Energy Efficient Windows
Selecting the right windows for energy conservation, layout and visual appeal are all discussed in detail by Myron Ferguson. In this article he looks into many of the potential problems or oversights when selecting what windows to use, plus their layout in the home. It is excerpted from "Build It Right: What to Look for in Your New Home," by Myron E. Ferguson.
Energy savings is an integral part of the design and construction of today's houses. And a very important part of that energy savings has to do with the number and types of windows that are used. In this chapter we discuss the relationship between energy and windows as well as form and function.
Windows and Aesthetics
You know the real estate agent's expression "curb appeal". It's the impression prospective buyers have when they are taken to look at a house that's for sale. Windows make up a big part of that impression. They should fit in with the overall style of the house and they should make the house look inviting - a place that gives you a feeling of being lived in. And it's different strokes for different folks - some like the large and multitudinous windows found in many of today's designs while others are happier with the conservative approach with more modest sizes and numbers of windows.
Inside the house, windows, probably more than any other feature, control the ambiance of a room. Correct windows are vital parts of how you will enjoy living in the house, and later on, of the resale value of the house.
Double-pane windows are made with and without a decorative grid between the panes. This grid makes the windows appear to be made of a number of pieces of glass, when in fact, they are not. More and more of these windows use vinyl-clad frames for its better insulation. The vinyl is usually white but colors are available.
There is, as always, some give and take between form and function, between how windows look in a house and their costs in terms of energy and dollars.
Windows are made of glass, which is made of silica, the same thing that sand is made of-- right? Well, yes. But there is a lot more to today's windows than that. The use of single pane windows is disappearing from new homes. The building industry calls windows single or double glazed rather than single or double pane.
Double-pane windows have these five things going for them:
- They make a house more comfortable.
- They don't weep or sweat (at least they shouldn't).
- They save energy in heating and cooling.
- Saving energy saves money.
- They make the house more saleable.
The downside is that they are more expensive initially.
There are three mechanisms by which heat goes through a window:
1.0 Radiation: This is heat you can feel, like the sun's rays or when you hold your hands in front of a burning fireplace. This heat goes through the window like light does.
2.0 Conduction: This is the heat a solid material carries from one place to another, like from one side of a window pane to the other.
3.0 Convection: This is the heat that is carried by air from one place to another. Heat is carried by convection between the panes of glass in a double pane window. Modern window construction tackles all three of these to keep heat loss down.
The panes may be low-e (low-emissivity) glass. This involves a special manufacturing process that reduces the ability of the glass to pass certain heat rays while still letting us see through the window. The idea of low-e glass is to let the direct sun's rays through for heating help in winter while stopping other heat rays so that the warmth generated in the house can't get out. In summer you have to depend on trees, drapes, shades, and roof overhangs to keep out the direct rays from the sun.
Conduction through the glass itself is a fact of life, but the materials around the edges of the window also conduct heat. This source of heat loss is tackled by using materials to mount glass that are not good conductors--vinyl for example. Wood and aluminum are also used.
The transfer of heat by convection between the panes of glass is inhibited by filling the space with a gas. Argon is the most commonly used but you may find other more exotic gasses. (A vacuum would stop convection entirely but it's not a practical solution.)
Putting argon between the panes of glass poses another problem--how to keep it there. In truth, you can't, at least not with cost effective manufacturing techniques. In time it will leak out, being replaced with plain old air. When this happens two things occur:
- The window will be a poorer insulator and
- Moisture will appear between the panes
At that point you'll have three choices: (1) if the window is covered by a lifetime warranty, (2) get the manufacturer to replace it, replace it at your expense, or (3) live with it.
Windows and Energy Codes
Windows are rated by a standardized process resulting in a U rating. This number is always a decimal fraction smaller than 1. The smaller the number the better the window in terms of energy conservation. Good windows will have a U-rating of less than 0.5. Better ones get down to 0.4 with manufacturers striving to get to 0.3. But note: a U-rating of 0.3 (very expensive) is not even twice as good as a rating of 0.5 (low cost). To spend a lot of money for a specially low-rated window is probably not effective in terms of either comfort (you'll never notice the difference) or energy savings (it'll take a lifetime to recoup the additional cost).
