Reglaze or Replace?

Reglaze or Replace?

How do you balance the historic value of character-defining existing single-glaze windows with their equally historic poor energy performance?

Increasing the insulative value of exterior glazing is usually the most impactful way to improve the overall energy efficiency of an existing building. In most cases, this means removing the old inefficient windows and replacing them with new energy-efficient windows and insulated glazing. In a non-historic building, the decision to replace can be fairly linear – window functionality, available budget and time, and minimum code requirements are the main factors. Historic buildings are another matter. In a building with landmark status, the decision tree has many more branches and is often subject to scrutinized review by various entities and stakeholders. Interventions are held up against the guidelines provided by “The Secretary of the Interior’s Standards for the Treatment of Historic Properties.” One of the main tenets of historic preservation is that there is intrinsic value to the historic building fabric. Not simply because it is old, but because it is the physical record of the building’s history including its uses, alterations, even everyday wear-and-tear. Features that are character-defining (which almost always includes the windows) are to be retained and preserved in order to maintain the integrity of what is significant to the building’s historic, architectural, and cultural value. Rehabilitation projects should be designed so that the defining characteristics of a building are not radically changed, obscured, damaged, or destroyed.

Model energy codes for new construction have been driving towards net zero performance for some time, however new construction only accounts for 1-2 percent of the building stock annually. With an estimated 5.9 million existing commercial buildings in the US, the real challenge lies in addressing the performance of existing buildings.¹ It is only recently though Building Performance Standards (BPS) have been introduced as a mechanism for regulating energy use in existing buildings. BPS policies are designed to reduce the carbon impact of the built environment by requiring existing buildings to meet an energy use or carbon emissions performance target by a specific deadline.

As awareness over the impact of climate change has increased, the push towards adopting building performance standards (BPS) targeted at existing buildings has gained momentum. This means owners and operators of existing buildings will have to meet ever more stringent performance targets for energy and carbon emissions. The following map and supporting data table shows the current status of BPS adoption at the state and local levels across the US:

It is well-established that glazed openings have an outsized role in the overall energy performance of any building, historic or not. Improving their thermal performance is often the single biggest opportunity to improve building energy performance.²

While most people usually think of wood windows when talking about a historic building, the legacy of Modernism is upon us. According to the 2018 Commercial Building Energy Consumption Survey (CBECS) more than half of U.S. commercial buildings were built between 1960 and 1999 with 21% being built prior to 1960. There is no doubt that a whole contingent of historic metal-framed windows and storefronts are out there waiting to be addressed.

Intervention approaches may include restoration (repair only), full replacement in-kind to match existing historic assembly, rehabilitation (repair + some thermal improvement), or full replacement with a new energy-efficient assembly). Each has advantages and disadvantages within various categories such as preservation value with regard to the retention of historic material and preservation of visual character, contribution to energy code compliance, as well as cost, time, and skill to implement. For instance, as shown in the example matrix below, a restoration approach may provide the highest preservation value, but offers little to nothing in the way of improved thermal performance. Full replacement with new glazing might offer the highest thermal performance, but at the expense of preservation value. In-kind replacement of poorly performing windows is the worst of both worlds.

It is for these reasons that rehabilitation is the most common approach. It allows for compromise and opportunity. Existing material can be preserved and repaired, and thermal performance improved by the introduction of higher u-value glazing typically in the form of an insulated glass unit (IGU).

When it comes to Modern architecture large expanses of glazing utilizing very narrow metal frames is common. In many instances, these frames are so lithe that reglazing with a thicker IGU is simply not possible without altering the frame and/or glazing stops. This is far from ideal from a preservation standpoint as it has the potential to alter/reduce sightlines and change the historic appearance of the windows. If additional metal is welded to the existing metal frames to widen the rebates, thicker IGU may be accommodated, but this type of alteration is irreversible and antithetical to the Standards.

With limited options for improving thermal performance of historic steel-framed windows, an all-or-nothing scenario is common. Either nothing is gained in the way of thermal improvement and the windows are restored with single-pane glass, or it is fully replaced with a new energy efficient assembly matching the original sightlines and profiles as much as possible.

