The overall energy index determines where the windows sits on the A-G scale and is calculated by taking into account:
Solar Factor - the amount of heat passing inside through the window from the sun
U Value - The amount of heat passing from inside through the window outside
Air Infiltration Factor - The amount of heat lost through air leakage through and around the window
The unit of the energy-index is energy transfer through the window (kilowatt-hours) per unit area (meters-squared) per year - kWh/m2/year. A negative number means overall heat energy is lost through the window, while a positive number means more energy is retained from the sun than is lost through thermal transmittance. The original A-rating was achieved at zero kWh/m2/year but as performance as increased, A+, A++ and A+++ ratings can be attained.
How Efficient Are Energy Efficient Windows?
The energy index rating cannot on it's own determine how much energy, and therefore money, that the window can save - it is a comparative figure that allows competing window products to be measured relative to each other.
In a typical house, the most heat loss will occur through the windows as these are the thinnest boundaries of the property. It makes sense therefore to focus energy-saving efforts here first.
Replacement windows are available in a range of energy-ratings and as you'd expect, an 'A'-rated window will perform better than a 'B'-rated window, and so on.
'A'-Rated Windows have a positive energy-index number - this means that more heat passes in from the outside than is allowed to escape outside through the window.
How Much Do Energy Efficient Windows Cost?
The higher the energy index of a window, the more production cost is involved in achieving that index and the higher the price will be. To determine whether energy-efficient windows are also cost-efficient windows, the customer must take into account:
- The number and size of windows in their property - the larger the area occupied by windows, the more potential there is for heat-loss
- The efficiency of their current windows - the biggest improvements will come from replacing single-glazed windows or windows installed prior to April 2002
-The pay-back period - Energy-efficient windows cost more up-front, but offer savings over the longer term.
As a guide, the BFRC suggests that an electrically-heated semi-detached house would save £461 (at December 2007 prices) per year by replacing single-glazing with 'A'-rated energy-efficient windows. It is worth noting that when it comes to selling a property, energy-efficient windows will improve the overall property energy rating and so increase it's saleability.
Do I Have To Have Energy-Efficient Windows?
No! At the moment having an energy-efficiency rating on windows is down to the manufacturer and completely voluntary. Windows that do not have energy-efficiency ratings have not failed any test, it simply means the manufacturer has not opted to have the windows tested.