Breathing clean, oxygen-rich air is easier on the respiratory system, and the vegetation on green roofs helps to re-oxygenate the air through the photosynthesis process of transforming carbon dioxide into oxygen. At higher temperatures, dry air can make breathing difficult, even when the air is clean. The natural evaporation of moisture from vegetation into the air can provide for easier breathing and a less stifling micro-climate.
Furthermore, some breathing troubles can be attributed to the dust that is constantly present in the air. In cases of allergies and asthma, breathing can also become aggravated by this atmospheric dust. Plant vegetation reduces atmospheric dust by attracting it to leaves and other plant surfaces and holding the dust until it is washed off into the soil by precipitation. The larger and more numerous the leaves, the more effective they are at removing dust from the air. In some parks and gardens, the air has been shown to contain 85 percent less dust than elsewhere.
Keep The Noise Down
There are advantages to the work environment, as well. Green roofs have been shown to allow less noise transmission into the buildings they cover. A higher level of external noise pollution is absorbed by the soil and vegetation of the roof garden than by a traditional roof system. Although the exact amount of noise reduction depends on the thickness of the substrate layer and other factors, such as existing sound leaks that can come from skylights, it is not uncommon to realize noise reductions of up to 50 dB from a green roof.
Green roofs can also protect the roof membrane from punishment caused by weather and the elements. The vegetation, soil, and walking surfaces, such as pavers, can help protect the waterproofing membrane from temperature extremes, solar degradation, thermal shock, wind, hail, and mechanical damage.
Insulation Requirements
Because of the presence of vegetation, it is important that green roofs be able to store water and not dry out too rapidly. If a green roof does not hold a certain amount of free water in the soil, then additional piping and watering systems must be used in order to keep the vegetation alive.
However, because the soil on a green roof is capable of storing water, problems can occur if excess moisture leaks into the insulation envelope. Although a tapered waterproofing layer can be recommended in order to drain this excess moisture, a flat roof system allows for greater water storage capacity. If a flat roof is chosen for the green roof, the membrane/insulation system must be highly resistant to moisture and its destructive effects.
Since the reliability of a green roof is highly dependent on the waterproofing system applied, an impermeable closed-cell insulation material is especially critical.
Combining a sealed insulation material, such as cellular glass, with the roof membrane is one option for flat roof systems. Since water cannot accumulate in cellular glass insulation, the danger of root invasion is eliminated. Plant roots follow paths of moisture and have been known to perforate insulation that has become damp; they are even capable of penetrating into the building itself in order to seek out whatever water source they can find.
Loss of thermal efficiency is a major problem associated with damp insulation. The layers of waterproofing material in a green roof are designed to keep moisture from entering the building, while the insulation is meant to prevent water already in the green roof system from extracting heat in the winter or cool air in the summer from the building. If an insulation absorbs only 4 percent moisture by volume, it can lose 70 percent of its thermal efficiency.
Another factor in insulation selection is that green roofs can add an enormous amount of weight to the roof of a building. The weight of the soil, drainage material, plants and walking surfaces will compact weak insulation, and loss of membrane support will occur. This will also result in local depressions where water will pond and accumulate on the flat roof. Cellular glass insulation, with its high compressive strength of 100 lb. per sq. in., will not compact under the weight of the overlying layers of the roof garden.
Julian Arhire is a Manager with DtiCorp.com - DtiCorp.com carries more than 35,000 HVAC products, including industrial, commercial and residential parts and equipment from Honeywell, Johnson Contols, Robertshaw, Jandy, Grundfos, Armstrong and more.
