Solar-Integrated Green Roofs: Enhancing Urban Sustainability with Alan Burchell at Urban Strong

What we can— I don't know if this would qualify technically as a solar integrated green roof, but we can definitely integrate solar panels onto the rooftop near a green roof in different configurations, a popular one being solar canopies. And so these can be installed, you know, 2 to 3 meters above the rooftop in a flat canopy and they serve as a pergola so people can sit underneath it because you don't, at least here in New York, I have no interest in being up on a rooftop for an extended period of time without any shade on a hot August summer day. You would absolutely roast. But if I was underneath a pergola and maybe had a drink in my hand and was looking out over a green grassy meadow, that could be quite beautiful, but also would be high functioning.

Welcome to Rethink What Matters, the podcast dedicated to aligning the economy and ecology with everyone for improved business performance, stronger families, and a greener, cooler planet. And today I'm joined by Alan Burchell, principal at Urban Strong in Brooklyn, New York, United States. And we're going to be discussing integrated solar green roofs.

Hi Paul, thanks a lot for having me.

It's great to have you here. Could you tell us about Urban Strong?

Sure thing. So Urban Strong is a full-service green building firm based out of Brooklyn, New York, as you said, since 2014.

What was your own personal journey then into solar green roofs?

I previously worked in the utility wind space. I worked for a Spanish wind turbine manufacturer at their North American corporate headquarters in Philadelphia. I fell very much in love with the world of environmental sustainability. I wanted to get a little bit more entrenched. And so I moved up to New York City where I studied at the Earth Institute out of Columbia University. And I was looking for, you know, a new industry to sink my teeth into. I had a strong concern about the global freshwater crisis. I recognized that if we're going to battle climate change efficiently and successfully, we're going to need to do so in urban centers where, as you know, more than half of the global population are now residing. And frankly, I just like big cities. I also love nature. I'm just fresh off of a week-long camping trip in northern Ontario with a bunch of my high school buddies. I just don't like the idea that nature and dense urban living are so separate. I don't think there's a need for them to be mutually exclusive. And in my studies at the Earth Institute, I came to realize that we actually can't afford for them to be so mutually exclusive. And in fact, we— a lot of the reasons we're suffering in urban centers is because we have very much pushed nature out. We've kept it at bay when in reality, we, we need, need nature. Now you don't have to be, you know, a camper, hiker, tree hugger in order to benefit mentally and physiologically from a connection with nature. There's been all sorts of white paper studies done that show that we, we benefit from having a connection with nature, sightlines to nature, a walk through a park, or even something as simple as being able to look up and see vegetation. And so I found the world of green roofs and realized that they checked a lot of boxes, both in terms of energy efficiency, solving urban issues that are climate change issues plaguing urban centers. And it had my, you know, the pet benefit of I get to work with plants and reinvigorating or reintroducing, sorry, nature into the built environment.

Well, why did you then introduce a solar panel onto your green roof?

So they started off separately at first. As much as I love green rooftops, vegetated roofs, they do have their limitations. They, they're not for every roof infrastructure. They're not for everybody's budget. And people sometimes underestimate just what a thorough, heavily engineered process green roofs are, the amount of maintenance that's required. They can offer extreme benefits for certain building types, some buildings more so than others. And so, I started offering solar as an alternative at first, a separate alternative for those folks who wanted to do something green with their rooftop, whether it's reduce their footprint or improve, you know, overall property values or improve building operating efficiency. And so we started selling solar panels, solar arrays. Then eventually people start asking, well, you know, is there a chance that we can do both? And I was aware that out there in the market, namely out in Europe in particular, there were some technologies that were integrating solar and green roof together in the same footprint. So we set our sights on that and fast forward a couple of years and we now have a line of solar integrated green roofs that we're offering to the North American marketplace.

Okay. So how popular are solar green roofs, say, in the US?

