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The Insulating Glass Manufacturers Alliance (IGMA) is a voluntary non-profit international
association whose members include certified insulating glass manufacturers and suppliers of related
equipment, materials and services. IGMA’s stated purpose includes establishing voluntary quality
performance guidelines for the industry.
This White Paper is for voluntary consideration and use of manufacturers and other interested
parties in their own independent business judgment, and IGMA disclaims any and all liability for the
use, application or adaptation of the information contained in this document. This document is
provided as a service to the industry and reflect the collective experiences and consensus views of
the members of IGMA.
This publication has been developed in accordance with IGMA’s due process procedures. They
reflect existing technology and are subject to periodic review and change.
The information contained in this publication is not intended to be exhaustive or d to exclude other
possible technologies. The information, however, reflect practices which have been developed over
the years. The information is intended to assist in the evaluation of vacuum insulating glass
technologies.
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manufacturer’s specific product or services. No person has the authority in the name of IGMA to
issue any such interpretations or imply directly or indirectly of an endorsement for a product.
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Insulating Glass Manufacturers Alliance
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1.0
BRIEF HISTORY OF VIG
1.1
Pre - Collins (starting with early patents, circa 1910)
Vacuum Insulating Glass (VIG) is not a new concept and has its roots at the turn of the
20th century. Zoller first described this technology in a 1913 patent. VIG technology is
strikingly similar to its parent technology, the Dewar flask.
Alternatives
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Like the Dewar flask, VIG utilizes two fixed solid surfaces with an evacuated gas
space between and a coated inner surface to control radiant energy transfer.
The evacuated surface serves to virtually eliminate conductive and convective
heat transfer of the interior gas, while the coating (usually containing one or
more silver layers for sputtered coatings, or doped tin for pyrolytic coatings)
reflects radiant heat transfer. The Dewar flask and similar insulating bottles have
been commercially available for more than a century. Conversely, VIG
technology remained non-commercialized until the late 20st century. There exists
a great body of pre-commercial work on the subject, evidenced by the work of
Richard Collins and the University of Sydney.
1.2
Collins (University of Sydney)
Collins recognized early in his efforts the basic construction requirements of a
commercially available VIG unit (circa 1989). The basic construction is best
described as two sheets of glass, hermetically sealed together around the edges,
encapsulating a narrow highly evacuated space. The separation of the glass
sheets under the influence of atmospheric pressure is maintained with an array of
small, high strength support members commonly known as pillars. Radiant heat
transfer between the internal surfaces of the glass sheets can be minimized by
incorporation of one or more transparent low emissivity coating(s) on these
surfaces1. Collins experimented extensively with the aforementioned basic
construction techniques for many years, and amassed a large body of work
describing most aspects of the product and production techniques and
economies of scale. In 1994, Collins partnered with Nippon Sheet Glass (NSG) to
further refine the basic construction methodologies. In his collaboration with
NSG, he learned how to commercially produce a sealed unit, refining glass and
pillar placement dynamics, evacuation (pump down) techniques, and gained
an understanding of product longevity. The resultant introduction of the NSG
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Spacia™ VIG (1996) technology spurred development efforts by others in order
to enter the market.
1.3
Post - Collins VIG Activities
VIG technology has been explored, with prototype efforts around the globe.
Australia, France, Germany, Switzerland, United States, China, Japan and Russia
all have patents and prototypes of VIG constructions. NSG in Japan has VIG
batch production lines producing the Spacia™VIG product for many years,
mainly for the Japanese market. St. Gobain (France) was among the first to
commercialize a VIG product, with a target market of freezer and cooler doors.
This VIG was produced near Paris, France, mainly for the freezer door market, but
technological hurdles put an end to limited production. QH Glass (Qingdao,
China), a spin off from Beijing Synergy, and has been producing VIG for several
years – although the product has been hand built using a batch oven process,
slow progress has been made to improve the VIG product in collaboration with
others. Beijing Synergy, near Beijing, has produced VIG in limited quantity, and
as of early 2014 has a continuous production line reaching viability stage.
