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Calculating Cool Roof Energy Savings

Posted on September 20, 2016March 29, 2022 by Candy McNamee

Whether it’s providing waterproofing, reducing thermal expansion and contraction, or supplying chemical and damage protection, cool metal roofing has much to offer. Of course, the most substantial benefit is the energy savings gleaned from reduced rooftop heat levels driving down air conditioning loads. In fact, the Lawrence Berkeley National Laboratory’s heat island group projects a whopping $1 billion reduction in cooling costs if cool roofs were to be implemented on a nationwide basis.

To assist architects in determining the kinds of energy savings that can be expected from cool metal roofing, the Oak Ridge National Laboratory (ORNL) has parlayed the data it gathered from a three-year evaluation of metal roofing products into a whole building energy savings calculator.

In addition to energy efficiency, cool metal roofs are known for extended durability and longevity.

Cool Roof Calculator

This calculator is called, simply enough, the Cool Roof Calculator. The easy-to-use tool is described as a quick way to compare overall energy costs and savings for a variety of roof and building conditions. Unlike some energy modeling calculators, which are limited to steep slope residential roofs with attics, ORNL’s tool models the typical low slope commercial roof with insulation placed directly over the deck and under the roofing membrane.

To calculate approximate energy savings offered by a cool metal roof, architects are instructed to input the building’s location, proposed roof R-value, roof reflectance and emittance, base energy costs, equipment efficiencies, electrical demand charges and duration.

While experts suggest that it may be difficult to accurately predict the base use and peak demand without detailed construction and cost information, tools such as the ORNL’s cool roof calculator can be a useful way to gather helpful performance estimations for a variety of building types and locations.

Attempting to do just that, the calculator outputs a number of values to offer an approximate estimate of potential energy savings, broken down into cooling energy savings—a calculation of air conditioning savings from base use and peak demand reductions—and cooling season demand savings, an estimate of the peak demand charge reduction enabled by enhanced roof reflectivity.

Accessible at http://rsc.ornl.gov, users can also compare the energy performance offered by a cool roof vs. a conventional black roof.

“It’s a nice tool to give people a feel for where a cool roof would actually help them and have the greatest impact in terms of energy use,” relates Robert A. Zabcik, PE, LEED AP BD+C, director, research and development, NCI Group Inc., Houston, in a Metal Construction News article.

Roof Reflectance Baseline

Roof reflectance and emittance, requirements and options, can be found in energy codes such as IECC, ASHRAE 90.1, California Title 24, and other local codes. Requirements may vary based on roof slope and climate zone, and may allow for either aged or initial solar reflectance, thermal emittance and/or SRI.

Fortunately, MBCI continues to stay current with individual testing and also maintains third-party tested and verified product listings through entities such as the Cool Roof Rating Council, and the U.S. EPA’s ENERGY STAR®.

Rooftop Solar Energy

Posted on August 18, 2015 by Brad Johnson

The “Sustainability begets resilience” blog ended with a nod to rooftop energy production. So, how will you respond when, not if, a building owner asks you about rooftop solar energy? An appropriate and accurate answer is, “The combination of a metal roof and solar energy is a recipe for a long-term, high-performance roof system,” or something like that. The fact is a metal panel roof is an ideal substrate for a solar energy system.

Installation Methods

Solar energy is the broad term for two sub-categories: photovoltaic (PV) systems (electricity) and solar thermal (hot water) systems. Besides the obvious differences, the rooftop attachment concepts for both systems are quite similar. PV panels and solar thermal panels are commonly rigid with metal frames. Attachment to metal roofing panels can be direct or include rails. Both methods use a customized clip that attaches to the metal roofing panel seam; then, metal-framed PV panels or rails are attached. The need for rails (think “purlins”) depends on the seam spacing and layout of the roof panels relative to the size and layout of the PV or solar thermal panels. Overall roof slope matters, too. Directly attached solar energy systems match the slope of the roof, which is not necessarily the optimum slope for energy production.

Structural & Performance Requirements

Other considerations include the structural load, fire resistance, wind resistance and the use of code-approved materials and components. A solar energy system adds weight to the roof. Does the structure need updating to carry the gravity load as well as any increased wind uplift loads? Adding panels to the roof will increase the sliding load (i.e., drag load) on the clips holding the roof panels to the substructure. And let’s not forget about the potential for snow retention or increased snowdrifts that will add weight.