They're definitely gaining traction. I mean, solar in the, in the one hour that you and I may be talking, approximately 70,000 solar panels will be installed around the world. And that the train has very much left the station on that trend. Solar is everywhere. There's highly generous subsidies at the city, state, and federal level here in New York and in many cities and states around the country. Green roofs have a long way to go to catching up to the popularity of solar, but they very much on their way. They're really trending upwards. People are waking up to the benefits of integrating green infrastructure into their urban centers. And, you know, plants are— plants were trending before COVID And now during COVID in particular, people started looking at their surroundings and evaluating how good their, their living environment and their working environment was for their overall mental health and physical health. And they started looking to integrate nature into their environment. And so the natural question started coming up, well, hang on a second, should I do a green roof or should I do solar? And we've seen a sharp uptick in the last, I'd say, 18 months for requests around solar integrated green roofs. Around the world, there's probably in the high hundreds, if not low thousands of solar integrated green roofs.

Okay. And does the solar benefit from being with a green roof? And does the green roof benefit from having solar with it?

It does. So contrary to what you would think solar panels actually lose efficiency as the air around them warms up. And given that you're, you know, ideally pointing them at the sun all day, you can appreciate that up on a hot sunny rooftop, the air there can get quite warm. And there's something on the spec sheet of every solar panel called the temperature performance coefficient. And this is a very small negative percentage number and it dictates the percentage of electricity production efficiency that that particular solar panel will lose for every degree of temperature that the ambient air underneath and around the solar panel warms up. And this is no small number. In warm summer months and summer evenings and afternoons, solar panels can see losses of, you know, up to 20, 23, 25% efficiency. Now, vegetation, we all understand inherently cools the air around it, which is why we all flock to a park or the forest forest on a really hot day, and you oftentimes feel the cooler air even before you get to the forest or the park. This happens through a process, what stage of photosynthesis called evapotranspiration. Basically, when vegetation is sweating out its excess moisture that it no longer needs, it ends up cooling the air around it. And if you vegetate underneath a solar panel, you're cooling the air underneath it, you're reducing that performance efficiency loss, a double negative becomes a positive. And you can often see, depending on the climate and the orientation and the vegetation, you can often see a solar panel's efficiency boosted by anywhere from, you know, 6 to 12% on these really hot summer days based compared to the base case of a business as usual where the solar panel might just be installed on top of a black tar bitumen rooftop.

Right, but doesn't all that moisture from, you know, all that green roof, doesn't that interfere with the solar panel at all?

No, no, it doesn't. The moisture is, it's vapor, it's in the atmosphere. It's not raining down on the panel or anything like that. It's just generally cooling the air around it. So it has no impact from that perspective. But one thing you would need to watch out for is when we're selecting which vegetation, which plant species to plant, plant in and around the panel, you have to look out for two things. Firstly, you want to make sure that the vegetation doesn't grow up so tall that it casts a shadow or shade onto the panel. So we specify low, low ground covering species that don't really have any height to them. And then secondly, you want to select plant species that don't shed any pollen dust because any kind of dust or debris on a panel could somewhat reduce its efficiency.

How are the green roofs benefiting from having the solar panels alongside them?

Yeah, that's a great question. So green roofs benefit from diversity. If you can have, find a way or have the opportunity to plant multiple different species of plants up on the green roof, then you improve its overall diversity and resiliency because, you know, you never see a forest with just one species because any one pest that could come along could then wipe out the whole forest. Similarly, multiple species when they're in close proximity can share nutrients with one another. And so when you integrate solar panels onto a green roof, you're going to have some areas of the green roof are always in bright sun, or, you know, between the rows and the spacing and around the perimeter. But then you're going to have some areas of the green roof that are partially, if not fully shaded by the solar panels. So different plant species prefer different levels of sun, just like in your apartment. Some plants like to be right on the windowsill in bright light, some like to be a little bit further back. So we'll plant shade-tolerant species underneath and around the panels, and then the bright sun tolerant or bright sun seeking species in the aisles and rows separate from the solar panels.

So if we're combining solar panels with green roofs, what percentage of the roof would be say, you know, what percentage of the roof would be covered by solar panels?

