The Physics of Windows
Physics 212
Rebecca Vaughan
The Alaska Cooperative Extension states that in a typical house, over 40% of the annual energy budget is consumed be heating and cooling.Ê The type and quality of windows in a building plays a large part in the expenses incurred during heating and cooling.Ê Because widows are basically just a big hole in an insulated wall we need to understand the importance of the windowâs individual thermal performance and how it affects our homes.ÊÊ We can begin to educate ourselves by understanding the difference between various glass types and frame materials.
Before going into what types of windows are available on the market today letâs define some terms commonly used in window lingo.
Terms
R-value: This is a measurement of the insulation effectiveness of the window.Ê The R-value for a particular material equals the materialâs thickness, L, divided by the materialâs thermal conductivity value, k.Ê The R-value also equals the inverse of the materialâs U-value, 1/U.Ê In short the higher the R-value, the better the insulating performance.
K-value: The thermal conductivity of a material is called a k-value and is a measured value that can be found in most structural handbooks.
U-value: The measurement of heat gained or lost due to the differences in temperature across the material.Ê Measured in Btuâs/hr./ft2 the U-value equals 1/R
Conduction: is the flow of heat through a material.Ê This occurs by one molecule transferring heat to the molecules next to it.
Radiation: is the transfer of electromagnetic waves such as infrared, visible, and ultra-violet.Ê Radiant heat transfer is what you feel when standing in front of a window on a cold day and feeling cold even though you are in a warm room.Ê Your body heat is being radiated from you to the window surface.
Convection: is a heat transfer process involving motion in a fluid, in this case air, caused by the differences in the density of the fluid and gravity.Ê Convection affects heat transfer from the glass to the room and between the panes of glass.
Emissivity: Describes a surfaceâs ability to transmit or receive radiation.Ê The E in low-E windows stands for emissivity.
Types
of glass
The
optimal energy efficient window
1)
External
radiated heat is reflected
2)
Direct
heat coming is reduced
ÊÊÊÊ 3) Internal radiated heat is reflected
Clear glass is what has been primary used in the past.Ê This glass consists of only a plain, uncoated, single pane of glass.Ê It is the cheapest of any window glass but it unfortunately allows enormous amounts of heat loss and condensation buildup to occur. Ê
Insulated glass is made up of two or more layers of glass separated by a spacer with the edges of the glass sealed.Ê This type of glass is usually called double-paned, triple-paned, etc. The space in between the layers of glass can be filled with air or a gas, the most common being argon or krypton.Ê By trapping air or gas between panes the window becomes an insulator by keeping heated air from passing through the window.Ê Using gases such as argon and krypton result in an enhanced energy performance of the window because the gas inside is denser than the air trying to pass through.ÊÊ
The greater the number of air spaces in an insulated window the higher the R-value and the lower the amount of heat lost through the window.Ê The width of the air spaces cannot become too large however or the flow of air within the window becomes unrestricted and convection transfers heat from the inner pane of glass to the outer pane at a much faster rate resulting in heat loss.ÊÊ For most windows an air space of 0.75 inches should not be exceeded.Ê The following table from the Alaska Cooperative Extension lists the R-values for the most common gases used and how the values vary with different air space widths.
Table 1: Gas R-values
|
Space widthÊ (inches) |
Air |
Argon |
Krypton |
||
|
0.25 |
|
|
2.21 |
2.5 |
3.68 |
|
0.375 |
|
|
2.79 |
3.31 |
3.81 |
|
0.5 |
|
|
2.9 |
3.59 |
3.72 |
|
0.625 |
|
|
2.9 |
3.4 |
3.61 |
|
0.75 |
|
|
2.9 |
3.4 |
3.61 |
|
1 |
|
|
2.9 |
3.31 |
3.42 |
Low-E glass ÊOne method in
improving the efficiency of an insulated window is to add a low-E coating to one
or more of the inside surfaces of the window.Ê
The coating usually consists of a layer of silver in between two layers
of metal oxides.Ê The purpose of the
low-E coating is to reflect the heat going out the windows back into the
building.Ê This type of glass reduces
heat loss through the window, which in turn saves on heating costs and reduces
that annoying draft during those cold winter months.Ê There are different quality levels of low-E coating used, such as
Heat Mirror, and each come with varying efficiency and costs.Ê
ÊÊÊÊÊ
Window Frames
Window frames are available in a variety of materials.Ê Frames can be primarily constructed of one material, or may include a combination of materials.Ê Each material has its structural advantages and disadvantages.Ê The most common frame materials offered today are wood, vinyl, aluminum, and fiberglass.Ê
Wood
Wood windows produce higher R-values and are less susceptible to the expansion and contraction problems associated with extreme temperatures.Ê Frames made solely from wood are becoming less popular in todayâs market due to their high maintenance.Ê If wood frames are not properly protected from moisture they can warp, crack, and stick.Ê This means that periodic painting of the frame is required to insure moisture from the surrounding area does not damage the frame.Ê Another drawback to wood frames is that they naturally degrade over time.
Due to supply and demand issues wood framed windows are generally more expensive than other frame types.ÊÊÊÊ
Vinyl
Vinyl framed windows are the most popular and least expensive type commonly available today.Ê Unfortunately they also are the most affected by expansion and contraction as can be seen from the following graph provided by Great Land Window
The thermal expansion and contraction of vinyl is over 8 times that of window glass.Ê This means that during thermal movement gaps will be created between the glass and the frame resulting in heat loss.
Aluminum
Aluminum frames were around before vinyl frames became available.Ê Aluminum can be placed between vinyl and wood in terms of popularity and price.Ê These frames have low R-values and produce some amounts of condensation around the edges of the frame.Ê Aluminum frames do not expand and contraction quite as much as vinyl, however gaps between glass and frame are still present and damaging.
Fiberglass
Fiberglass
frames are fairly new to the market and usually possess a much higher price
tag.Ê They are they most energy
efficient frame available today.Ê They
have the highest R-values of all frames, do not shrink, swell, rot, or
corrode.Ê The most important aspect of
the fiberglass frame is that its thermal movement is almost identical to that
of the window glass.Ê This means that
the window as a whole will expand and contract as one unit, greatly decreasing
heat lost through cracks and gaps in the window.ÊÊ To give a better idea of the difference between fiberglass and
common frame types consider that a reinforced 4â vinyl frame will warp
out of shape in an excess of a ¸ inch at a temperature of 40 degrees F.Ê At this same temperature an identical unreinforced
fiberglass frame will move less than an 1/8 inch over the same temperature
change.Ê
Many fiberglass frames contain a filling of insulation, which adds an even greater improvement to the thermal performance of the window.Ê
Fiberglass frames also possess a greater structural strength
than any other frame material.Ê
Fiberglass has over 8 times the strength of the most common frame
material, vinyl.ÊÊ This allows windows
containing fiberglass frames to hold a much larger expanse of glass as compared
to other types of frame materials.
References
Great Land Window Co.Ê Fairbanks, AK Ê(907)-479-8438 greatlandwindow@gmail.com
Inline Fiberglass.Ê Toronto, ON M9W 1K1ÊÊ http://www.inlinefiberglass.com
Seifert, Richard D. ÊBuilding in Alaska.Ê Alaska Cooperative Extension, March 1999.
Southwall Technologies.Ê http://www.southwall.com/cgi-bin/south.asp
Colt, Steve. 1991. Cost-Effectiveness of Alternate Window Systems in Anchorage, Fairbanks, and Southcentral Alaska.Ê Northern Building Science. July/August.