The energy cost of added glazing in hot, dry climates
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What is the energy cost for the extra glass?
Glazing and windows are one of the most important aspects of a new building! It defines the aesthetic of the building and improves the indoor environment for the people in the building. For office buildings looking for new leases, this is essential for attracting tenants.
For building designers, the impact of adding glass is also well understood:
1. More glazing = higher construction cost
2. If glazing > energy code max. ratio[1] = COMcheck failed, energy model needed[2]
3. More glazing = larger HVAC loads = larger, more expensive HVAC equipment
4. More glazing = higher energy bills
The last point on higher energy bills creates a follow up question:
How will energy cost change with glass area?
Designers know this answer, even though it can be unsatisfactory: It depends…
It depends on:
1. Thermal properties of the glass and how much shading the building gets
2. Where the building is and how it operates
3. The HVAC system
This article presents several scenarios to help designers answer this question on an example building in Phoenix, Arizona. The results will be approximate for buildings in similar climates, like Las Vegas and Tucson. The article will also provide initial guidance on energy code viability.
Figure 1: Rendering of example office building[1] Typically this is 40% but will depend on the energy code
[2] Refers to the “performance path” of energy code compliance (e.g. IECC C407, ASHRAE 90.1 Chapter 11)
Example
An example office building with the following details is used to evaluate the impact of glazing area:
95,000 sf, two-story core-shell office building in Phoenix, AZ
Walls / Roof slightly better than code (ASHRAE 90.1-2016[3])
Steel-framed walls with R-19 batt + R-5 exterior insulation (U-0.071)
R-30ci above-deck roof insulation (U-0.032)
No external shading
Code compliant[4] interior lighting (0.79 W/ft2) and exterior lighting[5] (2 kW)
Full daylighting controls (code compliance)
APS E-32M electric rate
The table below includes the evaluated options as well as the energy code equivalent:
[3] Most Phoenix municipalities use IECC 2018, however ASHRAE 90.1-2016 is a viable option and the only likely one for a core-shell office building with high glazing areas
[4] ASHRAE 90.1-2016 Building Area Method (Table 9.5.1) - Office
[5] Assumed to consist of bollards, entrance canopy can lights, and wall packs
Results
Simulation results are shown in Table 1, Table 2, and Table 3 below. The results indicate the following results for each HVAC system option:
1. Total building energy cost
2. How much the energy cost increases per additional square foot of glass area (by glass type)
3. If the design is expected to pass energy code
Additional results can be seen in Figure 2, Figure 3, and Figure 4 below. The results show a visual of how energy costs change with different glass and glazing ratios
Table 1: Simulation Results – Packaged & Split Heat Pumps (Single Zone Heat Pumps)
* Depends on overall building efficiency, including HVAC, lighting, and building envelope performance.
Figure 2: Building Energy Cost by Glass Type and Glazing Ratio, Single Zone Heat PumpsTable 2: Simulation Results – Water Source Heat Pumps (WSHPs)
* Depends on overall building efficiency, including HVAC, lighting, and building envelope performance.
Figure 3: Building Energy Cost by Glass Type and Glazing Ratio, WSHPsTable 3: Simulation Results – Chilled Water (CHW) VAV AHU System
* Depends on overall building efficiency, including HVAC, lighting, and building envelope performance.
Figure 4: Building Energy Cost by Glass Type and Glazing Ratio, CHW VAV AHU SystemKey Takeaways
The simulated glass options are all dual pane, low-E glass with similar U-Values. The main variable is SHGC. With this mind, we can see that lower SHGC mitigates the effect of high glass areas. This can be seen with similar energy costs with a 45% glazing ratio with Solarban 60, and 65% glazing ratio with Solarban 90.
The cooling efficiency of the HVAC system is very important to the impact of glazing. With an efficient HVAC system, the increase in building energy cost will be much less than with an inefficient system.
While more glazing makes it harder to comply with energy code, it can be made up with higher performing glass as well as improvements to HVAC and if applicable, lighting systems. These design improvements would be necessary if a particular design does not initially pass energy code, as some of the example simulations do.
What about your project?
Would you like your building’s design evaluated?
Contact G2 Energy Solutions today for a quote!
About Craig Green
Craig Green is the managing principal at G2 Energy Solutions, an energy engineering consulting firm based in Phoenix, Arizona. Over the last decade, Craig has evaluated dozens of buildings to help design teams make better decisions.
For questions on the article or to discuss working with G2 Energy Solutions, please contact Craig at 480-637-7900 or craig.green@g2energysolutions.com.
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