Thankfully, this is changing. The introduction of vacuum insulating glass (VIG) to the market is opening up a world of new possibilities when it comes to the performance of existing building envelopes.

IGU v. VIG

A conventional double-glazed IGU consists of two glass panes separated by a perimeter spacer bar with the edges hermetically sealed to form a unit. In a standard IGU, it is the still air space in-between panes that provides most of the insulating value. The overall u-value of the unit can be increased by using tinted glass, adding reflective or low-emissivity (low-e) coatings, replacing the air in space with argon or krypton gas, and/or using warm edge spacers. Using thicker glass and/or laminated glass for one or more of the glass lites provides higher u-values and higher acoustic performance.

The minimum air space for an IGU is 1/4” (6 mm), however the optimum spacing for best performance varies depending on what gas is filling the space.

An air-filled IGU performs best with a 1/2” (12.7 mm) gas space, whereas optimum gas space for an argon-filled IGU is 7/16” (11.1 mm) and a krypton-filled IGU is 5/16” (7.9 mm). This means highest performing and thinnest IGU option will be one with krypton gas fill.³

Such units are available and marketed for historic properties in the UK at least. Heritage Glass UK is one such manufacturer offering “Heritage double glazed units” which fit into 7 mm rebates. Their thinnest units with 4 mm cavity and krypton gas have a center pane u-value of 1.9 and those with a 6 mm and 8 mm cavity have 1.5 and 1.1 u-values, respectively.⁴

Vacuum insulating glass (VIG) has an even thinner form factor and provides superior energy performance. VIG is also a double glazing, but the space in between the two glass panes is minute and has been evacuated of air, leaving a partial vacuum. The edges are hermetically sealed to maintain the vacuum at the desired low pressure and an array of micro pillars support the glass and maintain separation.

While exchange by conduction and convection can cause up to 70% of the heat lost at a conventional double-glazed IGU with low-E coating, the vacuum all but eliminates convective and conductive heat exchange between the two glass panes in VIG.

Schematic diagram of a VIG unit⁵:

With a typical air space of 0.15–0.2 mm, the overall thickness of a VIG unit can be as little as 6.2 mm, with the energy performance a triple-glazed IGU. Performance figures of VIG versus conventional glazing⁶:

Several VIG manufacturers can now provide units with u-values below 0.5. W/m²K1.

VIG’s low thermal conductivity can also increase condensation resistance when paired with thermally efficient frames.

The improved sound reduction performance of VIG over a the conventional IGU is also notable. “particularly in the mid-frequency range where noise from traffic and public transport is prevalent.” Testing of the NSG Spacia shows the VIG is comparable to monolithic glass of the same thickness.

Vacuum-insulated glazing VIG is not a new glazing technology, but until very recently it required importing from Asia or Europe at a premium in cost and long lead times. Few projects in the US have been fully realized using VIG, and data on its performance in existing frames is practically non-existent.

Case Study

Introduction

The Master Apartments building (310 Riverside Drive) is a 28-story Art Deco tower located on the northeast corner of Riverside Drive (RSD) and West 103rd Street in the Upper West Side of Manhattan. The Master Building, as it was originally known, was designed by prominent skyscraper architect Harvey Wiley Corbett in association with the New York firm Sugarman & Berger as an apartment hotel with a museum dedicated to the Russian artist and mystic Nicholas Roerich. Today, the Master Apartments is a residential co-op with commercial tenants occupying the former museum spaces.

The building, completed in 1929, is a unique example of the Art Deco skyscraper typology that came to define the New York City (NYC) skyline during the first half of the 20th century. The property was designated an individual Landmark by the NYC Landmarks Preservation Commission (LPC) in 1989 and included within the boundaries of the Riverside-West End Historic District designated in 2015. It is also listed on the State and National Register of Historic Places (2016).

