“We are going to ban the classic glass and steel skyscrapers, which are incredibly inefficient,” proclaimed New York City Mayor Bill de Blasio, in a press conference in April. “They have no place in our city or on our Earth anymore.”

While the Mayor’s words are a stretch – no specific legislation has been drafted banning 100% glass buildings – New York City’s Green New Deal makes it clear: use less glass, or design your building to be efficient enough that it performs as it would with less glass.

Energy-efficient, fully glass buildings are not impossible. Check out 10 Hudson Yards, the first commercial office building in NYC to achieve LEED v3 Platinum certification. While not impossible, it is no easy feat, and it costs more to achieve the same low energy targets with a fully glass building compared to buildings that use less glass.

To explore the relationship between glass, energy use and costs, we built a case study to model a hypothetical 25 story, 200,000 sq. ft. building in NYC.

Using PlanIT Impact, we applied a three step approach to the analysis:

1. Benchmark the baseline energy use of a 100% glass building
2. Evaluate the energy savings and upfront building cost savings from a 75% glass building, and a 50% glass building
3. Discover the required design strategies to make the 100% glass building achieve the same energy performance as buildings with less glazing

Results Snapshot

Check out the following key conclusions from the exercise:

• The 75% glass building uses 27.0% less energy and costs $0.9 million less than the 100% glass baseline building
• To achieve at least the same energy performance as the 75% glass building, an additional $600,000 would need to be spent on efficient design strategies for the full glass building
• The 50% glass building uses 46.5% less energy and costs $1.8 million less than the 100% glass baseline building
• To achieve at least the same energy performance as the 50% glass building, an additional $2.1 million would need to be spent on efficient design strategies for the full glass building

The energy use, energy cost, added building costs are outlined as follows:

Read on for the full case study and results!

Project Walkthrough

Step 1: Benchmark the baseline energy use of a 100% building.

An ASHRAE 90.1-2013 building envelope and mechanical properties are applied for the baseline analysis in PlanIT Impact. The resulting EUI and total Annual Energy Costs for the fully glass building are below:

Step 2: Evaluate the energy savings and upfront building cost savings from a 75% glass building and a 50% glass building.

Next, the baseline is copied to two new design scenarios. In one scenario, the glass percentage is reduced to 75%. In the second scenario, the glass percentage is reduced to 50%. As shown in the following table, both scenarios significantly reduce energy use, total annual energy cost, and save on upfront construction costs. Win, win, win!

Step 3: Discover the required design strategies to make the 100% glass building achieve the same energy performance as the building with less glazing.

Now, let’s compare what design strategies are needed to make the 100% glass building perform equally, or better than the design scenarios for 75% and 50% glass buildings. Using PlanIT Impact’s Discovery Engine, the minimum energy savings and maximum payback period are set.

The design constraints are set such that the window:wall ratio (WWR) for each faced is fixed at 100%. This means that the Discovery analysis will not evaluate any other WWRs when considering possible design strategies to meet the energy savings target. The following image shows the 100% fixed WWR as well as other glass properties that can be evaluated.

Discovered scenarios are presented in the table below, along with two scenarios from Step 2.

Let’s compare Discovery B to the 50% Glass Façade results – the 50% glass building costs $1.8 million LESS than the 100% glass baseline. To achieve at least the same energy performance as the 50% class building, an additional $2.1 million would need to be spent on efficient design strategies.

The Energy & Cost chart below shows the recommended design strategies to meet the 52% energy savings. The most efficient strategy of using high-performance glass with a reduced U-value results in a 32.5% energy savings. It also adds $1.1 million to the total building cost.

Conclusion

In this exercise, we compared the energy use, annual energy costs, and upfront capital costs of a 100% glass building to two building options with 75% and 50% glass. The results show significant savings that can be realized in all areas for buildings designed with less glass.

We also compared the design strategies required for the 100% glass building to perform at the same energy efficiency level as buildings with less glazing. Considerable added building costs are required to implemented the strategies suggested for meeting the desired level of energy performance.

By using PlanIT Impact, you can easily show how your energy and costs are impacted, helping you make the case for less glass. Request a demo today for a deeper dive into how it works!

Pro-tip: Partner the results from PlanIT Impact with a daylighting analysis from another tool to add another level of confirmation to support the case for less glass!