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The Surprisingly Green Organic Solvent-Based Coating

According to a study by the University of Wisconsin, a developer could have saved more than $300,000, and spared the atmosphere more than 850 tons of carbon dioxide (1,885,500 pounds), had he used an organic solvent-borne air barrier rather than a water-based product.

Why is it then that architects and developers tend to steer clear of solvent-based coatings, whether for environmental, health or cost reasons?

Why is it that when it comes to exterior coatings, the two words solvent and based, when paired, tend to produce shudders nowadays?

And why is it that when you think “green” you automatically rule out the solvent-borne?

Although the knee-jerk reaction when the words solvent and based appear together may be to raise your fingers in the cross to ward off evil, that reaction is premature, for organic solvent-based coatings are not intrinsically bad or harmful.

Some—depending on the scenario—are in fact quite benign, and surprisingly green at that. Some may even—all things considered—be the best choice given the circumstances.

But in order to determine when and where an organic solvent-based coating might be called for, we need to look at the full picture.

What Is Green?

Green is a term that has come to mean environmentally friendly. Not so many years ago, if you told the man in the street that you were building “green” you also had to clarify that you were not talking about exterior color.

Sustainable vs. Green. Lately, however, the designation green has given way to sustainable, especially in the construction industry, as a way to indicate that we mean the building’s—or any of its constituent parts’—environmental impact over its full lifecycle.

Whereas green initially focused on the use of recycled materials, sustainable takes the concept of environmental impact much further.

Carbon Footprint Factors. When it comes to measuring sustainability, the term used most often these days is “carbon footprint.”

What is this footprint?

Loosely, the carbon footprint is a measure of the impact a product or activity has on the environment, and then in particular on the climate.

More specifically, the carbon footprint is the amount of greenhouse gas (GHG) emissions caused by an organization, event or product, which, for simplicity of reporting, is usually expressed in kilos or tons of carbon dioxide (CO2).

To gain the full scope of a product’s carbon footprint—from birth to death, so to speak—what factors must you consider?

Let’s take a look.

Raw Material. Locating, obtaining, mining or otherwise extracting the ingredients for a given product requires energy. The carbon footprint of obtaining the raw material for, say, a moonstone necklace (yes, made from stones collected at the moon) would be astronomical—pun intended.

The ingredient footprint of a wood product—say, a chair—would first entail robbing the Earth of a CO2 inhaler/Oxygen exhaler known as tree; add to that the energy consumed and GHGs emitted by the chain saw felling it and the human holding the saw, plus the energy consumed and GHGs emitted by the truck transporting the tree to the mill, plus the energy consumed and GHGs emitted by the mill cutting the tree into boards.

Transport to manufacturing. These boards now need to be transported to the chair manufacturer. On a good day, this factory is not too far from the forest and mill. On a bad day, the truck will travel across country to reach the chair-maker, consuming far more energy and emitting far more GHGs than truly needed.

Manufacturing. Depending on the efficiency of the manufacturer, the process of producing the product itself will consume little energy, or much, and will emit few or many kilos or tons of GHGs.

Transport. Once produced, the product now has to be transported either directly to a consumer, or—more likely—to a distributor or to a store for sale. All of which, again, consumes energy and emits GHGs.

Storage. In many cases, a product is warehoused for a time, long or short depending on demand. Many products are climate sensitive and require a heated, or cooled, storage environment. To maintain such an environment requires energy, and it will emit more GHGs.

Application/Installation. Some products—not chairs, obviously, but building materials, like paints or air barriers—have to undergo an installation process, which in itself requires energy and emits GHGs.

Paints and coatings take one more step when it comes to carbon footprints, since they must first dry, then cure, all the while not so much consuming energy as emitting GHGs in the form of Volatile Organic Compounds, or VOCs.

Longevity. The expected useful life of a product also impacts its carbon footprint. Something you have to replace weekly produces one much larger than something you only have to replace at turns of centuries.

Reuse. Can you reuse the product in part or in full? Important question. If you can, it certainly lessens the footprint; if you cannot, and if it—let’s hope not—is environmentally dangerous and has to be disposed of in sealed containers shot into space (or buried in the ocean), not a good report card.

All Factors Contribute. The thought to take away from this is that when you determine the green-ness of anything, including coatings, there is much more to it than meets the eye (or nose). All of the above factors contribute to the final footprint and must be taken into consideration for an accurate evaluation of environmental impact.

What Makes One Coating Greener Than Another?