You may not have the choice, however, depending on where you are building. In states where the idea of energy savings has a bureaucratic stronghold, there is continual pressure to reduce energy usage--even when it is not cost-effective. If you live in one of these areas, you may find low-e, low U-rated windows are required to meet energy code requirements. Because in most houses, windows, walls, floors, ceilings, or doors, some codes restrict the amount of glass, including skylights, you can put in your house. If you have these kinds of restrictions, some floor plans cannot be used.
On the other hand, these same codes may take into account that winter sun can be used to help heat the house and give "credit" for south-facing windows to reduce the amount of insulation you must have.
But don't be dismayed even if they cost more, because these windows make a house a whole lot more comfortable than the ones you were brought up in.
Windows and Fading
Today's low-e glass may be better then the plastic sheet but probably not much when it comes to slowing down the damaging effect of sunlight. It cuts out only a little over half of the ultraviolet. So plan on trees, drapes, and blinds to keep the sun from beating into your house--unless of course you don't mind multi-shaded furniture and carpet. And feel good about it--you're not only saving your furniture and carpet, you're lowering the heating and cooling bill too.
Windows and Decorating
When windows are selected for their appearance, whether from the inside or the outside of the house, don't forget energy loss and fading--to nothing about privacy.
Large windows will lose more energy than smaller ones with a resulting increase in heating and/or cooling bills. The more of the window you can cover with drapes when the outside temperature is too hot or too cold, the more comfortable the house and the lower your energy bills will be.
It is hard to get coverings for the upper parts of the two story high windows found in many modern home designs. And, of course, ultraviolet rays don't care if they come in through a round, or triangular window above a more conventional lower one--they'll cause fading in any case.
Most of today's house designs include skylights. They are used where windows would be inconvenient (as in small bathrooms), where windows are not possible (as in an interior room that has no outside wall), or where they let light in while still maintaining privacy. There are some penalties to pay when skylights are used. A windows, they are poof insulators. In general, it's not possible to shade them from the hot afternoon sun. When this is a problem, skylights will need to be made from tinted or low-e glass and/or they'll need some kind of interior blinds.
Where skylights are used in rooms like bathrooms that do not have vaulted ceilings, there is a shaft or well that brings the light down from the roof-mounted skylight. This well is often bigger at the bottom than the top thus spreading the light over a larger area in the room.
Skylights range from one to four feet on a side. Think carefully before using the smaller sizes. Many people don't like their cheaper appearance.
When choosing skylights for a well that can trap a lot of heat, get types that can be opened to let accumulated hot air escape during hot weather.
Relatively new for homes are metal tubes that funnel light from the roof down into a room. A small glass or plastic dome is mounted on the roof with appropriately shaped reflectors to capture the sunlight. At the bottom end a diffuser spreads the light around in the room. They are designed to mount in between existing structural members making them attractive for additions to existing houses where the original plans should have had a skylight.
The light that is delivered into the room is not the same as you get from a skylight, but rather has a distinctly cold metallic cast--which is not surprising since it depends on the reflection off the aluminum that makes up the unit. For new construction the tubes are less expensive than skylights with their wells. But, because of the non-conventional color of the light, they may detract from the saleability of the home from some buyers.
In some parts of the country, window openings are usually encased (wrapped) in wood while in others builders offer it as an option. In Arizona, California, and other areas with southwestern architecture, the usual approach is to provide a wooden sill or even no sill at all; the opening is simply handled as a continuation of the wallboard. These regional differences can be explained by the weather differences in combination with older single-pane windows. In climates with damper, colder winters old windows will sweat and wallboard or plaster is unsuitable around windows.
Where today's codes require double-pane windows that don't sweat (or at least not much), the functional need for woodwrap has been eliminated. Aesthetically, woodwrap has a richer, more finished appearance than does wallboard around a window and this has helped to keep its popularity in areas with more traditional architecture.
Woodwrap is an expensive proposition. For example, a tract builder offered this amenity as an option with a $2,000 price tag on it. You might give some thought to whether you really need it, particularly when you plan to paint the wood white against a white wall and then cover it with a valance and drapes so that only the sill shows.