The majority of people in the UK would think of double and triple glazed units, or IGU’s, as a way to keep heat inside a home and therefore reduce bills in winter. However IGU’s can also be used to keep the heat of the sun out of your house if that’s what you are looking for; it all comes down to the types and ratings of glass that you choose.
There are two main ratings to look for when you are choosing glass for your windows, the U value which indicates the insulating properties of a material; single pane windows are about 5, with newer double glazing at about 1.6. To put this in context external walls have a U rating of just 0,.3, which shows that your windows are the weak point when it comes to keeping heat in or out of your house.
The other important rating when it comes to glass is the solar gain, which is known as the G value in Europe, or the solar heat gain coefficient (SHGC) in the USA. This is a measure of how much heat from the sun is transferred through the window into your home. Depending on the time of year and the time of day, sunlight shining through your windows can really affect the inside temperature, especially on skylights or glass roofs. Sunlight hitting the glass at 20 degrees or less from the perpendicular (straight on to the window) will mostly pass through, whereas anything more than 35 degrees off this will generally bounce off and have little heating effect.
Low E (low emissivity) is a type of coating that can be applied to windows that help reflect solar rays; if you want to keep the heat in you apply this to the inside of the windows to reflect it back into the room. If you want to stop the heating effect of the sun then the coating should be applied to the outside of the glass.
All modern windows are rated by a national organisation; in the UK this is the BFRC, the British Fenestration Rating Council. All new window units have a label showing the rating that they have been given from A, which is the best, to G which is the least efficient. These labels also show the U and G values, as well as the L rating which is the amount of heat loss due to air penetration. The gas that is sandwiched between the panes of glass also effects how much heat energy is passed through the unit; inert gases such as argon xenon or krypton are better at stopping conduction than a plain air, and even the actual frame makes a difference; metal frames will conduct heat into your house much more readily than uPVC or wood frames. Learn more about frames here.
IGU’s can keep solar heat out, although this is not often a requirement in the UK. It is just a matter of knowing what you are looking for and choosing the right combination of coatings and glass panes to do what you need.