Heat flux roofs: Effect of colour rooftop on heat flow through roof during summer

Heat flux roofs: Effect of colour rooftop on heat flow through roof during summer

Gunnink, F.L.

Van den Dobbelsteen, A.A.J.F. (mentor)
Van der Ham, E. (mentor)
Prendergast, E. (mentor)
Schipper, H.R. (mentor)

Civil Engineering and Geosciences

Structural Engineering

2012-02-23

The main focus of this research was on the heat flow (density) through a roof in summertime. The effect of different roof toppings and construction types on the heat flow are investigated. Both a field experiment as well as a model were set up, to acquire data and generate results. Central research question was: What difference does roofing material and insulation make on the heat transfer/flow through a roof? With the following sub questions: What is the amount of heat transferred through a roof in summer conditions? How can this heat transfer be modelled? Key parameters that determine the heat flow through a roof are the reflectivity of the roof top, the insulation value and the (thermal) mass of the roof construction. Literature reports that change of the albedo of roofing reduces roof top temperature, inner temperature and energy usage for cooling. The higher the insulation value of the roof, the less reduction in energy demand for cooling when a cool roof is installed. Planted roofs give similar results. The soil and leaf layer function as an extra insulation layer on the roof. Nevertheless, plants cannot replace traditional insulation. A field experiment was performed in the summer 2011 on four locations. On three locations the rooftops were black, while a part was covered with white roofing. The Rc-value and mass of the roofs differs. The last location concerns a green (or planted) roof. The difference in outer surface temperature between black and white roofing was at maximum 25°C, the average on warm summer days was 9°C. The outer surface temperature of the green roof was of the same value as white roofing. The heat flow (qin) through roofs covered with white roofing was lower than in case when black roofing was installed. The size of the difference (?qin) depends on Rc-value and thermal mass of the roof construction. The peak in qin and ?qin were not measured exactly at the same time, but variance was not more than a hour. Due to insufficient data from the field experiment, the heat flow qin through a green (planted) roof was not determined. A model in CAPSOL is set up in order to gain results of the difference in heat flow through roofs covered with black and white roofing. Model and field experiment outcomes were in good agreement. In the model heat transfer coefficients ?e and ?i must be defined. It was found that a value of ?e between 15-20 W/m2K give good results when compared to the field experiment. For ?i a value between 5-9 W/m2K can be chosen, it doesn’t effect the model results of qin. White roofing (a=0,2) lowers the heat flow through a roof. With the model outcomes graphs are generated which give the relation between Rc-value and difference in heat flow (?qin) between black (a=0,9) and white (a=0,2) roofing. This is executed for light roofs (wooden construction, 20-50 kg/m2) and heavy roofs (concrete construction, 300-400 kg/m2). Thermal mass influences the moment in time of the ?qin. It was found that the peak in ?qin for light roofs with an Rc< 4,5 m2K/W occurs during office hours. In case of a heavy roof the peak ?qin is around midnight. Finally the daily Heat Flow Factor is introduced. It gives the daily difference in heat flow between black and white roofing in percentage of the daily incoming solar radiation. This percentage is translated to values for ?qin on a mean Dutch summer day (?qsun=7200 Wh/m2)

heat flux roofs black white roofing

http://resolver.tudelft.nl/uuid:d4c8dac5-5d3e-49b4-a740-0c833508d470

Student theses

master thesis

(c) 2012 Gunnink, F.L.