The
biggest radiator known to man is the sun and the biggest
absorber of radiant energy is the one we walk on everyday -
its called earth - and earth cools itself every night with
radiant energy. So does radiant heating and cooling
work? Well if it didn't - we'd all be dead.
Radiant heating and cooling is
essential for our survival yet in the world of HVAC it is
the most powerful yet least understood of all mechanism for
conditioning bodies and buildings.
This is
demonstrated by the fact that most HVAC systems in America
are designed to respond to the air temperature sensors,
which in fact do not reflect radiation problems,
particularly those caused by windows.
For our
purposes in building science we know this radiant energy by
two forms; that being shortwave and long-wave
electromagnetic energy (see
Fig. 5 ).
Shortwave
energy is the direct high intensity energy from the sun. You
see shortwave energy in colors and feel it as heat.
Shortwave falls in the categories of ultra-violet (UV),
visible light (VL) and infrared (IR). How important is
shortwave energy? Well the difference on earth between cold
and dark and hot and light depends on the flow of this electromagnetic
energy.
Long wave
energy is the indirect (diffuse) or filtered low intensity
energy that bounces off the environment. It falls primarily
under the category of far infrared. Body and room
temperatures (Fig. a & b ) are in the long wave spectrum.
Though these images appear hot, they are in fact cool - less
than 90°F in the case of the child and less than 75°F in the
case of the building.
Figure b. Thermographic image of a radiant
heated room. Image credit: (C) Bob
Rohr
Both short
and long wave energy is converted to heat when its strikes
an opaque surface of lower
temperature. Opaque means it's not transparent or
translucent.
Window
glazing is virtually opaque to long wave radiation but not
shortwave. It’s the shortwave energy passing through the
window that provides visible light and also heat’s up in the
interior space.
When you
read on the internet about problems with passive heating or
how solar gains and radiant floor heating can cause
discomfort, what you are reading about is the lack of
control over the two forms of radiant energy. This lack of
control comes from poor planning (ignoring building
orientation to south, length to width ratio's and excessive
window to wall ratio's) and inadequate
external solar control such as shading and blinds;
the last passive line of defense is the building enclosure
which windows are an integral part.
By their
very nature windows almost always create asymmetric thermal
environments. Symmetrical means the same, asymmetrical means
different. Asymmetric thermal environments means the two
different surface temperatures on opposites sides of the
human body. Think about you in front of a fireplace...one
side hot the other not...that's radiant asymmetry.
Specifically, a
window influences thermal comfort in these ways:
long-wave
radiation from the warm or cold interior glass surface to
the building interior and occupants
transmitted short wave solar radiationto the building
interior and occupants
induced air motion (convective drafts) caused by a
difference between the glass surface temperature and the
adjacent air temperature.
Comfort
effects from windows are fundamentally different based on
seasons.
The winter
effect is mainly driven by inside window surface
temperature, which are correlated with window U-factor (Fig.
3 and 4) and outside temperature.
The summer
effect is driven by a combination of the inside surface
temperature and transmitted short and long wave solar
radiation, which are heavily influenced by the optical
properties of the window; that being single, double and
triple pane with various fills (ex. argon) and coatings (ex.
low e).
Comfort aspects to consider:
the effect
of window surface temperature on long-wave radiation heat
exchange between the body and the window, and
the effect of solar radiation transmitted by the window and
absorbed by the body and interior surfaces.
Practical application
the closer
a person is to a window, or the larger the window, the
greater the impact on comfort.
Windows (fenestration)
are one of the weakest links in the building enclosure due
their inefficiencies as an insulator which also includes the
thermal bridging through structural elements that create the
opening for the window unit.
This topic is part of our
Professional
Development curriculum. Several on-line webinars and
multiday programs are offered through the year - many are at
no cost or available with government subsidies. Be
sure to also check out our new
Donate to Educate program.
Definitions
U Value:
measure of the rate of conductive heat transfer through the
glazing due to a temperature change between inside and
out-side surfaces also known as the overall heat transfer coefficient. The U values shown
in Figure 3. are for center of glass and for a complete window (glazing unit + frame). The
smaller the U value the better the window is at stopping
heat flow. The R value of a window is the reciprocal of the
U value (i.e. R=1/U). In cold climates (heating
dominated) low U value windows should be used on all
northward facing windows. Solar Heat
Gain Coefficient (SHGC): the solar heat gain coefficient of
the total window system represents the solar heat gain
through the window system relative to the incident solar
radiation. A value of 1.0 indicates that 100% of the solar
gain enters the building. A value of 0.0 indicates no solar
gain is entering the space. Low solar heat gain (LSG) coefficients can
significantly reduce cooling requirements. However in cold
climates (heating dominated) high solar heat gain (HSG) is
preferred on southward facing windows. Shading
Coefficient (SC): is a dimensionless number representing the
ratio of solar heat gain through the window system relative
to that through 1/8" (3mm) thick clear glass at normal
incidence. The lower the number, the less solar gain is
admitted. ex. SC = SHGC x 1.15. Low shading coefficients can significantly reduce
cooling requirements (see above regarding heating dominated
climates). Visible
Transmittance (VT or Tvis): percent of the visible
spectrum striking the glazing that passes through the
glazing. This value changes with angle of incidence. Visible
and Solar Reflectance: percent visible light or solar
energy that is reflected from the glazing. UV
Transmittance: percent transmittance of
ultraviolet-wavelength solar energy (0.30 to 0.38 microns).
High UV penetration will fade fabrics and can damage
sensitive artwork.
Background
Window
shading coefficient and visible light transmission are
governed mainly by the type of glass and coating used in the
glazing unit.
Low values
are indicative of the performance characteristics associated
with highly reflective glass and high numbers are indicative
of clear glass performance.
U values
should be evaluated for the complete window (glazing unit
frame) and take into account the impact of edge conditions
and frame construction on the window's performance.
Coatings
are hard coat or soft coat, and available with high solar
heat gain (HSG), moderate solar heat gain (MSG), and low
solar heat gain (LSG).
The differences between coatings affects what portion of the
IR spectrum is blocked.
Figure 1. Profile of a high performance
window (above)
Figure 2. Principles of radiant energy -
light and heat for LSG
Figure 3. Characteristics for a high
performance window. Compare the U values, SHGC and VT with
the windows below.
Figure 4. Characteristics for various other
windows.
Figure 5. Radiant energy
spectrum showing short wave and long wave energy.
Window installation is critical. Even the best windows if
installed poorly will leak.
Resources
1. Huizenga, C, Zhang, H., Mattelaer, P., Yu, T., Arens,
Edward A, & Lyons, P.(2006). Window Performance for Human
Thermal Comfort. UC Berkeley: Center for the Built
Environment.
2. Swinton,
M.C., Manning, M.M., Elmahdy, A.H., Parekh, A., Barry, C.,
Szadkowski, F., "Field assessment of the effect of different
spectrally selective low emissivity glass coatings on the
energy consumption in residential application in cold
climates," 11th Canadian Building Science and Technology
Conference (Banff, Alberta, March 22, 2007), pp. 1-16, March
22, 2007 (NRCC-49481).
3. NASA, Clouds and Earth's Radiant Energy System,
CERES
The
biggest radiator known to man is the sun and the biggest
absorber of radiant energy is the one we walk on everyday -
its called earth - and earth cools itself every night with
radiant energy. So does radiant heating and cooling
work? Well if it didn't - we'd all be dead. 