That's a great question. So there are typically many things up on a rooftop that are creating shaded areas where you cannot install solar panels regardless if they're integrated with green roofing beneath. So typically the parapet or the edge walls are, you know, about a meter or 3 or 4 feet tall. They create some shading. Oftentimes there's the stairwell bulkhead itself, rooftop mechanical equipment. They're typically casting a lot of shade. And then there can sometimes be fire department rules and regulations that say that there need to be clear pathways set back from the front streetside edge of the building in case they want to jump up with their ladders. So already solar panels are— you're never filling the entire roof area with solar panels. Green roofing, we typically throw in a factor of about, we say about 87% of a roof area, just for a crude rule of thumb, can be vegetated. and this is similarly accounting for space around rooftop mechanical equipment and parapets and the bulkheads. Some waterproofing manufacturers don't want you to vegetate right to the edge of a wall. Similarly, when you integrate the two systems together, you typically can't have the same density of panel row spacing as you could if it was not vegetated because you end up having the panels a little bit more steeply angled in order to allow sunlight to get partially to the vegetation that's underneath the panels. For example, here in New York, if it's not vegetated, then many people designing a solar array for a rooftop may lay the panels flat or at a 5-degree tilt. But if we're going to be integrating it with vegetation, we may say, look, at the minimum, you're gonna want to tilt it 10 degrees. When you tilt it up though, that panel, each row of panels creates a bit of a longer shadow behind it, which means you've got to move the next row back a little bit further. So you do lose a little bit of row spacing there. But oftentimes, you know, each individual panel, its efficiency can be boosted if it's a bit more steeply pointed, particularly here in the north.

We have reduced the area of the solar panels and we have reduced the green roof as well, haven't we? So it would seem like, you know, we've got less energy, so less money and less cooling, less efficiency. But this is still a good idea. This still works for people.

So I'll say two things to that point. One, although you do reduce row spacing, as I mentioned earlier, the evapotranspirative cooling effect of the vegetation does help those solar panels that are integrated produce at a higher rate of efficiency. So yes, you lose some on the increased row spacing, but but you gain some of it back by the increased electricity production from each of those panels that does make it onto the system. And then secondly, with the green, there's not so much green roofing lost because we just have, the panels themselves don't take up much footprint and eliminate any greenery. As I mentioned, we're greening right up underneath the footprint of the solar panel and all around it. You could argue that you're losing some you know, insulation benefit only in that the photons are striking the panel rather than striking the membrane or striking the green roof instead of the membrane. But the green roof, a lot of, a lot of insulation and cooling benefits a green roof offers is when they're retaining stormwater. And so holding a bit of water provides more evaporative cooling and more insulation. So you're still getting the bulk of the thermal insulation benefits. Of the green roof, whether it's integrated with solar panels or not.

Brilliant. So you are really getting the benefits, aren't you, of both? You're getting the benefits of adding the solar panels as well as adding the, you know, the cooling of the green, the green roofs as well. So they do sound like they're made for each other in a way, don't they? Solar, solar panels and green roofs work really well. They do.

I mean, they— people assume that they're sworn enemies battling for real estate. And photons up on the rooftop, but in reality, they play very nice together when designed and installed correctly.

So installing them correctly then, do their installation requirements, do they differ? Do they conflict with one another?

One benefit before we get to the conflicts or potential conflicts, one benefit is that, you know, where possible, solar installers like to not penetrate the roof membrane when they're laying down the racking for the solar panels. So commonly they're ballasted down with a simple cinder block. I get a little kick out of the dichotomy there of the very old rock technology weighing down the cutting-edge technology of solar panels. But that really is the modern-day current business-as-usual approach to ballasting down the panels and the racking so that they don't blow away. When you install a solar integrated green roof system, you can use the weight of the green roof and the growing media, like the engineered soil and all the different layers of the green roof, can actually act as the ballast and weigh down the racking of the solar panels. And so that's a great opportunity. This then prevents the need to penetrate the roof membrane with any mechanical bolts or fasteners, which could then otherwise lead to a leak. Now, the fact is, one of the benefits of green roofs is that they sponge up and they manage stormwater and they hold it up on the roof in, in limited quantities that are highly engineered and very predictable. But when you are holding that much water up there, you do have to be extra careful that the roof is absolutely watertight. So in the event that you did have to make some penetrations in order to weigh down or fasten the racking or run any conduit, electrical conduit or something like that, you need to make extra special attention to these penetrations to make sure that they're waterproofed. And we're always going to look to make sure that the roof drains are completely free and clear of any debris because you want the excess water beyond the maximum carrying capacity of the green roof to be able to flow down these drains freely.

Okay, so you've got vegetation and plants that can grow underneath these solar panels. So is that creating a new sort of environment that otherwise wouldn't exist on these green roofs that perhaps is benefiting biodiversity?