The first seventeen floors of the tower consist of a rectangular shaft with exposed brick walls of varying color, brick patterns on the spandrels and along the window bays, as well as terra cotta copings, and the corner windows for which the building is widely known. A series of setbacks and chamfers with similar patterned brickwork and terracotta copings define the cross-shaped floor plates and terraces between the 18th and 25th floor levels, and from there up to the 28th floor level where the water tank room is located. A non-functional sheet metal-clad lantern room graces the tip of the building.

On the ground floor, the former museum entrance on Riverside Drive (RSD) is marked by a double-height monumental window above a cantilevered rectangular marquee. Granite steps lead up to the recessed entrance landing and steel-frame window wall with flanking steel-frame doors. The steel frame monumental window has tripartite fixed lites with a blue glass and decorative ferrous ornaments along the jambs extends from the marquee roof up to the 2nd floor lintel.⁷A similar recessed entrance with stepped return brick walls, steel-clad marquee, and a steel frame transom window with clear glass on the fixed panes and blue glass sidelites defines the entrance to the residential lobby on the West 103rd Street facade.

AYON Studio was commissioned by the Master Apartments board of directors in early 2021 as part of a larger rehabilitation program to assess and make recommendations for the repair, improvement and/or possible replacement of the existing steel-framed single-glazed monumental transom windows above the marquee entrances on Riverside Drive (RSD) and West 103rd Street, as well as the storefront entrance on RSD.

Existing Conditions

Riverside Drive Entrance:

The existing steel frame storefront assembly at the RSD entrance was in poor condition overall. Many of the steel framing elements were corroded, including the bottoms of the steel mullions and the surface-mounted horizontal bars installed at mid-height of the mullions. The steel panels adjacent to the doors at the return walls were also corroded. The original travertine base panels on the exterior side of the storefront assembly were covered with metallic paint. Several of the stone panels at the exterior and interior were damaged, cracked and/or displaced with open joints above the stone. The exit panic hardware and closers on return wall doors were barely functional, making the doors difficult to operate.

Riverside Drive Monumental Window:

The existing steel frame monumental window was in poor condition. Most of the framing was structurally sound except for the sill frame and bottom of the vertical mullions, which were severely corroded and had little structural integrity remaining. The sill of the window was located sitting directly above the marquee roof surface, with no raised curb or flashing. Numerous previous attempts to repair/waterproof the sill had resulted in layers upon layers of bituminous/mastic material present on the exterior.

All of the framing members had surface corrosion visible from the interior, with the lower third of the window being worst affected. Surface corrosion was also apparent to a lesser degree on the exterior side of the framing. Although the sill of the Riverside Drive transom window sits below the first-floor lobby ceiling, it is separated by an enclosure and is not actually visible from the first-floor lobby, Its lower portion is accessed by removing a metal floor grille adjacent to the window on the second floor.

All of the single-pane glass was intact with the exception of the bottom center lite of clear glass that was cracked. One of the textured blue glass panes had been stabilized from the interior with an applied horizontal piece. Plastic had been installed at the 2nd floor level presumably to prevent draft and condensation. This was removed to allow for inspection of the window condition.

West 103rd Street Monumental Window:

The existing steel frame monumental window on 103rd Street was also in poor condition. Like the Riverside Drive transom, most of the framing was structurally sound with the exception of the sill frame and bottom of the vertical mullions, which were severely corroded and had little structural integrity. Surface corrosion was visible in various areas throughout the exterior side of the framing and decorative elements, particularly at the non-original out-swing casement sash located on the east side of the window. Surface corrosion was also visible to a lesser extent on the interior side.

The sill of the transom window was located sitting directly above the marquee roof surface, with no raised curb or flashing. Multiple previous repairs of the sill area were apparent by the layers of bituminous/mastic material present on the exterior.

This transom window is installed within the rough masonry opening and there is a short, finished interior return at the top and sides of the window at both levels (unlike the Riverside Drive transom window which appears to have been installed to the interior face of the wall covering the masonry opening instead of sitting within). This meant removing the window would likely disturb the interior finishes around the window which would need to be repaired following the reinstallation.

