Windows

Windows

Heat naturally flows from warm to cool.  In the winter, it flows from heated living spaces to the outside and to unheated spaces in your home such as attics.  In the summer, it flows from the outside into your home.

When heat is lost or gained due to poor insulation, your heating and cooling systems must work harder to maintain a comfortable temperature in your home, resulting in higher energy costs.

Both your body and the room lose heat to the cold surface of your windows, causing you to feel cold in the winter time even though you are surrounded by warm air.  More energy-efficient windows can prevent this discomfort due to their insulating properties and higher interior surface temperatures.

Glazing. The glass panels in window frames are also called glazing. In order to ensure the energy efficiency of your existing windows, be sure to check the glazing of your windows for any cracks.  Small cracks can be repaired with glazing compound, found in most hardware stores.  Always replace glazing with large cracks or holes in it as these openings can lead to significant air leakage.

Quick Links:
How to buy?
What are my Options?
Energy Saving Tips
How much can I save?

How to buy?

Windows are rated according to their performance as whole structures (frame + glazing), but in selecting a window, you may be wondering which types of frames, glazings, or operating types are right for your home.  The following are some things to consider

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What are my Options?

Single Pane Windows. Inefficient windows (e.g., single pane with aluminum frames) or high moisture with inadequate ventilation can result in condensation, frost, or pools of water on windows and sills. Moisture in the air condenses when it touches a cold surface. (The same effect causes a glass of ice tea to “sweat” on a hot humid day.) Continued excess moisture can lead to mold, mildew, and deterioration of your windows and sills.

Single pane windows provide minimal insulation for your home from the elements outside. They are great to see out of, providing maximum visibility, but one layer of glass will not cut down on the massive heat gain you experience in the summer or the tremendous heat loss in the winter.   The size of the window area should not exceed 15% of square footage of the room’s floor area.

Switch to Double Pane. Double pane windows provide much more than just another layer of glass.  What you get when you replace the wood or aluminum framed single pane windows with energy efficient vinyl framed double pane windows includes the following:

A vinyl frame and sash components reduce the transmission of heat significantly from that which you would find on a wood or aluminum window (which is what every single pane window employs). Often times, vinyl window manufacturers will even fill their windows’ frames and sashes with insulated foam to further reduce heat loss/gain. 

Glass thickness is much greater in double pane windows thus slowing down heat transference. 

The space between the two panes of glass in a double pane vinyl window can be filled with a dense, inert, gas (usually either argon or the much denser krypton) to slow down the convection of heat between the panes of glass. Don’t be fooled, even with double pane insulated vinyl windows, you will still gain or lose heat.   But the point is to reduce the rate at which heat is gained and lost in your home every year and a dense gas between the panes reduces that rate by large margins.

Low-e and spacer systems. Both of these elements are also common in a vinyl double pane window.  Low-e (low emissivity) is a coating that actually goes on the glass in the windows to reflect heat back to its warmer side or its source.  A spacer system, is the material that is between the two panes of glass that also reduces heat transference and heat loss because of its non-conductive properties.  Make sure you avoid spacers that are metallic, particularly aluminum. Though they do not rust, they are one of the most conductive substances you can have in a window. “Warm-edge” spacers are used in vinyl windows and some recognizable, reliable names are Super Spacer, Intercept, Dura-Seal.

Those are some of your most basic improvements from a wood frame/sash (or aluminum frame/sash) single pane window to a double pane window. It’s more than just an added sheet of glass and the energy savings provided can be tremendous.

Frame Materials. Window frames are made from a variety of materials, including aluminum/metal, fiberglass, vinyl, wood, and composite materials.  Each material has its advantages and disadvantages.

Aluminum/metal frames. These are durable, light, and easy to maintain.  However, metal is a poor insulating material.  Aluminum or metal frames should have a thermal break, a plastic strip placed between the inside and outside of the frame and sash.

Fiberglass frames. These have air cavities for insulation that make them more efficient than wood or vinyl frames.

Vinyl frames. Good insulators and resistant to moisture.  They do not require painting.  However, they may expand in the heat or crack in cold temperatures.  For high efficiency, insulated vinyl frames are also available.

Wood frames. Good insulators, but they require the most maintenance of all the frame materials.  They may also expand or contract due to weather conditions.

Composite frames.  These are very stable, have the same (or better) thermal properties as wood frames, and are more resistant to moisture and decay than wood frames.

Glazing. The type of glazing or glass you select can play an important role in improving the energy efficiency of your windows.  Glazing technologies include gas fills, heat-absorbing tints, insulated (double-glazed, triple-glazed), low emissivity (low-e) coatings, reflective coatings, and spectrally selective coatings. The NFRC label on a window lists the type of glazing used, so you can see how each type performs.
 
Two features to look for are low-e coatings and gas fills.  Low emissivity, or low-e, coatings allow light to come through the window while reflecting the sun’s heat.  A double-pane window with a low-E coating can insulate as well as a triple-pane window with no low-e coating.

Gas fills are a technology that involves filling the space between each glass pane with harmless, inert gases (typically argon) that act as insulators. This improves the thermal performance of the window.

Operating Types. Windows have a number of different operating types, some of which have lower air leakage rates than others.  The following diagram illustrates traditional operating types:

In general, the awning, casement and hopper types have lower air leakage rates, while the single-hung, double-hung, single-sliding, and double-sliding types have higher air leakage rates.  Fixed panes are airtight but can be inconvenient since they cannot be opened.

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Energy Saving Tips

Upgrading Your Existing Windows. Caulking and weather stripping are two simple improvements that reduce air leakage from your existing windows.  Caulk comes in two basic forms: rope caulk (caulking cord) and tube caulk. Rope caulk is ideal for temporary seals.  It can be pressed into place with your fingertips and later removed.  For permanent air seals (e.g. around your window frame where the frame meets the wall), tube caulk should be applied with a caulking gun.   Weather stripping comes in the form of a strip (usually made of foam, felt, vinyl, rubber, or metal) and can be placed around windows and doors for air sealing purposes.

Installing storm windows can also reduce air leakage from your existing windows.  Storm windows work by creating an air space between the storm window and your existing window, allowing for better insulation.  Storm windows come in a variety of forms, from plastic sheets to triple-track glass windows, and can be installed either on the interior or the exterior of your existing window.

The U.S. Department of Energy recommends interior storm windows over exterior storm windows as they are easier to install, remove, and maintain.  They are also more effective when it comes to preventing air leakage.  When installing a storm window, the recommended air space between the storm window and your existing window is ½ inch. 
Some storm windows can double the insulating value of your windows and cut heat loss by 25 to 50 percent per window, saving you energy and money.  Storm windows have major comfort benefits as well, including:

Increasing the interior surface temperature of your windows
Reducing condensation
Alleviating drafts
Reducing street noise
And protecting furniture, carpets and other valuables from ultraviolet damage.

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How much can I save?

WHY BUY ENERGY-EFFICIENT WINDOWS?
Save Money, Save Energy, Save the Planet. If all windows in the United States were replaced with ENERGY STAR windows, we would save $7 billion in energy costs over the next 15 years, reduce carbon emissions in the equivalent to taking 336,000 cars off the road, and save enough energy to light two-thirds of all the homes in Pennsylvania.