Yeah, so when you, when you have these shaded areas underneath the solar panels, oftentimes that's not possible on a on a plain, open, bright, sunny green roof. So you are creating these zones, if you will, where you can install select different plant species that otherwise wouldn't have been possible. And then when you have a variety of plant species up on the roof, this, it's sort of like hedging your bet and improving the overall resiliency of the system. So you can end up, yeah, overall improving the biodiversity, getting different bugs, or it could attract different pollinators. Or migratory species or whatnot if you have a variety of plant species up on the rooftop.

But what about, so what about the vegetation of the plants underneath the solar panels that now, you know, aren't receiving any rain?

So built into the design of the base of the solar integrated green roof racking, water that runs down off the solar panel and trickles down off the leading edge of the panel falls directly down, trickles down through a a gravel strip that we install in the vegetated area right beneath the leading edge of the panel. And this water is collected into the base tray of the integrated racking system. And into the tray are these designs, these little channels that allow the water to then flow backwards underneath the solar panels so that plants that are under there can receive irrigation and have their irrigation needs met underneath there.

Does the maintenance increase though for the solar panels and the green roofs when they're combined like that?

I would definitely say that if you— solar panels are notoriously low maintenance. Really, there's no moving parts. There's nothing you have to do. When you integrate vegetation into a rooftop solar array, there is some added maintenance because all living systems do require some degree of maintenance. We design a solar integrated green roof to be as low maintenance as possible because a lot of the time the you know, they're not looking for high-quality aesthetics with the vegetation as you would say for an amenity deck green roof. You're looking for performance, low maintenance, drought tolerance, and high levels of evapotranspiration. So we also, we select vegetation that's not going to grow up, but it's still going to require a certain amount of weeding and maintenance, particularly in the beginning. Okay.

And do you think it takes any special skills to install a a solar panel on a green roof?

I would say with a little bit of training, they could get there. We basically, when we first meet a new solar installer, we make it very clear to them that, you know, we representatives from our team and our, and our typical green roofers definitely need to be there for the first few installs to train them and get them up and running. It's, it's green roofing is not just your run-of-the-mill grade level landscaping. It is a very special art and science. There's several factors that you need to be careful of that maybe aren't really an issue down at grade. And so typically we have, we on a project, we'll have a solar installer, the green roof installer, and then we will come in as the integrators and do a bit of construction administration and make sure that the systems are being installed in the right order because the trades do have to come sort of stacked you know, first the green roofers, then the solar, then the green roofers again, then the solar folks come back. So there's a bit of orchestrating that goes with that. And typically that's not done by a single trade.

So are solar integrated green roofs becoming a part of legislation in New York, if not elsewhere as well?

They are not directly, but it's happening indirectly because rules, for example, like here in New York City, we have the New York City Department of Environmental Protection's Unified Stormwater Rule. And without getting into the details, this is basically ratcheting up the requirements for not only for building owners to manage an increasing amount of stormwater that lands on their property, but it's stipulating that first consideration must be given to vegetative solutions for managing the stormwater, not just your typical gray infrastructure of underground cisterns and whatnot. So as certain rules are mandating, or at the minimum financially incentivizing, vegetated rooftops and then, and or they're mandating, you know, high levels of energy efficiency in a building or onsite renewable energy production, you're going to see these two opportunities or these two solutions clash or come together where people are going to start realizing, wait a second, I can check two boxes at the same time, or I can have two different opportunities for energy saving or property value boosting or overall building performance improvement. Happening in the exact same footprint. So they are, as green roofs and solar are being driven up in popularity, the intersection of the two of them is raised as well. Okay.

So is installing a solar green roof, does that take a lot longer than just installing a green roof by itself? Can these things be done in parallel?

Much of the work can be done in parallel. I wouldn't say that it takes much longer, although it really cannot be done at the same time. So I would say that the the sum of the parts is probably equal to however long it would have taken you to install the green roof plus however long it would have taken you to install the solar system. They do share materials. So in the list of materials of a solar roof and that of a green roof, there's some overlap in their Venn diagram there. So you could say that there's a reduction in time from not having to install entirely 100% separate systems. However, any time that's saved there is maybe then gobbled up by ensuring there's certain integrated elements that would have to be cared for and additional construction administration. So I think it all nets out to about the same time. Okay.

