You may have heard about cool roof coatings or cool roof paint as a way to significantly reduce the temperature of your container.
What are these coatings exactly, how do they work, and how do they differ from regular paint?
As you may already know, one of the challenges of living or working inside a container home is maintaining a comfortable interior temperature. For many geographic locations, the biggest challenge is keeping the container cool for most of the year.
In other articles, we discussed how air conditioning, ventilation, and insulation are ways to reduce the temperature of a container home. However, there is another method worth discussing: cool roof coatings.
These coatings allow you to economically reduce the surface temperature of your container by dozens of degrees and take only a few hours to install. And we are not just talking about cool roofs.
Energy Star, the US Green Building Council’s LEED certification program, and California’s Title 24 Building Energy Standards all reference cool roofs as a way to save energy.
A cool roof coating is more applicable when the top of your container is your roof. But for those who choose to build a roof over their container, depending on the design, a cool roof coating may not be necessary.
If there is an air space beneath your roof that is open to the environment, you will see a small improvement compared to a cool roof coating. But if your roof forms a closed attic space over your container, a cool roof coating may still be relevant.
A cool roof coating may look like paint, but it actually has specific ingredients with properties that make it very effective at reflecting and emitting thermal radiation. For more information on what that means and how it benefits you and your container home, keep reading!
What is a cool roof coating?
A cool roof coating has several similarities to paint:
- It can usually be applied in the same way, with rollers, brushes, or a spray rig (depending on the product type)
- It comes in a liquid form in a container similar to what you’d use to buy paint.
- Once applied, it looks more or less the same.
However, there are several important differences between a cool roof coating and paint:
- While it also has an aesthetic purpose, the primary goal is not appearance, but thermal performance.
- It is much thicker and better able to withstand natural and artificial wear.
- It contains special pigments designed to reflect and emit specific wavelengths of electromagnetic radiation (specifically, thermal radiation).
The last bullet is the most important one, as you may have guessed! While there are other cool roof products, such as tiles, we’re focusing here only on liquid-applied coatings.
How does a cool roof coating work?
Like insulation, a cool roof coating is passive, meaning it doesn’t require any additional energy to work. Compare this to something like air conditioning, which is active and requires the addition of energy in the form of electricity to function.
All other things being equal, maximizing passive cooling is an excellent way to be more eco-friendly and save money. We talked about how thermal energy is a type of non-ionizing radiation in our article on heat transfer.
In summary, the sun is sending energy (called solar radiation) at a variety of wavelengths, some of which correspond to light, some of which correspond to heat, and others to other things.
Cool roof coatings target the wavelengths that transmit heat, which we’ll call thermal energy. The coatings are specially formulated to reflect and emit thermal energy, and little attention is paid to their effect on other types of radiation (such as visible light, radio waves, and so on). Before we continue, let’s visualize the topics we’re going to discuss, and then break down each type of energy transfer.
Solar Radiation
As mentioned earlier, this is the energy that comes directly from the sun, also known as Solar Irradiance.
Reflected Radiation
- Incoming: every object around you reflects radiation from other objects and the sun, and some of that is reflected towards your container.
- Outgoing: a cool roof coating has a performance measure called reflectivity or reflectance. It tells us how much incoming energy can be reflected back, like a mirror, but for heat. The majority of the benefit of a cool roof comes from its enhanced reflectivity.
Emitted Radiation
- Incoming: every object around you emits radiation, and some of that is emitted towards your container.
- Outgoing: a cool roof coating has a performance measure called emissivity. It tells us how much internal thermal energy absorbed can be emitted to the environment. For example, if you were in a dark room, sitting in front of a hot box, you would feel more of the heat from that box radiated towards you if the box were black instead of white because black objects tend to have higher emissivities.
Note: the Solar Reflectance Index is a number you may also see. It is a composite measurement calculated by an algorithm that includes both the reflectivity and emissivity of a material.
Convection
- Incoming: at certain times in certain geographic areas, hotter air can touch the container’s coating and transfer heat to it.
- Outgoing: at other times and places, the container’s coating can transfer heat to cooler air.
Conduction
- Incoming: heat from the container’s coating flows into its insulation (and eventually the container’s interior) if the interior is cooler than the exterior.