As VIG technology continues to evolve from a laboratory success into
commercial reality, other manufacturers and start-ups are developing VIG
solutions of their own. As well, commercial glazers and window and door
manufacturers alike are anxious to implement this emerging technology into their
products. As of early 2014, the landscape of start-up efforts includes:
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EversealedR Windows, Inc. (Colorado) offers licensed VIG technologies to the
window, door, and glazing industries for a flexible edge VIG product.
Guardian Industries is working to commercialize a continuous VIG production
platform.
A consortium in Europe, Pro-VIG, led by Grenzebach, worked for 10 years to
commercialize a product with flexible edge seals; obtaining significant grants
from the German government, however, in 2011 the Consortium broke up
due to technological and production hurdles.
Landglass (China) is working to commercialize VIG production line
equipment.
Good Technology and Engineering (South Korea) is working on VIG
equipment as well.
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There is also government interest in development of a commercially viable VIG
product. For instance, in the United States, the Department of Energy has a
history of funding certain research and exploration efforts to develop a VIG
product and appropriate testing methods. In 2012, NREL began actively working
on design and build of a sophisticated testing platform that will rapidly cycle the
surrounding environment in order to stress a VIG sample.
2.0
GENERAL INFORMATION ABOUT VIG
2.1
Overview of VIG Technology
The general desire of Vacuum Insulating Glass (VIG), much like conventional
non-evacuated Insulating Glass (IG) technology, is to improve the overall
insulating properties between an external environment and an internal
environment by slowing the rate of thermal energy transfer across two or more
sheets of glass. Like conventional IG’s, a small amount of energy is still transferred
across the VIG unit by means of conduction through the edge seal, as well as
across the pillar array, and by radiation from opposing surfaces within the space
between the panes. However, VIG differs from conventional or even gas-filled
IG units in that VIG significantly limits convection and conduction within the
space between the glass sheets by significantly reducing the amount of residual
gas between the panes to a high vacuum such that the volume of residual gas
remaining approaches zero. Convective heat loss is not a factor in VIG due to
the lack of gas within the cavity. Figure 1 illustrates how gaseous conduction
and convection is minimized in a VIG unit.
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Figure 1
Though construction methods and materials may vary, VIG units on the market
today are generally comprised of two or more planar glass sheets, of which each
sheet of glass is separated from the next sheet of glass by a distance typically on
the order of one-millimeter; an array of spacers or pillars to maintain the void
between the sheets of glass; a hermetic edge seal – typically a glass frit, which
joins together and encircles the entire perimeter of the VIG unit; often a getter
material is employed to control residual gas build up; and a hermetically
sealable portal, or pump out tube, through which to evacuate residual gas (See
Figure 2).
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Figure 2
To attain the most desirable insulating properties of the VIG unit, the residual gas
between the glass sheets is evacuated to a level such that “the pressure is low
enough that the distance between [molecular] collisions is about the same as
the distance over which the gas is contained” (Kocer, 2006). The level of
vacuum attained greatly influences the residual conduction of air within the unit.
Once the proper level of vacuum is attained, the portal is closed and sealed
hermetically to “lock in” the vacuum. Another version of VIG incorporates a
flexible metal edge seal which allow the two sheets of glass to decouple. This
design allows for expansion contraction effects of the glass panes which helps
reduce the stresses at the hermetic edge seal of VIG units. There is a version of
VIG which does not utilize a pump out tube, which requires the evacuation of
the space in which the VIG is also sealed about its perimeter.
2.1.1
Glass and Coatings
The cornerstone of the VIG unit is the actual glass itself; the glass sheets,
typically comprised of annealed float or heat-treated glass, serves the
function of creating two sides of the vacuum construction. The gauge or
thickness of glass used to construct the unit is dependent on the size of
the pillar, spacing of the pillar array, resultant forces on the glass due to
the vacuum and the overall size of the VIG unit. Utilizing a low-emissivity
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coating on surface two or three of the unit minimizes the effects of
radiant heat transfer through VIG unit. Without a low-e coating, the VIG
would be no more energy efficient than a clear / Low e coated
standard IG unit.
2.1.2
Array of Pillars or Spacers
The purpose of the pillar or spacer array is to prevent the two opposing
surfaces between two sheets of glass from touching when the VIG unit is
evacuated and sealed.
