Window Retrofits and Attachments

Window Retrofits and Attachments

Analyzing Coverings

Window blinds
Window blinds in test facility

Window attachments represent a wide range of products that are commonly attached to windows in a house as an "add-on" device.

The most common and widely used types of attachments are window coverings and fashions that are typically used to control glare, to provide privacy, and for aesthetic purposes. Drapes are a typical example of this attachment type. Some types of coverings are traditionally used to control solar heat gain in conditioned and unconditioned houses. When they are used in unconditioned houses, they are used to create more comfortable living conditions. Examples of these products are outdoor-mounted louvered shutters, roller shutters and awnings. For the most part, indoor-mounted coverings and window fashions are used more for aesthetic reasons, privacy, and glare control, although some energy benefits of their use is recognized.

While we understand that different types of coverings have varying effects on energy performance of the windows they are installed over, generally their energy performance is not as well understood as that of prime windows. This is because their mounting and occupant use impacts their performance, and their energy impact can be orientation and climate dependent. Some limited measurements were done over the past few decades for both indoor and outdoor-mounted shades. These measurements were done for specific window combinations and in specific climates. Due to the high cost involved in measurements and the uncertainty and lack of standardization of these measurements, it is only possible to observe broader trends from them.

Simulation methods for window coverings and other optically complex systems, including complex glazing, have been steadily advancing over the past decade and now cover a large range of products. With support from the Department of Energy, we have significantly expanded the modeling capabilities of a suite of energy performance assessment tools originally developed for the window industry. Berkeley Lab researchers completed the development and implementation of a generalized Bi-Directional Scattering Distribution Function (BSDF) methodology, which enabled SHGC and Tv of an expanded range of products and devices to be modeled.

The BSDF methodology was first implemented in our WINDOW and THERM programs for calculating thermal and optical performance of windows with coverings and later was extended to EnergyPlus to allow for the calculation of energy impacts of these products on a commercial or residential building.