And can you just give us a few ideas of the types of plants, the species of plants that, you know, work well? I think you mentioned it earlier, that work well on a green roof if there are solar panels.

Yeah, they're all going to fall in the category of sedums. And so there's, we have a, I don't know them off the top of my head, but there is a definite list of sedum species that as a, that do not shed a lot of pollen dust and do not grow vertically. It still very much changes region to region. So wherever you're thinking of planting a green roof or even an at-grade garden anywhere in the world, typically we consult this map called a plant hardiness zone map, and this will basically tell you, you know, how severe are the conditions for vegetation in your area of the world. And these maps get pretty finite, and I think there's about 15 different zones in the US alone. And we, your plant hardiness zone will dictate which plant species are possible for at-grade gardening or up on a green roof. We typically bump up a couple of levels when it's a green roof compared to at-grade because the conditions up on a roof tend to be colder and windier and just generally more harsh than the grade-level landscaping on the same property.

And are these, are the solar panels always integrated with the greenery of the roof or are the solar panels also used in other ways on the green roof, maybe for shading of where people are sitting, for example?

What we can, I don't know if this would qualify technically as a solar integrated green roof, But we can definitely integrate solar panels onto the rooftop near a green roof in different configurations, a popular one being solar canopies. And so these can be installed, you know, 2 to 3 meters above the rooftop in a flat canopy, and they serve as a pergola so people can sit underneath it because you don't, at least here in New York, I have no interest in being up on a rooftop for an extended period of time without any shade on a hot August summer day. You would absolutely roast. But if I was underneath a pergola, and maybe had a drink in my hand and was looking out over a green grassy meadow, that could be quite beautiful, but also would be high functioning.

So what's the, what is the ROI on a solar green roof thing?

That is a tricky question because it very much depends on several different factors. It's going to, whether it was just the solar array or the green roof alone, each one depends on many different factors. So it's going to, They benefit or suffer from economy of scale. The bigger the system overall, things get cheaper either per square foot or per watt, however you want to size it or gauge it. Sorry. It also depends on the incentives that are available. Green roofs are incentivized in some parts of the world, some cities, but not in others. And that can make a substantial difference in the return on investment. I'd say similarly, The other issue would be, what is the building type exactly? Is it a commercial entity, an institutional entity? Is it a single-family home? Is it a multi-family building? And I say this because they in particular will benefit differently from a green roof. So, green roofs offer an array of benefits. I like to refer to them often as a Swiss Army knife of benefits, but not every building type can benefit from each of the features or benefits that green roofs offer. So for example, a commercial entity that's looking to improve its branding, improve its overall HR hiring ability and employee retention. If you have a green area that employees can walk out onto or enjoy sightlines to as they're working along, they can see productivity boosted, overall company morale boosted. All of the things being equal, people prefer to work in a company that is visibly green and putting out a reputation of sustainable conservation. And when you have a vegetated rooftop on your building, this is really great for branding and overall company morale and employee retention. You don't have branding and company, you know, employee retention on a single-family home. You don't necessarily have that in small companies or multifamily buildings either. So there are certain benefits that don't really apply to certain building types.

So are there integrated solar green roofs in both intensive and extensive green roofs?

Roofs?

So they're used in both applications?

They can be. However, we typically will only install extensive shallow, simple, low-rise vegetation green roofs underneath and around the solar panels. Intensive green roofs tend to be quite lush and beautiful. You get bushes, grasses, big flowery vegetation, and you don't want that in the immediate vicinity of the green roof. But oftentimes, you know, my ideal green roof has some amenity decking with lush intensive green roof around it. The rest of the area is covered with solar integrated green roof where possible. So you'll get different pockets of vegetation on different parts of the roof, but in the immediate vicinity of the solar panels, you're only going to want extensive green roof.

And so an integrated solar green roof, is that helping with a company's ESG score as well?