The spacing and pattern of the array is
dependent on pillar size, pillar geometry and physical properties of the
spacer material. The pillars themselves must be strong enough to
withstand the compressive forces exerted on the unit as a result of the
pressure differential created between the high vacuum pressure on the
inside of the unit and standard atmospheric pressure on the outside of the
unit, along with any other environmental stresses. The shape of the pillar,
often cylindrical, is dependent on the material properties of the spacer,
and is designed to minimize stresses on the glass. The pillars are
constructed of a material such that the inherent properties do not
significantly degrade the vacuum over time. Also, the composition of the
pillar material ideally minimizes the conductive transfer of thermal energy
from one glass sheet to the other. Pillar materials include metal alloys and
ceramics and may incorporate special coatings.
2.1.3
Hermetic Edge Seal
The function of the perimeter hermetic seal is twofold: It joins the two
opposing sheets of glass together; and it forms a barrier that prevents
degradation of the vacuum within the unit by resisting the inward
permeation of gaseous molecules. The seal may be melt flowed as in a frit
such that the glass sheets and the perimeter seal are one, or bonded to
the sheets of glass and may utilize either homogenous or heterogeneous
materials to complete the seal. The seal may be formed between the
sheets of glass as in a ceramic frit, metal solder, or applied around the
edge of the unit as in a flexible metal ribbon seal, or a combination of
methods. The seal is designed to accommodate differential thermal
expansion between the two glass panes, such that the maximum
expected movement, in both distance and cycles, does not cause the
seal, bond, and thus the VIG unit to fail.
2.1.4
Hermetically Sealable Evacuation Portal
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Many designs incorporate an evacuation portal; a small passage way
through which the residual gas present between the glass panes is
removed. A typical embodiment of the evacuation portal is a stepped
hole, drilled in one corner of one of one of the glass sheets, with a small
glass tube placed in the hole to the exterior of the glass sheet and then
sealed with ceramic frit. Once all of the residual gas is evacuated
through the portal by means of a high vacuum pumping system, the glass
tube is hermetically sealed using a melt process to prevent any backflow
of gas into the unit. The small sealed tube is generally protected with a
small button or cap to prevent breakage.
A prototype version of VIG (Russia) has the pump out tube mounted in
one of the corners along the edge between the sheets of glass. In this
design the pump out tube is not visible behind the window framing, and
there is no protective cap.
If the VIG is evacuated then sealed about its perimeter within a locked
vacuum chamber, a pump out tube is not necessary. This design was
pioneered by Baechle (Switzerland) and the ProVIG consortium lead by
Grenzebach (Germany) in the early 21st Century.
2.2
Importance of Vacuum Stability
The crux of the VIG unit is the level to which residual gas is evacuated and the
stability of the resulting vacuum in the system. Kocer (2006) notes, that the
thermal conductivity of the residual gas is dependent on the interrelation
between the pressure of the gas and the mean-free-path of the moleculemolecule collisions. Thus pressure within the unit must be reduced to a level such
that the distance between molecular collisions is greater than the shortest
distance of their encapsulation. Figure 3 shows the correlation between pressure
and gaseous conductance. Note the sharp drop off in efficiency as the pressure
increases from 10-3 Torr to 10-1 Torr. The graph illustrates the importance of the
material selection and method used to seal the perimeter of the unit as well as
the evacuation portal. Even microscopic leaks at a molecular level can
significantly degrade the performance of the VIG unit over a short time.
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Figure 3
Manufacturers have worked on incorporating devices or systems that help with
the longevity of the VIG by means of capturing excess, volatilized or free radical
gas molecules. This technique was well known to the vacuum tube industry in
the early 20th Century. Such compounds are commonly referred to as getters.
They are capable of removing select gasses from the interior of a vacuum space
through a process such as oxidation. VIG units may incorporate getter materials
in order to extend life or actually improve the vacuum level within the cavity.
