Preservation in Action

By Ted Muntz, Owner of the Lovness Estate

Mitered glass corner windows are a signature feature of Frank Lloyd Wright designs. This innovation doubles down on his desire to inject nature into his buildings by removing obstructions. His window walls and bands of windows serve to blur the space between the house and its surroundings. And by eliminating the center mullion from corner glass, the effect is all the more exciting.

When renovating the Lovness Studio in 2017, we were presented with four existing examples of corner glass windows on the main floor. Of these four, one was mitered to the outside and three were mitered to the inside. Originally, all windows in the 1955 structure were constructed of single-pane glass. However, insulated glass had replaced all flat panel (non-mitered) glass sometime in the past. In 2017, we replaced all these dual-pane, flat panels with Low E, argon gas-filled insulated glass (also known as insulating glazing units or IGUs) – not a difficult task. We also became aware of the more recent development of suspended-film insulated glass, a clear polymer film that is positioned within two glass panels but is weightless. This technology, an outgrowth of films developed for smartphone glass, may have some future for flat panel installations, but not currently for corner window applications.

So the corner glass presented us with problems. First, the main floor corner glass already had insulated, corner glass in place, but two had failed. Upon investigation we discovered that Don Lovness had these made by Cardinal Glass, a local glass manufacturer. This was accomplished through his personal friendship with Cardinal’s owner, who agreed to do this work in 2000 as an experimental project. Although we asked, Cardinal would not repeat this for us as the R&D personnel had retired and the past attempt had evidently failed.

Expanding our search to the wide world of window manufacturing proved disappointing. No glass company would agree to warrant and, therefore, produce insulated corner glass without a center mullion. A sealant suitable for both inside and outside miters seems to have been the obstacle in securing a warranty.

Therefore, our choice for replacements was either an insulated, center-mullion window or to preserve the aesthetic with mitered single-pane glass. We really wanted to replicate the experience of living in Wright’s total design. We had invested freely in that endeavor for other aspects of the house. Consequently, we opted for the aesthetic: single-pane, mitered corner glass. The next winter provided us with proof that our selection would not stand the test of time.

In Northern climates insulated glass is a must. It is much more efficient than single-pane glass. It increases the R-value (a measure of insulation) and therefore reduces heating costs. It reduces draft and increases the interior comfort level.

But the major concern for us was the freeze and thaw cycle that causes condensation to accumulate and subsequently frost interior glass surfaces in cold weather. Think of how a cold glass of water sweats on a hot summer day and how a coaster absorbs the dripping water. When the frost warms on window glass in the winter, the resulting moisture drips to the bottom wooden sill where on successive days it will thaw and freeze again and again. In the first year after renovation, we measured ice as thick as three-quarters of an inch at the base stops of these windows, rising up the glass as high as ten inches. When exterior temperatures moderate, the ice melts onto and into the interior wooden stops and sill, accelerating the deterioration of both.

This video gives a good overview of how condensation develops.

We had to find a solution.

On a visit to another well-preserved mid-century house, I noted a similar situation with single-pane glass, but it also included a mid-century solution. At the bottom of the glass panel, a concave aluminum trough 2” wide and ¼” deep was placed to collect the condensate and allow for natural evaporation.

While this seems to have dissipated the condensate for this house, it did not do so much to maintain glass clarity. Our windows did not have sills wide enough to accommodate the 2” wide aluminum collectors. This is likely a system that must be designed for and installed initially and not retroactively. As I have not seen this in other locations, the system may not have worked well enough to survive the 1950s.

We learned from the Conservancy of successful installations of storm windows on Prairie-period buildings that had significant numbers of single-pane art-glass windows. These included both exterior storm windows and interior windows.

Of all these possibilities, the interior option appealed to us. Specifically, we were looking at compression-fitted, aluminum-stiffened acrylic insert panels manufactured by Indow Windows, Inc. These have a proven track record of successful installations on flat panel windows of all shapes and sizes. Even though Indow’s engineering department had addressed the issue sometime in the past, the company’s experience with corner window solutions was limited. After discussion with Indow, we decided to work together to not only solve our problem, but also formalize a successful and repeatable process for corner windows for other clients.

Compression fit systems require meticulous measurement. Indow provides a laser measurement tool, complete instructions, and a personal, online project manager to assist. Each window panel requires six measurements. Concave and bow conditions can also be considered. All measurements are entered into a chart supplied in the measurement kit. Customer measurements, entered online, are reviewed by Indow for accuracy and continuity before they are put into production.

Three colors are available – white, black and brown. Window inserts are each permanently marked at the factory with location and position provided by the homeowner on the measurement chart. Windows are shipped with substantial protection and complete instructions for installation, storage, and maintenance.

We began our relationship with Indow by addressing the most pressing issue – the two corner windows in the bedrooms. These windows were relatively recent replacements. They were also the most vulnerable to damage. One of these was mitered to the outside and the other mitered to the inside.

The key to using two compression inserts in corner window applications is to create a uniform, perpendicular surface at ninety degrees to the window plane for the compression ends to meet. Finding three sides in a corner window frame set is easily determined. Finding the fourth, where the two inserts meet, requires some innovation. An additional frame can be designed and built of wood or other solid material – a removable mullion, if you will. However, Indow developed a formed, aluminum F-bar for that purpose. The F-bar accepts window inserts in both ninety-degree directions.

Either type should be designed to be easily removable – utilized only during cold weather months to eliminate condensation – just like storm windows. When removed during warm weather months, the “invisible” aesthetic is restored.