Fire and wind resistance should be discussed with the manufacturer or designer of the PV or solar thermal system. Fire and wind design are incredibly important, and there are very specific code requirements to meet.

Layout Considerations

Rooftop layout of solar systems, especially PV, should not block drainage or impede roof maintenance. Also, clearance at roof perimeters and access to critical roof areas (e.g., drains, rooftop units) is necessary. Last but certainly not least, check with the metal panel roof system manufacturer about warranty issues regarding a rooftop solar energy installation.

While there are many things to consider when installing solar energy systems on roofs, the long service life of metal panels and the ease of installation certainly make metal roofs and solar energy a great combination!

When It Comes to Roofing Expertise, It Doesn’t Hurt to Diversify

Posted on May 5, 2015 by Amy Crenan

As is often the case when it comes to your investments, it’s always a good idea to diversify. This also applies to the investment of your construction expertise as a roofing contractor. Even with the mild uptick in new construction activity of late, contractors are smart to explore the additional revenue stream that can come from roof renovations and retrofits.

Most metal roof retrofit work entails adding slope to an existing flat- or low-sloped roof. According to a 2013 article in Metal Construction News, about 25 percent of U.S. commercial, institutional and public buildings are 55 years old or older and consist of flat-roof stock that has reached the end of its service life. Two years later, that percentage is surely higher.

To transition from a flat roof to a sloped roof is a good move, because it will result in lower energy and maintenance costs for years to come. It is also environmentally smart, because metal is one of the most recycled materials used in construction, and metal roofing is 100 percent recyclable at the end of its service life. A metal roofing system provides for additional insulation, as well as the installation of solar panels that reduce reliance on electricity. And in most circumstances, a new metal roof can be installed without having to remove the existing flat roof. A metal retrofit may carry a higher initial cost, but when total life-cycle cost is considered, a metal retrofit will end up being the lowest cost alternative.

A large number of buildings with flat membrane or built-up roofs require a framing system to produce an adequate slope. But this particular type of retrofit can be challenging. In general, the retrofit market is more specialized and much more technical than what roofing contractors are likely used to in the existing metal building market. At the same time, the retrofit market can be very profitable and is worth getting up to speed on.

Whether you’re doing a small retrofit project or a complete renovation, MBCI can assist you with developing a preliminary budget, estimating, engineering, as well as providing a complete set of shop drawings for your retrofit project.

Stay tuned for future posts where we’ll provide some guidelines on how to successfully navigate the design process of retrofitting a flat or inadequately sloped built-up or membrane roof. Adding this diversity to your portfolio of roofing skills will likely net a high return on investment.

Solar Roofing: Overcoming Misconceptions

Posted on September 24, 2012 by MBCI

As an engineer who works for a metal roofing company that also sells roof-mounted photovoltaic (PV) equipment, I have the incredible opportunity to help people turn their metal roofs into money making machines. That’s more literal than you think, especially if you have good incentive programs available to you or your electricity costs are high or varying over the course of the day. I’ll leave the environmental green reasons for another blog. I’m talking cold, hard cash here, folks. I’ll be very honest: A PV system is a sizable investment. But they can also have rates of return associated with them that make day-traders salivate.

However, I do find that the vast majority of my time is spent educating potential owners of PV systems on how these systems operate and how that translates into cash flow. Along the way, I have discovered many misconceptions that very smart people have about PV. I have listed them below and this is my attempt to put some of those things to rest. Are you ready? Let’s power on.

1) I have to put holes in my roof to support a PV system

Call me old school if you want, but from where I come, putting a hole in your roof is a bad thing. Fortunately, if you are blessed with a metal standing seam roof, there is a very good chance you can mount a PV system on it with ZERO roof penetrations. Zero, zip, nada, or as the soccer folks say, nil. There are some very good mechanical mounting systems out there and many of them do not require expensive aluminum railing because they attach directly to the roof seam. That’s a huge cost savings but in my opinion, the risk it mitigates is even more important. I may sound like Captain Obvious when I say this, but every time you penetrate a roof, you increase its chances of leaking in the future. This just in: Roof leaks are bad.