2.3
Finished Product Characteristics
2.3.1
Size Range
VIG technology is not limited by dimensional boundaries; the reduction in
gaseous conduction in a small unit is the same as it is in a large unit. Like
conventional IG units, edge effect will impinge on the efficiencies that are
realized in the center of the unit; thus, units with smaller lateral
measurements are not as efficient overall as units with larger lateral
dimensions. The only limiting factor to the size of the VIG unit may in fact
be the equipment used to produce the unit itself.
2.3.2
Testing
In 2012, IGCC/IGMA in North America issued a bulletin for durability testing
of VIG, which allows use of ASTM E2190 standard for testing. However, the
ASTM E2190 tests are designed to exacerbate organic and inorganic seal
failure, check for volatile fog and measure argon loss. VIG has a highly
hermetic seal, has an insignificant amount of gas inside (including volatile
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compounds), and essentially has no argon. Further testing is being
developed by National Renewable Energy Laboratory (NREL), Golden,
Colorado, USA, to insure the stability and performance of this emerging
technology. Additionally, there is a standard in China for VIG, however
much of it is devoted to VIG specifications, rather than testing. Gas
Content Initial and After Weathering (GCIA) testing is not applicable for
VIG unless it is a hybrid unit
2.3.3
Shipping Considerations
Care must be taken when packaging and shipping VIG units due to the
higher residual stress on the glass sheets as a result of the point loading at
the pillar(s) and rigid edge seals locations. Rigid edge seals and pillar
locations may be more sensitive to shock or loading than standard IG
units. A breach in the VIG, even on a microscopic scale, can degrade
the level of vacuum in the unit, resulting in a loss of insulating efficiency. A
VIG with atmospheric pressure within may allow unsecured pillars to drop
to the bottom of the unit. The resultant efficiency is approximated by a
monolithic piece of glass with the same thickness and Low e coating.
3.0
Applications
3.1
VIG Development Influencers
Building envelopes for North American residential and light commercial buildings
typically have insulated walls with insulation batting between the wall studs.
These buildings have an average exterior wall conduction of U-0.1 (R-10). The
U.S. Department of Energy (DOE) has published the need for windows to be as
thermally insulating as the building walls to achieve cost-effective energy
efficient buildings. The DOE also states that windows should have a U-factor of
0.1 or better for cost-effective net-zero energy buildings (“ZEBs”) to be feasible
and practical. VIG may be one way of achieving this goal.
3.2
VIG Use
Vacuum Insulating Glass (VIG) can typically be used in most applications where
traditionally sealed insulating glass units are glazed, although this can be
dependent on the type of VIG edge sealing technology utilized. Current
technologies include both rigid and flexible edge sealing systems where specific
uses or applications will dictate which edge sealing system may be most
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appropriate. The fenestration designer should contact the VIG manufacturer for
information related to glazing design options.
VIG can be installed as an individual assembly, or it may be incorporated with
another monolithic lite and traditional spacer, desiccant and sealant system of a
standard IGU assembly to further enhance performance or aesthetic
characteristics. This type of construction is known as a Hybrid VIG (see Figure 4).
A double VIG separated by traditional spacer, desiccant and sealant system is
also known as a hybrid VIG, but the U-factor performance can achieve
performance levels much less than 0.10 (center of glass). Actual whole window
performance may change depending on the framing system used, VIG edge
seal and the size of the VIG. Insulating glass unit enhancements such as
spandrels, silk-screens, laminates, pyrolitic and sputtered coatings, or internal
components such as grids may be incorporated in hybrid VIGs. As an
alternative, coatings or decorative elements that are bonded to the surface of a
glass lite may be incorporated into a VIG’s interior cavity, provided they do not
cause any out gassing.
Figure 4
With special consideration to the framing system and glazing methods, VIG is
suitable for most traditional applications of vision, spandrel, acoustical, and
security glazing. However, additional testing may be necessary to determine
how a VIG component will perform in more extreme environments of blast
mitigation, hurricane, differential temperature extremes and seismic activity.
