Vacuum Insulated Glass for High Thermal Performance Curtain Walls and Skylights

Envelop, LLC is developing R-25, very long service life, high reliability, large structural vacuum insulated glass units for curtain wall and skylight vacuum systems.

Glass curtain wall vacuum systems comprise large structural vacuum insulated glass (VIG) units that are connected through tubing and valves to each other and to a pumping station that establishes and maintains service vacuum pressures within a façade by removing the gas load from permeation and outgassing (patented and U.S. and foreign patents pending). Depending on their design, they can operate in the medium or high vacuum range.

Based on finite element analysis, these types of façades can have area weighted average façade U-values as low as 0.23 W/m2K or 0.04 Btu/hr °F ft2 (R-25 hr °F ft2/Btu) with a window to wall ratio of 100%.

Outgassing occurs at ambient temperatures after installation. Estimated pump down times are based on time to remove total amount of adsorbed and absorbed water at effective pumping speed given a pressure that is final service pressure attributed to outgassing. This results in a worst case scenario that is modified by scaling to an inverse power law curve to obtain a more realistic time frame.

Outgassing times can be significantly reduced by venting windows with dry nitrogen to remove water (patent pending). The nitrogen and or windows may be heated during venting.

A polymer sheet with nanometer or micrometer close packed voids or nanoporous or microporous material may cover the inner surface of the outboard lite with a gap between it and the inboard lite. The gap can operate in the viscous flow regime with the mean free path of gas molecules greater than the void or pore dimensions. Under this scenario, a façade can achieve maximum insulation in the medium vacuum range with relatively fast pump down times.

Vacuum system façades make glass clad net zero energy buildings feasible in climates with a high number of heating degree days. It has been estimated that the revenue from net zero energy buildings will reach $690 billion by 2020 and $1.3 trillion by 2035.

Above: An example of a VIG unit that would operate in the high vacuum range from 10^-3 to 10^-4 Torr - viscous edge seal, 8 mm unobstructed gap between lites, polymer spacers designed for rapid automated assembly in 50 mm array, and ionoplast laminated glass. The pictured viscous edge seal is fifty to one hundred times more effective at preventing gas and water vapor transmission than a thermoplastic spacer system and that effectiveness cannot degrade over time.
Above: A conceptual design that illustrates a VIG unit that could operate in the viscous flow regime and medium vacuum range - thermoplastic spacer (TPS) primary seal, structural silicone secondary seal, ionoplast laminated glass, cross shaped polymer spacers in 50 mm array embedded within low density hydrophobic monolithic silica aerogel, 2.5 mm gap between aerogel and inboard lite, and 25 mm gap between lites. It needs to be noted that monolithic silica aerogel is extremely expensive. We have no indication its cost can ever be low enough to make it feasible for window applications and no indication that an economy of scale exists that can make it feasible.

Vacuum glazing has a vacuum between its glass sheets; regular insulated glass (IG) has an inert gas. Vacuum glazing operates on the same principal as vacuum insulated thermos bottles: heat conduction and convection is reduced or practically eliminated in a vacuum with the level of reduction depending in part on the level of the vacuum.

Vacuum glazing requires some type of physical spacer system between the glass sheets that maintains the vacuum space by resisting the compressive load of atmospheric pressure. These spacer systems conduct heat between the glass sheets and tend to negate the ultra high insulating value of the vacuum.

Permanently sealed VIG units, which are evacuated once at time of manufacture in a bake-out and outgassing procedure, use discrete metal spacers because they require ultra-low outgassing materials in order to maintain vacuum service pressures over decades. Polymers, which have relatively low thermal conductivity compared to metal, cannot attain the low outgassing rates of metals and therefore cannot be used as spacers in permanently sealed VIG.

