Article No: 266

2009-12-22 10:19:23
Myth Busting Concrete Home: Affordable and LEED for Homes Platinum Certified
By: Donn Thompson


Photo by Donn Thompson

Habitat for Humanity has provided affordable homes for working class families for many years. They have a thorough vetting process that evaluates the ability of selected families to afford all of the costs of home ownership. Until now, energy costs associated with conditioning interior spaces, hot water and electrical loads have been a significant part of the budget that would be used to determine which family could qualify for the donated house. At an estimated operating cost of only $32 per month, a fortunate Habitat family recently moved into an affordable, cutting edge concrete home built in Phoenix, Ariz. Among the many innovative products and systems incorporated into the home, the latest cement based technologies helped to contribute to this home earning LEED for Homes Platinum, the green building program’s highest certification level.

Construction was spearheaded by Salt River Materials Group, a local supplier of portland cements, fly ash and aggregates. They partnered with Phoenix-area contractors and suppliers to demonstrate “green” concrete building technologies. The 1,320-square-foot home with detached garage is the first concrete Habitat home in Arizona and was able to earn 114.5 out of 136 total points available for LEED for Homes certification.  

Completed in November 2009, the single story, slab-on-ground home was constructed with above-grade concrete walls, a conditioned attic space and energy-efficient windows and HVAC equipment. Combining these elements with a roof integrated photovoltaic system allowed the home to reach the coveted “Net Zero” energy designation. Exceeding Energy Star certification, the project’s additional “green” products and features will enable the home to meet other local green building program certifications including the National Association of Homebuilders National Green Building Standard as well as the Environment for Safer Living Certified Green program.

The home’s walls were constructed with HercuWall, a panelized insulating concrete form (ICF) system that combines the performance benefits of EPS foam and reinforced concrete. This wall system provides the homeowners with a strong, safe, quiet, comfortable environment that is also mold and termite free. Before factoring in solar power generation, the thermal building envelope was measured to reduce utilities’ costs by more than 40 percent. Mechanical equipment could be smaller, with only a 2-ton air conditioning system installed. This will result in reduced operating emissions associated with the energy needed to effectively condition the interior of the home.

Today’s concrete wall systems contribute to significant reductions in energy consumption through the combined benefits of thermal mass, high insulating value and reduced air infiltration. Called mass walls, they utilize less energy to heat and cool than similarly insulated buildings with wood or steel frame walls. Concrete’s thermal mass has the capacity to store warmth or cold. This results in moderate indoor temperature fluctuations, slower transfer of heat through the building envelope and the ability of a building to store energy and shift peak energy requirements. With continuous layers of insulation and concrete, these wall assemblies have fewer seams and pathways for unconditioned outside air to penetrate to the interior.

“This home provided an opportunity to demonstrate and showcase cement-based products in ‘green building’ applications,” Scott Palmer, chair of the Portland Cement Association’s residential committee, said. “Innovative ‘green’ concrete wall building systems, which could transform homebuilding across the country, install quickly and are affordable even for entry-level housing.”

Home builders in Phoenix, like many areas of the United States, are now faced with increasingly stringent energy code and performance requirements. Achieving these higher standards with conventional construction means needing more sophisticated installers, with more knowledge and skill. Exterior envelopes must be built with more complex and meticulous detailing, and more time consuming coordination. Frame technologies are being stretched beyond their limits to perform. This project helped local builders recognize the simplified approach to high performance construction that concrete systems offer. With more continuous construction, fewer “parts and pieces,” a well-insulated, tighter wall assembly can be built with fewer headaches, fewer steps, in less time, with less complexity.

The energy performance of the home was evaluated using the HERS Index, a scoring system established by the Residential Energy Services Network (RESNET) in which a home built to the specifications of the HERS Reference Home (based on the 2006 International Energy Conservation Code) scores a HERS Index of 100, while a Net Zero energy home scores a HERS Index of 0. The lower a home’s HERS Index, the more energy efficient it is in comparison to the HERS Reference Home. The HERS Index score for the Phoenix home was finalized at -4. Depending on family usage, they may actually be putting energy back on the grid.

Getting to a net-zero classification involved the installation of a specially designed roof system as well as the passive benefits of concrete roof tile installation. Eagle Roofing Products concrete roofing tiles with a solar reflectance index of 99, combined with a building integrated photovoltaic (BIPV) solar roof system, allowed the roof to meet Eagle’s Energy Saving Roof criteria. The solar panels have about the same size and shape of the individual roof tiles so they blend in well to enhance aesthetics. There are no unsightly solar collectors on the roof or in the yard of this home, yet the integrated array provides supplemental electrical power resulting in a home able to generate more electrical power than it consumes.

Concrete tile roofs are typically constructed over wood batten strips that create an air gap between the underside of the tiles and the roof sheathing. During the day, as sunlight shines on the roof, the temperature under the tile increases. The heated air rises toward the ridge of the roof, drawing cooler air into the system through vented eave risers. This natural convective loop helps to draw heat off the attic, lowering attic temperatures compared with direct to sheathing installed conventional shingles. As a result, attics beneath concrete roof tiles stay cooler, reducing mechanical loading inside the living spaces of the home.

Another sustainable feature of the project was a pervious concrete driveway, running from the street, alongside the home, to the detached garage at the back of the lot. When rain falls, it can be quickly absorbed by the pavement, which allows stormwater to flow through the paving layer and into a specially designed stone sub-base, the key to the proper performance of these systems. A properly designed base will have the required capacity to absorb anticipated runoff. Here the rainwater is held until surrounding soils can naturally absorb it to recharge underlying aquifers. In addition, the absorptive qualities of the pavement mean no standing water following a storm.    

Concrete was used to enhance sustainable performance on the inside of the home as well. In the kitchen concrete countertops featuring recycled content and integral color accent the decor. These tops are attractive and extremely durable, for long lasting performance. Unlike the off-gassing that can be emitted by the cores of conventional laminate counters, these rock hard tops are inert, and will not degrade indoor air quality.

Concrete slab on ground is a common occurrence in new residential construction in Phoenix. Typically, the concrete slabs are finished with padded carpet or other resilient finishes. To eliminate the waste generated when old carpet or other floor finishes must be pulled up, thrown away and replaced, the concrete used for these floors was mixed with integral color to provide a beautiful, long lasting finish. Extensive use of fly ash and recycled aggregate further enhanced the sustainability of the slab.

The new homeowners are thrilled with their new home. They know they were blessed to qualify for the generosity of the program. But, with benefits of concrete, this home will continue to pay them back, month after month. Now they understand how truly fortunate they are to be living in a high performance concrete home, one that is providing even more affordability and comfort.

Donn Thompson, AIA, CGP, LEED AP, is director of low-rise buildings at the Portland Cement Association. As a licensed architect, Thompson has more than 20 years of design and construction experience in commercial and residential projects.