3.3
VIG Applications
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In theory, VIGs may be used anywhere double-pane IGs are presently used. In
practice, whether a VIG can be used in a particular application will depend
more on the applicability of the particular VIG’s environmental capabilities and
glass type (annealed, tempered, laminated impact-resistant, etc.) than the type
of window or window system. Some manufacturers’ VIGs have limitations on the
temperature differential between the building’s outdoor temperature and its
indoor temperature, solar heat gain, and outdoor weather conditions (e.g.,
windless day). Some manufacturers specify conditions under which their VIGs are
best used. Any limitations imposed by a supplier or producer for use of a VIG
must be carefully reviewed prior to use.
3.4
Main Benefits of VIG
The primary benefit of VIG is the thermal performance in combination with a
reduced VIG thickness and potentially a lower weight.
Thermal benefits of
constructing buildings with VIGs are that their use may allow smaller HVAC
systems (an initial or non-recurring cost of construction) and lower use of energy
to cool and heat the buildings’ interior spaces, and by having the same or better
thermal performance as the building’s exterior walls, architects can use more
windows to allow more natural daylight into the buildings. For some applications,
VIG can be made thin enough and light enough to replace existing monolithic
glazing without the need to replace costly framing systems resulting in lower costs
for window renovations.
Aside from the superior thermal performance, VIG has other inherent properties,
which can be beneficial to the end user and thus may be incorporated for
situations where these properties are exploited. For instance, acoustic properties
of VIG are better than monolithic or standard IG at certain frequencies due to
the lack of air within the cavity to conduct sound waves. Testing shows that VIG
in general is better than monolithic 8mm glass below 250 Hz and above 1000 Hz,
or situations where low frequency noise is prevalent such as near an airport or in
the city.
There is a beneficial aspect to sound transmittance with VIG. Since sound is not
transmitted in a vacuum, the sound waves are not completely transmitted
through a VIG. However, the sound can travel through the pillars, and thus at
certain frequencies sound is transmitted. Outdoor-Indoor Transmission Class
(OITC) is a standard used for indicating the rate of transmission of sound
between outdoor and indoor spaces in a structure that considers frequencies
down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower
frequencies. The OITC rating describes the sound attenuation in decibels
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achieved through the measured partition. When comparing similar designs, the
higher the number, the better the sound attenuation. Testing commissioned by a
primary glass manufacturer measured the following values for various IG
constructions:
Need to show make-up in more detail Glass only.
Sample
OITC
28” x 32” std. IG
½" air space
22
28” x 32” VIG
27
28” x 32” Hybrid-VIG + ½"
air space w/ 3mm lite
14” x 20” std. IG
3/8" air space
14” x 20” VIG
26
19
24
Figure 5
VIG may be better suited as a spandrel component since it can withstand high
temperatures, which may be detrimental to standard IG edge components. VIG
may also provide a design strength element to certain framing constructions,
since a VIG panel made with two 4mm lites may have some of the same
strength characteristics as a monolithic 8mm glass in frame construction
One of the least studied areas of characteristics for fenestration products is
actual occupant comfort. Loosely stated, it is the comfort one feels as they
occupy a space adjacent to a window or door. Several factors come into play,
including cold draft (convection) and radiation. For instance, in a cold climate,
as a window’s U factor decreases (R value increases), the occupant does not
“feel” the cold adjacent to a window. A major factor to reducing this feeling of
cold is by decreasing the center of glass U factor – in order to minimize the
convective loop of cold air along the glass surface.
3.5
Potential Limitations
A potential limitation to using rigid edge seal VIG, is an edge seal that due to its
rigidity, may limit use where severe temperature differentials are possible.. With
this consideration, a hybrid VIG which adds a third lite using conventional spacer
and sealants could be utilized. The addition of a traditional IG-like unit facing the
outside of the building provides a thermal buffer to the VIG. The thermal
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performance of the unit would approximate the sum of the R-values of the
interior-facing dual-pane VIG and the exterior-facing conventional IG.
02-27-2014 call ended here
Certain designs may incorporate a flexible edge seal system. This system will
require special glazing methods, and may incorporate radiused corners and
polished edges to the glass lites. Since the lites are allowed to slide relative to
one another, the pillars will be moving along the surface of the glass, thus stresses
and friction coefficients between pillar and glass become important, along with
low-e coating technology that is resistant to degradation from repeated
movement of the pillar at its surface.
