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The Right Window Means a New Lease on Life … and New Leases

March 27, 2015 by · Leave a Comment      Print Page Print Page

By Matt Baker

The elder sibling of Prudential Plaza turns 60 this year. As a birthday gift, it’s getting a $100 million upgrade which includes new mechanical systems, a 10,000 square foot outdoor deck and other improvements. The most extensive change, however, is hidden in plain sight: the replacement of every window in the building’s façade.

PrudentialThe new windows may be tough to detect at street level, but they certainly improve the view from the inside. And while their allure to new and existing tenants was a strong selling point, they also offer much better energy efficiency.

When constructed in 1955, the 41-floor One Prudential Plaza was the tallest building in the city. “The story goes that the window washers union wanted more money to clean the windows because it was so tall,” said Greg Prather, Senior Vice President and Group Manager at JLL, which manages the property. “So Prudential made the decision to put in these rotating [windows].”

It was an unusual design for the time—rather than send maintenance crews over the sides, pivot the windows in and clean them from inside the building. After several years, however, the gaskets on the windows failed, allowing air and water to invade the office space. The window washers union won in the end as the pivoting windows had to be sealed shut.

Over the years, One Prudential Plaza’s vacancy rate declined to a low of about 70%. Prather ascribes that, partially, to the state of the windows. “If the first thing a prospective tenant sees are old, single-pane windows that are caulked shut,” he said, “their first impression is going to be that they are in an old space with old infrastructure.”

The windows’ age was evident not just in their disrepair, but their design. The original window casings were rounded at the corners. Though perhaps imperceptible from the outside, from a tenant vantage, the spectacular view of Chicago’s lakefront was almost as if through a ship’s porthole.

And that amazing view, looking over the city’s great urban parks, provided another issue, as a broad southern exposure with no buildings to blunt the sun’s impact can lead to pretty severe heat gain. It was more than the 60-year-old, uninsulated, single-pane windows could manage. For aesthetic, maintenance and performance reasons, they had to go.

JLL selected a new window model and started installing. But new is not always enough. There are a number of factors to consider when choosing your window, especially in structures the size of One Prudential Plaza.

Prather and his team spoke with Apogee Enterprises, parent company of Wausau Window and Wall Systems. The first thing they noticed was that the new windows weren’t properly rated for the building. “For commercial buildings there are different types of ratings for windows,” said John Bendt, Vice President of Building Retrofit Strategy at Apogee. “Greg and his team were looking at a window that typically wouldn’t be installed in a high rise building and we were able to educate him on the different performance level of windows, and what that means in terms of wind load and code.”

In this infrared image, the difference between  the heat transfer of the existing and new  windows is easy to spot. On the day that this image was taken, the interior temperature was 71ºF while outside it was 33ºF. The new Wausau unit on the right lost 3ºF through the glass and 4ºF at the frame. By contrast, the  existing windows were losing more than 7ºF through the glass and over 11ºF by the frame.

In this infrared image, the difference between the heat transfer of the existing and new windows is easy to spot. On the day that this image was taken, the interior temperature was 71ºF while outside it was 33ºF. The new Wausau unit on the right lost 3ºF through the glass and 4ºF at the frame. By contrast, the existing windows were losing more than 7ºF through the glass and over 11ºF by the frame.

Commercial-grade windows, like the ones first chosen for the One Prudential Plaza retrofit, aren’t designed for the stresses of a tall building. They are appropriate up to about ten stories; the higher you go, however, the more wind loads a building façade has to contend with. These loads can lead to more air and water finding a way inside if the correct windows aren’t in place.

Architectural-rated windows, on the other hand, go through a tougher regimen, testing thermal capabilities, life cycle, air penetration and water infiltration. For example, the American Architectural Manufacturers Association requires architectural windows to resist water penetration to a minimum of 8 pounds per square foot while the minimum for commercial windows is only 4.5 psf. Air leakage is similarly strict. Architectural windows allow a third of the draft of commercial products at four times the pressure differential.

The performance contrast between the new windows and the original, 1955 ones, is drastic. The casings are constructed of 70% recycled content and a fiberglass thermal break reduces heat transfer whereas the originals had no such insulation, as is evident in the infrared image. And while double-paned windows are the standard in virtually every new window manufactured today, it’s important to note that for 60 years, tenants of One Prudential Plaza had a single pane of quarter-inch glass separating them from the elements.

The new windows also offer better overall thermal protection. Solar heat gain coefficients and U-values are important things to pay attention to when selecting windows, but they shouldn’t necessarily be the driving factors. “What’s often not thought about, and it’s not just building owners but also at the architectural level,” said Wausau’s Terry Iczkowski, “is that it’s so key to have a balanced design and not march forward with just one value.”

With different window products, thermal performance and condensation resistance tend to play against one another. Choosing a window with a lower U-value may leave you with one that allows for condensation build-up. The ratio between the amount of glass and frame in the entire fenestration drives how the window will perform. The more aluminum you put into the frame, the higher the U-value will be. With condensation resistance, it’s the opposite; the more interior metal you have, the better.

With this in mind, one of the first things that Apogee did was to energy model the building envelope. This evaluation determined the heat load that the windows were likely to endure as well as where the dew point could be on the windows to reduce any chances of condensation. The modeling also informed what type of glass and low-e coating to use.

While refurbishing the building envelope can be a costly upgrade, building owners and managers can realize savings in other areas. “One of the biggest things we find in our industry is that people start with low payback items, which are lighting, HVAC second and envelope third,” said Bendt. “The problem with that process is that you leave a lot of energy savings on the table because the envelope can really affect the sizing of the HVAC.”

One silver lining to One Prudential Plaza’s low occupancy is that the window installation went in rather quickly. Currently, about 95% of the building’s windows have been exchanged, with full replacement slated by the end of the year.

The improvements to the property, including the new windows, have driven occupancy rates back up to between 70% and 80%. “Quite frankly, I’m not sure we could have gotten new tenants to move in with the old windows,” Prather said. “When you walked onto the floor and saw those old windows, it was like the TV show Mad Men should have occupied this space back in the ‘60s.”

There are more subtle advantages to the new high-performing windows than just their comeliness. A changing trend in office space has been the shrinking amount of square footage allotted per employee. Offices typically had about 225 square feet per employee years ago while today’s standards are closer to 150 square feet. Employees are not going to want to work near the drafty windows of an older building, meaning a floor’s space isn’t being used to its potential. “But now with new windows, you can more efficiently use the space,” said Bendt.

Building sustainability is all about managing efficiencies. Installing the latest and greenest air handling equipment will make less of an impact if that air is going out the windows. A property manager or owner looking to create a comfortable, efficient space to attract new tenants should start by looking in from the outside.

Images: Christopher Arndt, Infrared Inspections

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