- Outgoing: heat from the interior flows into its insulation (and later, into the container’s coating) if the interior is warmer than the exterior.
Absorbed Heat
- When we take the incoming energy directly from the sun plus the reflected and emitted energy from other objects, then subtract the reflected and emitted energy from the container’s coating, and also subtract (or add, depending on the direction of heat flow) the heat conducted out of or through the insulation, we are left with the absorbed energy or heat.
- The absorbed heat is directly related to the increase in temperature of the container.
When not to use a cool roof coating?
We’ll be straightforward: if your roof is already white or light-colored, clean, and in good shape, any small benefit you might get from a cool roof coating may not be worth the time and cost of adding it. But for everyone else with a darker roof or a dirty and peeling roof, cool roof coatings may be a worthwhile idea to consider.
In addition, it’s pretty logical to think that if you live in a place where it’s cold most of the year, a cool roof coating may not be for you. Instead, you want to maximize the heat absorption of your containers so that you can minimize the additional heat needed (via heaters, stoves, etc.) to keep it warm.
However, there are other factors at play that can minimize the negative impact a cool roof coating would have on your heating bill.
First, the sun is at a lower angle of incidence in winter, which means it has significantly less radiant energy than in summer. It also means that more sun is hitting trees, buildings, and sides of your container instead of directly falling on your roof.
Second, if you live in an area with snow, the accumulated snow will make the performance of your roof coating irrelevant, as it will be covered. If you live in a mixed climate, the decision of whether a cool roof coating is right for you is more nuanced.
It may be helpful to determine the amount of heating days versus cooling degree days (HDD vs. CDD) that apply to your location as you analyze the decision.
Degree days measure how cold (HDD) or hot (CDD) a location is by comparing the average outside temperatures recorded for a location to a standard or base temperature, usually around 65 ° F (18.3 ° C). For example, if the average temperature at your location on a particular day was 80 ° F, that day would equate to 15 CDD.
More extreme outside temperatures mean more degree days, which in turn means a higher level of energy use for heating and cooling.
Should you use a cool roof coating on the side walls of your container?
Depending on your latitude and the shade your container home has at different times of the day, using a cool roof coating on the walls may also have a measurable impact. However, any potential benefit (if there is one) is quite unique to each home and construction site.
The more direct sunlight a particular wall receives, and the closer the angle of incidence is to 90 degrees, the greater the impact it will have. Regardless of the potential benefit, you should certainly consider aesthetics. A coating on the roof may not be noticeable, but a coating on the walls will be obvious.
Therefore, you may be able to get by with a white coating on the roof, but you may need a colored coating on the walls. Fortunately, cool coatings are also available in many colors, but before paying a premium, make sure you have enough sun exposure for it to be worthwhile.
Why not use white paint instead of a cool roof coating?
As we just mentioned, cool roof coatings don’t have to be white. Due to their use of special pigments and the fact that they primarily deal with radiation outside the visible light wavelength, cool roofs can be many different colors.
However, lighter colors perform better than darker colors under equal conditions. Another thing to consider is the visible reflection of ordinary white paint. If your home is multi-level or if you have multi-level neighbors nearby, strong visible reflections may occur at certain times of the day onto the second and third-floor windows, which can be quite problematic.
Using a non-white cool roof coating can provide some of the performance benefits with less of the annoying reflections. Finally, regular paint is thinner and less weather-resistant than a cool roof coating, which is certainly something to consider.
A roof is an extreme environment with one of the harshest exposures to the elements, so a thick, durable, and specially designed coating is generally a better choice. And let’s be honest, getting up on your roof to paint (or paying someone else to do it for you) is not something you want to do more often than you have to.
How to save money on cool roof coatings?
Cool roofs offer you the opportunity to save money in two ways:
- Under the right circumstances, the cool roof effect could be enough for you to install a smaller air conditioning unit than you would otherwise need. However, knowing when you have the right circumstances to do so is quite complicated. Experienced air conditioning companies have access to expensive software that can model the AC requirements for your home and tell you what size unit you need, but this is really beyond the scope of a DIY project that isn’t a trial-and-error experiment.
