Summer is upon us and that means the heat is on! Installing energy efficient windows are a great way to not only keep your energy bills low, but keep your home’s temperature low, too. Everyone knows that these types of windows do a great job of keeping cold air in, but just how do they keep warm air out? What keeps an energy efficient window cool? Let’s take a look!
Components Of Energy-Efficient Windows
First of all, at a minimum, energy efficient windows should include the following:
- Double-pane insulated glass
- Heat-resistant (Low-E) glass coating
- Airtight frames
- ENERGY STAR rating
Perhaps the most important facet of what keeps windows cool is found in their construction. Windows that are truly energy efficient are great at adjusting accordingly to both hot and cold temperatures.
Glass: Dual-pane designs use an air or gas-filled space between two panes of glass. This insulates much better than a single pane. As far as keeping the heat out, more importantly, special Low-E coating on the glass blocks infrared light which keeps cold air inside while keeping warm air outside. In addition, this special coating also filters damaging UV light to help protect interior furnishings from fading over time.
Spacer: A spacer keeps the dual glass panes of a window the correct distance apart which also allows for optimal airflow between them. Excessive airflow or restricted airflow can affect the efficiency of the insulating glass. The design of and the material the spacer is made of can also make a big difference in a window’s ability to handle expansion (on hotter days) and contraction (on colder days) and can also play a role in reducing condensation.
Frame: A very important consideration is what material comprises the frame. Energy efficient materials like vinyl and fiberglass do an excellent job reducing heat transfer and contributing to insulation value. Like the other components listed, the right frame can keep cold air in while repelling warm air and vice versa.
Finding The Right Glass Is Key
Focusing on the type of glass found in any window is crucial. The glass panels contributes to the overall energy efficiency of a window of course but what should you look for? There are three main factors to consider including Visible Transmission (VT), Solar Heat Gain coefficient (SHGC), and U-value.
- VT – amount of natural light that is transmitted through the glass
- SHGC – amount of solar heat (outdoor heat) that can pass through the glass
- U-Factor/U-Value – amount of heat that can escape your home through the glass
- High VT rating means that the glass will allow a significant amount of natural light into your home, reducing the need for indoor lighting during the day therefore lowering your electricity bill. The recommended VT value is 60 to 80 percent. For warm climates the SHGC must be as low as possible, 0.40 or below is recommended. The lower the SHGC the lower amount of solar heat (outdoor heat) will enter your home through the glass, helping to reduce your air-conditioning bills. Glass with a low U-Value is ideal for all climates.
The Importance Of Climate
The different ratings and numbers in each of these factors varies and is determined primarily on where your home is located. The U.S. Department of Energy (DOE) and the Environmental Protection Agency (EPA) have developed ENERGY STAR designations for windows that meet certain energy performance criteria (efficientwindows.org). Windows that have the ENERGY STAR designation are labeled showing the zones in which they are qualified for. Below are the requirements (for SHGC and U-Factor) as well as a map of the zones that are used to determine the ratings:
The great thing about energy efficient windows in this day and age is the fact that they work well in either warm or cool temperatures. On hot sunny days, these windows work to keep warm air outside while keeping cool air inside. On cold days, they keep cool air outside while keeping warm air inside. Energy efficient windows are also rated for specific climate zones. This can make the decision-making process easier for new homeowners or those that wish to upgrade their older, energy-wasting windows. The bottom line is simple: energy efficient windows are designed keep you comfortable year-round.