Keep the heat where it belongs and be stylish too!
Using energy saving curtains is one of the least expensive ways to save energy in the household.
In order to choose the right curtain, its important to understand how they work.
If a home has bare windows there are a few ways heat can be lost:
Light transfer
Involves the sun’s rays passing through the window and transferring to heat. This is great during winter when it is cool inside but will make cooling the home difficult during summer.
Radiative transfer
This is unwanted during winter or summer because it involves heat transferring to cool air and vice versa. The warmth inside during winter transfers through the glass out to the cold and during summer the warmth transfers in. Curtains provide an effective barrier against radiative transfer.
Convection
The process of heat moving through gas, water, air or liquid. As the heater warms a room it circulates through the room and past the window. The heat is absorbed by the window, warming the window and cooling the air. Since cold air falls, the cooled air at the window flows away from the window, which draws warm air down behind it, meaning you lose more and more heat to the outside.
Conduction
The process of heat moving through solids such as windows. The warmth from the room will not only travel through the glass of the window, but also the window frame especially if it is metal.
When helping your parents choose curtains, keep this in mind: 
Ensure the curtain is nice and thick to provide insulation for your home.
Light coloured curtains reflect heat from sunlight during summer and provide partial lighting in the room.
Remember to cover as much of the window as possible to prevent air flow around the edges.
Heat gain through an unshaded window in summer can be 100 times greater than through the same area of insulated wall.
Most importantly…close your curtains during summer and keep them open during winter!