Concrete Thinking Think Concrete
Case Studies  > Napa Valley Home Exemplifies Sustainable Design in Hot, Fire-prone Region
Print   eMail

David Horobin, principal of Estudio Verde Architects in Napa, Calif., designed a fire-resistant, sustainable home for his family in 2008 with the help of contractor Patrick Bentley of Bentley Construction and Concrete. The design and construction of the home took into account the effect on the neighbors, placement, profile and mass of the structure, building materials, utilities, and aesthetics.

Energy and Resource Efficiency

Beginning with site placement, careful attention was given to energy and resource efficiency. The home’s footprint is shaped like the letter “L” and angled to accommodate the original land contours. This minimized the need for grading and rock excavation. The site placement also plays the role of good neighbor by mitigating obstruction of the upper neighbors’ view of the San Francisco Bay.

The 2,700-square foot home is oriented specifically to maximize views, reduce solar gain, and ensure privacy. The kitchen windows primarily face south and have extended overhangs to accommodate winter solar gain at a lower sun angle while shading the same windows from a higher angle of sun in the summer. Most of the bedroom windows face east to accommodate morning solar gain and ambience, using low-E glass. There are minimal windows on the west side to limit the evening solar gain in the summer, which can be very strong and overbearing.

The entire envelope of the home was designed to deliberately avoid the use of mechanical cooling equipment. The north wing has concrete floors incorporating plenty of thermal mass for the radiant system. The use of a controlled ventilation (CVC) system in the basement area of the east wing connects the thermal mass of the earth to the interior of the house, thereby taking advantage of the earth's constant 55-60 degree temperature to both heat the house partially in the winter and help cool it in the summer.

Except for the upstairs cross walls, all exterior walls of the house are built with insulated concrete forms (ICFs). The form chosen consists of a six-inch concrete core with just over two inches of expanded polystyrene (EPS) on each side. The equivalent R-Value created by the thermal mass of concrete encapsulated in two layers of R-12 EPS amounts to approximately R-65. Compared to conventional framing where R-19 fiberglass insulation may be reduced to an average of 13.5 because of the inconsistencies of wood framing, the ICFs provide excellent energy efficiency. 

ICFs limit the potential infiltration or exfiltration from electrical boxes and plumbing punctures in the envelope, which decrease the efficiency of conventional walls. The walls of the garage are constructed with ConForm ICFs, a product designed by the Horobin and manufactured in Utah. The garage is unheated but always remains at a constant temperature because of the Conform technology.

Except for the garage roof, the home’s roofing is constructed with a structural insulted panel system (SIPS).  With an R-Value of 45 compared to a conventional framed system, which may only achieve an R-22, the decision was easy. With the added benefit of being able to assemble entire hip roofs (three of them on the east wing) and the gable roofs on the garage floor, SIPS cut short-term construction costs as well as long-term energy consumption.

The combined energy elements of the design and construction, including the choice of hot water radiant heating, provide a maximum heating load of 40,000 BTUs. This amounts to approximately 25 percent of a similar design using conventional materials such as wood studs. A 50-gallon water heater provides all the heating and domestic hot water used in the home and at a greater comfort level than conventional heating systems. The master bathroom is heated with an in-floor electric radiant system. This was installed prior to applying the tile floor finish and is controlled by a timed thermostat, which provides a toasty ambience for bare feet.

There are two operable and remote-controlled skylights in the entire house. They are specifically placed to provide thermal venting for the highest ceiling elevations of both the one-story wing (in the kitchen) and in the master bathroom for the two-story wing. The bathroom skylight is located directly above the main shower and tub to allow for moisture venting as well as heat, which stratifies at the higher points. The kitchen operable skylight is specifically placed above the double ovens and the stovetop to vent unwanted heat gain.

The choice of hot water radiant heating was an automatic for the Horobin residence. In the concrete floors of the north wing, which includes the kitchen, office, dining room and entrance lobby, Pex piping is placed in the floors. These zones all have two inches of EPS foam faced with reflective foil placed between the concrete and the compacted gravel base, providing an R-8 insulation value to essentially reflect the energy up through the slab into the rooms above. The ICF walls provide perimeter insulation of R-65 for all concrete slab areas.