Although a VIG panel may incorporate a variety of glass types in its construction,
it is important to acknowledge that VIG panels have pillars that separate the
individual lites of glass, which may be visible to the naked eye and may not be
acceptable where a completely unobstructed view is required.
Also, highly insulating IG systems such as VIG may retain exterior condensation
longer, due to the outer lite being closer to the exterior temperature conditions
and below the dew point of the moisture in the air. This is actually a sign that the
window is working correctly. Surface !
3.6
Glazing VIG
VIG by nature has a much thinner overall profile than conventional IG due to its
small vacuum air gap. Typically VIG has a profile thickness starting at 1/4" - 5/16”,
but can be somewhat thicker depending upon factors such as; overall size,
design loads, any specialized performance, etc. Conventional IG is typically
5/8” to 7/8” thick for a double unit and 1” to 1-1/2” for a triple unit. All VIG other
than hybrid-VIG is unaffected by pressure differences caused by elevation
differences between point of manufacture and installation.
Often VIG’s can be glazed into an existing single glazing rabbet, for instance in
historical buildings, thereby enhancing the overall building thermal and acoustic
performance. Monolithic glass rabbets were often 7/32”, and for this condition,
care must be taken with respect to the VIG sealed edge sightline. VIG’s may
also be used to replace older IG systems due to its reduced overall thickness.
With removal of the IG to be replaced, typically the glazing pocket is deeper
than required by VIG. Users should consult with the VIG manufacturer to ensure
that the conditions of the vintage product are adequate for VIG installation.
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When using VIG’s for new products, the thickness of current IG, glass bite, sealant
compatibility, water management systems, etc. must be considered. This may
require new stops, shims or other means of making up the depth. Care must be
taken with metal clad systems, such that there is a means of insulating between
the glass and cladding in order to maintain the thermal benefit of the VIG and
reduce the likelihood of thermal stress cracking.
New window designs must take into account and compensate for the following:
1)
The majority of thermal loss from a VIG is at the edges and through the pillar
array due to conductivity in these locations. To minimize this effect, in some
designs more of the edge may be incorporated deeper in the framing
system below the glazing stop or bead.
2) VIG edge designs may be stepped, flush edge or may incorporate external
wrap around componentry. For external wrap around designs, special
setting blocks, no setting blocks, or altered sash designs are required.
Uniform support of all lites is recommended per IGMA Glazing Guidelines
document, TM-3000-90(04).
3) Flexible edge seal designs may require that glazing sealants and adhesives
be adhered to the glass, not the wraparound components.
4) Flexible edge seal designs incorporating a corner radius should take the
rabbet depth design into account.
5) Some VIG designs incorporate a protective button to cover the vacuum
evacuation port, and clearance between the protective button and glazing
stop, moveable sash, obstructing meeting rail or other components should
be considered.
6) Traditional glazing methods are applicable to VIG, with care taken for edge
profile considerations and excessive blunt force or bending during
installation. Care must be taken while installing the VIG into the glazing
cavity to avoid undue stresses such as prying, impact or excessive edge
loads which may compromise the VIG during installation or its lifetime.
Consult with the VIG manufacturer for proper glazing and handling methods.
3.7
Fenestration Design and Manufacturing Considerations
There are several key considerations that must be considered when
incorporating a VIG unit into a window product. They include, but are not
limited to:
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Clearance around the VIG perimeter and setting block location.
Uniform loading, since under typical uniform loading, many VIG
designs behave similar to a monolithic lite of equivalent overall
thickness. For example, a VIG comprised of two 4mm lites behaves as
if it were a monolithic 8mm lite of the same glass type under uniform
loads. These typical uniform load limits on VIG will differ from
monolithic glass due to thermal delta conditions introducing bi-metal
stresses affecting the loads on the individual glass lites.
External stresses such as edge flexure of the VIG, wind load, snow
load, temperature differentials, etc.
Cushioning or damping of closure.
o
The VIG assembly may be affected by excessive impact
loads. Special consideration may be given to cushioning the
VIG within the glazing cavity so that it is not subjected to high
impact energy (door slam, single or double hung sash slam).
