Green Roofs and the Price of Oil

March 12, 2008

The problem with this hot-applied built-up roof was that asphalt weighs about 8.2 pounds per gallon, so the flood coat was equivalent to applying 7.3 gallons of oil, the moppings contributed another 2.4 gallons of oil per ply, and the saturant in each felt layer roughly another gallon. If you are counting, that’s over 22 gallons of oil per roofing square!

The financial world has recently been turned upside down, and news anywhere in the world seems to impact the construction industry. A recent Wall Street Journal article on oil shipments from Venezuela brought back bad memories of the oil embargo of 1973-1974. For those of you too young to remember, there were long lines at gas stations, inflation was rampant, and the Nixon Administration had imposed price controls – a step just short of rationing. In the years following the embargo, the United States and (and the world) seemed committed to not having this happen again, and this led to increased exploration of fuel sources, interest in alternative energy, and, of course, energy conservation.

The Roofing Industry’s Impact on Energy Consumption
In the years preceding 1973, typical commercial roof construction used very little thermal insulation. One (1) inch of wood fiber, perlite, or 15/16-inch Fiberglas® with thermal resistances (R-value) of roughly 3 (hr/BTU/ft2/h) were used as substrates over most roof decks. The roof membrane itself was typically bituminous in nature, with hot asphalt used to adhere the insulation to noncombustible decks. The membrane usually consisted of four layers of asphalt-saturated roofing felt mopped together with hot asphalt. Surfacing was generally a “flood coat” of about 60 pounds per square (i.e. 100 square feet) of hot asphalt into which aggregate was embedded. Since asphalt was cheap and plentiful, this roof system was dominant even though some single-ply systems were already appearing.

The problem with this hot-applied built-up roof was that asphalt weighs about 8.2 pounds per gallon, so the flood coat was equivalent to applying 7.3 gallons of oil, the moppings contributed another 2.4 gallons of oil per ply, and the saturant in each felt layer roughly another gallon. If you are counting, that’s over 22 gallons of oil per roofing square!

The roof insulations were really no better. From the mining and expansion of perlite or production of glass fibers to the asphalt used in binding or surfacing the boards, and especially the BTUs needed to dry the wet-formed boards and/or to set the binders in glass fiber boards, were not very energy efficient on a cradle-to-grave basis.

Getting Better
Cellular foams now dominate the commercial roofing market. Not only are they more energy efficient in manufacture, but they also provide the high thermal resistances (an R-value of 20-plus) that our energy codes mandate. This transition has had some bumps, but the fire-rated assemblies that use polyisocyanurate (isoboards) and expanded polystyrene are now the accepted norm.

What the Industry is Doing Different this Time
Using even more insulation is certainly a consideration; however, there is an inverse relationship between R- and U-values. According to the U.S. Environmental Protection Agency’s ENERGY STAR® program, R-value is the measure of a material’s ability to resist heat flow, while U-value characterizes the rate of heat loss.

In a recent newsletter from HOLT Architects, calculations were made for upgrading a building under construction at Ithaca College, Ithaca, NY, from 4-inch isoboard with an R-value of 25 to a 6-inch isoboard with an R-value of 36. HOLT found that the upgrade would save only $31 per year (for a 58,000-square-foot building), but carried an additional construction cost of $14,000. This certainly reflects the law of diminishing returns (and would hardly qualify as a green roof system).

Getting Back to Venezuela
Prior to 1973, the physical and chemical properties of asphalt (in roofing) were very consistent. This is because asphalt is a bulky product, so the closest refinery was generally the supplier, and their source of petroleum crude usually came from a single source.

When the embargo began, this situation suddenly turned upside down. Refiners obtained whatever petroleum they could, but with asphalt being a natural product, each crude can vary in its ratios of oil, resin, and asphaltenes.
Consider an asphalt shingle plant back in 1973 that unloaded tank cars full of molten asphalt each and every day for saturating and coating shingle felt. What if the material just received was different enough to affect the quality of the shingles? They would be out the door in a day and probably installed on a roof in a week to a month.

What about modified-bitumen systems, where compatibility between polymer and asphalt were especially critical in forming the polymer network?

The Asphalt Roofing Manufacturers Association put into place a three-step research program to address this variability. First, a rapid test was needed to determine if the material just received was identical to yesterday’s material. Secondly, if it was different, could we predict if the weathering would be better or worse? And, lastly, if it was worse, how could we modify the material so that it would perform for its expected life?

The successful part of this story was that these goals were met; however, with our supplies having been relatively consistent since the 1970s, we have again become complacent. For those of us who are not experts in asphalt chemistry, Venezuelan crudes are considered some of the best weathering materials (compared to Alaskan material, for example).

For producing modified bitumens, it’s possible to vary the amounts and types of polymers and oils to obtain the needed polymer dispersions, so this, too, may need to become a day-by-day process.

As China and other emerging nations become major petroleum consumers, availability of suitable asphalts for the roofing industry will become a concern again. Biodiesel and Gasohol are not candidates for asphalt substitution, although they may free up some of the demand for asphalt. One would hope that tar sands and oil shale may prove to be suitable for roofing use as well.

Vegetated roofs may also become part of the petroleum solution. It has long been recognized that protected membrane roofs (PMRs) – roof systems with thermal insulation on top of the roof membrane – contribute to long life. Tests have revealed that asphalt’s principal enemies are exposure to ultraviolet energy, oxygen, and ozone, plus thermal variations. In a PMR or vegetated configuration, none of these agents of aging is present. Today’s vegetated roofs may bring a revitalization of PMRs, with the potential to double the durability of a bituminous roofing system.

Using the roof to host photovoltaics seems to be another opportunity for energy conservation. Hopefully, these systems will prove to be durable, resistant to impact damage, and less complex than the solar collectors of the 1970s. While liquid systems have proven economical in some countries (at least to provide hot water), they lack the durability and flexibility needed for whole-building heating and/or cooling.

We need to reduce our reliance on petroleum for many reasons, but the special needs of the roofing industry require us to find how to reduce our dependence on it.

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