If your windows are ancient and aren't doing a passable job of keeping the warm air inside your house in cold weather (or keeping it out in warm weather), it might be time to consider having new, energy saving windows put in. But new windows, especially good quality energy saving ones, can be very expensive. As a result, their payback period can be quite long. For the amount of cash you would pay to redo the windows in a single big room, you can get almost the same energy efficiency with some basic weather stripping and by using energy efficient window coverings to eliminate heat transfer between your home and the out of doors.
Let's first look at how windows help keep the cold out in winter, and the hot air out in summer. Windows block heat transfer in three ways: convection, conduction, and radiation.
Windows eliminate or reduce the convection airflow between the inside and outside, blocking heat from passing through the window along with the airflow. A leaky window, or one with cracked glass or broken putty, allows air through these gaps, so heat gets out in winter, and heat leaks in during the summer.
Even a very thin sheet of glass has some insulating properties, but if the glass is double-glazed and the space between the panes is an inert gas such as argon, the panes provide extra insulating value, which reduces heat transfer through conduction. Conduction is what causes the metal handles of a pot to heat up when you boil water in the pot; so you can guess that a metal window frame, if not properly built, can conduct a lot of heat. While you can't easily add extra glass sheets to a window, there are other techniques to create additional still air spaces between the window glass and the room, which will add insulation and reduce conduction losses.
Radiation, the third type of heat transfer, usually occurs when light in the infrared spectrum passes through windows, heating the air inside, or when heat inside the room radiates out through the glass. Home energy auditors sometimes take infrared photographs of a home to show where heat losses are most significant, and windows are typically one of the largest sources of heat escaping from houses in winter.
How does this knowledge about heat transfer through convection, conduction, and radiation help you cut energy losses through your windows?
The first problem to address is convection. If your windows have cracked panes, get them repaired. If you still have the old wood-framed windows with putty holding the glass in, inspect the pane for any peeling or missing putty. It's fairly easy to pull old putty out with a putty knife and put a fresh layer of putty on in its place. If the wood of the window itself is broken, or if the glass is hard to get out for replacing, you may not be able to put off getting new windows, but if you can cut the small air leaks, you'll have gone a long way towards minimizing energy losses and should feel some relief in your utility bills.
You may be surprised to learn how much heat can travel out of a home in cold weather through the wood trim around a window. Just wait for a really cold day, put all the exhaust fans on in your house( kitchen range vent, bathroom exhaust fans, etc.), and run your hand along the edges of window and exterior door trim on the inside of rooms. Anywhere you feel cold air coming in, you have a draft that should be sealed. It won't hurt to run a thin bead of clear or white caulking around window and door frames to cut this convection heat transfer.
The last thing to try to reduce convection heat transfers is to use tape-on or Zip-Loc type energy saving window kits to seal any windows that are particularly drafty, or windows that really need replacing but that can't be replaced because of your budget (or because you are renting the place). These kits are a great way to rapidly reduce your heating losses in winter: the kits normally come with several sheets of three by five foot transparent plastic, and a roll of double sided tape. (If you have a large number of windows to cover you can purchase a 48" roll of the plastic and buy the tape separately.) You measure and cut plastic rectangles slightly wider than the window, run the tape around the window frame, peel off the protective tape from the double sided tape, then set the cut plastic over the window, sealing along the tape line. Blow dry the plastic for five minutes, and it shrinks to form a tight, flat extra pane of 'glass' that is hardly unnoticeable. This plastic can hold its taut shape for months, although you may find it needs an occasional short blow dryer blast to pull up the odd wrinkle in the plastic.
The next heat loss you'll want to address is conduction - heat being conducted through the solid materials of the window. In terms of energy efficient window coverings, your goal here isn't actually to avoid this conduction - you can't usually change the materials the window was made of - but to add barrier layers between the window and the room to slow the conduction down. The plastic window insulation kits stop convection heat loss by eliminating drafts into the room, but they also eliminate conduction, by providing a layer of trapped air between the window and the room. A curtain can perform the same task: when closed, the curtain traps a small amount of air between the curtain and the window, so that on a cold day the air behind the curtain naturally gets cold but the room itself stays cozy.
When you install curtains on windows to reduce conduction heat loss, it's important to think about convection currents inside the living area. Hot air rises, and cold air falls, so if you install curtains you should make sure the air currents are stopped, preferably at both the top and bottom of the curtain. Otherwise, in winter, the cold window will cool the air between the window and the window covering, and that colder air will fall to the floor, pulling warm air from above the window covering down in front of the window in a continuous cycle. In summer, the flow runs the other way, with the air between the curtain and the window warming from sunlight, rising out the top opening, and drawing cool room air up towards the glass where it gets heated.
You should block these air flows by having the curtains flush against the wall at the top and bottom, or by having the bottom touch the floor and by closing in the curtain rod area at the top.
If you have cloth blinds that cover the window, you can use Velcro tabs along the sides of the blinds that you then press into Velcro tape along the window frame; this completely seals the air space between the window and the blind, providing an above average insulation layer. Cloth blinds as well as curtains can use a similar technique but with magnetic tape in the fabric, and metal on the framing, so that the cloth sticks to the walls on either side of and below the opening.
Another way you can try to add an insulation barrier to conduction through the window is to buy cellular blinds, which are made in a cellular or honeycomb cross section, or other blinds that incorporate a hollow space within the blind. Cellular blinds when fully pulled up use almost no space and the cells fold shut; when extended, they can add insulating value to a window from R-2 (for single-cell thickness) to R-5 (for double-cell).
Window blinds can help address the final type of heat transfer, radiation, by preventing heat from radiating through the glass (into the room from sunshine in summer, and towards the outside from the indoor radiant heat in cold weather). The most effective blinds use light, reflective colors on the outside, so that sunlight in summer is more or less completely reflected away from the room. Good window blinds can reflect sufficient solar energy away from a room to reduce heat gain by as much as 45%, but they do very little to the R-value of the window, so have little effect in winter.
Roller shades, which have a wind-up axle and can be pulled up or down (we used to call these blinds when I was young) are a great radiation reflector, and also provide superior insulating value to reduce convection air flows around the window that lead to conduction losses through the glass or window frame. Roller shades, because they are placed so close to the glass, do well at reducing convection currents, especially if their side edges and bottom are attached to the window frame (side tracks are used to accomplish this). Loose-sided shades can reduce heat transfer by up to 28%, while attached-edge shades reduce it up to 45%. Shades that can be reversed, with one side dark and the other light, are even better, because you can use the light side to reflect the heat where you want it (keep it out in hot weather, inside during winter).
Awnings and overhangs are a good way to cut direct light from entering a home and heating it up in summer. Because the solar angle is lower in the winter, such window coverings only block the sunlight you don't want, allowing the lower-angled winter sun in to help heat your home.
Storm windows - which were added to many older homes - can be up to 50% more energy efficient than single-pane windows, provided the storms are properly sealed against air leakage. So if you have old wooden-framed storms sitting around in your basement or garage, you should put them on each fall and take them down once winter has passed. If you have aluminum framed storms you can typically leave them up year round; just don't forget to slide the glass sash shut when autumn comes.
Windows form such a small percentage of the surface area of a house, yet they are one of the biggest potential sources of energy waste in a house being heated or air conditioned. So you should strive to do anything you can to restrict heat transfer through convection, conduction, and radiation. Just remember to set aside a portion of the money you save on reduced utility bills, so you can replace any old, energy wasting windows with brand new, energy efficient ones when the time comes.