Repair Options

Three approaches – repair, rehabilitation, or replacement – were presented to the client for consideration, representing increasing degrees of both intervention and cost.

Repair, expected to be the least expensive option, also offered the least thermal improvement. The approach entailed removal of the existing historic transom windows and entrance storefront (including single-pane glazing, steel frames, mullions, transom bars, muntins, and ferrous ornaments) for off-site paint removal and repair. Existing blue glass from the transom windows would be salvaged for reuse. The existing transom window sill height would be modified to increase the base height, allowing for proper waterproofing and flashing. Severely deteriorated steel sections would be replaced to match existing.

The fully-repaired assemblies would then be reinstalled and painted to match the original paint color (as determined through paint analysis). The existing single-pane glass at the entrance storefronts would be replaced with new single-pane laminated/tempered glass as required by safety requirements. The travertine stone base would be repaired and cracked and/or deteriorated units replaced to match existing historic stone. At the transom windows, the salvaged blue glass would be reinstalled at the side lites and replacement single-pane clear glass (laminated/tempered as required) would be installed at the center panes.

Rehabilitation was expected to be more expensive than Repair but offered the potential for substantial thermal improvement over the existing single-pane glass. The scope would be identical to the Repair approach with exception of the glazing. In lieu of single-pane glass, vacuum-insulated glazing (VIG) would be installed at the entrance storefronts. At the transom windows, where retaining the historic blue glass was a priority, the following options for a hybrid approach utilizing VIG were proposed:

Option 1: Reinstall salvaged blue glass. Provide replacement single-pane clear glass (laminated/tempered as required) at the center panes. Provide operable/removable secondary glazing sashes over the full interior area of the transom window with double-pane vacuum-insulated glazing (VIG).

Option 2: Reinstall salvaged blue glass. Provide double-pane vacuum-insulated glazing (VIG) at the center panes. Provide operable/removable secondary glazing sashes with double-pane vacuum-insulated glazing (VIG) at the interior side of the blue side panels only.

Replacement offered the most potential thermal improvement and was likely to be the most expensive. This approach proposed to completely remove the existing steel frames and glazing and replace them with a new thermally-broken steel frames and double-pane insulated glazing units (IGU) with low-emissivity (low-e) coating and argon infill to match existing historic sightlines and profile appearance.

Evaluation

The decision-making process involved a comprehensive evaluation of various factors in order to arrive at the most suitable solution for improving the thermal performance of the building while preserving its historical character and minimizing environmental impact.

The initial feedback from contractors regarding the financial implications of each option was generally in line with expectations. From a cost-benefit perspective, the Repair approach was the least attractive option and was quickly eliminated. The difference in estimated coast between the Repair and Rehabilitation scopes was not large enough to justify reinstalling poorly performing single-glazing.

The Replacement approach was appealing from an occupant comfort and maintenance standpoint, but undesirable from a preservation perspective. Not only would existing historic material be removed, but the historic appearance would inevitably be compromised as new energy-efficient assemblies cannot faithfully replicate the historic appearance and sightlines of the existing narrow steel frame windows and entrance storefront.

The prospect of using VIG in rehabilitation was very intriguing from the owner and architect side for all of the reasons cited earlier in this paper. However, open questions about cost and the potential for long lead times were early concerns that AYON Studio was able to address working directly with the manufacturer and their only licensed distributor in the US. Once the material and shipping costs of VIG were understood, the benefits of increased occupant comfort and energy savings outweighed the concerns around cost and extended lead time for this client and it was agreed that the project would move forward with the Rehabilitation approach.

The focus of discussion then turned to technical feasibility and the potential risks and challenges inherent in the various options. With so few projects realized using VIG in the US, obtaining physical samples to aid with evaluation was critical.

VIG In Situ Monitoring

In March 2023, prior to the work implementation, a sample of replacement VIG was installed in an existing side lite frame where blue glass was removed. Building Envelope Testing LLC was contracted to perform on-site testing and thermal evaluation for the purpose of comparing the performance of the window system before and after glass replacement. Monitors recording heat flow at 1-minute interval were installed on the interior side of the window glass and a Multi Point Heat Flux Measurement was conducted over the course of several days per a modified ASTM C1046 procedure.

