Sash and frame

The figures below illustrate the general development trends for window sash and frame in Scandinavia within the last 40 years with examples from Winsmart project partner IDC. Up until the 1970's window frames were largely produced in wood, primarily heartwood from Scots pine. The purpose of the sash/frame was to hold the glazing in place and allow the window to be opened. Insulation properties was not a major driving force for development at this stage.


In the 1980's, maintenance and service life became important development drivers. In windows, wood is viewed as a high maintenance material. Wood is susceptible to biodegradation by decay fungi (rot) and to dimensional instability due to its hygroscopic properties. Wood windows therefore require a relatively high level of maintenance - in the form of surface treatment (painting) at regular intervals - to be kept sound. Consequently, window producers gradually began to limit the amount of wood in the parts of the window exposed to the weather. The figure below shows a window with an aluminum profile attached on the outside of both sash and frame to protect the wood from direct exposure to the elements. The aluminum profile increases the durability of the window and lowers the maintenance requirements for the consumer. However, the main part of both sash and frame is still wood which was typical for the Scandinavian window market at the time. Elsewhere in Europe, especially in in Germany, the development trend went towards eliminating the use of wood in windows altogether, and to produce window sash/frame exclusively in PVC or, later on, aluminum.


Mainly driven by architects, sash and frame design became an important development factor from the 1990's onwards. The sash and frame part of the window should be as slim as possible to increase the amount of light per window opening and to make the window look more elegant from the outside. This is evident in the figure below, which shows a considerably slimmer design compared to the figures above. The sash is now designed to be an extension of the frame rather than closing in on top of the frame. The trend of limiting the use of wood is continued and the sash is now constructed of pure aluminum with no use of wood. Still, insulation properties and energy efficiency were not the main development drivers.


From the turn of the millennium to the present day, energy efficiency has grown increasingly important as a development factor for window sash/frame design and is now perhaps the main driver for sash and frame development. The development has been partly market driven because the ever-increasing energy costs coupled with increased environmental consumer awareness has resulted in an increased demand for energy efficient windows. In addition, legislative action has put minimum demands on the energy efficiency of windows in newly constructed buildings.

The figure below shows a profile of FUTURA+, a state-of-the art Scandinavian window from IDC. The entire sash and the outer part of the frame is made of rigid poly-urethane foam (PUR) with a density high enough to give it the structural properties needed to support the weight of the triple glazing and to give the whole window structural stability. At the same time, PUR provides good insulation properties due to the relatively low thermal conductivity (λ-value) of PUR foam compared to wood, PVC, and aluminum. Two versions of FUTURA+ is produced: one for the Scandinavian market opening out and another for the central European market opening in. The Scandinavian version (below) is designed with wood on the interior part facing the consumer while the central European version has aluminum facing the customer.


The U-values of the FUTURA+ window as a whole (Uw) is 0.87 for the Scandinavian version opening out and 0.83 for the central European version opening in with triple glazing installed.


Sash and frame in Winsmart


As described above, current state of the art windows have a Uw-value in the range of 0.8. With this background, the winsmart objective of reaching a Uw-value of 0.3 is very ambitious, especially when you consider the additional objectives of lower weight, reduced embodied energy, and lower production cost compared to state-of-the-art.

The window sash and frame design should enable the overall winsmart objectives as well as provide structural support for the VIG and the switchable technology. These goals will be achieved through use of the latest technology materials with extremely low thermal conductivity.



This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 314407.