Glazing materials, gaskets or other means may be employed
to mitigate such impact forces. Special door or window
dampened closure devices might also be considered..
Care and training of personnel with respect to damage during
shipping, installation and construction phase
Weep and water management system
o
Adequate water management / drainage systems should be
incorporated into the pocket design in order to keep the
edge of the VIG dry, in order to avoid long term corrosion of
certain VIG design components, such as wrap around
components if present..
Edge seal systems:
Check with the VIG manufacturer for
compatibility of the VIG edge seal system with adhesives, or other
coatings adjacent to the VIG. Marine or boot glaze systems should
be analyzed for forces around perimeter and distance between
edge of glass and lip, as well as adequate weeping system designs.
If external SDL (Simulated Divided Lite) muntin bars or grilles are
installed on Surface #1, Surface #4 or both, pressure to apply the
bar should be a uniform and dispersed load. Using any blunt force
such as rubber mallets to install the muntin bars or grilles may
compromise the VIG integrity.
Hybrid-VIG is suitable for internal components, such as spacer bar
or shadow box for SDL or GBG (Grilles Between Glass). For a hybrid
VIG, more traditional glazing methods and glazing stops may be
used.
TB – Vacuum Insulating Glass
Version 02-7-2014
This draft document is the exclusive property of the Insulating Glass Manufacturers Alliance.
Reproduction of any part of this document is strictly prohibited.
Page 17
With respect to hybrid VIG installations, the following Considerations should
be accounted for:






Orientation of the additional single lite for optimal thermal
(condensation) and structural performance
Impact loads may be mitigated with the monolithic lite on the
exterior.
If a laminated lite is used in the VIG hybrid construction, the
laminated lite is installed against the glazing surface for impact
products.
Location of the evacuation port.
Aesthetic considerations; reflected color, deflection, etc.
Low-e, solar control, tint, and other factors for the monolithic lite
The center of glass U-Factor for VIG takes into account the conductive
heat flow through the pillars themselves.
The path for heat transfer is the following:

Boundary Conditions adjacent to the VIG on the warmer side (e.g.
temperature, air-flow, physical location, etc.).
Surface
contaminant or film (e.g. condensation) and boundary air film
conditions on the warmer side.

Conduction through either of the two glass lites.

Contact area and surface conditions between warm side glass lite
and pillar affecting heat transfer.

Conduction through the pillar.

Contact area and surface conditions between pillar and cold side
glass lite affecting heat transfer.

Conditions adjacent to the VIG on the colder side (e.g.
temperature, air-flow, physical location, etc.).

Surface contaminant or film (e.g. condensation) and boundary air
film conditions on the colder side.
Of these, the pillar size, spacing, surface morphology, material of
construction and associated thermal conductivity properties dictate the
total amount of conductive heat transfer through the pillar system at the
TB – Vacuum Insulating Glass
Version 02-7-2014
This draft document is the exclusive property of the Insulating Glass Manufacturers Alliance.
Reproduction of any part of this document is strictly prohibited.
Page 18
center of glass area. Properly designed and built VIG units take the
following into consideration:



Lowest conductivity pillar system that can withstand the high
compressive stresses imposed upon it by atmospheric air pressure.
Minimized pillar cross sectional area.
Maximized pillar-to-pillar separation.
All the above contribute to minimizing heat flow across the VIG.
Include illustration showing pathways of thermal conduction and transfer. (need
volunteer)
4.0
References
1
T. M. Simko, A.H. Elmahdy and R. E. Collins; DETERMINATION OF THE OVERALL HEAT
TRANSMISSION COEFFICIENT (U-VALUE) OF VACUUM GLAZING
2
Dr. Cenk Kocer (2006); Vacuum Insulating Glazing Part 1; An Introduction to Design
and Performance http://www.glassonweb.com/articles/article/816/
3
http://ars.els-cdn.com/content/image/1-s2.0-S0011227501000339-gr8.gif
TB – Vacuum Insulating Glass
Version 02-7-2014
This draft document is the exclusive property of the Insulating Glass Manufacturers Alliance.
Reproduction of any part of this document is strictly prohibited.
Page 19