Glass curtain wall vacuum systems make it possible to use polymer spacers and edge seals by removing their continuously produced outgassing products (primarily water molecules) that would otherwise cause the vacuum pressure to rise above service pressure. One polymer suitable for discrete spacers has a thermal conductivity less than one-hundredth that of stainless steel.

Common to all vacuum glazing is an edge seal that seals the vacuum from the atmosphere. Envelop has pioneered very long service life and very low stress viscous edge seals (patented and U.S. and foreign patents pending). Viscous seals eliminate the long-standing performance problems related to edge seal stress that continue to plague other designs and that have prevented commercial viability of VIG in cold climates and placed limits on its size.

Unlike edge seals for permanently sealed VIG, which must be either metal or glass, viscous seals allow the large windows, tempered glass, heat strengthened glass, chemically strengthened glass, laminated glass, and glass coatings that are essential for modern glass curtain walls and compliance with international building standards and codes.

Vacuum glazing requires tempered or chemically strengthened glass in order to prevent breakage from thermal stress and to take advantage of insulated deep inset interior edges that minimize edge conduction on very large units. Insulated inset interior edges increase temperature gradients across the face of the inboard lite and reduce those across the outboard lite. Given the exceedingly low U-value of vacuum system façades, inset interior edges that minimize edge conduction tend to make spandrel space counterproductive for minimizing area weighted average façade U-value, regardless of the method used to insulate the spandrel space. Therefore, the full thermal advantage of the windows is attained with an uninterrupted total glass exterior.


Glass curtain wall vacuum systems may include banks of up to 250 large VIG units all connected to one small turbomolecular pumping station by one to four inch diameter 6061-T6 extruded aluminum tubing with a one-eighth inch wall and thin internal oxide layer. Alternatively, low coefficient of thermal expansion borosilicate glass tubing may be used (patent pending). If the system is designed to operate in the medium vacuum range smaller diameter polymer tubing can be used.

The cost of one pumping station distributed over 250 windows that are eight by twelve feet in size is $20 per window. Pumping station power consumption is less than 250 Watts.

The vacuum gap between the glass sheets can be maintained by cross shaped polymer spacers. The spacers shown undergo less than one percent strain in a 50mm array.

The size, shape, and composition of polymer cross shaped spacers (U.S. and foreign patents pending) relieve the high contact stresses that could cause contact damage in VIG with small glass or metal spacers. Elastic deformation of the polymer under load allows a spacer to conform to glass flexure, reducing contact stresses. Polymer deformation evenly distributes compression loads between spacers, compensating for uneven glass or roller wave if tempered glass is used. These attributes are critical for resisting wind, snow, and impact loads.

Above: Viscous edge seal, 8 mm unobstructed gap between lites, polymer spacers in 50 mm array, and ionoplast laminated glass. The spacers shown above are designed for rapid automated assembly and strong bond to inner lite. Polymer spacers can be produced by multi-shot injection molding. This allows the disk portion of the spacers shown above to be glass filled with the remainder unfilled, thus preserving the low thermal conductivity of the unfilled resin. Glass fill improves the high strength low outgassing epoxy bond between the round disk base and the inner lite (U.S. and foreign patents pending).
Above: Viscous edge seal, 8 mm unobstructed gap between lites, polymer spacers in 50 mm array, and ionoplast laminated glass. Spacers are designed for automated assembly and strong bond to inner lite.

The expected service life of a pumping station exceeds twenty years, and their eventual replacement with newer and increasingly more efficient units with longer service lives must be weighed against the alternative of replacing large IG units with service lives that are limited by permeation or seal failure. Even permanently sealed VIG with metal or glass seals has a service life that is limited by outgassing, making replacement inevitable. This problem is exacerbated for permanently sealed VIG units that are produced at lower assembly temperatures than those that undergo a high temperature bake-out.

If a power outage is anticipated or threatened, or a climatic event is forecast, the system as a whole and each window individually can be isolated by special purpose mini gate valves and or the pump switched to generator or backup power. Unanticipated and sudden loss of power de-energizes a solenoid that releases a fast actuating spring driven protection valve at the pump.

