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Green roofs and urban heat island

LukeInClimateZone7 | Posted in Green Building Techniques on

A policy-oriented question for the physics-minded folks out there (Allison?)

I’ll try to be brief here.
The question I have here is: How does a ‘green roof’ performance towards the urban heat island effect compare to a Cool Roof in a hot/dry climate?

–Let’s stipulate that there’s no formal definition of a green roof other than some kind of membrane with soil and plants planted at the time of Final Inspection. Since most green roofs are installed on low-slope roofs, then the comparable ‘cool roof’ performance is somewhere in the ballpark of Reflectance-0.8 and Emittance of 0.8.

I’m on a green building committee that’s tasked with advising the local municipality on a new green building code that would be adopted and implemented locally. This code features mandatory measures as well as a requirement to fulfill a number of ‘electives’. One of the electives on the list is ‘green roof’, and I’m just thinking about whether or not this is actually a sensible idea to include on the list. I worry that it’s actually expensive and counterproductive, though it obviously has good green ‘optics’ from a PR perspective.

references on GBA–
There a bit of good discussion already on this forum about Green Roofs. I note that the benefits cited by advocates are usually poorly defined, or can only be ascribed to specific green roof compositions in certain climates. And there’s vigorous counterpoint about whether or not the amenities of stormwater management and energy efficiency could not be better and more cheaply provided by more appropriate materials and assemblies. (I’m largely convinced by these views).
So, the question is actually taking on the last ‘benefit’ of green roofs, and wondering if it can be shown to be better than a Cool Roof at lowering Urban Heat island. I’ve narrowed the question a bit by saying I’m thinking about a hot/dry climate (e.g. many parts of California)

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  1. LukeInClimateZone7 | | #1

    (addendum) I know this is probably a question that's better answered with regional climate modeling, but I'd still be interested in getting some intuitions from this forum.

  2. STEPHEN SHEEHY | | #2

    A typical roof requires no maintenance and no attention after installation. A planted roof needs watering, weeding, and is almost certainly more expensive to install. It probably also needs a beefier structure to support it. Any roof leak is usually hard to find, but a leak in a roof covered with soil and plants would be harder to find and much harder to fix.
    Do a white roof and grow your pot inside like everyone else.

  3. LukeInClimateZone7 | | #3

    Thanks Stephen, I generally agree. Any thoughts on my question?

  4. Expert Member
    Dana Dorsett | | #4

    The raw base line albedo of a green roof is lower than a mid-SRI "cool roof", but it's still quite a bit higher than typical roofing materials. But evapotranspiration is your friend:

    The evapotranspiration benefit makes a real difference on the heat island effect, and it can even trump the high albedo of a titanium-white cool roof finishes (particularly in dry climates). But of course it comes with water & maintenance costs that the cool-roof doesn't have.

  5. LukeInClimateZone7 | | #5

    Excellent Dana-- this is helpful. Your comments about albedo are the kind of thing I'm looking for.

    I suppose I am wondering-- is evapotranspiration our friend? After all-- aren't we just trading sensible for latent heat? In the overall energy balance, it still seems to me that we'd be better reflecting the visible light back out into space as such, instead of thermalizing those photons into sensible and latent heat for the surrounding environment.
    The best cases I can make for it in my mind are as follows:
    -- hot/dry climates where it's really dry, there's plenty of room for evapotranspirational (evaporative) cooling which en masse won't end up putting significant extra load on cooling coils.

    -- Evapotranspiration becomes a way of more quickly dispersing the heat out of the urban core and into mesoscale atmosphere surrounding rural areas.

    of course, with the mass of green roof systems, there is the effect of providing more thermal capacitance in the urban environment which as I understand it, would result in moderating the 'peaks' of urban heat islands. But the overall energy balance is still roughly the same-- right?

    Dana (or anyone else): if you have any insights that would set me straight about the overall energy balance of what's going on here, that would be really interesting to me.

  6. LukeInClimateZone7 | | #6

    I just found an article which seems to give a little credence to my energy balance question, and still concludes that there are indeed meaningful benefits as Dana described. My worry that the evaporative cooling from a green roof would just cycle back onto the AC coil is misplaced. I'm way off in how 'contained' the latent heat would be in the local airshed, and thus the cooling effect of both white and green roofs is pretty clear.

    “Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs,” said [Art] Rosenfeld. "

  7. STEPHEN SHEEHY | | #7

    In a dry climate, where does the moisture that evaporates from the roof come from? I'd be concerned you'd need to water the roof to keep the plants alive and in a dry climate, you'd be fighting against evaporation and need to water even more to get the benefit of any cooling.Sprinklers on the roof? I guess you could use drought tolerant plants, but that sounds really pointless.

  8. charlie_sullivan | | #8

    “Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs,” said [Art] Rosenfeld. "

    That sounds about right and they are quoting a very reliable source.

    The water is the other consideration. If you have problems with storm water management the green roof helps; if you have problems with water shortages, it hurts.

  9. Expert Member
    Dana Dorsett | | #9

    Stephen- Yes, in dry climates you have to water the plants, and that's part of the operating expense.

    Swamp coolers are also pretty effective in CA type climates. Dumping that latent heat to the local outdoor air does not increase the latent cooling load in the conditioned space the way a swamp-cooler would, and it doesn't impede the performance of other types of air conditioning.

    Green roofs are both more expensive and much higher maintenance than cool roofs, which is why I've never been a big fan. If rooftop gardening or urban rooftop farming floats your boat, great- have at it! But for the other 99% of the population a cool roof would be a better choice.

