TechPractices: Hillside Oaks, Carl Franklin Homes, Dallas, TX

TechPractices are outstanding housing projects throughout the U.S. where innovative technologies are implemented. Builders and remodelers can use these examples as models for projects of their own.

Summary

What happens when you combine the technology of structural insulated panels with a geothermal heat pump system that costs less to operate and maintain than conventional HVAC systems? The result, according to builders Steve and Dick Brown in Dallas, is an energy-efficient housing product that goes up quick and offers home buyers something different.

Details

This subsidized housing project offers single and two-story two- to four-bedroom homes in Dallas. There are four models in sizes ranging from 1,239 sf to 1,554 sf for a four bedroom, two-story home. These homes are stellar energy savers thanks to two very efficient technologies: geothermal heat pumps and structural insulated panels (SIPs).

Geothermal heat pumps utilize the earth's constant moderate temperatures. Like refrigerators, heat pumps move heat from where its undesired to a place where it can be expelled. A small amount of electric energy can harness a large amount of constant ground temperature energy. A closed loop, water-filled pipe buried in the ground facilitates the heat exchange process. Hillside Oaks uses a Hydro Delta geothermal heat pump system to supply the energy for heating, cooling, and hot water. Since these devices consume up to 70% of household energy use, improvements in efficiency have a big impact on monthly utility charges.

The small, 50-foot by 100-foot lot sizes at Hillside Oaks meant that only deep, vertical closed-loop pipe shafts could be used. (On large lots, pipe loops in 6-foot trenches work as effectively as vertical shafts and are often less costly to install.) During the heating season, fluid in the loop transfers heat from the ground which maintains a constant 68°, to the house. In cooling, the 80°+ heat is extracted from the house and, via the pipe, is transferred to the ground where it is cooled and returned to the house to repeat the cycle.

Like the ground source heat pump, SIP wall and roof systems are energy efficient. Hillside Oaks uses 4 ½-inch R-15 wall panels and 6 ½-inch R-23 roof panels. SIPs are energy efficient in part because of the R-values per inch of foam and partly because of their airtight assembly. This results in very tight exterior walls that permit little air infiltration and few thermal bridges when compared to stud wall construction. Although SIPs are designed to accommodate wire and pipe runs, the builder ran most of the electrical and all of mechanical drops inside interior stud walls.

After each house is completed, it is blower door tested to determine Energy Star compliance. Ventilation is provided through the air handler and is automatic when the system is in heating or cooling mode mixing outside air with filtered return air. Between seasons, when outside air is temperate, the air handler runs in ventilation mode only.

Installation/Construction

The first step in this project is drilling and placing the heat pump closed loop wells. Once the well stubs are in place concrete slabs are poured with tensioning sleeves installed at 6 feet on center in both directions because of the low bearing value of the soils. Then the SIP building skin is assembled.

Because of the nature of the panel joinery, entire building shells from the foundation up can be completed in a single week. SIPs combine outside wall, inside wall, structure, and a solid and stable insulation medium within a panel. SIPs are usually 4 feet wide with lengths up to 24 feet. At Hillside Oaks the panel openings and joints are precut at the factory.

The SIP wall and roof sections are joined by splines of various thickness and strength depending on the stresses involved. The splines serve two purposes: to stiffen the wall at the joint making it stronger than the regular section and to join panels. EPS insulation cores are inserted behind or between webs of the splines to prevent thermal bridging and to fill up all possible insulation voids. Once the walls are complete the ridge beam is set into beam pockets cut into the gable end panels. The exterior wall is strong enough to accept the roof loads without reinforcing. With walls and ridge beam in place, the roof panels attach at the ridge and eave, each successive panel glued and nailed to the previous panel. The Hillside Oaks roof panels require a double 2 by 6 spline beam (longer spans or higher roof loads might require laminated I-joists or even composite lumber to reinforce the splines and stiffen the panel).

Because the panels are faced with OSB, installing wall board is easy. The interior partitions are non-bearing (except some in two-story units) so they are lighter weight and easier to install. The 4-foot by 8-foot wall panels are light enough for two people to handle. Once properly joined with roof and wall panels and anchored to the treated 2 by 6 sill plate (attached to the slab with ½-inch anchor bolts at 4-foot centers) the composite structure is remarkably strong and rack resistant. SIP houses have performed very well in high wind areas.

The foam at the perimeter of the rough opening is removed to a depth of 1 ½-inches with a EPS cutting tool and 2x4s are then caulked, glued, and nailed into the channel, creating a clean, reinforced opening. Stanley insulated exterior doors are foam sealed to the rough opening to create an airtight fit. Aluminum frame, double-pane windows with low-e coating are installed in a similar fashion.

Benefits/Costs

The builder claims that, at $77,000, the houses at Hillside Oaks are half the median home price in Dallas. Residents receive a $2,000 down payment from the Dallas housing department if they meet assistance qualifications. Low operating costs are likely based on the performance of the exterior skin and heating system. If the heating or cooling cost for any single month exceeds $34, the builder will pay the difference. To provide specific data on heating and cooling use, a separate meter is installed on the heat pump circuit.

Code/Regulatory

Local building authorities were not familiar with SIP construction or with geothermal heat pumps. The approval process was aided with a series of mock-up demonstrations showing how panels are assembled and how ground loop pipe works with the heat pump. Once the credibility and quality of the products were demonstrated, inspections were hassle free. The energy performance of the first few models was so good that subsequent iterations of same models needed little new testing to receive the Energy Star rating.

Feedback

Residents report that their homes are economical to heat and cool, and the builder has yet to pay a rebate on utility bills. Even during the summer 1998 heat wave, Lupe Garcia, a resident at Hillside Oaks, told the builder she was "very pleased" with her low summer electric bill.

The builder, Carl Franklin Homes, constructs 50 to 100 homes a year. Eighty percent of their business is derived from the affordable housing market. Even though the profit margins are low on a unit basis, project volume and the construction simplicity of affordable housing make this a profitable building approach. Because they develop chunks of affordable housing at one time, they are usually partnered with a charitable housing organization that acts as developer. The success of Hillside Oaks has given them a reputation for delivering a quality house at affordable prices, with low operating costs. This track record makes them attractive to non profits who are often obligated to build a certain number of houses per year. SIP and geothermal homes are built to higher standards and this is what makes them desirable. Non-profits want homeowners who can afford to operate their homes as well as pay for them. Higher quality and lower operating cost are worth paying a little more for from the lender/developer's point of view. The high demand for homes at Hillside Oaks makes good business sense.

The company is starting a similar multi-family affordable housing project in Commerce, Texas, and sees the combination of SIPs and geothermal heat-pumps as an ideal "affordable" package. Although it costs them more to install the heat pump system over a cheaper furnace, and more to build with SIPs, the builders say that home buyers like it and the units are pre-selling before construction.

According to Franklin Homes co-owner, Dick Brown, the numerous referrals they get from the satisfied home owners at Hillside Oaks is making it easier to sell new SIP geothermal homes. That, coupled with the Energy Star rating (which, Brown says, adds 5% to the resale value) makes these houses easy to sell.

Contact(s)

Do you have a specific question? Try the contacts listed below:

Carl Franklin Homes
16800 Dallas Parkway, Suite 250
Dallas, TX 75248
972-248-2511
www.carlfranklinhomes.com

Hydro Delta Corporation
1000 Rico Road
Monroeville, PA 15156
412-373-5800