Experimental Characterization and Modeling of Advanced Polymer Composite Window Frames

Even though the window frames cover a relatively small fraction of the entire building envelope, they are responsible for a major amount of heat loss, due to their poor insulation properties compared to the other envelope elements. With the current trends to reduce energy use in buildings, it is obvious that the thermal performance of contemporary frames needs to be improved, so that buildings can fulfill the new, more rigorous demands.

This study will focus on improving the thermal performance of window frames made of fiberglass reinforced plastic (FRP). This material has been recently introduced to the window frame market and is becoming an increasingly interesting choice, due to its fine thermal and mechanical properties which can lead to a significant improvement of the insulation properties of a window frame.

There are several ways to improve the thermal transmittance of a window frame. Heat transfer through a frame is a combination of conduction, convection and radiation and each of these mechanisms of heat transfer can be diminished in certain ways. An example could be subdividing the cavities to reduce convective heat flow or using low emissivity materials to limit the thermal radiation. These, and several other solutions, are investigated in the first part of the study. Simulations on simple geometries are conducted to evaluate the potential of different solutions. The results show that by applying different modifications the thermal transmittance of a frame can be significantly reduced, but only to a certain extent. To reach further improvement, more drastic changes need to be done.

Therefore, an extensive study is done on how to optimize the frame geometry on a broader level. Contemporary frames can be crafted into various shapes, giving the engineers a large freedom of design. Almost an infinite number of window frame geometries can be created, but not all of them will result in a frame with satisfying thermal properties. It is important to determine how a proper design of the frame can contribute to the improvement of its thermal performance. Afterwards, the entire frame geometry can be optimized in terms of the thermal transmittance, size and material use.

The optimization process can be conducted by designing very large number of window frame geometries and evaluating their thermal properties, preferably in an automated way. A large part of this study is devoted to the development of a parametric design tool that could be used for this purpose. Afterwards, the final tool is used in a process of optimizing the frames. Optimization is done with regard to several objectives, what makes the task more complicated. Two approaches to this problem are suggested in this study: a weighted sum method and a genetic algorithm.

Another objective of the project was to develop a new combination of materials that could be used in improved window frames. This combination consists of a high strength composite laminate on the outside and highly insulating foam on the inside. As a result, a functionally graded material with spatially varying properties was developed. This unusual construction creates a challenge for experimental characterization of this product. This issue can be solved by using laser flash method, which can be adapted for these specific purposes. Small modifications have been done in the LFA 447 laser flash apparatus that allow for derivation of the local transport properties of various materials, with a fine resolution. The method was validated and successfully used for this purpose.