Maximising the amount of natural light in your home - through good use of windows, roof lights, sun pipes, etc - will help to reduce your need for artificial lighting. Windows and doors form a part of the building shell so the use of performance timber windows and triple glazing can really help the building's overall thermal performance. Modern performance windows can be very efficient with whole window U values as low as 0.8W/m2K. 'Solar gain' or heat captured through windows from the sun can be used to help heat a building and combined with thermal mass to help reduce heating bills. The fitting of windows is a key part of ensuring long lasting building performance, as they form a critical part of the building shell. This is another area where buying too cheap is a false economy.