If a glass sheet breaks and its ionoplast interlayer is breached, a very high reliability bidirectional valve, which is just a very simple light weight flapper at the 25 millimeter tap to the window, is forced closed in less than a millisecond by the atmospheric wave front (patented and U.S. and foreign patents pending). This occurs without sensors, springs, electrical energy, or compressed air. Any temperature rise in air that rushes into the system can only cause a several degree Celsius temperature rise in the glass sheets because of their very much higher heat capacity. Pressure rise in the system and against the spacers, which are bonded to the inner lite, can never exceed atmospheric pressure.

If there is a breach in the vacuum system, service pressures are reestablished by outgassing at ambient temperatures. Depending on the extent of the breach, outgassing times can be significantly less than those at time of installation.


The pictured viscous edge seal comprises low molecular weight low viscosity polyisobutene (PIB) constrained between inner and outer gaskets of medium modulus structural silicone adhesive sealant (in the rendering the PIB is opaque white, in actuality it is optically clear). An 8mm vacuum gap provides low silicone percent elongations for low stress thermal strain between the glass sheets. This gap is over twice the structural glazing minimum glueline thickness for thermal dilatation of the silicone for a twelve foot high window on resting pads and a plus or minus 140 °F temperature differential between inner and outer lites.

With 6.5 pound-force per lineal inch on the inner structural silicone seal, which includes atmospheric pressure and PIB hydrostatic pressure at 12 feet, there is 3.25 pound-force of shear force per lineal inch of silicone for each lite. The structural silicone is rated at 40 pounds per inch (ppi) peel strength as determined by the more severe conditions of the ASTM "Standard Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants" designation C794-10.

Above: Viscous edge seal

Liquid PIB, which is a Newtonian fluid, is not subject to the failure modes inherent in elastic materials and bonds. It accommodates relative movement of the glass sheets by undergoing viscous shear. At 20 degrees Celsius, the PIB has the consistency of honey and cannot experience the cohesive, adhesive, thermal degradation, or bond failures that can ruin all IG primary seals, and it cannot experience strain hardening, which could cause brittle fracture in metal or foil VIG edge seals. This, in combination with the medium modulus structural silicone and ionoplast interlayer laminated glass, makes the entire unit resistant to failure from seismic and extreme climatic events.

A patent pending U-shaped spring-like element made of very thin stainless steel is embedded within the PIB to minimize the area that the PIB presents for gas permeation. The floating spring element maintains two 0.0015 inch gaps filled with PIB between itself and the glass sheets by riding on roller wave or by stand off means if chemically strengthened glass is used. The pictured viscous edge seal is fifty to one hundred times more effective at preventing gas and water vapor transmission than a thermoplastic spacer system. Because of the minimal use of very thin stainless steel with the bulk of the seal comprising low thermal conductivity polymers, viscous edge seals, like thermoplastic spacers, represent warm edge technology.

The PIB, silicone, and spring element comfortably accommodate long-term spacer creep and differential expansion and contraction of the polymer.

The silicone and PIB degas out of the bulk relatively fast. Silicone has a high permeation rate for nitrogen so that if nitrogen venting is used, the nitrogen will outgas from the silicone relatively fast.

A patent pending flexible metalized polymer diaphragm at the top of the VIG unit allows the fluid PIB to rise and fall like water in a glass in order to accommodate movement and thermal expansion and contraction of the PIB and unit. The diaphragm seals the PIB from moisture condensation. Heated and degassed PIB is loaded into its cavity through a sealable port in the diaphragm while the cavity is placed under vacuum through a second port in the diaphragm. In the unlikely event the diaphragm fails, the embedded spring element sequesters condensation, which will eventually permeate out of the seal through the outer gasket of silicone.


March 29, 2015, Delafield, Wisconsin, USA

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