Absolutely. So they check a lot of boxes with, with the environmental and social and governmental qualities that you're looking for in a system. So socially speaking, they improve mental health, physical health, connections with nature. And it's even been shown that when a certain amount of green infrastructure, meaning green walls and green roofs, are installed in certain neighborhoods, they've seen the crime rates actually come down. Green infrastructure, as opposed to gray infrastructure like sewage and pipes and tanks and cisterns, green infrastructure tends to create a lot of local jobs, local ongoing jobs that require ongoing maintenance. So that's great. There's, of course, all the environmental benefits. The green roofing on rooftops is sequestering carbon. It's cleaning and cooling the air. It's providing habitat for local and migratory species of birds. Bees, bats, butterflies. And yeah, it's a, it's a great way for a building to green its portfolio of companies and building assets.

How are integrated solar green roofs, how are they a part of different building standards?

Got it. So two building standards that they contribute to immensely would be the LEED building standard and the WELL building standard. Solar and green roofs on their own check a lot of boxes across several different categories within both both LEED and WELL. And when you integrate the two of them together, not only do you get the credits from both technologies that are, you know, transcending multiple categories, but you can sometimes qualify for the innovation in design category. So they're great for very— a green roof alone can skyrocket a building's rating from maybe bronze all the way up to gold or platinum because of all the different credits that it can qualify for.

If you have an existing green roof, maybe that green roof has been there for the last 5 years or 10 years or so, does it present any particular challenges to install solar panels afterwards?

Unfortunately, I mean, anything's possible with the right amount of money. Unfortunately, most solar integrated green roof solutions, including ours, very much require that they be installed together at the same time because there's a bit of a layered effect. If you had an area of a green roof that you wanted to upgrade into an integrated system, we could, depending on the green roof system that's installed, we could remove a portion of it, install the foundation for the solar integrated green roof racking, and then place the green roofing back on. It would just mean a little bit of extra labor.

Are there any other trends in play at the moment with integrated solar green roofs that you're seeing?

We're seeing an increase request in solar integrated green roofs. From both multifamily buildings and schools for very different reasons. The multifamily buildings are looking to reduce their electricity bills and improve their building's energy efficiency. And so they're very much looking at solar, but COVID has woke them up to the idea that they want to be able to access nature, maybe without going into public. And so the idea of having an exclusive rooftop park or rooftop garden is you know, very appealing to them. And so they start to ask, hey, can we do a little garden sitting amenity area in one corner, but put solar across the rest of it? And this naturally leads to the, to a discussion around the opportunity to have green roofing throughout so that they're not just looking at a solar array, but instead looking at a solar array surrounded by greenery, which is more aesthetically pleasing. Plus you get all the operational benefits of both systems. Schools are increasingly contacting us about solar integrated green roofs because The particularly for schools, solar integrated green roofs have the benefit of tapping into a long list of different curriculum subjects. And so they'll often turn the rooftop into a living laboratory or learning opportunities. So we'll integrate a deck and some pathways, some walking pathways around the system where the students can be led and they can do little studies and, you know, everything from physics to nutrition to life sciences. So we've definitely seen an uptick in that trend there.

And how does wind figure in the installation of a solar green roof? I can imagine the wind might want to pick up the solar panels and blow them away.

And believe it or not, they would also pick up the green roofs, particularly in the corners of the roof. So wind uplift is a crucial part of the due diligence and the permitting that our building engineers need to consider when filing with the Department of Buildings, particularly here in New York, because we're in a hurricane zone. So extensive structural engineering and wind uplift studies must be done on whether it's a solar roof or a green roof, or particularly a ballasted solar green roof, because you— we are relying, and one of the benefits, as I mentioned earlier, of a solar integrated green roof is that the weight of the green roofing can act as the ballast and weigh down the solar array. So you don't need the cinder block ballast and you don't need to be penetrating the roof membrane with mechanical fasteners, which could potentially lead to leaks. However, some very careful engineering calculations must be done by a registered, you know, professional engineer to show that when the green roof is dry between rainfall periods, show that it's still heavy enough to weigh down the solar equipment so that it doesn't get picked up like a kite and blown off the building. But then similarly, when the green roofing is at its maximum saturation, when it's holding the greatest amount of stormwater that it was designed to, we need to make sure that it's also not so heavy that it's going to put any additional or unwanted or stress on the structural capacity of the roof.

And thanks very much for your time on this podcast. I really appreciate it in helping us to understand the benefits of integrated solar green roofs.

Paul, thank you. It's been an honor to be on here. Thank you for all your excellent questions. I appreciate your time and the opportunity to spread the word about the benefits of solar integrated green roofs.