The short answer is: A coating that over its lifecycle produces a smaller carbon footprint than another is the greener of the two.

A longer answer would be: A coating that when you consider everything—raw material extraction and collection, to-factory transport, manufacturing process, to-distributor transport, short- or long-term storage, to-retailer transport, to-building site transport, product application, longevity, and reuse—produces a smaller footprint than another, is the greener coating.

VOCs—A Closer Look

Volatile Organic Compounds is a family of chemicals that tend to react photochemically with Nitrogen oxides and UV light to create ozone (a common GHG).

Ozone is a good thing in the upper atmosphere where it screens harmful UV light, but in our homes and in the city air we breathe, it can be toxic in higher concentration.

VOCs and Ozone. Ozone is also a major contributor to smog. That is why much, if not all, current state VOC legislation is in place to reduce the environmental impact of VOC generated ozone.

California, for one, in an effort to reduce smog and after discovering that large amounts of ozone-creating VOCs were generated by what they termed architectural coatings, began regulating VOCs about 30 years ago. Indeed, architectural coatings became a major target for state legislators, and they have since set strict standards for allowable VOC contents in such coatings.

An important point to keep in mind in this context is that VOCs are currently regulated not because of threat to human health per se, but precisely for their photochemical reactivity, which generates ozone. In other words, VOCs are regulated to minimize the carbon footprint of architectural coatings.

VOCs and Health.VOCs are not unlike sunlight. The sun, under the right conditions and in the right amounts, is beneficial indeed. The same sun in other circumstances and for too long can be deadly.

Some VOCs are very harmful, but not all of them are harmful.

VOCs emitted from solvent-based indoor coatings into unventilated areas (where they can reach concentration of up to 1,000 times that found outdoors) can be outright deadly.

However, when a solvent-based architectural coating is applied outdoors, there is a constant supply of fresh air, which naturally lowers exposure concentrations.

The Occupational Safety and Health Administration limit for xylene exposure—xylene is a common VOC ingredient in solvent-based products—is 100 parts xylene per million parts of workplace air (100 ppm) for 8-hour shifts and 40-hour workweeks.

Similarly, OSHA has limited workers’ exposure to ethylbenzene—another common solvent-based VOC—to an average of 100 ppm for an 8-hour workday with allowed periods of exposure to be short term at 10- to 15-minute intervals during a 40-hour workweek.

Applying a xylene and ethylbenzene based exterior coating and wearing readily available respiratory protection, will keep workers well below the OSHA stipulated limits of exposure and would not pose a hazard to workers.

Water and Winter—Possible Nightmare

Minnesota. February.

Deadline: The water-based air barrier has to be applied, dried and mostly cured by the end of the month.

The tent enclosing the structure (with its own not insignificant carbon footprint) is in place. The heaters (with their substantial carbon footprint) are on and doing their best to keep the building at or above the manufacturer’s recommended 50 degrees for application.

Three days of intense work—and literally hundreds of tons of carbon dioxide released into the atmosphere by the small army of propane heaters—sees the exterior nicely covered.

Day four sees very bad weather. It is so bad, in fact, that winds rupture the tent and in effect freezes the structure. The not-yet-dry water-based coating turns to ice.

Day five sees better weather and the tent repaired.

Days six through 10 see removing the now ruined water-based coat in preparation for reapplication. Now it’s three more days of intense application along with much hope for better weather.

Water and Summer—Son of Possible Nightmare

Louisiana. July.

No rain is in the forecast for the next three days; that should be enough to get the coating on and dry enough to repel water.

Now in order for a water-based thermoplastic paint to form a good film, the water carrier must evaporate as fast as, or preferably faster than, the organic solvent filming-aids in the paint. However, when the air is too humid, water does not evaporate well and the majority of the filming solvent may indeed evaporate first.

Such circumstances will often find the applied film partially cured, water swollen and susceptible to further damage by the elements. In some instances the quality of this film, even when fully dried, is so poor that it cannot fulfill its function as an air barrier, thus leaving complete removal of the applied coating as the only option.

And that’s the one thing the weatherman forgot to mention: The humidity reached near triple digits, and three days of work were indeed wasted.

So, you try again, still no rain in the forecast. But this time the weatherman lied, the rain came before the final coat had dried, and literally washed it off.

Clean up, try again.

Nightmares. Both of these scenarios—winter and summer—are potential construction nightmares. They do not happen every time, but they do take place.

And here’s the rub: An organic solvent-based coating would have worked just fine in either case, obviating rework while producing a significantly smaller carbon footprint, especially in the winter scenario.

