2. Heat Loss - "U" values

The conductive heat loss of a material is expressed as its "U" value. If we measure the amount of heat lost through a pane of glass one metre square when the temperature on the outside of pane is one 1ºC. cooler than the inside, we get a figure of around 5.4 watts. That is its "U value."
So now we can solve a simple problem;-

What is the heat loss for a single-glazed window 2.07m x 1.63m in a room heated to 21ºC. against an outside temperature of -1ºC.?

Answer; 2.07 x 1.63 x 5.4 x 22 = 400 watts.

(For interest, a modern, argon-filled double glazed panel with a "U" value of 1.6 would bring the heat loss down to 119 watts.)
XXThe "U" values for a wide variety of building materials can be found in any text book on the subject. Or you can find an abbreviated table on this website by clicking here.
XXAs we have seen, the arithmetic involved is simple but rather tedious for a whole house since it becomes necessary to go round calculating the surface areas for each type of material used in each room. Most heating engineers now use a slide-rule or computer program designed to simplify the process.