Robin Green

Cutting heat losses with energy saving window coverings

Energy efficient window coverings keep warm air inside in winter, and keep hot air outside in hot weather. Curtains, shades, awnings, and blow-dry window insulation kits can all make a difference.

If your windows are ancient and aren't doing a decent job of keeping heat inside your house in cold weather (or keeping it out in summer), maybe it is time to think about having new, energy saving windows put in. But new windows, especially high quality energy saving windows, can be prohibitively expensive. This means that their payback period can be quite long. For the cash you would pay to replace the windows in a single large room, you can achieve almost the same energy savings with some basic weatherproofing and by using energy efficient window coverings to reduce heat transfer between your home and outside.

Let's first look at how windows keep the cold out in winter, and the hot out in summer. Windows reduce heat transfer in three ways: convection, conduction, and radiation.

Windows stop or reduce convection airflow between a room and out of doors, preventing heat from travelling through the window with the air. A leaky window, or one with cracks in the glass or damaged putty, allows air through these gaps, so heat gets out in winter, and heat sneaks in during hot weather.

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 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. Although you can't readily add extra glass sheets to a window, there are other ways to create extra still air spaces between the glass and the room, which will add insulation and minimize conduction transfers.

Radiation, the last kind of heat transfer, usually occurs when light in the infrared spectrum travels through windows, heating the air inside, or when heat inside the room radiates out through the glass. Home energy auditors can actually take infrared photos of a house to illustrate where heat is escaping, and windows are usually one of the biggest sources of heat loss.

How does knowing about heat transfer through convection, conduction, and radiation help you reduce energy losses through your windows?

The first problem to tackle is convection. If your windows have broken panes, get them repaired. If you still have the old wood-framed windows with putty holding the glass in, check the pane for any cracked or missing putty. It's fairly straightforward to pull 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 cracked, or if the glass is hard to replace, you may not be able to put off getting new windows, but if you can cut the minor air leaks, you'll have accomplished a lot towards reducing energy waste and should feel some relief in your home heating and cooling 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 chilly 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. Anywhere you feel cold air coming in, you've got a draft that needs to be sealed. It probably doesn'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 losses is to use shrink-wrap or Zip-Loc type window kits to seal any windows that are very leaky, or windows that really should be replaced but that can't be replaced because of your budget (or, for example, because you are renting). These window insulation kits are a great way to rapidly cut your heating losses: the kits typically 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 get the tape separately as you'll pay far less than buy buying several kits.) You measure and cut plastic a little bigger than the window, tape around the edges of the window frame, remove the white 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 a few minutes, and it shrinks to form a tight, flat extra pane of 'glass' that is hardly invisible. This plastic can hold its flat shape for months, although you may find it needs an occasional short blow dryer blast to take in the odd wrinkle 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 saving window coverings, your objective here isn't really to avoid this conduction - you can't really change the materials the window was made of - but to add barrier layers between the window and the inside of the room to slow the conduction down. The plastic window insulation kits stop convection heat loss by cutting drafts into the room, but they also cut conduction, by adding a layer of still air between the room and the window. 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 air in the room stays warm.

When you put curtains on windows to cut conduction heat transfer, you need to consider convection flows within 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 or blind. Otherwise, in cold weather, the cold glass will cool the air space between the window and the window covering, and that colder air will fall to the floor of the room, drawing warm air from above the window covering down in front of the window in a continuous cycle. In summer, the flow runs backwards, with the air between the curtain and the window warming from the sunlight, rising out the top opening, and drawing cool room air up towards the glass where it gets heated.

You can block these air flows by having the curtains flush with the wall above and below, or by having the bottom touch the floor and by enclosing the curtain rod area at the top.

If you have cloth blinds that cover the entire 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 barrier. Cloth blinds as well as curtains can use a similar technique but with magnetic tape in the fabric, and metal on the window frame, so that the cloth holds to the walls on either side of and below the opening.

Another method 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 (for example, air-foil shaped blinds). Cellular blinds when fully pulled up take up almost no space and the cells are empty; 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 house from sunshine in warm weather, and towards the outside from the indoor radiant heat in cold weather). The most effective blinds are light-colored on the outside, so that sunlight in summer is more or less completely reflected away from the room. Well-made window blinds can reflect enough heat away from a room to reduce heat transfer by as much as 45%, but they do very little to the insulation value of the window, so have little effect in winter.

Roller shades, which have a spring mechanism and can be pulled up or down (we used to call these blinds when I was young) are an effective barrier against radiation, and also provide good insulating value to reduce convection currents around the window that lead to conduction losses through the glass or frame. Roller shades, because they sit so close to the glass, are better at cutting convection currents, especially if their side edges and bottom are attached to the window frame (for example, by using side tracks). Unsecured shades can cut heat transfer by as much as 28%, while attached-edge shades cut it by up to 45%. Reversible shades, with a dark and a light side, are even better, 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 can help cut sunlight from entering a room and warming it up in summer. Because the angle of the sun is lower in the winter, these window coverings only block the sunlight you don't want, allowing the lower-angled winter sun warm 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 to eliminate air leaks. So if you have the old wood-framed storms gathering dust in your basement or garage, you should use them each fall and take them down each spring. If you have aluminum framed storms you can usually leave them up year round; just remember to slide the glass sash shut when autumn comes.

Windows form such a small percentage of the surface area of a house, yet they can be among the biggest potential sources of energy waste in a house being heated or air conditioned. So it's important to do anything you can to restrict heat transfer through convection, conduction, and radiation. Just remember to put aside a portion of the money you save on lowered energy bills, so you can replace any worn-out, leaky windows with new, energy efficient ones when the time comes.

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