How Energy Flows

To appreciate the improvement in today’s windows, it is helpful to understand three ways that energy can flow through them.

• Air can carry heat in or out of a window. Intentional air flow is called ventilation. Unintentional air flow – leakage – is called infiltration.
• Heat – or cold – can flow through the frame and the glass.
• Solar radiation – sunlight – can pass through the glass and can heat whatever is inside the building.

Ventilation and Infiltration
Well-placed windows allow for natural ventilation, which can significantly reduce cooling costs in the summer. But infiltration – unplanned air leakage through a window’s joints, cracks, frames and sashes – can account for as much as 15 percent of a home’s heating and cooling losses. It can make a home much less comfortable and more costly to operate.

Different types of windows can allow more infiltration. For example, a horizontal sliding window may not be as airtight as a window with a swinging sash. In addition, the overall quality of the window can affect infiltration – some windows are built better than others.

Modern Windows – Think of Them as Thermos Bottles
When there is a difference between inside and outside temperatures, heat transfers through a window. It’s lost to the outside during the heating season and is gained from the outside during the cooling season.

A window’s thermal performance – which can be measured at the center of glass, the edge of glass and the frame – is rated with a U-factor, its overall ability to resist heat flow.

Have you ever tried to drink hot coffee from a drinking glass? If so, you know that glass is a very good heat conductor. The outside of the container can quickly become too hot to hold. Using two layers of glass with an air space between – the ideas behind the Thermos bottle, incidentally – dramatically cuts the heat flow.

Single-pane windows can act like that drinking glass, conducting heat to the outside. Dual-pane windows, with a 1/2 inch to 3/4 inch air space between sheets of glass, work like a Thermos bottle to cut down the flow of heat. If you replace the air between the panes with an inert gas like carbon dioxide, argon or krypton, the window will transfer even less heat and be even more efficient.

The Frames
Before dual-pane windows, the material used for window frames was not of great concern, since a single pane of glass conducts heat about the same as a solid aluminum frame. With more efficient glazing, however, new materials and new designs were used to make window frames themselves less conductive. Today, wood, fiberglass, vinyl or vinyl-clad wood window frames will generally perform better and provide more comfort than metal frame windows that do not have a thermal break – a piece of non-conductive material sandwiched between the metal parts to cut down the flow of heat.

The Glass
Even in dual-pane windows with less conductive frames, the type of glass and type of spacers used to create the air space can improve energy performance. Low emissivity (low-E) coatings can help to decrease the U-factor; the lower the U-factor, the more efficient the window becomes. A low-E coating is a microscopically thin layer of metal or metal oxide deposited on window glass. The coating reflects warmth back into the home in the winter and prevents unwanted heat from entering the home in the summer.

When shopping for windows, look for the overall U-factor rating. The lower the U-factor, the better the window’s energy performance will be.

Windows that Help Block the Sun
Sunlight passing through a window adds heat to a room – desirable in winter, but generally unwanted in the summer. Solar heat gain through windows may account for 30 percent or more of a home’s summer cooling requirements.

The amount of heat from direct sunlight through a window is measured as Solar Heat Gain Coefficient (SHGC). The lower the coefficient, the less solar heat comes through the window.

Window tints such as bronze and green keep out more of the sun’s heat than clear glass does. Low-E coatings used to lower window U-factors may also reduce a window’s Solar Heat Gain Coefficient. Keep in mind, however, that by tinting your windows to keep out heat, you may make it harder to see through them. Consider a window’s orientation to the sun before selecting window tints. South facing windows gain the most beneficial heat in winter, so their Solar Heat Gain Coefficients could be higher and therefore allow more heat gain. If these windows are properly shaded – say, by deciduous trees – they will allow little heat gain in summer. Note: careful consideration should be made to what climate zone in California you are in and orientation of the window before selecting fenestration.

Solar Heat Gain Coefficients for north windows can also be higher, since they get little direct sun and do not contribute much heat any time of the year. East and west facing windows should have low SHGCs, since they get direct morning, mid-day and late afternoon sun and are responsible for most of the solar heat gain during summer.

Often the best strategy to reduce heat gain in the summer is to provide exterior shading through overhangs, awnings, shade screens and trees.