Abstract
Pitched wooden roofs are ventilated through an air cavity beneath the roofing in order to remove heat and moisture from the roof construction. The ventilation is driven by wind pressure and thermal buoyancy. This paper studies ventilation driven by thermal buoyancy in the air cavity of inclined roofs. The influence of air cavity design and roof inclination on the airflow is investigated. Laboratory measurements were carried out on an inclined full-scale roof model with an air cavity heated on one side in order to simulate solar radiation on a roof surface. Equipment to measure temperature was installed in the roof model, while air velocity in the cavity was determined by smoke tests. Combinations of different roof inclinations, air cavity heights and applied heating power on the air cavity top surface were examined. The study showed that increased air cavity height led to increased airflow and decreased surface temperatures in the air cavity. Increased roof inclination and heating power applied to the roofing also increased the airflow. The investigations imply that thermal buoyancy in the air cavity of pitched roofs could be a relevant driving force for cavity ventilation and important to consider when evaluating the heat and moisture performance of such a construction.
Highlights
Figure 5with gives examples of how the airroof temperature profileAalong the cavity from inlet outlet changed cavity height inclination complete overview from inlet to to outlet changed with air air cavity height (h) (h) andand roof inclination (θ). (θ)
A complete overview of fromofinlet to outlet changed with air cavity height (h)inand roof inclination (θ)
Experimental investigations of thermal buoyancy in the air cavity of pitched ventilated roofs were performed to answer the following research questions: (1) How are the temperature conditions in the air cavity related to the air cavity design? (2) How is the airflow through the cavity influenced by the air cavity design? (3) To what degree may thermal buoyancy drive airflow in the air cavity? The temperature (1) and flow conditions (2) in the cavity were observed to be dependent on both cavity height and roof inclination
Summary
The use of wooden roofs poses certain challenges, as building envelopes are subject to large weather strains and related damage risks [2]. To ensure their durability, pitched roofs are designed to be naturally ventilated by allowing air to flow through an air cavity beneath the roofing. This ventilation is essential for removing moisture and heat from the roof construction, which is necessary to avoid damage and durability problems [1]. The increasing focus on CO2 -emissions from buildings and the favourable carbon footprint of wood make wooden roofs attractive to use Buildings 2020, 10, 8; doi:10.3390/buildings10010008 www.mdpi.com/journal/buildings
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