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If your windows are worn out and aren't doing a passable job of keeping the warm air inside your home in winter (or keeping it out in warm weather), it might be time to consider having new, energy efficient windows installed. But new windows, especially high quality energy efficient ones, can be very costly. This means that it can take a long time for them to pay for themselves in energy savings. For the amount of cash it would cost you to redo the windows in a single big room, you can achieve almost the same energy efficiency improvement with some simple weather stripping and by using energy saving window coverings to reduce heat transfer between your home and the out of doors.


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


Windows stop or reduce the convection airflow between the inside and out of doors, blocking heat from travelling through the window with the airflow. A leaky window, or one with cracks in the glass or damaged putty, allows air through these gaps, so heat escapes in cold weather, and heat sneaks in during warm weather.


Even the thinnest 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 what 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 allow a lot of heat through the window frame. Although you can't readily add extra glass sheets to a window, there are other ways to add still air spaces between the glass and the interior, which will increase insulation and reduce conduction transfers.


Radiation, the last kind of heat transfer, typically occurs when light in the infrared spectrum travels through windows, heating the air inside, or when infrared radiation inside the room radiates out through the glass. Home energy auditors sometimes take infrared photos of a home to illustrate where heat losses are most significant, and windows are typically one of the largest sources of heat escaping from houses in winter.


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


The first issue to tackle 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 glazing for any cracked or missing putty. It's quite easy to pull cracked putty out with a putty knife and put a fresh layer on in its place. If the wood of the window itself is broken, or if the glass is hard to replace, you may not be able to put off buying 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 home heating and cooling bills.


You may be surprised to know how much heat can travel out of a house in winter through the wood trim around a window. Just wait for a chilly day, put all the exhaust fans on in your house, 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 have a draft that needs to be sealed. It probably doesn't hurt to run a thin bead of clear or white caulking such as Alex Plus around window and door frames to cut this convection heat transfer. You can also caulk exterior window frames with outdoor caulking material if there are major gaps betweent the frames and the exterior wall material.


The last technique 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 need replacing 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 quickly cut your heating losses: the kits usually come with several sheets of 3 by 5 foot transparent plastic, and a roll of double sided tape. (If you have a large number of windows to cover you should buy a 48" roll of the plastic and get the tape separately.) You measure and cut plastic rectangles a little bigger than the window, tape around the edges of the window frame, peel off the protective tape from the double sided tape, and 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 practically unnoticeable. This plastic can hold its taut shape for weeks, although you may find it needs an occasional short blow dryer blast to pull up the odd wrinkle in the plastic.


The next problem you'll want to address is conduction - heat being conducted through the solid materials of the window. In terms of energy saving window coverings, your goal here isn't really to stop 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 providing a layer of trapped air between the window and the room. A curtain can accomplish a similar task: when closed, the curtain traps 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 room itself stays cozy.


When you put curtains on a window to cut conduction heat transfer, you need to consider convection flows within the living area. Hot air rises, and cold air falls, so if you install curtains you should ensure that the air currents are blocked, preferably at both the top and bottom of the curtain or blind. Otherwise, in cold weather, the cold window will cool the air space between the window and the window covering, and that colder air will fall down onto the floor, pulling warm air from above the window covering down in front of the window in a continuous flow. In summer, the process is reversed, with the air between the curtain and the window warming from sunlight, rising out the top opening, and drawing cool room air up to the glass where it gets heated.


You can block 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, you can put 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 along the framing, so that the cloth holds to the frame 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 (for example, air-foil shaped blinds). Cellular blinds when fully pulled up take up 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 third kind of heat transfer, radiation, by preventing heat from radiating through the window pane (into the room from sunlight in summer, 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 is more or less completely reflected away from the house. Well-made window blinds can reflect sufficient heat away from the window to reduce heat gain by as much as 45%, but they do very little to the insulation value of the window, so don't hold heat 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 pane or window frame. Roller shades, because they are placed so near the glass, do well at reducing 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 exchange by up to 28%, while secured-edge shades reduce it by as much as 45%. Shades that can be reversed, with a dark and a light side, are even better, because the light side can reflect the heat where you want it (keep it out in summer, inside during cold weather).


Awnings and overhangs are a good way to cut sunlight from shining into 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 need, letting the lower-angled winter sun help heat your rooms.


Storm windows - which are found on many older homes - can save up to 50% of the heat loss compared to single-pane windows, as long as the storms are properly sealed against drafts. So if you have old wooden-framed storms sitting around in your basement or garage, be sure to put them on each fall and take them down each spring. If you have aluminum framed storms you can usually keep them on all year long; just don't forget to close the glass when autumn comes.


Windows form such a small percentage of the surface area of a house, yet they can be among the biggest potential energy wasters in a home being heated or air conditioned. So it's important to do anything you can to limit heat transfer through convection, conduction, and radiation. Just remember to put 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.

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