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Green Roofs - Energy Savings
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Thermal performance
The green roof system helped keep the building cool during summer. The rood membrane was cooled by direct shading from the plants, evaporative cooling from the plants and soil, and additional insulation values from the plants and growing medium.
Energy efficiency
Heat flow through a building envelope creates energy demand for space conditioning in a building. Solar radiation and snow coverage strongly affect heat flow through roof systems. Figure 4 compares the average daily energy demand for space conditioning caused by heat flow through the green and reference rood systems.
A rooftop garden can stop UV radiation from attacking its roof membrane and minimize aging of the membrane from heat exposure. An exposed membrane absorbs solar radiation during the day, and its surface temperature rises. It reradiates the absorbed heat at night, and its surface temperature drops. These daily temperature fluctuations create thermal stresses in a membrane, affecting its long-term performance and ability to protect a building from water infiltration. Figure 3 shows the daily membrane temperature fluctuation (daily maximum temperature/daily minimum temperature) and daily ambient temperature fluctuation of the reference and green roof systems.
Storm Water Management
The garden absorbs and uses rainwater in an efficient way. It can retain 75 percent of a one-inch rainfall there is run-off into the sewers. In addition to decreasing run-off, which overburdens the sewer system and pollutes the lake, the garden both cleans and cools the rainwater by filtering it.
Longer Tile
(Green roofs can last 30 years and typically carry 15-year warranties, compared to conventional roofs which last up to 20 years and carry 10-year warranties.) (Building owners, imagine the financial implications of extending the life of your roof to 50 years, while making it easier to cool in the hot summer months.) That’s just
Surface temperature readings of the rooftop garden show that on a 95-degree day, temperatures range from 90 degrees in the planted areas to 115 degrees on the reflective walking paths. In contrast, the adjoining Cook County Building rood, which is of black tar, has reached temperatures as high as 165 degrees.
The urban heat island effect is caused by the large amounts of asphalt and dark-colored roofs that absorb the sun’s energy during the day and release the stored heat in the evening. This stored heat causes urban areas to be 6 to 8 degrees higher than outlying areas. The increased temperature causes an increase in energy use for air conditioning. The combination of elevated pollutants from energy production and higher air temperatures creates smog, and decreases the air quality in the urban center.
Heat-flow measurements show the average daily energy demand caused by the heat flow through the green roof system was less than that of the reference roof system in springs and summer.
The green roof system’s energy efficiency was slightly better than that of the reference roof system in the fall and early winter because the growing medium acted as an insulation layer. However, as the growing medium froze, the green roof system’s insulation value greatly diminished. Snow coverage provided excellent insulation, but the snow coverage on the roof was not uniform because of the influence of wind and a high parapet. Snow ranged from 8 inches to 24 inches (200 mm to 600 mm) across the rooftop. Once snow coverage was established on the roof, heat flow through both roof systems was almost the same.
The green roof system significantly outperformed the reference roof system in spring and summer with regard to energy efficiency. Solar radiation strongly influences heat flow through a roof system. The reference roof system’s membrane, being exposed to the elements, absorbed solar radiation during the day and reradiated the absorbed heat at night, creating a high daily energy demand for space conditioning. On the other hand, the growing medium and plants enhanced the green roof system’s thermal performance by providing shading, insulation and evaporative cooling. The green roof system acted as a thermal mass, which effectively dampened the thermal fluctuations going through the system.
Definitions
green roof \’grēen ‘rüf\ n: an area of planting/landscaping, built up on a roofed/waterproofed structure at any level, that is separated from the natural ground by a man-made structure.
Garden Roof TM \'gärd-n rüf\ n: American Hydrotech, Inc.’s specialized system of drainage, water retention and root barrier components, utilized in the construction of a green roof over MM 3125 roofing.
Extensive Roofscape \ ik-'sten-siv 'rüf-,skāp\ n: low-to-no maintenance green roof landscaping consisting of shallow soil depths (>3” [76mm]) with plant varieties restricted to primarily mosses, herbs and grasses capable of withstanding harsh growing conditions (i.e., drought, wind, frost).
Intensive Roofscape \in-'ten-siv 'rüf-,skāp\ n: green roof landscaping requiring regular maintenance (i.e., watering, fertilizing) consisting of deeper soil depths (>6”[152mm]) with a wider variety of plants, including shrubs and small trees.
The Garden Roof ® Assembly combines Hydrotech’s superior waterproofing technology with ZinCo’s patented, engineered system of drainage/water retention components. Supported by ZinCo, the world leader in green roof system technology, Hydrotech can offer detailed solutions to the architect and owner to bring the structure back to life.
A brief description of some of the Garden Roof ® components:
Roofing Membrane- Monolithic Membrane 6125-EV a high endurance waterproofing membrane, no VOC’s, 25% post-consumer recycled content.
Protection Course/Root Barrier-Hydroflex 30/Root Stop WSF 40 or Hydroflex RB. Light weight or heavy-duty root barrier sheets.
Insulation-Dow STYROFOAM® moisture resistant, thermally stable, reusable, CFC free. (optional component)
Drainage/Water Retention Elements-FD40 or FD60 patented, three-dimensional, 100% recycled polyethylene panels provides water storage, drainage, and aeration for substrate soil. SSM45 a specially designed polypropylene mat for added water retention.
In addition to providing the Garden Roof® components, Hydrotech can work in tandem with the landscape architect to provide technical guidance on the selection of an appropriate blend of our LiteTop® lightweight soils and vegetation.
The Garden Roof® Assembly by Hydrotech is a sustainable system design backed by over 35 years of combined experience in premium waterproofing and green roof components. For more detailed information regarding the planning of your next “Garden Roof”, contact a Hydrotech representative to request a Planning Guide.
