Windows are made from glass, and all glass is created equal, right? WRONG. Have you ever stood by a window on a hot day and felt the solar heat bearing down on you? That is because glass offers very little resistance to radiant heat transfer.
You may ask, “What is radiant heat transfer?” To answer that let me give you a little background science. There are three ways that thermal energy moves from one place to another:
- Conduction – heat transfer through direct contact (i.e. a metal rod in the fire has its other end heat up since the what travels through the rod itself)
- Convection – heat transfer through the movement of a fluid (i.e. heat move upward in a pot of water on your stove as the water flows in a circular pattern)
- Radiation – heat transfer through infrared or UV rays (i.e. a warming lamp at a restaurant transfers heat to the food through its light)
The goal of low-e window coatings is to lower the amount of thermal energy passing through your window by way of thermal radiation. The means by which it does this can be understood by examining the ‘e’ in low-e: EMISSIVITY.
Emissivity describes the ability of a material to absorb or emit thermal energy. Materials with high emissivities will absorb a large amount of energy from sunlight and consequently re-radiate it as heat. The simplest example of this is the choice you make about shirt color when going outside on a hot sunny day. A dark colored shirt will absorb solar heat and feel hot whereas a light colored shirt will not. The dark shirt is said to have a high emissivity. For more information on emissivity, check out this site.
Low-e glass is glass with a very thin coating added to one side to reduce the glass emissivity by reflecting some of the radiant energy which is incident upon it. Depending on the goal of energy saving, the coating will either be on the inside or the outside of the pane: the inside if you want to keep energy in the house (i.e. in cold climates) or on the outside if you want to keep heat out of the house (i.e. hot climates). For more information on low-e coating, check out this article on glass.com.
Another aspect of low-e glass can be the addition of the noble gas Argon between panes. This is a means of reducing the convection of heat through the gas that is between the panes of glass. Argon has a higher density than the gasses in normal air (i.e. nitrogen, oxygen, etc.). This makes it slower to be set in motion and change directions which is the essence of convection currents. For this reason, Argon is an excellent thermal insulator. For more information on Argon filled windows, check out this article.
The combination of low-e coatings on glass and Argon filled spaces between panes, will reduce energy transfer through the window by attacking both heat flow through radiation and convection. While there are many more aspects of Low-e glass, I hope this helps give you the general idea of the science behind it and why it might prove beneficial for you and your home. Give us a call if you would like to explore how new windows could help reduce the energy bills in your home.