On the Path to a Zero Energy Home: Highly Insulated Windows and Doors

Windows and doors can be the big holes in what is otherwise a very well insulated home with an airtight building envelope. You can better control heat loss and heat gain by using the right doors and windows in your new home. An often overlooked aspect of windows and doors is selecting the right size and correctly orienting them in the home based on the home's siting on the building lot. Here is a primer on windows and doors for an affordable zero energy ready home. 

WINDOW PERFORMANCE (A.K.A. ENERGY EFFICIENCY) 

Windows are extremely important when it comes to designing energy efficiency into our home. We need windows to provide light and ventilation into our homes. But windows are also the weakest point in a home’s exterior. The primary way we measure a window's energy efficiency is with U values. 

U values are most commonly displayed on windows and doors of a home. You will find it on the labels required to be affixed to each of those products. U is simply the symbol for internal energy. The U value of a building element is the inverse of the total thermal resistance of that element. The U-value is a measure of how much heat is lost through a given thickness of a particular material but includes the three major ways in which heat loss occurs – conduction, convection, and radiation (Air infiltration is managed by the home’s thermal envelope). 

RELATED: ON THE PATH TO A ZERO ENERGY HOME: IT STARTS WITH DESIGN

Typically, windows with the lowest U-values (lower is better) have a double glass with one low-e surface, argon gas, low-conductivity glass spacers, and frames. High-performance double-pane windows can have U-factors of 0.30 or lower, while some triple-pane windows can achieve U-factors as low as 0.15. 

The solar heat gain coefficient (SHGC) tells you how much of the sun’s heat is allowed to penetrate the glass. The SHGC is also expressed as a decimal from 0 to 1. Lower numbers mean there is less solar heat gain. The term visible transmittance (VT) tells you how well you can see through the glass. These numbers also range from 0 to 1. The higher the number the better visibility. Air leakage (AL) shows how well the window seals against drafts around the operable sash. Lower numbers mean less leakage. 

U-value, SHGC, VT, and AL are important to understand when making your window and door selections for your Zero Energy Ready home. 

LOW U-VALUE WINDOWS 

Window technology is improving rapidly. Select the most cost-effective windows with a heat loss rate of U-0.25 or lower. In some situations and climates, this may be advanced double-glazed windows, while other situations may call for triple-glazed windows. Let window specifications and your energy model be your guide. 

Low U-values are achieved using three important elements: 

• Low-emissivity (Low-E) coatings are microscopically thin layers of metal applied to the inner surfaces of the glass or on layers of clear film suspended between the glass panes. They limit the amount of heat that jumps from one glazing layer to the other. 
• Inert gases fill the space between the panes and allow less heat transfer than air does. The most common gas is argon, but krypton is also used. While many people have expressed concern that inert gases will leak out over time, research has shown this to be an extremely slow process, less than 1% per year. 
• Spacers that hold the panes apart are typically made of aluminum and conduct heat rapidly. High-performance glazing units use thermally improved plastic spacers or a plastic “thermal break” in the spacer to reduce heat loss around the edges. 

OTHER WINDOW AND DOOR CONSIDERATIONS FOR YOUR NEW HOME 

Window Sizes – More heat flows through the frame than the insulated glass unit. This has two implications. First, look for windows that have the smallest frame profile, because the frame has a higher thermal transmission than triple-pane glazing. Second, it is more energy-efficient to use fewer, larger windows with the same glazing area than more, smaller windows, because larger windows have a higher glass to frame ratio. 

Natural Ventilation – Operable windows can be strategically placed to allow for optimal summer cross ventilation, which will reduce the summer cooling load. Make use of the “stack effect” to ventilate and cool the building, by placing operable windows near high points to allow warmer air to escape and near low points to draw cooler air in. All operable windows should be casement or awning where possible because they seal better than sliders or single-hung windows. 

Fixed Window – Select fixed windows for locations not specifically designed for natural ventilation. Fixed windows, followed by casement and awning windows, are the most airtight and have better U-value compared to similar sliding or single hung windows. Fixed windows also cost less than casement. 

Window Orientation – Where possible, place 50% to 60% of the window area facing south. Most of the common living areas should be on the south side where it receives light and heat from the sun. Unless shaded in some way, windows facing east and west tend to gain too much heat when it isn’t wanted, even in northern climates. 

Solar Heat Gain Coefficient – Ideally, windows facing south would have a high solar heat gain to gather valuable heat in the winter. Windows facing east and west would have a low solar heat gain to reject heat during summer. Experts have suggested that designers specify the SHGC accordingly, and this makes sense.  

Window Overhangs – The goal is to create a shading strategy that allows the sun to heat the building when needed and reject heat to avoid overheating when it’s not. Fixed overhangs must be a compromise between similar sun angles in spring and fall when the heating or cooling requirements are much different. Consider using a shorter fixed overhang of 12 to 18 inches along with moveable shading, such as awnings, sunscreens, or vegetation. This will allow greater heat gain during spring and less heat gain during fall.  

Cost-Effective Energy-Efficient Doors – Insulated fiberglass doors are currently the best option for the price and are the most cost-effective. Be sure that they have tight gaskets and air seals. The glazing on the door can be a source of air leakage and should be checked during the blower door test. If possible minimize glazing on doors, specify that they have high R-value glass, and verify that there are no air leaks around the glass. Consider a multi-point latching mechanism that will hold the door tight against the weather-stripping. This is especially valuable for doors that are exposed to sun and weather. 

MODULAR MEANS MORE 

When designing a Zero Energy Ready home, modular construction just makes a new home more efficient. Modular construction means you are starting out with a home that is built tighter and with virtually no moisture trapped in the home to begin with. Insulation is placed in a controlled environment which means you are starting with a better-sealed building envelope. After you build an airtight home and super insulate it, using the best windows and doors is probably the 3rd area to see the biggest improvement in energy efficiency.

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