Based on the data collected from this case study, the calculated U-value of the clear VIG panel was 0.19 Btu/h·ft2·F f and the U-value of the clear glass panel was 0.72 Btu/h·ft2·F. This indicates that the proposed replacement VIG panes decrease the U-value of the system by about 73% compared to the original single-pane clear glass and existing non thermally-broken steel frames. That represents a nearly 4-fold improvement over single-pane glass.

Technical Challenges

AYON Studio began the process of finding replacement blue glass for the monumental transom windows in January of 2023. Multiple color and texture samples provided by Wissmach Glass Company were reviewed against the existing blue glass. An acceptable matching color and translucency was eventually found by March but matching the nuances of texture proved to be much more difficult. Even when the interior face was an acceptable match, the texture of the exterior face could be quite different compared to the original glass. A color/pattern combination matching the original blue glass was finally agreed upon in June (Wissmach color Blue 243 with pattern English Muffle), but the process again hit a snag when the manufacturer came back with minimum quantity requirement that was much larger than needed for the project and would have added significant cost as well. Alternate combinations were sought from Wissmach Glass and Bendheim as well.

During this time, AYON Studio began to work with WHTB Glass to find a viable solution that would enable a custom VIG unit to incorporate the replica blue glass for installation in the monumental transom windows. WHTB endeavored to manufacture a VIG unit in line with those proposed by AYON Studio in the sketches below but was ultimately unsuccessful. A stable VIG could not be achieved once the glass was subjected to the vacuum forces due to the uneven nature of the textured glass.

Implementation

Ultimately, the existing historic steel frame transom windows and entrance storefront were repaired in situ. Severely deteriorated steel sections were replaced to match existing. The existing transom window sill height was modified to increase the base height, allowing for proper waterproofing and flashing.

The entrance storefront was reglazed with clear VIG as were the center lites of the transom windows. At the transom window side lites, a viable solution for blue glass replacement with VIG was not possible with the current technology. Initially, the salvaged blue glass was to be reinstalled at the side lites and any missing or damaged glass would be replaced in-kind, however the owner later decided to proceed with replacement cast glass from Lucid Glass in order to achieve a more uniform appearance.

Conclusion

VIG is a viable alternative to wholesale window replacement in existing buildings. Reglazing with VIG has the potential to significantly move the needle when it comes to the performance of existing building envelopes..

VIG contributes to a more sustainable future by:

• making our existing building stock more sustainable/perform more sustainably
• increasing the service life of these assemblies
• reducing the carbon impact of the built environment.
• Increasing preservation opportunities to retain existing fabric (whether historic or not);
• Allowing reglazing of frames in situ that would otherwise be too narrow for IGUs, thus saving on replacement/removal costs, and labor costs, in addition to energy use savings and meeting building compliance requirements.
• Installation of secondary glazing with VIG is a more attractive option as VIGs take up less space and have less visual impact while offering comparable increases to overall u-values.

As costs become more competitive in the US, reglazing with VIG will become more accessible and be appealing for a wider range of projects. Further development of standards and long-term data on real life application is needed.

Other Work to be done:

• Availability, Manufacturing capabilities in the US
• Lack of established testing standards
• Limited data on performance or life expectancy in lab or field conditions
• Lack of long-term data
• Lack of regulatory guidelines on usage/performance. Regulatory – what entities/jurisdictions have acknowledged/allowed/limited uses;
• Should we expect code impacts in the future codes
• Working with installers/contractors – Guidelines for practitioners needed.

VIG is a paradigm shift for the entire building industry when it comes retrofitting existing buildings. As preservationist, VIG is the tool we’ve needed and has long been missing in our efforts to rehabilitate more problematic glazing assemblies, allowing middle-ground of options and allowing a higher degree of both preservation and energy-efficiency while working toward the collective goal of higher building performance.