The effect of the ventilated air layer in the new open joint facade on energy performance of the building

Horizontal development of the cities has resulted in much destruction environmentally and economically. The building’s façade acts as a barrier between exterior and interior climatic conditions. There are various methods for improving the energy performance of the building. One of these solutions is the ventilated facades. Open joint ventilated façade (OJVF) is a group of ventilated facades in which the exterior coating material (metallic, ceramic, stone or composite) is arranged in slabs separated by open joints that enable exterior air to enter and leave the cavity all along the wall. This ventilation leads to decreasing moisture and problems of condensation and ensure the health of the wall. In addition to aesthetic and constructive reasons, the main interest in open joint ventilated facades is their ability to reduce cooling thermal loads. This is achieved by the buoyancy effect induced by solar radiation inside the ventilated cavity, where the air can enter or leave freely through the joints. The OJVF is usually classified among the “light weight” or “Advanced Integrated Facades”. They are replacing the conventional facade in many new buildings and particularly in the refurbishment of old ones.
In light weight facades, the exterior light coating material is hanged over the interior wall (insulation, perforated brick or concrete, finish) by means of a metallic-frame structure or metallic bindings, leaving an air gap. The air chamber height can be the whole building height. The main difference between the OJVF and other advanced facades is that, as a rule, the ventilated air chamber is only open to the exterior at the top and at the bottom while, in the OJVF, the exterior coating is placed in an arrangement of tiles or slabs and a series of thin gaps (joints) shaped from slab to slab.
The improved thermal performance of the OJVF under radiation conditions relies on buoyancy: The slabs of the exterior coating are heated up and produce an ascending mass flow of air (by natural convection) that enters and leaves the cavity through the joints. This flow removes part of the heat loads. This phenomenon takes also place if the openings are only at the bottom and top of the facade, but the efficiency is not as high due to the reduced flow and the higher temperatures attained at the upper section of the air gap.
The main question is the effect of the open joints on the thermal performance of the façade in the various times of the year. The main objective of this work is to investigate the thermal and fluid dynamic phenomena taking place in a typical OJVF under solar radiation, and to appoint a methodology to quantify the energy savings produced by an OJVF in contrast to a conventional facade.
The determination of the linked thermal and fluid dynamic behaviour of the flow in the open joint air gap is quite a challenge, compared to the sealed cavity or even the top and bottom ventilated facades. The inlet and outlet flow through the joints all along the facade faces the analytical methods, making compulsory the use of CFD tools to obtain a detailed model. To achieve this aim, both of the open joint and the sealed facade have been simulated in the “Fluent” software.
Two series of tests have been carried out for both OJVF and sealed cavity facade. The first ones are steady state simulations directed to understand the phenomena involved, and the second ones are quasi-steady (unsteady or transient) state simulations to compare the energy performance of both systems. For the first group, two temperature conditions were selected, representing summer (T_room=24℃, T_ext = 30℃) and winter (T_room=24℃, T_ext = 8℃) weather. The (absorbed) solar radiation ranges from 0 to 800 W/m2. The radiation values above 400 W/m2 are high for summer, but not for winter, because the sun inclination is lower. The second group comprises two sets of simulations with the exterior temperature and the solar radiation varying hourly: one for a typical day of summer and another for a typical day of winter. The data used for simulations is produced by the Energyplus software.
Due to the extent of the subject, the study has been developed over a particular set of comparable geometries, and with specific climatic conditions; however, special care has been taken to avoid assumptions limiting its application.The OJVF simulated geometry comprises of four cement board tiles and five joints. Each slab is 70 cm that are separated by joints of 8 mm. The exterior coating layer is separated 0.05 m from the massive wall by the ventilated air cavity. The massive wall is composed of gypsum, a brick layer and exterior insulation with a total thickness of 21 cm. To study the heat transfer problem in the case of a conventional wall, a 3D model with the same dimensions has been created to simulate the convective loop inside the sealed air cavity. The only difference with respect to the OJVF model relies in the exterior coating which is continuous (without joints between slabs).
The CFD model developed to simulate a typical OJVF has enabled a better understanding of the ventilation effect induced by the solar radiation in the air gap of the facade. Velocity profiles, together with temperature and heat flux distributions have been compared with those obtained in a conventional sealed cavity facade.
Velocity profiles show that the air flow in the OJVF is ascending in the whole width and does not form a convective loop as in the sealed cavity facade. Moreover, the profiles show much higher velocity values in the case of OJVF. These two characteristics favour the heat removal from the cavity walls, which is one of the most claimed advantages of OJVF under radiation conditions. The air temperature in the cavity remains lower than in the conventional wall, and the heat transferred to the room is therefore lower.
The model has been also used to compare the thermal performance of both facades for the specific climatic conditions of Tehran and Yazd. The results of the simulations conclude that open-joint ventilated façade _in Tehran and Yazd city which are selected with respect to their solar radiation- can help to achieve substantial energy saving. Therefore using the OJVF in the south facade of the building in the aforementioned climates, considering their simple technology, is more suitable than conventional sealed facades.
The comparison of the energy performance of the specific OJVF and conventional façade analysed in this article shows that the open joint façade results in %20.5 energy saving for Tehran and %12 for Yazd through the south facade
At this point, it is not possible to give a definite criterion, because it is still necessary to evaluate the overall year performance of the specific OJVF geometry for each climate conditions, taking into account building costs and the price of the energy used for heating and cooling. Nevertheless, the data found in this study show that the OJVF could be a more energy efficient system than the conventional sealed facade, and help to reduce the cooling needs, mainly for south orientations in places with hot summers and mild winters.