Also known as the U-factor or coefficient of heat transmission, a measure of
the rate of non-solar heat loss or gain through a material or assembly.
U-Values gauge how well a material allows heat to pass through. U-Value
ratings generally fall between 0.20 and 1.20. The lower the U-Value, the
greater a product's resistance to heat flow and the better its insulating
The inverse of (one divided by) the U-Value is the R-Value.
U-Value is expressed in units of W/m2 °C or Btu/hr-sq ft °F. In the US,
values are normally given for NFRC/ASHRAE winter conditions of 0°F (-18°C)
outdoor temperature, 70°F (21°C) indoor temperature, 15 mph wind, and no
solar load. U-Values are often quoted for windows and doors.
U-Value & Windows
In the case of a window, for example, the
U-Value may be expressed for the glass alone or the entire window, which
includes the effect of the frame and the spacer materials.
It is extremely important for owners and designers to make sure that they
are comparing U-Values for the entire window unit, (not just the glass) when
selecting windows, and to realize that though the numbers are small, the
differences can be quite substantial. (A .22 U-Value is 12%
more thermally efficient than a .25 U-Value, and 36% more efficient than a
.30 U-Value, which is the current standard required for the federal tax
To reduce U-factors, some manufacturers apply a low-E (low-emittance)
coating to glazing surfaces. These low-E coatings reduce heat loss,
improving both heating and cooling performance. Windows can also be
assembled to improve thermal performance. Some assembly strategies include
using two or more layers of panes or films, low-conductance gas fills
between the layers, and thermally improved edge spacers, which are placed
between the panes.
The sash and frame of a window represent 10% to
30% of a window's total area, depending on the window size and design. The
material used to manufacture the frame can thus impact heat loss and related
condensation resistance. In colder climates, in non-residential buildings,
where aluminum frames are used, thermal breaks should be specified in order
to minimize heat transfer and condensation on the frames. In colder
climates, with residential buildings, most products use wood, vinyl, or
other non-metallic frames. Some door frames will also conduct heat readily.
For solid doors, insulated metal or fiberglass doors are usually the best
choice. Window coverings, such as shades, shutters, and insulating or storm
panels, can help reduce heat loss too.
Calculating and Converting U-Values|
Metric U-Values / factors are defined as Watts per square
meter per degree Celsius. To convert metric
U-Values to Imperial inch-pound U-Values, divide
by 5.678. Example: A metric U-Value listed
like the following "Standard:Uw 1.2 W / m2K,"
simply divide 1.2 ÷
5.678 = .21 The resulting U.S. U-Value is .21
To convert inch-pound Imperial
U-Values to metric U-Values, multiply by 5.678.
To convert Imperial inch-pound R-Ralues to metric
R-Values, multiply by 0.1761. One inch = 2.54 cms.
One (RSI) U-factor coefficient = 5.678; one R-Value
= .1761 RSI.
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