Green Roof Resource

The energy literature on green roofs developed in the 1990s. The majority of research has focused on the insulative abilities of green roofs in summer. In summer months, green roofs behave as high quality insulation reducing the flux of solar radiation in a building [1,2]. Insulation layers may retard heat flux in situations that are undesirable. While insulation reduces cooling load for hours when outside temperature is higher than inside temperature, the insulation retards heat loss for those hours the internal temperature exceeds external temperatures [3]. For a dark roof, the negative impact on AC usage during hours where external temperatures were below internal temperatures was greater than for a white roof suggesting that both roof insulation and roof reflectivity should be assessed to minimize energy use in buildings.

A recent study on the surface heat budget on a green roof and high reflectivity roofs revealed that the sensible heat flux is small compared to concrete roof surface on both a highly reflective white paint surface and a green roof [4]. The heat flux is small on the white roof due to the low net radiation. In contrast, the green roof had a large net radiation. The small sensible heat flux for the green roof was attributed to the large latent heat flux by evaporation [4].

The two main parameters that influence the solar radiation that reaches the roof deck are leaf foliage and soil thickness. The leaf area index (LAI) and leaf angle affect shadows [1]. The larger the foliage development of a particular plant, the smaller the heat flux through the roof [1,2,5]. Roof surface temperatures also decrease according to increasing LAI [6].

Soil thickness also plays an important role in heat transfer. Thick soil layers reduced cooling needs during summer months while thin substrate layers resulted in little to no cooling benefit [5]. Roofs with substrates between 7.5 cm and 10 cm reduced the average daily heat flow throughout the year although greater in summer months [7]. Generally, heat transfer is greater on roof surfaces that are not vegetated [2,6] although the vegetated roof should not serve as a replacement for insulation [8].

Sources:
[1] Del Barrio, E. 1998. Energy and Buildings, 27:179-193.
[2] Niachou, A; Papakonstantinou, K; Santamouris, M; Tsangrassoulis, A; Mihalakakou, G. 2001. Energy and Buildings, 33:719-729.
[3] Akbari, H. 2003. Energy, 28:9:953-967.
[4] Takebayashi, H and M Moriyama. Building and Environment, Volume 42, Number 8, p.2971-2979.
[5] Theodosiou, T G. 2003. Energy and Buildings, 35:909-917.
[6] Wong, N; Chen, Y; Ong, C; and A Sia. 2003. Building and Environment, 38:261-270.
[7] Liu, K; Minor, J. Proceedings for 3rd Annual Greening Rooftops for Sustainable Communities. Washington, DC.
[8] Eumorfopoulou, E and D Aravantinos. 1998. Energy and Buildings, 27:1:29-36.

As green roofs become more established within the US building community, the need for standardization of good building practices increases. Green Roofs for Healthy Cities is establishing an accreditation program to provide proper training to professionals. The National Roofing Contractor’s Association (NRCA) has recently published a green roof guide. The American Society for Testing and Materials (ASTM) has published four Green Roof Performance Standards and one standard guide. Two additional standards are under consideration. What follows is a brief overview of the ASTM standards. Links to the summaries of the standards as provided by ASTM are below.

ASTM E2396-05 Saturated Water Permeability of Granular Drainage Media
This standard details the procedure for assessing the permeability of granular materials used in the drainage layer. The permeability under low-head, horizontal flow conditions that exist on green roofs is also addressed. This method for determining water permeability assists in determination of the dead load in the ASTM E2397-05.

ASTM E2397-05 Determination of Dead Loads and Live Loads Associated with Green Roof Systems
The dead load and live load determination standard provides a procedure for predicting the system weight of a green roof system. The weight assessment accounts for components that are typically encountered in green roof systems. The weight is determined under two conditions. Dead load is the weight of the system under drained conditions and the weight of retained water or other precipitation. The second scenario assesses the weight when precipitation is actively occurring and the drainage layer is saturated. The difference in weight between the first (dead load) and second conditions is considered a live load.

ASTM E2398-05 Water Capture and Media Retention Standards of Geocomposite Drain Layers for Green Roof Systems
To determine the saturated weight, the standard method for water capture and media retention of the drainage layer would be used. This standard is applicable to geocomposite drains layers that retain water and media in cup-like receptacles on their upper surface (e.g. shaped plastic membranes).

ASTM E2399-05 Maximum Media Density for Dead Load Analysis of Green Roof Systems
The standard method for maximum media density assists determining the dead load by providing a measure of the moisture content and the water permeability measured at the maximum media density.

ASTM E2400-06 Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roofs
The plant selection, installation, and maintenance guideline is applicable to both extensive and intensive green roof systems.

Two additional standards are currently under development. ASTM WK575 Practice for Assessment of Green Roofs will include technical requirements and sustainable development considerations. Assessment of some technical requirements may refer to existing green roof standards. Fire safety will likely need additional standards developed for proper assessment. Assessment for sustainability considerations may include energy efficiency, water management, and biodiversity.

The other standard under development is the ASTM WK7319 Standard Guide for Use of Expanded Shale, Clay, or Slate (ESCS) as a Mineral Component in Growing Media for Green Roof. This standard describes the characteristics of the material to be a mineral amendment and covers the sampling appropriate for the procedure.