The building envelope is only as effective as its weakest element and all too often these areas the doors and windows in a home which can account for up to a third of the overall energy 
loss according to the APA. Not only do these openings lead to energy loss, but as a result of less than appropriate levels of insulation, condensation can occur, which can lead to damage and can cause mold and mildew. Technological advances have seen the creation of advanced window and door systems which prevent energy loss and condensation.
Advanced window systems help to maintain indoor temperatures through spectral selection (low e-coatings) which keeps out hot summer sun rays while letting desirable warming winter rays in. Double or triple pane windows help to reduce thermal energy loss in the colder months, while also keeping the heat out in the summer.
In colder climates, select windows with a U-factor of 0.35 or less and in warmer climates, opt for windows with a low U-factor and solar heat gain co-efficiency (SHGC) which should fall in the 0.39 to 0.65 range. The AL rating refers to the rate at which air leaks from the window system; choose one below 0.30.
New advances in phase-changing windows mean that these windows can reduce energy consumption by 30-50%. Here the window is constructed of a material that stores heat by melting when it’s hot and then radiates it back into the building at night, as the material cools and re-solidifies. Phase-change windows enable designers to incorporate window systems that allow natural light in through a translucent material, while providing insulating properties comparable to that of a wall.
Vacuum insulated glass is another emerging technology. Here two panels are separated by small ‘pillars’ and a vacuum is created between the panes for a high R-10 value.
Using gas-filled panes can contribute significantly to the efficiency of your energy envelope. The most common gases are Argon and Krypton. Argon is inexpensive, non-toxic, inert, clear and odorless with a thermal conductivity of around 67% which is why it is a common choice. Krypton is more expensive, but it does insulate better.
Advanced window and door systems do cost more, but they can save you more than their upfront cost through utility savings and can help to reduce the size of your required HVAC equipment. According to a 2007 US Department of Energy study, Energy Star windows can reduce HVAC bills by $465 annually in some residential applications. The design professional should take into account the orientation of the windows and their shading to maximize solar heat gain in the winter, while rejecting it during the summer.