Skip to main content

Energy Conservation and Microclimate Moderation

Seasonal Heat Flux Properties of an Extensive Green Roof

Green roofs, or vegetated roofs, can reduce heat flux magnitude through a building envelope as a result of insulation provided by the growing medium, shading from the plant canopy, and evapotranspirational cooling from the plants and substrate. This study quantified thermal properties of an inverted 325 m2 (3500 sq ft) retro-fitted extensive green roof versus a traditional gravel ballasted inverted roof in a Midwestern U.S. climate characterized by hot, humid summers and cold, snowy winters. 

The roof was installed in May 2004 on a portion over the headhouse of the Plant and Soil Sciences Building.  On the green portion of the roof, thermocouples were located inside of the building; on top of the roof membrane, insulation layer, and water retention fabric; under the plant canopy on the growing substrate surface, and at a height of one meter above the foliage. Heat flow sensors were placed above the insulation layer and soil moisture probes were installed in the moisture retention fabric and in the growing substrate. On the conventional portion of the roof, thermocouples were located inside the building, on top of the roof membrane, above the insulation layer, on top of the gravel ballast, and one meter above the roof surface. Heat flow and moisture sensors were also present on the conventional side. Sensors were replicated three times at separate locations in the ballasted and vegetated roof sections for a total of 36 thermocouples, 6 heat flow sensors, and 9 moisture probes. Ambient temperature, irradiance levels, wind velocity, relative humidity, and precipitation were continuously recorded by a weather station located on the roof. Data was recorded every five minutes, 24 hours a day using a Campbell Scientific CR10X datalogger with peripheral multiplexers, switch closure modules, and storage module.

Results demonstrate how roof temperatures and heat flux are influenced by an extensive green roof in Michigan during different seasons of the year.  The greatest impact was during the summer as the green roof reduced heat flux through the building envelope by an average of 13% in winter and 167% during summer.  Summer cumulative monthly heat flux values show a net heat gain into the building for the gravel roof while the green roof showed a cooling effect on the building.  In terms of temperature, maximum and minimum average monthly temperatures over the course of the year were consistently more extreme for the gravel ballasted roof than the green roof and the gravel roof was up to 20oC warmer during the summer.  The transition seasons (autumn and spring) showed similar responses between the green and gravel roof, as well as the winter when there was snow cover on both roofs.  This study agrees with other reports that heat transfer and thermal differences between green roofs and gravel roofs appear to be primarily influenced by solar radiation, ambient outside temperature, and volumetric moisture content of the growing medium.  The presence of snow was also a controlling factor. For all variables measured, the gravel roof generally exhibited larger fluctuations than the green roof.

Results suggest the potential for energy use reductions for green roofed buildings with associated monetary savings but also environmental benefits as carbon emissions are reduced by lower energy consumption.  The magnitude of these savings will be influenced by many factors including climate, type of roof, the amount of insulation provided during building construction, growing substrate depth and composition, plant selection, and whether the roof is irrigated.  This particular study was conducted on an inverted roof which would minimize the benefits because the insulation is located above the roofing membrane. It also took place in a location that experiences hot, humid summers along with cold, snowy winters on a sedum dominated shallow depth roof without irrigation.  Increasing substrate depth or supplying irrigation would allow the use of plants with greater biomass and leaf area, which in turn would lead to higher evapotranspiration rates.  In an area with a completely different climate such as the tropics, plant selection would be completely different making these results difficult to extrapolate.

Complete results are published in:

Getter, K.L., D. B. Rowe, J.A. Andresen, and I.S. Wichman.  2011.  Seasonal heat flux properties of an extensive green roof in a Midwestern U.S. climate.  Energy and Buildings 43:3548-3557.

Plant and Soil Sciences Building (July 2006) showing location of instrumentation on green and gravel ballasted roof
Plant and Soil Sciences Building (July 2006) showing location of instrumentation on green and gravel ballasted roof.

Effect of Substrate Depth and Vegetation Type on Heat Flux Properties of an Extensive Green Roof

The green roof on the Molecular Plant Sciences Building was installed in 2011 with substrate depths ranging from 5 cm (2 in) to 20 cm (8 in).  The shallow areas contain an assortment of sedum, whereas the 10 cm (4 in) and 20 cm (8 in) deep sections were planted with 17 native herbaceous perennials and grasses.  In May 2013, the various roof sections were instrumented with heat flow sensors, thermocouples, and moisture sensors to determine the effect of substrate depth and vegetation type on heat flux into and out of the building.  Data will be collected for at least one year.

Two men maintaining the MPS green roof
Molecular Plant Sciences Bldg (4, 10, and 20 cm depths)

Differences in temperature, moisture content, and plant performance under conditions of extreme slope and solar incidence.  The objective of this study was to examine the effect of slope and solar incidence on roof temperature, moisture content, and plant performance. In addition, we will compare these factors at the top, middle, and bottom portions of the slope on the north and south sides relative to a flat roof.  The research site was a timber frame barn located on the Old Mission Peninsula, Grand Traverse County, Michigan. The barn, a non-insulated, steep-roofed structure with a 45% slope, was oriented in a due east-west direction along its main axis. The roof surface covers approximately 98 m2 (1050 ft2) on a side. A flat roof was constructed adjacent to the barn to compare results to a normal flat extensive green roof.

A drainage layer, filter fabric, water retention fabric, and substrate were placed and then seeded with eight species of sedum (S. acre, S. album, S. floriferum, S. hispanicum, S. kamtschaticum, S. pulchellum, S. reflexum, and S. spurium) at a rate of 0.5 g/m2.  Prior to sowing, seed was mixed with a dry sand mix to ensure even coverage.  Shade cloth was placed and the roof was irrigated as needed until established.  At this point irrigation was terminated so results are based on natural conditions. Prior to seeding, soil moisture sensors and thermocouples were laid out on the top, middle, and bottom section (three of each sensor per section) on the north and south slopes, as well as the flat area.  In addition, pyranometers were placed on each side to measure irradiance levels. Temperature, substrate moisture, light levels, and weather conditions are being recorded continuously on a datalogger.  In addition, plant coverage and diversity are being measured periodically with a point frame transect.

a barn with a green roof - two ladders are mounted to assist with maintenance
North side of the barn (July 2010)

The PSSB headhouse before Green Roof installation
Roof over the headhouse of the Plant and Soil Sciences Building prior to installation of green roof in May 2004

A sensor setup under the substrate
The roof is instrumented with heat flux sensors, thermocouples, and soil moisture probes at various locations inside the building and in the roof profile.

a man wiring the MPS datalogger
Connecting wires to the datalogger

Liatris Aspera on the MPS roof - mostly green with purple flowers
Molecular Plant Sciences Bldg (20 cm depth)

sensor wiring under sedum mats that have been pulled back
Burying wires under sedum mats

a man digging a senson trench on a green roof
Installing sensors in herbaceous perennials and grasses

sensors under sedum
Thermocouples and heat flux sensors on roofing membrane

a barn roof being prepared for a green roof install
Installation of sensors prior to addition of substrate on barn located on the Old Mission Peninsula, Grand Traverse County, MI

three strips of shade cloth over a green roof, cloth is green in color
Shade cloth to promote germination

Seeds germinating through erosion control fabric
Seedlings emerging through jute erosion control fabric

a man atop a green roof, collecting data
Data collection using point frame

a point frame on the barn green roof
Point frame transect