- Using the same air conditioning unit, with a cool roof, the unit may end up needing to run less frequently to keep your home at the same temperature, saving you money on your electric bill. However, remember that for humid areas, you may need to run your AC to keep the indoor relative humidity low, which could somewhat negate the benefit.
This calculator (only applicable to US locations) can help quantify the financial impact of a cool roof. To help offset the initial cost of adding a cool roof, many utility companies have started offering rebates to homeowners.
Some are listed here, but we recommend also contacting your local utility providers and checking if they offer incentives.
How can I know which cool roof coatings to buy?
The performance of cool roof coatings is driven by the criteria we discussed earlier: Reflectivity and Emittance, sometimes combined into a measure known as Solar Reflectance Index. Any reputable manufacturer should be forthcoming about sharing these numbers for their product.
You also have a choice of coating types. The two main options are acrylic and silicone. While products from both materials have similar thermal performance, they differ in several other important ways:
Acrylic (sometimes called ‘elastomeric’) cool roof coatings
- Physical properties: water-based (easy to clean up with water), highly durable and resistant to dirt buildup, but can lose thickness due to weathering over time
- Application: weather-sensitive at the time of application to achieve proper adhesion and requires multiple layers to achieve the required material thickness
- Ponding: cannot withstand ponding water (typically defined as standing water for more than 48 hours) and will eventually delaminate and peel off the metal roof below
Silicone cool roof coatings
- Physical properties: Not water-based (cleaning requires a solvent such as mineral spirits), and typically includes volatile organic compounds (VOCs). Less durable. More prone to collecting dirt and grime, and can sometimes turn slightly yellow in the sun over time (all of which reduce thermal performance).
- Application: Can typically be applied in a single layer at once, but is a bit more sticky and messy. Known to ‘set up’ on humid days while sitting in an open bucket prior to application, so it is also weather-sensitive at the time of application.
- Ponding: While not completely impermeable, it is notably more water-resistant than acrylic products and is more capable of resisting water accumulation or ponding.
What we recommend
In summary, acrylic is better than silicone in most cases except for handling ponding water. Ultimately, a container roof is a flat roof, so ponding water can be a concern. Pour some water on your container roof and see how it drains, especially if it is a used container and may have some dents in the roof. You may need to use a hydraulic jack or a hammer to push out some of the dents. Due to the narrow width of containers, even if some accumulation did occur, it is expected to not be much. For applications where the roof is fairly flat and uniform, and there is no substantial accumulation in your water test, a high-quality acrylic cool roof coating should work well.
If you have multiple containers joined together and it doesn’t drain between the containers, you have effectively made your roof much wider and more prone to ponding. For these situations, or in environments with heavy rain, you may want to consider a silicone cool roof coating just to be safe.
Either way, we recommend using a premium cool roof coating from an established manufacturer and avoiding unknown brands. Henry Enviro-White is an excellent choice for an acrylic product. If you decide you need a silicone cool roof coating, Henry Tropi-Cool or something similar would be a good choice.
Does ceramic bead roof coatings offer higher performance?
If you do a little research on cool roof coatings, you’ll start to see a number of products advertising the addition of small ceramic spheres or beads in the coating. Supposedly, the beads work to increase the reflectivity of the coating while reducing absorbed heat through less conduction.
Like with other categories of building products, there are some dishonest or unknown suppliers trying to sell you products that don’t work as advertised (we’re looking at you, bubble wrap with aluminum face). A study by researchers at the Oak Ridge National Laboratory found that ceramic beads basically did not offer any improvements in reflectivity or durability.
In fact, the highest-performing product in the test didn’t have ceramic beads. Based on the results of the study, we do not recommend paying extra for a product with ceramic beads.
Conclusion
As you can see, a cool roof coating is definitely an option you should consider when planning your container home project, or even contemplating an upgrade to an existing container home.
With just a little bit of money and a few hours of work, you can significantly lower the external temperature of your container. Exactly how this can correlate to a reduction in utility bills depends on a variety of factors such as your geography, house design, amount of insulation, etc. However, we encourage all current and potential container homeowners to consider at least a cool roof coating for their project.