2) I need a battery system to work with my PV system

I run into this misconception every day. Unless your building is in a place where your electricity service is questionable or non-existent, batteries are not required or even advisable. Why? They are expensive, maintenance intensive and a power drain. Case-in-point: Your cell phone battery. Granted, they are typically different technologies than PV system batteries but the situation is very similar. If you’ve ever priced them, you know they are expensive. If you’ve ever had one that wouldn’t charge all the way because you haven’t been draining it all the way, you know they are maintenance intensive. And if you ever felt one get very hot as it charged that last 10% or while you were using your GPS, you know they waste power. (That heat energy has to come from somewhere, right?) They also only have about half the life expectancy as a PV system. By contrast, the electrical grid is like the world’s most perfect battery. It costs you nothing (I mean the grid itself, you obviously pay for it indirectly when you buy the electricity), it basically lasts forever and someone else is responsible for maintaining it and fixing it when it does break. But most importantly, when you put power into the grid, you get 100% price credit for that electricity, provided your utility supports net-metering. It’s really a no-brainer.

3) PV Systems only make electricity during the day

OK, this is not a misconception; it’s true. But so what? You don’t use electricity during the day? Your building probably uses more electricity during the day than any other time. And even if that’s not true, if you are a net-producer of electricity and your utility supports net metering, your meter simply runs backwards during this time, offsetting the cost incurred when you are using electricity. All of this with no battery involved. It’s why there are more grid-tied systems being installed now than battery systems and that trend is not likely to change. Furthermore, if you live in California or other places where your electricity rate is higher during the day than it is off-peak, PV systems can really have a huge impact on your bottom line because they are producing the most when demand is the highest.

4) My roof doesn’t face south, so it’s not worthwhile to put PV on it

Au contraire, roof azimuth has less effect than you might think and it is certainly secondary to what your incentive and electricity cost situations are. I won’t go into a deep technical explanation here, but I’ve learned one thing after years of running payback calculations on PV systems: If the money is right, the building is right. You may not have the absolute lowest payback or highest ROI theoretically possible, but the cash flow will still be very favorable. Also, don’t fall into the same thought process because you’ve heard that that the PV modules have to slope the same angle as your latitude. Oh look, that’s our next misconception!

5) My roof doesn’t slope enough to hold a PV system

While it is true that PV systems theoretically produce more electricity when they are pitched at an angle equal to your latitude, much like azimuth, this effect is far less than you think. Obviously, it’s not advisable to mount a PV system on the northern slope of a 12:12 (45 degree) roof. But like most things in life, PV electricity production operates on a sliding scale and the end result is a function of many factors working together. If you focus too much on any single factor, you’re missing the point. (To refresh, the point is to put money in your pocket.) And, THIS IS IMPORTANT, there are other more serious issues with racking and tilting systems that you have to contend with. Keep reading, this is getting serious.

6) I need to have a rack system on my roof to support the PV

Not true. PV systems work very well on flat or near flat surfaces. Besides, if you live in certain parts of the country where heavy snow or high winds are a concern, I’d highly advise you to stay away from rack systems. Think about this: What does a rack system look like to you? Spanish Armada-era sailboats sitting in a harbor, perhaps? Do you REALLY want an air foil on your roof if you live within reach of a hurricane? I’m guessing not. Perhaps even more dangerous is the potential snow accumulation that can happen under a rack-mounted PV system in snow country. This is a situation we engineers call aerodynamic shade and it is a serious concern from a structural standpoint. After alternating heavy snows and freeze-thaw cycles, hundreds of pounds per square foot of snow can gather under and around a roof obstruction and I doubt your building was designed for that. I highly encourage you to call your structural engineer and have this discussion with him or her before you put a rack-mounted PV on your roof. I’m being very serious here; the consequences are severe or even life-threatening.

So, there you have it. There are many more aspects of PV than I can cover in this forum but hopefully, this charges your brain and sheds some light on a subject that is just starting to heat up. Since I’ve obviously blown my pun quota, I’ll have to cut it here. But if you want more information, please visit www.mbci.com/solar.

