News
August 20, 2014
Cool Roof Calculator Paper has been accepted for IC2UHI Venice Conference
December 25, 2013
The Cool Roof Calculator has been updated for parametric simulation, radiant barrier and simulation caching.
December, 2012
The Cool Roof Calculator is being upgraded to EnergyPlus 7.1.
April, 2012
The Cool Roof Calculator is being upgraded to EnergyPlus 7.0.
December 20, 2010
The Cool Roof Calculator has been updated.

External Links


Cool Roof Technology

       Cool Roof is a roofing technology which reflects the solar radiations and emits heat back into the sky instead of transferring it into the building below. This property of coolness of the roof is measured by two major properties of the roof surface.

       A reflective roof is typically light in color and absorbs less sunlight than a conventional dark colored roof. Less absorption of sunlight lowers the roof.s surface temperature, reducing the heat transfer through the roof. This property of the roof makes it a Cool Roof.

       When the roof surface is painted white or treated white, the surface temperature of the roof decreases. This further decreases the temperature difference between the outdoor and the indoor thereby reducing the heat gain into the rooms through conduction. This heat transfer can be measured using the formula:

Q= UA(ΔT)

       The solar reflectivity is measured according to ASTM E903. Traditional roofing materials have a Solar Reflective Index (SRI) of between 5% (brown shingles) and 20% (green shingles). White shingles with SRI around 35% were popular in the 1960s, but they did not gain enough popularity later, since they are more prone to getting dirty which would reduce their performance. The current trend is to make white shingles more reflective. The other property of a roof material, emissivity, defines the ability of the material to radiate out the absorbed heat. All common roofing materials except metals generally have an emissivity of 0.9. Emissivity of metal roofs is around 0.5, due to which rooms with metal roof have less possibility of being cooler and comfortable. Materials with less emissivity are appropriate in cold climates.


Tool Description

       There are two modes in this calculator, one the simple and other the detailed. The tool works on EnergyPlus v7.1 to run the hourly simulations and calculate the resulted annual savings. Broadly the inputs can be put under the heads of building envelope, internal loads, HVAC, building geometry, and the location.

       In the simple form, details such as roof type, roof finish, roof area and HVAC system type are the inputs. In the detail form, the building envelope data includes details such as: the window to wall ratio on each of the four major orientations of the exterior walls, roof insulation detail, the normal roof solar reflectivity and the cool roof solar reflectivity. Internal loads such as the equipment and the lighting density are also the inputs which the user needs to provide. In HVAC, system type and COP is the data which the user needs to input. By default a square block with perimeter and core zoning is considered for the simulation. Location of the building is another major input which is to be selected by the user. The city in which the building is located can be selected from the list.



       A detailed explanation of the model can be found here.

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