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Solar, Safety and Security Window Films: Tech Bulletin
Understanding U-Factor
The key to communicating the value of low emissivity films
to customers is a solid understanding of U-factor. This
Technical Bulletin provides a description of what U-factor is,
how emissivity affects the U-factor of a window, and provides
examples to explain real world effects of the concept.
U-Factor Defined
U-factor is the property of a window that represents the
amount of total thermal energy (heat energy) lost through it.
[It is expressed in units of watts per square meter, per degree
Kelvin (W/m2 ºK). The temperature indicated in this expression
is simply the difference between the outdoor temperature
and the indoor temperature.
U-factor indicates the amount of heat “lost” through a window
under a given set of conditions. In any climate, the greater
the temperature difference between the inside of a building
and the outdoors, the more heat is lost to the outside. This
heat loss is directly proportional to the temperature difference
in degrees. So if the outdoor temperature is twenty degrees
Figure 1: single pane clear glass
Low e
Radiative loss
Conduction
Conductive loss
Insulation
Outdoor
Indoor
window, such as air gaps and extra glass panes. These extra
layers are conductive insulators, and serve to reduce the
amount of heat conducted through the glass to the outdoors.
cooler than the indoor temperature, twice as much heat is
lost through the window than if the outdoor temperature
were ten degrees cooler. If the temperature is the same
both indoors and outdoors, there is no room heat loss or
gain. This is why U-factor is highly important to cold
climates, since temperature differences are greater.
Approximately 60% of the heat lost through a window is
The heat lost through windows in a building relies on three
things:
Approximately 40% of the energy lost through windows
•Window
U-factor
which is to say long wave infrared radiation from indoor heat
•Amount
of glass
•Indoor-outdoor
temperature difference
Emissivity and U-factor
Energy is transferred through three mechanisms: convection,
conduction, and radiation. Convection is not a significant
source of energy transfer for windows, so it is not directly
incorporated into the calculation of U-factor. Conduction
and radiation are the two significant sources of heat loss
through windows.
The majority of heat lost through a window is conductive,
and can only be affected by adding insulating layers to the
conductive, though this varies by window construction and
frame type. Window film does not improve the conductive
insulation of a window, and therefore the approximately 60%
of the energy conducted outdoors remains unaffected by the
addition of solar control window film.
from the indoor environment to the outside is radiative,
radiates through the glass pane. Solar control coatings are
designed to affect radiation, and low emissivity coatings
are specifically engineered to affect this type of infrared
radiation. The emissivity of a window describes how much of
the indoor infrared radiation that strikes it is reflected back
inside. For example, a single pane window has an emissivity
of.84, meaning 84% of the radiation is allowed to pass to the
outdoors. However, when coated with Solar Gard® Silver AG
50 Low E film, the emissivity improves to .37, meaning only
37% of the infrared radiation is allowed to pass outdoors.
Unlike conduction, the type of frame a window has does not
affect its emissivity.
Emissivity contributes to less than half of the U-factor.
Therefore, even if a film existed with an emissivity of zero (0),
and no infrared radiation passed through the windows, the
problem of conduction still remains. This is why emissivity
alone cannot help a customer understand the total benefit
of retrofit window film. Rather, the U-factor communicates
the appropriate effect.
Figure 2: Glass with low e film applied
It is easiest to understand how U-factor works by examining
some practical examples.
A homeowner has 40 square meters of window with a
U-factor of exactly 5.5. It is 22 degrees inside the house,
and 0 degrees outdoors.
Since we know the temperature difference between the
indoors and outdoors is 22 degrees C, and we know the area
of the window (40 square meters), we can multiply all four
of these numbers into the simple equation to determine
the total heat lost.
5.50 U x 40 sq. m. x 22 C x = 4,840 watts
Low e
Radiative loss
Conduction
Conductive loss
Insulation
Outdoor
Indoor
This simple calculation is a great tool for demonstrating to
customers the exact impact of their glass on heating. In the
example above, the customer requires 4840 watts of heating
to simply keep up with the heat lost through the windows. To
take this exercise one step further, dealers can then use the
potential U-factor of installed film to communicate the exact
benefit and savings to a customer. For example, if the same
home owner decreases the U-factor of the windows by half,
it would reduce the heating requirement by 2420 watts.
This is identical to adding a 2.4 k W heater to the home.
The insulation value of walls is largely dependent only on
Key Points
conduction. As a result of this, as well as traditional methods
• U-factor
of determining wall insulation values, the term “R-value” is
• U-factor
is a measure of total heat lost through windows
used instead of U-factor. R-value is simply the reciprocal of
includes heat lost by both conduction and radiation
U-factor (1/U = R). So a wall with an R-value of 5.0 would
• Emissivity
have a U-factor of 0.20. Although technically U-factor and
• U-factor
R-factor can be interchangeably converted for use with
both windows and walls, traditionally window specifications
affects only heat lost through radiation
can be used to describe exact customer heat loss
• Demonstrate the benefit of low e window film to customers
are published in U-factor, and wall insulation is published
using R-values.
Communicating the Value
A simple equation can be used to tell a homeowner how
much energy a home is losing at any given moment, by using
the following equation. The energy lost can be determined
multiplying three simple numbers.
[U-factor] x [square meters of window] x [temperature
difference (C)] = watts lost
Emissivity and U-Factor Table
The following table shows the relationship between emissivity
and U-Factor for three types of windows. For instance, a
filmed window with an emissivity of .45, applied to a double
pane window, will have a U-factor of 2.33 W/m² °K (which is a
solid improvement over double pane clear).
Note: It is physically impossible for any window film to
increase the performance of a single pane window to equal
that of a double pane window. As Table 1 shows, even a
single pane window with a perfect emissivity of zero has
poorer U-factor performance than that of a double pane
clear window with an emissivity of .84.
Table 1:
Winter U-factor: Btu/hr/ft2 ºF (W/m2 ºK)
e*
Single
DoubleATripleB
0.84
1.04 (5.91)
0.49 (2.78)
0.31 (1.76)
0.75
0.99 (5.62)
0.47 (2.67)
0.31 (1.76)
0.65
0.93 (5.28)
0.45 (2.56)
0.30 (1.70)
0.55
0.87 (4.94)
0.43 (2.44)
0.29 (1.65)
0.45
0.81 (4.60)
0.41 (2.33)
0.28 (1.59)
0.38
0.77 (4.37)
0.40 (2.27)
0.27 (1.53)
0.33
0.74 (4.20)
0.39 (2.21)
0.27 (1.53)
0.15
0.64 (3.63)
0.35 (1.99)
0.25 (1.42)
0.07
0.59 (3.35)
0.34 (1.93)
0.24 (1.36)
0.00
0.55 (3.12)
0.32 (1.82)
0.24 (1.36)
*emissivity of innermost glass surface
A
3 mm + 12 mm + 3 mm
B
3 mm + 12 mm + 3 mm + 12 mm + 3 mm
www.solargard.co.uk
Saint-Gobain Performance Plastics
Unit 13, Ball Mill Top Business Park
Grimley, Worcestershire WR2 6LS
United Kingdom
Tel: +44 (0) 1905 640 400
[email protected]
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