The living room has two runs of piping that are placed under the subfloor and reflected upwards into the room. The master bathroom and shower has an electric radiant cable system placed in the mortar bed below the tile controlled from the master bedroom. All controls, pumps, thermostats and manifolds were tested by the manufacturer to ensure the entire system balanced.

Fire Resistance

The Horobins lost a house to the Lexington Hills fire above Los Gatos in 1985. Since then, David has encouraged clients to follow his example and build responsible, safe homes that mitigate fire damage. The use of ICFs provides an entire concrete structure with a minimum of four-hour protection in the walls.

The entire roof and overhangs are covered with GoldBond, a thin fire-protective fiberglass and gypsum sheathing. The use of clay tile and careful placement of bird-stops to prevent thermal fire-chimneys under the tiles adds to the fire resistance of the entire structure.

The use of SIPs completely eliminated the need for ceiling or roof vents, a huge concern for fire crews and homeowners of conventionally constructed homes. Fires usually travel uphill and the downhill walls of the home are constructed with no windows on the first floor. With most of the windows on the east side—windows being the weakest link in fire consideration of any structure—there is little danger from fire as the trees and vegetation are limited to the other side of the property lines, far away from the house.

From an interior standpoint, the entire structure is has sprinklers per the City of Napa code requirements. All floors are tile or hardwood as well as all interior window and door detailing – this eliminates the use of wood trim, providing simple and elegant aesthetics as well as minimizing potential of fire. Ninety percent of all interior walls are pre-manufactured steel frame panels from Codding Industries, few of which are load-bearing walls, thereby mitigating the potential of structural damage in the event of a fire.

All three exterior decks are finished with a fire-resistant composite decking material.

Seismic Consideration

The original structural design offered the alternative of using Helix Fiber, a helical galvanized steel fiber designed at the University of Michigan to enhance the reinforcement of concrete in seismic conditions. Openings, corners and foundations still require conventionally designed steel reinforcement but the fibers get mixed in the concrete at the batch-plant. The advantage is that they make every cubic inch of the concrete into tensile concrete, as well as eliminating many hours of steel reinforcing fabrication and material on site. Otherwise, the six-inch concrete walls provide compliance with all structural and seismic codes as required by the Uniform Building Code.

Fortified for Safer Living

As a result of the structural and fire aspects of this home, the owners were awarded the first west coast "Fortified for Safer Living" certificate by The Institute for Business & Home Safety (IBHS). IBSH promotes safe building construction in any natural disaster-prone zone in US.

Recycled Materials

The designer specified a concrete mix of 40 percent fly ash. Adding fly ash increases the strength of concrete and is less porous than a conventional mix.

Additional benefits of using fly ash include:

  • Delays the heat of hydration and reduces thermal cracking
  • Improves the workability of concrete
  • Makes the mix homogeneous and reduces segregation and bleeding
  • Finish (for slabs) is improved due to perfectly spherical fly ash particles
  • Permeability is substantially reduced which enhances the life of the structure
  • Contributes to the long-term strength in concrete 
  • Fly-ash is a byproduct of combustion chamber and otherwise destined for a landfill

Bedroom number three has recycled wine-cork floor tiles and a notice-board wall. The material returns the corks to their place of origin in Napa and provides a unique and beautiful finish. It provides additional acoustic and thermal insulation as benefits.

Additional Benefits

The home’s interior walls are constructed with steel beams by Codding Industries instead of wood. Panelized from 18-gauge steel C-channels, the contractor was able to lift each panel into place from the stack delivered to site. All first-floor interior walls were placed and attached within a 48-hour period.  Steel walls are generally very reliable in tolerance compared to wood studs, which delights drywallers who have a much easier time as they can rely on the straightness of the walls.

The house is extraordinarily quiet. Unless the kitchen windows are open, all street noise is eliminated. This is a deliberate design element caused by a combination of the placement of the garage, the use of ICFs and SIPSm and few west-facing windows. The owners used leftover SIPS polystyrene foam between the bedrooms to mitigate sound transfer.

To support the driveway and path to the main entrance, native large boulders from the property were dry-stacked to create a natural and rustic retaining wall.

The main driveway and parking area are made with “ginger rock,” a beautiful and locally mined fine rock material. As a pervious pavement, the need for perimeter and foundation drains was eliminated.