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developed by the Athena Institute for Green Roofs for Healthy Cities (GRHC)
See note below and tutorial guide for a more indepth discussion and interpretation of financial results
The discount (hurdle) rate is to be entered by the user. This rate may be equivalent to the financing or mortgage rate, the accepatable rate of return (hurdle rate)
reflecting the inherent risk of the project or the average weighted cost of capital (debt and equity ) for an organization. An LCC yielding a zero present value at
the desired time period for the investment indicates that the initial hurdle rate was attained, a positive present value indicates that the hurdle rate was surpassed
while a negative PV indicates that the hurdle rate was not achieved. The LCC calculator takes a very long term view from a private investment perspective and provides a before tax Net Present Value.
Financial Results Summary - the results calculator worksheet and the LCC results summary above provide the Net Present Value for a single investment at the selected
discount factor and investment time period intervals (up to 60 years). Any one investment must be compared to at least one other investment alternative providing the
same function and utility at the same investment discount rate and over the same time period. The investment having the lower LCC is the preferable option. The user
can compare or contrast up to three investement scenarios and save their individual files by various names to reuse the calculator for assessing the LCC of other
alternative investments. The results summary above also provides a simple payback period (in years) for the total installed cost and calculates a before tax internal
rate of return (IRR) or return on investment (ROI). An IRR message “very negative” will be given when no positive cash flows are generated over the life of the investment analysis.
In addition, the calculator allows the user to input other annual costs or benefits. For example, water use savings or costs.
An increase in waterproof membrane durability accrues cost savings to a building owner through a diminished need for re-roofing over a building‘s lifetime. N. American roofs have an
average life span of 10-15 years (Hutchison, 2001), whereas membranes in greenroof applications in Germany typically last 30 to 40 years. Admittedly, the life span of green roofs in N. America
may deviate from the German value, varying by region, climatic zone and waterproofing membrane type. Protected membrane roofs (PMR) assemblies, those where the membrane is covered by insulation, have
similar performance attributes as a green roof in terms of membrane durability. These assemblies typically have an average lifespan 20-25 years in N. America and may be more indicative of the rate of
re-roofing required for green roof applications (Mutton, 2004). For purposes of LCC assessment, GRHC suggests using a 35 to 45 year life span for green roofs. Green roofs prolonged life span postpones the generation of end-of-life roofing materials going to landfill.
Roof Durability Statistics
Maintenance, Repair and Replacement Effects for Building Envelope Materials (2002) prepared by Morrision Hershfield Ltd. for the Athena Institute
Cash, C.G. (1997) The relative durability of low-slope roofing. In Proceedings of the fourth International Symposium on Roofing Technology
Roofs are subject to the greatest amount of heat loss in the winter due to rising heat and, as one of the hottest surfaces in urban centers, the largest amount of heat gain in the summer. Various studies have shown that green roofs can significantly moderate the heat flow through a roofing system and thus, reduces the average daily energy demand for space conditioning (see some typical values below).
Factors affecting the degree of energy savings associated with a green roof include:
cost of heating fuel and electricity, thickness of roof insulation + thickness of the green roof growing medium, the efficiency of the HVAC system, positioning of air intake vents on roof, percentage of green roof coverage, building height (roof area as a ratio of wall area), plant selection and coverage, amount and timing of irrigation (if any) and climate zone.
Items not considered in this version of the tool include:
- Building integration synergies - e.g., employing a green roof in combination with PV panels to keep the panels cooler and thus operating at a higher efficiency.
- The pre-cooling of air afforded by green roofs when roof top air handles intakes are located in close proximity to the green roof surface.
Annual Savings/sq.ft. of Roof Area
Source: Allen Lee, Quntec LLC
Annual energy savings for three different cities for a “typical” green roof on a single story building. Savings estimates per sq.ft. of roof are fairly constant with building height, though they vary a moderate amount for heating energy. The analysis assumes natural gas heating and electric cooling.
Other studies have indicated a 1% decrease in heating energy and a 6% decrease in cooling energy associated with green roofs.
Dense urban areas have a limited capability to accommodate surface water run-off due to a low percentage of permeable surface area; resulting in heavy usage of storm water infrastructure. For many cities storm water infrastructure is in need of renewal and expansion. In the interim, some cities offer one time development grants/subsidies to developers and building owners who employ on-site storm water management methods (e.g., cisterns, bioswails and green roofs that limit the storm water burden on existing infrastructure.
