Towards the aeraulic characterization of roof windows?

Low energy buildings, being highly insulated, are subject to important overheating risks. Thermal simulation as well as experimental studies have shown the large potential of ventilative cooling. One barrier against this approach is the difficulty of evaluating air flows. Appropriate calculation methods and characterization of openings are needed, so that these systems can be dealt with in design, regulation and certification tools.
The present study is based upon the monitoring of a 135 m2 zero energy house situated near Paris. Temperature profiles have been measured when varying the ventilation pattern, i.e. opening or closing vertical windows, roof windows and internal doors. A dynamic thermal simulation tool is used to evaluate temperature profiles in the house under the climatic conditions corresponding to on site measurements (external temperature and solar radiation). The model accounts for the conductive, radiative and convective heat transfer, as well as energy storage in the building envelope related to solar and internal gains. In this highly insulated new construction, the most uncertain parameter is the natural ventilation flow rate. This parameter, and the related aeraulic characteristics of the openings, can be calibrated by minimizing the discrepancy between calculated and measured temperature profiles. Given the small window height, and the large height between ground floor and roof windows, a one way flow model is considered.
The roof window characteristics will also be evaluated in a laboratory benchmark. A cell (3m x 3m x 2m) is divided into two compartments by a slanting wall including the window. A ventilator blows air into one space and the pressure difference is measured between both sides of the window. Varying the air flow rate allows a relationship between the flow rate and the pressure difference to be identified. This relationship may depend on the pressure difference between both sides of the openings, therefore calibration using on site measurement is helpful. The air exchange rate estimated by this method will be compared to measurements using tracer gas, performed in the house as well as in the laboratory benchmark. The possibility to use anemometers will also be tested.
The method proposed here, combining a benchmark in a laboratory with numerical simulation and on site monitoring may bring a supplementary input, complementing the existing knowledge in the field of passive cooling of buildings. The feasibility of using this method in order to prepare appropriate input data for numerical models implemented in regulation, design and certification tools will be studied.