Forgiving. The key here is that the evaporation rate of a solvent is not affected by temperature and humidity—whereas the evaporation rate of water is.

That is why solvent-borne products are more forgiving, and why they allow you greater flexibility.

Which Is Greener: Water-Based or Solvent-Based?

Naturally, the knee-jerk answer is water-based. And in many circumstances, that is no doubt the correct answer.

However, during the winter months in our Northern states, when the full picture is taken into account—even without windy mishaps, there is little doubt that the greenhouse effect of solvent-based paints and coatings is considerably less than that of water-based paints and coatings, which now require tenting and heating for their application and drying.

This conclusion was recently confirmed by a 2008 University of Wisconsin study, which, in part, determined that during the project in question—according to estimates by Ryan Laube, a project manager from Hanson Masonry and Concrete, LLC—1,900,000 pounds of carbon dioxide was released into the atmosphere from burning propane to heat the project. That, by the way, is the amount of carbon dioxide emitted by 112 cars in a year.

Compare this to the estimated 14,500 pounds—roughly the equivalent of only one car in a year—of VOCs that would have been released by a solvent based coating, had it been used instead. This shows a ratio higher than 100:1 in favor of a solvent-based product.

The study concludes: “The choice is solvent based coatings.”

When and Where Is Which Coating Greener? When all carbon footprint factors are considered, there is undeniably a time and a place for the solvent-based coating. It may not be prevalent, but it will occur often enough to warrant serious consideration if your aim is to build as greenly as possible.

Given all carbon footprint factors, the following grid offers a proposed overview of when to use what product for exterior coating:

Water vs. Solvent—An Ethical Equation

Assuming that you care for the environment, and assuming, too, that you care both about cost and about the health of those who will do the actual work: What coating should you choose in a given scenario, water-based or organic solvent-based?

Let’s examine the three things you care about: cost, environment and health

Cost. The cost may not be an issue if you are in a climate/season where water-based is a given; in fact, the solvent-based product might cost you a little more.

However, if you find yourself in the <50°F predicament—especially in a below-freezing Northern winter—the costs involved in deploying a water-based product stack heavily in favor of an organic solvent-based application.

In fact, the study performed by the University of Wisconsin noted the following:

According to Laube, the actual water-based coating costs $0.25 less per square foot than its solvent-based equivalent.

However, whereas the water-based coating required the surface to be heated prior to spraying, a solvent-based coating would not have. This meant additional money for scaffolding, tenting, labor and heating in order to apply the water-based coating during the Midwest winter months of November through March.

The bottom line is that a solvent-based coating, in this scenario, would have cost approximately $300,000 less than did the water-based coating.

In above 50°F climates that threaten rain or suffer above 80 percent relative humidity, the case is not so clear-cut. If you luck out and the rain does not arrive to wash out your water-based coat, you’re ahead; should, however, inclement weather force you to reapply one or more coats, you are definitely spending more money on time and material than you would have with a solvent-based, less rain-sensitive product.

In an above 80 percent RH climate, you again take your chances. Should the water-based coating begin to cure before all of the water-vehicle has fully evaporated, you stand to remove the now ruined coat and reapply—again, at a cost far above using a solvent-based product begin with.

Environment. In a climate/season that sees dry weather with a temperature above 50°F and a relative humidity below 80 percent, choose the water-based product. To be sure, this will be your scenario a large majority of the time.

During the Northern winter, however, the carbon footprint of the water-based solution may in fact be more than 100 times larger than the solvent-based product. The green way is going solvent.

As mentioned, in an above 50°F rainy scenario, or in an above 80 percent RH climate, you take your chances. Should the water-based coat work fine the first time, it would be the greener choice. However, if you have to re-do the work for any reason, the energy spent on removal and re-application will quickly eat up any carbon footprint advantage of the water-based product and may soon favor the solvent-based one.

Health. Seeing as we are only considering solvent-based products for exterior coatings—and with the effective respiratory protection available today, the impact on worker health is negligible either way.


When it comes to choosing an exterior coating, whether paint or air barrier, the knee-jerk reaction says water-based. And for good reason, it is usually the best solution.

But not always. And that is the lesson to take away from this: You cannot afford to jump to this conclusion. Instead, you should objectively consider all factors as outlined above, and base your ethical choice on those.

And so, may your project be green indeed.

Coeur d’Alene, Idaho–based Ulf Wolf writes for the construction industry as Words & Images.

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