was making the roof sheathing colder). I have seen many roofs where the snow-covered ridge vents did not function until the snow melted. Attic and roof venting must be addressed on a case-bycase basis. Venting can cause as many problems as it solves if we are not careful.
It is essential that materials and systems be durable in green building, so installing moisture barriers (including rain screens, on sidewalls) under the finish roofing is critical. It is also vital to install flashing on all interface points between the various components of the roof, and between the components of the roof and the walls of the structure.
Fortunately, homeowners are starting to understand and appreciate these issues, and are requiring those of us in the building industry to do better.When the marketplace finally demands that we improve,we had better be ready to respond thoughtfully, in a manner that reduces energy costs, improves the longevity of materials and systems, and has the least impact on the environment over the life of the roof.
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