  10. LukeInClimateZone7 | | #10

    In this particular case, the water comes from Yosemite national park. I just visited the reservoir last summer and it's worth the trip! The reservoir would probably be nicer without the reservoir, but that's another story. :)

    And I believe the recent amendment to the landscape ordinance compels projects to use drought tolerant landscapes. Of course, most of my projects have those, but they still manage to vastly overwater them. It turns out that people think I drought tolerant plants are ugly when they actually experience a drought, so people run their irrigation systems to make sure they don't.

    Is a green roof pointless in the midst of all this? Well, going back to the Original Post, that was essentially my question. I wanted help pointing out all the pointlessness of this particular 'point' (credit) in the city's green building program.

  11. LukeInClimateZone7 | | #11

    Thanks for a great discussion guys. I appreciate the feedback. Especially Dana-- as always you give thoughtful and meaningful feedback. You should have some sort of online tip jar-- with the amount of rigorous analysis that you and Martin both provide to this forum (and probably others), I often wish there could be a way you could convert your considerable diligence into something more fungible than mere respect (like bitcoin?).

    I still have one question remaining that I believe is beyond this forum-- at what point does extensive deployment of well irrigated green roofs actually start measurably effecting the local relative humidity and turn into the urban 'latent' heat island effect. I'm just curious where that threshold is for my general numeracy. This question/model pertains more to the biosphere, and less to building science, so I will take it to another forum.

  12. JC72 | | #12

    " what point does extensive deployment of well irrigated green roofs actually start measurably effecting the local relative humidity and turn into the urban 'latent' heat island effect. I'm just curious where that threshold is for my general numeracy. This question/model pertains more to the biosphere, and less to building science, so I will take it to another forum."

    I don't think it's possible to answer that question but I doubt you could move the needle very much where it would be noticeable since humidity levels are regional. Cities just aren't that big and the air can only hold so much moisture at a given temperature.

    On a side note depending on your climate has any thought been given to increasing ground absorption of storm water instead? Maybe using water permeable materials for parking lots and driveways if there is such a thing.

    On a larger scale perhaps implementing a wetland system used as a final treatment of sewage. This system would be placed upstream of the municipal water intake of course. Doesn't get much "greener" than that.

  13. gusfhb | | #13

    I think from a policy point of view there is little negative about allowing them as an option, but I would be cautious about any policy that would corner people into using them when it does not make sense for their structure.

    I studied them a bit as I wanted to be able to use one on my personal addition and found the concept wider than I anticipated, varying from underground parking garage[like under the Boston Common] to lightweight trays on the roof systems

    Ideally a residential system would not require much maintenance if planted with the correct plantings.

    I see green roofs more from the perspective of water control than energy efficiency, meaning in an urban area if all of the roofs could handle the first 1/4 inch of rain it would have a huge impact on water control in the city, and if it is a positive in other aspects, awesome.

  14. LukeInClimateZone7 | | #14

    Re: Urban latent heat island:
    The only thing that keeps me wondering about this is when I look at the psychrometric chart, I note the difference in changes in wet bulb and dry bulb have on enthalpy.
    Let's figure on an extreme/unrealistic scenario-- what if every roof in a 50 square mile metropolitan area is required to install some kind of shallow depth green roof that requires intensive watering in the summer for optimal performance. And the result in that broad deployment is that on a hot dayt the sensible temperature drops 1 degree, but the wet bulb increases 0.01 degree (through a lot of ambient noise). My back-of-envelope says this a net energy loser.
    Of course, i have no idea what realistic values would be, but I have been to Yuma, AZ in the summer when all the sprinklers are on.

    To the policy question: this forum has elicited enough information to convince me that the policy I should propose is indeed a prohibition on green roofs. (Don't worry-- this proposal won't win) The first key factor is water-- the city in question is in a deep drought, and they are really trying to move towards banning landscape irrigation now and in the future.
    Secondly, the city is among those that is trying to be as aggressive as possible in demonstrating its leadership towards climate change. Green roofs come with an opportunity cost of having a higher albedo cool roof, which is already required locally (though high-mass roofs are exempted).

    Stormwater management is not relevant for discussion since all projects are required to manage all of their water onsite already. The policies are strict enough that they don't even allow french-drains around basement walls-- since that would make it easier for a homeowner to keep their basement walls dry by pumping the water out into the street (the intent is to force strong waterproofing details).
    And lastly-- the prohibition would only apply to 'roofs' strictly defined, not patios or terraces, or whatever that act as roofs but also provide outdoor spaces for people.

  15. LucyF | | #15

    I'm envious that you live in a city that asks such questions and makes a commitment to "leadership towards climate change". We need more cities like that. Thank you for asking that question here and stimulating this discussion.

  16. LukeInClimateZone7 | | #16

    Thanks Lucy--
    Be careful what you ask for-- you just might get it! Municipalities are stumbling towards success when it comes to these goals. Currently, the way the rules are written pretty much make all-electric NZE homes near impossible to permit. The game is rigged to all but coerce projects to move over to natural gas, instead of using electricity from the grid.
    This effect of the policy (which was publicized in this here website on April of last year) is generally apparent only to people like me and the few homeowners that want to build a NZE home. For those who have participated in a lot of local governments, this is probably par for the course. For my callow optimism, the expectation of good governance continues to be frustrating.

  17. LucyF | | #17

    At least where you are they believe in government. Or at least they can conceive that government is capable of doing some good.

    But you did get me thinking, I really should be more involved in directly encouraging energy efficiency face to face - rather than just by being online with people whose opinions are close to mine.

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