Timber roof constructions are commonly ventilated through an air cavity beneath the roof sheathing in order to remove heat and moisture from the construction. The driving forces for this ventilation are wind pressure and thermal buoyancy. The wind driven ventilation has been studied extensively, while models for predicting buoyant flow are less developed. In the present study, a novel analytical model is presented to predict the air flow caused by thermal buoyancy in a ventilated roof construction. The model provides means to calculate the cavity Rayleigh number for the roof construction, which is then correlated with the air flow rate. The model predictions are compared to the results of an experimental and a numerical study examining the effect of different cavity designs and inclinations on the air flow rate in a ventilated roof subjected to varying heat loads. Over 80 different test set-ups, the analytical model was found to replicate both experimental and numerical results within an acceptable margin. The effect of an increased total roof height, air cavity height and solar heat load for a given construction is an increased air flow rate through the air cavity. On average, the analytical model predicts a 3% higher air flow rate than found in the numerical study, and a 20% lower air flow rate than found in the experimental study, for comparable test set-ups. The model provided can be used to predict the air flow rate in cavities of varying design, and to quantify the impact of suggested roof design changes. The result can be used as a basis for estimating the moisture safety of a roof construction.

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.

Connecting Life Cycle Assessment (LCA) to parametric design has been suggested as a way of facilitating performing environmental assessments in early design stages. However, no overviews of potential approaches and tools are available within recent research. Also, no characterisation frameworks adapted for parametric LCA tools are present. In order to guide the development of workflows for environmental analysis aimed at the early design stage of buildings, the goal of this paper is to provide such a framework, and to demonstrate its use by characterising a number of available LCA plug-ins for the commonly used parametric design framework Grasshopper® (GH). First, a framework for classification and characterisation of tools based on workflow, adaptability, and required user knowledge was developed. Second, a tool inventory was performed, identifying 13 parametric LCA plug-ins for GH. Finally, four of these plug-ins were further investigated using the developed evaluation framework, a user persona approach, and a simplified test case. It was found that the characterisation framework was able to differentiate tools based on the level of LCA expertise integrated in the tools, and the allocation of responsibility for data entry and interpretation. A contrast was found between streamlined tools, and tools which provide more versatility. The characterisation framework, and the resulting overview of approaches can be used to guide the future development of parametric environmental analysis frameworks.

Implementing Building Performance Simulation (BPS) in a parametric design framework is a prevalent way of facilitating environmental assessments in early design stages. However, no up-to-date overviews of potential approaches are available, and no characterisation frameworks adapted for parametric BPS tools are present. In this study, such a framework was developed and its use demonstrated through an investigation of eleven available BPS tools for the parametric design framework Grasshopper®. It was found that the framework was able to successfully differentiate tools based on the level of BPS expertise integrated in the tools, and the allocation of responsibility for data entry and interpretation. A contrast was found between streamlined tools, and tools which provide more versatility. The characterisation framework, and the resulting overview of approaches, can be used to guide the future development of parametric environmental analysis frameworks for buildings.