Glass and glazing
Before you specify………
Consider both heat loss and solar gain.
Detailing
The type of high-performance glazing you specify will be useless unless
you first ensure that your building is well-sealed with a minimum of cold
bridging. When detailing windows pay particular attention to:
• Cold bridging across the frame
• Air leakage around the frame within its opening
• Edge sealing between the glazing unit and the frame
The proportion of glass to frame makes a difference as the framing is often a worse (higher U value) than the glass. Warm edge spacer technology is a marginal help but worth considering. The bigger the window the less significant the spacer bar conductivity is. Some profile makers claim zero air leakage.
U - values
Be aware that the U-value of a glazing unit varies across its face. The
centre of the unit will perform best with the lowest U-value whereas the
perimeter will perform less well due to the conductivity of the spacer
units. There is as yet no standard by which U-values are presented within
the industry. If there is any doubt, ask the manufacturer.
Typical available U-values:
Single-glazing 5.0
Double – glazing 3.0
Triple-glazing 2.2
Double-glazing with low-e coating 1.7
Double-glazing with low-e coating and Argon filled 1.3
Triple-glazing with multiple low-e coatings and Xenon filled 0.4
Established technologies
Low-E (low-emissivity) coatings
Low-E glazing has special coatings that reduce heat transfer through
windows. The coatings are thin, almost invisible metal oxide or semiconductor
films that are placed directly on one or more surfaces of glass or on
plastic films between two or more panes (Heat Mirror Glass). The coatings
typically face air spaces within windows and reduce heat flow between
the panes of glass.
When applied inside a double-glazed window, the low-e coating is placed
on the outer surface of the inner pane of glass to reflect long wavelength
energy (heat) back into the living space whilst permitting short wavelength
solar energy (light) transmission through from the outside. This same
coating will slightly reduce solar heat gain during the summer season.
There are two types of Low-E (low emissivity) glass available - Pyrolytic
(hard coat) is considered to be a medium performer, and sputtered (vacuum
deposition or soft coat) is considered to be the highest performer. Soft-coat
low-e films degrade when exposed to air and moisture, are easily damaged,
and have a limited shelf life, so they are carefully applied by manufacturers
in insulated multiple-glazed windows. Hard low-e coatings, on the other
hand, are more durable and can be used in add-on (retrofit) applications.
The soft coats (eg Pilkington Optitherm SN) have low U values but because they are so effective they also stop more short wave radiation. The hard pyrolitic coats (eg Pilkington K Glass) are more transparent to short wave radiation. The net effect is that the energy equation is about the same for both types.
A new option is to use low iron glass (eg Pilkington Optiwhite) in the outer pane to improve the g value (total solar heat transmission).
Gas filling
Filling the gap between the glass panes with low conductivity gas such as argon or krypton (as well as the more expensive xeon) at atmospheric pressure improves the window performance by reducing conductive and convective heat transfer. They are mostly used in conjunction with low-emissivity coatings. One drawback though might be the long-term integrity of the fill.
‘Superwindows’
‘Superwindows’ combine technologies for solar control and
heat loss reduction. They were originally developed for residential applications
in cold climates. Three- or four-pane glazing units with multiple low-e
coatings, gas fills and special frame construction reach U-values of 0,4-0,5
W/m2K for centre-of-glass values and 0,7-0,9 W/m2K for the total window
U-value. Their overall energy performance provides net energy benefits
for any orientation.
Emerging technologies
Vacuum double glazing (Evacuated windows)
Working much in the same way as a Thermos flask, the "fill" strategy with the lowest conductance is the use of vacuum between low-e coatings. Only a very small distance between the glass panes is necessary, but the long-term integrity of seals and the structural stability of the unit (due to pressure differences) are difficult to master in a cost-effective manner.
Switchable glazing technologies ("smart" windows)
These materials have variable solar-optical properties, which can
be passively or actively altered. Their application in architectural
glass allows dynamic regulation of solar energy transfer through the
fenestration for visual comfort, thermal comfort, peak load management,
the control of glare, privacy and daylight.
There are different types of chromogenic materials
under consideration for use in buildings, based on different physical
principles. The most relevant are:
• Photochromic coatings: change the solar transmittance
as a function of light intensity (like sunglasses). Primary benefit:
improving visual comfort.
• Thermochromic materials: change their optical
properties as a function of their temperature (like liquid crystal temperature
indicators), from transparent when cool to a white, reflecting/diffusing
state when heated. Liquid and gel based materials as well as thin-film
solid state devices are investigated.
• Electrochromic materials: are multi layer films
whose optical properties can be controlled using an applied voltage
(like flat display panels). These have the greatest versatility since
their transmittance can be controlled at any moment actively by occupants.
However, they are potentially more complex and difficult to fabricate
and more expensive than single layer chromogenic materials.
• Gasochromic glazing: in these a low concentration
of hydrogen in a neutral gas such as nitrogen is dissociated by a catalyst
and intercalated into a tungsten oxide layer. This turns deep blue,
but does not block visibility. Bleaching is achieved with oxygen.
Standards
Glass manufacturers are starting to use the new European Standards (EN). Performance figures generated to the more demanding EN standards are more accurate.
