Green Roof Plants; Insulation & Thermogenesis Issues

All plants possess biological systems that directly impact our ecology and the immediate environment surrounding our day to day activities.

After purchasing a truck load of plants on Saturday we unloaded most, however forgot to remove all of the plants from the cab.

Though the night air was cold (6C) when I opened the truck door and climbed in to drive to the market yesterday evening, after dark, I was enveloped with warm, moist air and confused as to why - with the cold dry air outside - the truck windows were fogged over with moisture.  Then I realized the plants were still in the truck, taking in CO2 and pumping moist O2 back into the air.

After spending much of Saturday evening outside taking temperatures with the ExTech IR thermometer, the oxygen and moisture filled truck cab emphasized what I already knew - plant's biological process are complex and have definite effects on their surroundings.

Sometimes we forget just how much plants impact our environment.

However in addition to the wonderful visual greenery (again we sometimes take for granted), plants sequester CO2, produce O2, provide habitat for wildlife in the Urban Core, provide food, fiber and medicine, clean stormwater and provide a myriad of other functions.

All of these factors and processes impact green roofs.  Understanding how these factors interact with the building is important.

This weekend I wanted to gather additional data on heat and green roofs.  My questions were many and included;

* Do green roofs really act as insulation?
* Do green roofs act as a heat sink - storing heat - instead of being an insulator?
* Does green roof plant selection impact the energy efficiency of green roofs?
* Does green roof soil composition impact energy efficiencies of green roofs - and if so, how?
* and a host of other questions.

After spending several hours with the IR, examining plants and green roof systems after dark - and in 6C ambient air, I can say much data needs to be collected, many studies completed and analysis done before we really understand the dynamics of green roofs.

Just as with the fertilizer and irrigation issues (I am always amazed at how some promote green roofs as ecologically friendly and important yet insist for the inclusion of potable water irrigation systems and fertilizer applications), the insulation or heat sink issues just don't seem to be adequately answered.

After collecting temperature data from under green roofs we see a green roof behavioral trend pointing to a heat sink rather than an insulator type system.  In other words, green roofs may tend to absorb heat during the day and then slowly release it back into the atmosphere and building during cooler evening hours.

Yet the complexities of plant species, plant growth characteristics, root systems, stomata to leaf surface area ratios, soil media specific heat qualities and other issues all contribute towards a complex model.

Getting back to the IR thermometer field  foray, some of the more interesting observations we noted were;

* Night time green roof plant leaf temperatures were approximately the same as ambient air temperatures,
* There were variable levels of warmer temperature readings found in the air space under the green roof plant leaves and above the green roof soil media, depending on the time of night and wind exposure - suggesting a level of insulation occurring as a result of leave structure
* The underside of an extensive green roof (3" soil media) stayed 10F warmer than a similar roof with no green roof system - and stayed warmer all night -- up until 5am the next morning,
* Banana plants stayed considerably warmer than ambient air for up to three hours after dark - unlike other plants,
* and other observations.

The banana plant elevated temperatures pointed us in the direction of thermogenesis in plants.  Thermogenic plants are those plants that can generate heat as a result of biological processes. The voodoo lily, Sauromatum guttatum, can generate temperatures of up to 110F - 32C!

There is a great video on thermogenic plants here.

However, the banana plant is not a thermogenic plant and the reason the banana plant stayed warmer than ambient air for several hours after sunset was the plant's high water content.  Water has one of the highest specific heat values of any compound or substance - four times than of limestone for instance.  Because the banana tree was full of water, the solar heat gain experienced during the day only slowly dissipated after nightfall.  Banana trees stayed warmer than most plants after dark because of the heat stored in the large volume if interstitial water within the plant.

It is possible the succulent filled extensive green roofs we are studying that emanate heat throughout the night are behaving like the banana plants.  The combination of green roof soil media and the water therein is absorbing heat during the day - maybe quite a bit of heat - then slowly releasing the heat at night.

The factors involved in modeling this complex heating and cooling dynamic are many and not well documented today.

We think the heating behavior of the extensive green roof is due to water in the extensive green roof plant root systems.  Because the system studied was non-irrigated (nature only irrigation), the soil media was rather dry.  However for heat to continue to be released for long hours, the heat source probably was water - and probably water stored in the underground parts of plants.

We ask ourselves many questions - if water is a significant heat sink and heat source, then do green roofs really act as insulating systems?

If green roofs are heat sinks then how much heat do they dissipate back into a building at night?

Are irrigated green roof systems actually hotter than non-irrigated vegetated roofs or reflective white roofs?  If so by how much?  How much cooling does plant transpiration and evapo-transpiration on irrigated green roofs?

There are many questions to be answered.

As an industry we need to sponsor and encourage more study of green roof thermodynamics.