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Getting Beyond Zero Net Energy

Getting Beyond Zero Net Energy: Software Helps Reduce Energy Use In Homes

From ASHRAE Journal Newsletter, October 24, 2017

By Mary Kate McGowan, Associate Editor, News; and Dane Christensen, Member ASHRAE, Senior Engineer at NREL

If you had $5,000 to invest in your home’s energy efficiency, what would you do?

“You have many options. For example, you could upgrade your furnace or air conditioner,” said Jon Winkler, Associate Member ASHRAE, and a senior engineer at the National Renewable Energy Laboratory (NREL). Improvements to water heaters, attic insulation, light bulbs, appliances and windows are also potential options, he added.

Because there is more than one way to achieve zero net energy, software can be used to help identify cost-effective design solutions. The BEopt™ (Building Energy Optimization) software provides a simulation-based analysis on building characteristics such as envelope, equipment, size, occupancy, location, utility rates and more, according to NREL, which developed the software in support of the U.S. Department of Energy Building America Program and makes it available at no cost.

The software can suggest solutions including improvements to insulation and HVAC systems, so that the utility bill savings more than compensate for higher first cost. The software has been used on new and existing building designs for both single- and multifamily constructions.

As cities and states move toward zero net energy—including California, where all newly constructed homes must be zero net energy by 2020—BEopt can help architects and engineers decide what combination of efficiency measures will best meet their design and construction cost goals.

BEopt uses another U.S. government tool, EnergyPlus, which was also developed at NREL using ASHRAE Standard 140, Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs, according to NREL.

A Building as a System

Zero net energy requires the integration of energy efficiency and renewable energy generation so a building produces as much energy as it consumes each year. Cost-effective solutions for achieving zero net energy performance require that different systems in a home are designed and installed to work well together.

“What is interesting is that any one of those investments changes the cost-effectiveness of other investments,” explained Winkler. “If you reduce the thermal load on the air-conditioner by improving your windows or your attic insulation, then upgrades to the air conditioner SEER become less cost-effective because you’re using your air conditioner less. Switching to LED lights reduces the thermal load as well, and that changes the opportunity for insulation and HVAC improvements. All of these things interact. Look at the building as an overall system that operates and adds up to a utility bill and how these things interact with each other.”

The software accounts for these interactions as part of its analysis, examining individual efficiency measures and including the most cost-effective ones at each step in the process. This is how it identifies the best set of efficiency measures without double-counting.

“It really does require building energy simulation to account for these complex interactions,” Winkler said.


Some simple solutions are apparent early in the process, but they are climate-dependent or building type-specific solutions. A few general trends have been seen, but they may not apply in every case.

Winkler said tightening the building envelope, using LED lights and including EnergyStar appliances are some of the first cost-effective efficiency solutions the software usually shows.

Closed-cell spray foam is relatively inexpensive and seals up cracks around a home very successfully. This improvement over minimum-code construction requirements usually pays for itself quickly and increases thermal comfort, too.

Other solutions to reduce a building’s energy use depend on where the building is located. In colder climates, adding attic insulation can be more cost-effective than adding wall insulation to achieve the same energy savings. In hot climates, sealing and insulating ducts in the attic are cost-effective tactics that reduce energy use, he said.

One of the better strategies to reduce an existing home’s energy footprint is to make improvements at opportune times. Winkler recommends re-siding a house as a great time to consider air-sealing, improving wall insulation and maybe adding exterior foam insulation.

BEopt can be used to examine the cost/benefit of these upgrades for any given house.

Identifying ways to achieve zero net energy in a new home is fairly straightforward and more cost-effective, compared to the process of retrofitting a home to the same energy performance.

“You may not have enough roof area to install enough (solar) PV, and it might not be as cost-effective to integrate efficiency because you have to tear other things out. You have to remediate problems like infiltration or open combustion appliances, which can create a hazardous situation if you air-seal the home without addressing them,” Winkler said.

Deep retrofits are also intrusive on occupants, which is not an issue with new-construction zero net energy homes.

The software provides insights in several ways.

One output is a graphic showing all the analyzed combinations of efficiency measures, which are compared in terms of annual energy savings versus annual energy cost. Cost includes both the home’s utility bill as well as the financed cost of the upgrade based on the user’s inputs.

There are typically a number of upgrade packages that deliver comparable savings and cash flow, so designers and engineers can select among those to find a package that best fits their site, local trends and incentives or other considerations.

For each set of efficiency measures, BEopt also shows where the energy savings occurs. So an insulation upgrade results in energy savings predominantly in the HVAC category, whereas upgraded lighting saves energy in both the lighting category and HVAC.

BEopt has been used to design many efficient and zero energy homes across the country.

For more information, see NREL’s BEopt website, where the software, documentation and tutorials are available for free. The U.S. Department of Energy Building America website includes numerous case studies by program participants who have applied the software to advance building science in every climate region nationwide.