Energy-efficient windows for green buildings

For it to earn its stripes, a green building must fulfill two basic criteria. It must be built with energy-efficient materials and make efficient use of energy.  Among the green window options is Fenesta, which has introduced a range of green and monsoon-ready windows.

Windows can conserve energy through two different ways. The window material should not consume high energy to produce or it should not put much strain on our energy reserves and natural resources. The window, once installed, should prevent any energy losses through it. The most popularly used material for window frames are wood, steel, aluminium and PVC or vinyl. Using wood puts a lot of stress on our already depleting forest reserves.  

Basic aluminium is extracted from aluminium ore or bauxite. This process consumes about 56kWH of energy per kg of aluminium by the most efficient process of extraction. Some windows are made of PVC, which is 57 per cent salt and the rest is natural gas or petroleum produced through a chemical process which consumes the least energy even among all the highly consumed polymers. Natural salt is abundantly available and total PVC production consumes only 0.3 per cent of annual oil and gas consumption and 10 per cent of salt consumption.

The window frame section of both aluminium and PVC are produced by a process called extrusion. The PVC extrusion is a very low-energy consumption process, operated at a temperature of 185C, while aluminium extrusion is done at a temperature above 700C.  
How can a window can be efficient in conserving energy in a building? Heat loss / gain through a window can happen through any of the following four ways

Heat flow through a window: This is measured by ‘U value’.
The U value of a typical aluminium single glazed window is about 6.591 W/m2K and that of Wood/Vinyl 4.77 W/m2K.  A study says that nearly 60% of all the residential buildings in 15 EU countries are still single glazed with average U Value of 5.7 W/m2K
Amount of heat gained from direct solar heat as a factor of total heat falling on the window: Affected by the glass used in the window and less affected by the framing material.

Amount of visible light allowed through a window as a factor of total visible light falling on the window: Affected by the glass used in the window and less affected by the framing material.

It is the amount of air that passes through a window: This is affected by how the window is made and installed, and depends on the design, type of material used and technology used to fabricate the window.

PVC has the lowest U value which implies it is the preferred material for energy-efficient windows.  Insulation is further improved by the use of sealants between the wall and frame, double or even triples glazing and fusion welded joints that leave no gaps.  
Resource reuse: UPVC is 100 per cent recyclable. A report by British Plastics Federation, states that PVC can be recycled in a relatively straightforward manner. It is now common practice to recover and recycle factory wastes and / or off-cuts after the window has been fabricated.

Indoor environment quality: Today, UPVC windows are designed to perfection and fabricated in modern state of the art factories provide windows with excellent sealing to keep environmental pollutants like sound, dust and even tobacco smoke isolated. With these windows a sound insulation of more 30 per cent is always possible.

As per British Plastics Federation, the life expectancy of a PVC window is over 40 years. Low emitting materials (adhesives and sealants) – The silicone sealants used for sealing window around the periphery have nil VOC content and are safe to use. Low emitting materials (paints and coating) – The PVC window is extruded pristine white and does not require any maintenance. It can be easily cleaned with soap and water and does not require any painting.

Indoor environment quality: Today UPVC windows are designed to perfection and fabricated in state-of-the-art factories provide windows with excellent sealing to keep environmental pollutants like sound, dust and even tobacco smoke isolated. With these windows a sound insulation of more 30 per cent is always possible.