We chose to use the Indow F-bar. For outside mitered corner glass windows the F-bar required no attachment to any woodwork in our single main-floor installation because the compression within the F-bar is forced toward the surrounding woodwork. The F-bar was simply cut to length and friction fit on the chosen frame step.

Once a center mullion or F-bar is installed all four frame stop surfaces are established. To determine the insert dimensions, Indow provides clients with a digital laser measurement tool and complete instructions for its use. The tool provides height, width, and diagonal measurements to accommodate out-of-plumb or warped window openings. Each insert is permanently marked with its location and position to aid in installation.

Inside mitered installations present more difficulty because their solutions are dependent on the configuration of the window casing, frame and stops. A removable center mullion or F-bar must be positioned to be a continuous match of the selected frame stop and then secured in a manner to be both rigid enough to accommodate the inward movement of the compressed surfaces, but also be removable. For this bedroom installation we used 90-degree frame clips, provided by Indow, on the inside of the F-bar, which centered and secured it from movement in all directions. This was possible because the window height was relatively short. We leave these clips installed permanently, but can easily remove the friction-fit F-bar from them.

After both the F-bars were cut, fit and permanently identified for location, they and the clips were sent out to be powder-coated the same color as the Indow inserts.

The results were encouraging. Neither of these locations experienced condensation during the following winter of 2021. This successfully concluded our first phase of engagement with Indow.

Following the bedroom installations, we addressed the larger interior mitered corner windows in the living room. These are substantially larger with each panel measuring roughly 75” high by 23” wide. Since these were the most challenging, we addressed them last in order to utilize the knowledge we had learned from the previous project. The challenge came from their height and the small ¾” sill covered by a 1 ¼” beveled stop.

While we were able to prepare the frame stop work for the previous project, we needed the help of a professional contractor on this one. The contractor removed the sharply-beveled stops and replaced them with 1” stops with a smaller bevel. Indow requires a 5/8” minimum stop width and suggests their compression joints will work with as much as an 11-degree stop bevel.

The location of the frame stop on these windows required the use of the F-bar but that had to be modified at both ends to clear the stops which protruded beyond the frame stop. Small spacers were fashioned to be installed above and below the stops for stability. The F-bar and the spacers were drilled to accept 1 ¼” long #10-24 stainless machine screws. The casement into which the F-bar was to be secured received stainless #10-24 grommets to facilitate annual installation and removal. The grommets are painted to match the color of the casement and are nearly invisible when not in use.

Measurement proceeded as described in the earlier project with the two height and two width dimensions determined by the laser tool. The two diagonal measurements, however, presented problems. The increased height (75”) and narrow width (23”) combined to render the laser tool (used in conjunction with the adjustable extension) entirely useless. Indow’s adjustable extension should be substantially longer to reduce the interference encountered in our installation.

Our Indow fit specialist recommended to manually measure the diagonals, but doing so with conventional tapes was difficult and inaccurate. Conventional tape measures have a clip at the beginning that prevents them from getting fully into the 90-degree corner. In addition, conventional tapes are slightly curved to enable retraction into the reel housing, making them difficult to read.

Eventually, we used a combination OD/Inch tape that was ¼ inch wide and flat. Since this type of tape measure has excess length at the start, it could be bent at a 45-degree angle opposite the beginning of the inch scale.

Using this tape requires help. One person fits the bent portion of the tape into the upper corner using a long shaft slotted screwdriver (to reduce hand interference and maintain the end in place). The other person pulls the tape tightly to the lower opposite corner and reads the dimension accordingly. This procedure proved to be repeatable and, therefore, accurate.

Installation instructions supplied with the Indow window inserts are explained in great detail. We followed them to successful conclusions on all locations in the initial project. However, we did note that on these taller, inside-mitered inserts, the instructions should suggest that the insert on the uncaptured half of the F-bar be installed first, then the captured side. To do otherwise may permit the uncaptured side insert to wiggle inward subject to the movement during installation of the other side. As this can happen regardless, it is better to be able to push the uncaptured panel back into position from the inside and then try the captured side again. It is also advisable to have a second person assist by applying some pressure to the mid-point of the F-bar to prevent the slight bowing that can occur due to the compression exerted.

The winter of 2023-24 was not severe as Minnesota goes. These installations exhibited no leakage or condensation at 0 degrees Fahrenheit. At minus 10 degrees we noticed very slight leakage on the Northern location. This seemed to have been caused by the slight bowing of the F-bar over the 75” of the height dimension. Adding some strips of cork insulation to the F-bar solved the problem.

In the winter of 2024-25, we experienced more normal temperatures. At temperatures to -10 degrees we experienced no leakage or condensation. At -17 degrees (our coldest so far) we noticed very slight leakage on the Northern location, but that had evaporated by the next day.

Besides the two projects referenced earlier, we also addressed four smaller clerestory exterior-mitered windows with similar results.

After three projects and almost four years of winter, we have judged our experience an unqualified success. In our very demanding climate, we are able to enjoy seven months of the clear-view design aesthetic, while in the cold months we experience minimal to no condensate problems. In more moderate temperature areas, the preserved aesthetic can be viewed even longer.

Please Note: Each restoration/maintenance/repair situation involving a Wright building requires analysis and research to identify the correct approach. The Conservancy is sharing the information so it may be of use to others as they evaluate their own specific situation and may consider these and other approaches. The Conservancy strongly recommends that owners consult with a design professional and an experienced contractor to determine which method is best for their specific projects.

Further, the provision of this information or mention of a specific product or products does not constitute endorsement, recommendation, preference or approval by the Conservancy.