PV and Metal Roofing: The Perfect Combination

Posted on August 1, 2012 by kbuchinger

In an effort to cut electricity costs and to help the environment by burning fewer fossil fuels, photovoltaic (PV) arrays are becoming very popular in spite of their significant installation costs. To further stimulate this trend, the DOE’s SunShot Initiative has been created to make solar energy costs competitive with other forms of energy by the end of the decade in an effort to drive widespread adoption of this renewable energy technology.

At the same time, in many areas of the country, electric energy providers are struggling to meet the growing energy demand during peak times of use. While they need to be able to provide sufficient capacity during these times, they don’t want to spend billions of dollars to build additional capacity that is only needed for a small portion of weekdays in the summer (and at night in the North during the winter). To help reduce electrical usage during these peak times, smart meters have been introduced and time of use (TOU) electrical rates are now offered, which charge more for electrical use during peak times and less during off peak times. Currently, in most cases, TOU is optional but, in a growing number of areas, is becoming mandatory.

Save Money with a Solar Energy

Obviously, if you are on a TOU plan, you will want to minimize your electrical usage during the peak times. But, another way to save considerable money on your electric bill is to install a solar array on your metal roof. During the summer peak time when the electric company charges you the highest rate for your electricity, your PV array will be generating the most electricity to offset your usage. This helps both you and the electric companies save money – you save on your electric bill, and the electric companies hope to reduce the need for additional generation capacity.

Installing a PV Array

The best place to install a PV array is generally on your roof, be it a home or commercial building. This keeps the equipment out of the way yet reasonably close to the building’s electric meter. So, if this is the best place to install a solar array, will my roof handle it? What do I mean by that? A flexible thin film PV array weighs about ¾ pound per square foot and a glass-based crystalline PV array weighs 3-4 pounds per square foot so your roof must be capable of supporting this additional weight. Your roof should also have minimal shading and a life expectancy equaling or exceeding that of the solar array. Otherwise the roof will have to be replaced one or more times during the approximate 30 year life of the PV array. This is especially true if the existing roof is 5-10 years old when the PV array is installed. Removing and installing the PV array adds a huge additional cost plus to the cost for the roof replacement(s).

Installation of glass-based crystalline on an SSRM

Is Your Roof Solar Ready?

So, what should a person do? Whether you are a homeowner or a commercial building owner, you should seriously consider ensuring that you have a “solar ready” roof. You may not want a PV array now, but if you decided to install one in the future, the roof would be capable of accepting it, and, just as important, last as long as the PV array. This would be easy to do with new construction but can also be done when it’s time to reroof. This is a hot topic in California where the California Energy Commission recently approved a new energy standard that includes a requirement for all residential and commercial buildings to have “solar ready” roofs beginning January 1, 2014. So what is the best roof type for a “solar ready” roof. In my opinion, it is a standing seam metal roof (SSMR), which is ideal for both commercial and residential applications.

Standing Seam Metal Roofs

Metal standing seam roofs are lightweight (great in seismic zones), fire resistant (excellent in wildfires), impact resistant (think hail), easily engineered to withstand winds of 130 mph and more and are extremely long life. The greatest attribute of the SSMR, however, is its longevity. Recent studies indicate that Galvalume^® coated steel standing seam roofs are lasting 40-60 years. This means that even if your SSMR is 15 years old, it can still outlast a PV system, eliminating the roof replacement expenses previously mentioned.

Both thin film and crystalline arrays can be installed on an SSMR without penetrating the roof.

Another valuable attribute of the SSMR is the ability to attach either the flexible thin film or the glass-based crystalline PV module to the SSMR panels without penetrating the roof or using an expensive racking system. Roof penetrations are both costly to waterproof and have the potential for leaks. Racking systems are generally required on other roof types. The material and installation costs of racking systems in addition to the penetration costs can often exceed the cost of the SSMR. In many cases, this means that by going with an SSMR in lieu of other roof types and eliminating the racking system for your PV array, you can save enough money on your PV installation cost to pay for your roof.

If you want to install a large PV array on a commercial building with a flat roof, you can often pay for a retrofit framing system with a SSMR that will far outlast other commercial roof types, increase your building energy efficiency and provide the perfect platform for you PV array.

With a multitude of benefits for a standing seam metal roof, coupled with the long term money savings of a PV array, it’s easy to see how solar and metal are truly the perfect combination.