If your windows are old and aren't doing a decent job of keeping the warm air inside your home in cold weather (or keeping it out in warm weather), it may be time to think about having new, energy saving windows installed. But new windows, especially high quality energy efficient ones, can be prohibitively costly. This means that it can take a long time for them to pay for themselves in energy savings. For the money you would pay to replace the windows in one large room, you can obtain almost the same energy efficiency improvement with some simple weather stripping and by using energy saving window coverings to cut heat transfer between your home and the out of doors.



Let's review how windows help keep the cold air out in winter, and the hot out in summer. Windows block heat transfer in three ways: convection, conduction, and radiation.

Windows halt or reduce the convection airflow between the inside and the exterior, blocking heat from passing through the window along with the air. A leaky window, or one with cracked glass or broken putty, allows air through these openings, so heat escapes in winter, and heat sneaks in during the summer.

Even a very thin sheet of glass has some insulating properties, but if there are two panes of glass 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 the type of heat transfer that causes the metal handles of a pot to heat up when you boil water in the pot; so you can imagine that a metal window frame, if not properly built, can conduct a lot of heat. While you can't easily add extra glass panes to a window, there are other methods to create additional still air spaces between the glass and the interior, which will increase insulation and cut conduction transfers.

Radiation, the last type of heat transfer, typically happens when light in the infrared spectrum passes through windows, heating the air inside, or when infrared radiation inside the room radiates out through the glass. Home energy auditors can actually take infrared photographs of a house to illustrate where heat losses are most significant, and windows are typically among the largest sources of heat loss.

How does this knowledge about heat transfer through convection, conduction, and radiation help you minimize energy losses through your windows?

The first problem to tackle is convection. If your windows have cracked panes, get them repaired. If you have the old wood-framed windows with putty holding the glass in, inspect the pane for any cracked or missing putty. It's fairly straightforward to pry cracked 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 assuming you can cut the small air leaks, you'll have gone a long way towards reducing energy losses and should feel some relief in your heating and cooling bills.

You may be surprised to learn how much heat can escape from an old house in winter 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 fingers along the edges of window and exterior door trim on the inside of rooms. Wherever you feel cold air coming in, you've got 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 minimize convection heat transfers is to use tape-on or Zip-Loc type energy saving window kits to close off any windows that are particularly leaky, or windows that definitely should be replaced but that can't be replaced because of your budget (or because you are renting). These window insulation kits are a good way to quickly reduce your heating losses: the kits typically come with several sheets of 3 by 5 foot clear plastic, and some double sided tape. (If you have many windows to cover you can purchase a 48" roll of the plastic and get the tape separately as you'll pay far less than buy buying several kits.) You measure and cut plastic rectangles a little bigger than the window, tape along the window frame, remove the protective tape from the double sided tape, and then place the cut plastic over the window, sealing along the tape line. Blow dry the window 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 a bit of slack in the plastic.

The next problem you'll want to tackle is conduction - heat being drawn through the solid materials of the window. In terms of energy efficient window coverings, your objective here isn't really to avoid this conduction - you can't usually change the materials the window is made of - but to add barrier layers between the window and the room to slow that conduction down. Plastic window insulation kits stop convection heat transfer by cutting drafts into the room, but they also stop conduction, by adding a layer of still air between the window and the room. A curtain can accomplish the same task: when closed, the curtain holds a small amount of air between the curtain and the window, so that on a cold night the air behind the curtain naturally gets cold but the air in the room stays cozy.

When you install curtains on a window to cut conduction heat transfer, it's important to think about convection flows inside the room. Hot air rises, and cool air falls, so if you add 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 glass will cool the air space between the window and the window covering, and that colder air will fall down onto the floor of the room, pulling warm air from above the window covering down in front of the glass in a continuous cycle. In summer, the flow runs backwards, 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 stop these air flows by having the curtains flush with 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 entire window frame, 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 barrier. Cloth blinds as well as curtains can use a similar technique but with magnetic tape in the fabric, and metal along the window frame, so that the cloth holds 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 honeycomb cross section, or other blinds that incorporate a hollow space within the blind. Cellular blinds when fully retracted 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 dual-cell).

Window blinds can help address the final kind of heat transfer, radiation, by stopping heat from passing through the glass (into the room from sunshine in warm weather, and towards the outside from the indoor radiant heat in winter). The most efficient blinds use light, reflective colors on the exterior-facing side, so that sunlight in summer is most thoroughly reflected away from the room. Good blinds can reflect sufficient heat away from a room to reduce heat transfer by up to 45%, but they do very little to the insulation value of the window, so have little effect in cold weather.

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 an effective radiation reflector, and also provide superior insulation to reduce convection air flows around the window that lead to conduction losses through the pane or window frame. Roller shades, because they are placed so close to the glass, do well at cutting convection airflows, especially if their side edges and bottom are attached to the window frame (side tracks are used to accomplish this). Loose-sided shades can cut heat exchange by as much as 28%, while secured-edge shades cut it by up to 45%. Reversible shades, with a dark and a light side, are ideal, because you can use the light side to reflect the heat where you want it (outside in summer, keep it in during winter).

Awnings and overhangs are a good way to cut direct light from entering a room and warming it up in summer. Because the angle of the sun is lower in the winter months, such window coverings only block the sunlight you don't want, letting the lower-angled winter sun in to help heat your home.

Storm windows - which are found on many older homes - can be up to 50% more energy efficient than single-pane windows, as long as the storm windows are sealed against air leaks. So if you have the old wooden-framed storms sitting around in your basement or garage, be sure to use them each fall and take them down each spring. If you have aluminum framed storms you can usually keep them on year round; just remember to slide the glass sash shut when autumn comes.

Windows make up such a small portion of the surface area of a home, yet they are among the biggest potential sources of energy waste in a home being heated or air conditioned. So it's important to do whatever you can to restrict heat transfer through convection, conduction, and radiation. Just don't forget to put aside some of the cash you save on reduced utility bills, so you can replace any old, energy wasting windows with new, energy efficient ones when you can afford it.