Does the municipality in which your project is to be built provide a storm water management subsidy or development grant? Do any of the scenarios you are considering meet the minimum storm water management requirements and hence are eligible for the grant. If so, use the worksheet below to calculate the grant value. This value will be subtracted from the initial capital cost of your project scenario.
There a number of other benefits associated with storm water control which are excluded from this tool at this time. These include:
– downstream erosion control;
– reduced municipal infrastructure requirements;
– reduced downstream water treatment requirements;
– reduced combined sewer overflow into water bodies;
– reduced potable water demand through reuse of captured storm water;
Storm Water Worksheet
This worksheet has been devised to calculate a one time grant/subsidy for the mitigation
of storm water run-off. The grant is applied as a reduction to the Initial Capital Cost
to arrive at the Net Capital Cost for the project. If the grant or subsidy program in
your location is an annual grant or subsidy - do not use this worksheet!
Instead, calculate the amount applicable annualy and enter it as a positive value in the line “Other Annual Costs or Benefits” under the applicable scenario.
The urban heat island (UHI) effect is the temperature increase in urban centers caused by the replacement of “natural vegetation” with pavements, buildings and other structures. The impermeable surfaces convert sunlight to heat reducing natural cooling processes provided by vegetation. The UHI effect can have important ramifications for human health and energy use.
Does the municipality in which your project is to be built provide a UHI effect mitigation subsidy or development grant? Do any of the scenarios you are considering meet these mitigation requirements and hence are eligible for the available grant. If so, use the worksheet below to calculate the grant value. This value will be subtracted from the initial capital cost of your project scenario.
Warmer UHI temperatures can also exacerbate air pollution contributing to the formation of smog and ozone. These higher levels of air pollutants increase the risk of health complications. Green roofs may help alleviate these air pollution problems via their filtering of pollutants (NOx, VOCs and particulates) and mitigation of UHI. Air pollutant effects are not directly considered in this version of the tool.
To be developed in the next phase.
Employing various types of roof systems may help meet or exceed municipal greening and sustainablility goals. This worksheet can be used to determine the extent to which the various roofing systems being considered may be in line for possible development cost relief or bonuses. These development elements may include:
1) land purchase cost avoidance to attain stated municipal storm water control measures;
2) a development density bonus due to effect mitigation;
3) “fast tracking” of the development process resulting in reduced project development cost associated with obtaining various permits; and,
4) approval to build on more sensitive sites.
All of these possible benefits need to be apportioned to the roof system employed as opposed to the “other” green building elements of the development.
Various types of roof systems have the potential to increase rental income by attracting tenants and reducing vacancies. If a roof offers recreational or gardening opportunities, or abates overhead noise, rents may rise to reflect these added values. Alternatively, rooftop amenities may provide additional rentable commercial space (such as a plant nursery or lawn bowling).
Food garden production calculation guide.
Green roofs can increase the sales value of a property by boosting the quality of life to residents in the building, reducing noise from overhead sources, and attracting media attention. Quality of life improvements can flow from having access to a green roof for passive and active recreation. In cases where the green roof is on a shoulder of the building and is visible from upper floors, further property value increases can be expected due to quality of life enhancements for those units with a view of the green roof. Green roofs can also enhance property values by helping to abate noise from overhead sources such as aircraft. Finally, green roofs can attract media attention and provide free publicity for a development. This attracts buyers and reduces the cost of advertising the building.
These benefits are summed as a one-time capital benefit that is included in the calculator for the year in which the building is sold.
See calculating the public relations benefit.
Providing accessible and viewable greenspace can positively affect worker health and productivity. Studies have shown that workers with viewable greenspace from their workstations are generally more productive and are less prone to take sick days. This worksheet is very preliminary and requires careful consideration when used as this effect is very site specific. For example, a building with a conventional roof membrane may still have sight lines from windows looking out on pleasing vistas on higher floors or ground level greenspace on lower floors. Similarly, a building with a greenspace roof that is not viewable or accessible may not have any influence on worker productivity or absenteeism. Users of this worksheet should ask themselves to what extent do greenspace views or access effect their calculations on a relative basis and use this incremental value when completing their analysis.