TechPractices: Healing House, BBH Enterprises, 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

While guaranteeing a "dollar a day" heating and cooling costs, this super tight, energy and resource efficient house also provides a healthy, indoor environment for a severely asthmatic client. Located in a moderate income Dallas neighborhood, this house achieves its health and energy goals on a moderate ($65/SF) budget by pairing an innovative mechanical and ventilation system with a novel floor plenum design.

Details

The Healing House is the latest version of a line of energy and resource efficient homes built by BBH Enterprises. This one offers energy conservation and near pollution-free indoor air quality. The new owner, who suffers from chronic asthma and multiple chemical allergy conditions, has found relief in her new house and has given it the name, Healing House.

There are a few strategies here for energy efficiency worth looking into. Air infiltration (.3 ACH) is minimized by carefully sealing all gaps in the exterior skin, there are extra levels of insulation, and the house is sited to admit winter sun and reduce overheating from summer sun. Most of the windows are on the south elevation, few are to the north and east elevations, and there are none to the west. Overhangs and thermal mass at south walls modulate and store solar gain.

This house has a hot air system but no ducts. An electric heat pump uses a loop of circulated water buried deep in the ground, teamed with a novel air distribution system. 16" open-web floor joists create a hollow floor plenum for distributing air throughout the house with no ductwork. Conditioned air is pumped into the plenum from a centrally located air handler. This system works well because the house envelope is very tight and well insulated. Infiltration loss is low, and air temperature stratification is minimized. Conditioned air is not forced through at high velocity, but seeps out at a comfortable, barely perceptible flow.

Return air is collected on the second floor through a dropped corridor ceiling and at the first floor through the air handler closet louver door. Air is supplied by an air-to-air heat exchanger that modulates the incoming air with temperate exhaust air from bathrooms and kitchen.

The one-and-a-half ton, geo-thermal heat pump made by WaterFurnace operates at an impressive 16.2 EER cooling capacity rating. Two 250 feet deep wells (drilled by the local utility company) circulate water to assist in heat pump operation. During heating or cooling, the constant 65°F ground water exchanges its heat to the refrigerant at the refrigeration coil. In winter the ground water helps the refrigerant absorb heat as it is compressed into a gas; in summer the ground water helps dispel heat from the refrigerant gas as it reverts back into a liquid. The ground loop's tempering effect is what gives the system its efficiency. An added plus with this heat pump installation is the free hot water that is produced as a by-product of both heating and cooling cycles.

Working in tandem with the heat pump is the Life Breath heat exchanger by NuTech Energy Systems. Before it provides makeup air to the air handler, the heat exchanger mixes with the outgoing exhaust air and transfers 70% of its heat to incoming air. The heat exchanger works continuously throughout the year, providing a constant rate of 0.3 air changes per hour. The heat exchanger works during the cooling season but not as efficiently as it does in heating.

Installation/Construction

This is a modified stick-built house with a few new twists. Exterior walls are framed with 2 by 6s 24 inches on center. Double top plates are not needed because joists and rafters, align so that they never land mid span on a wall plate. Exterior corners are fastened with metal cleats nailing plates like those used in trusses and this saves redundant studs. Insulated headers prevent heat loss. This Optimum Value Engineering (OVE) method saves lumber without sacrificing structural integrity. Floor trusses support the second floor deck and exterior walls bear on a thickened edge, post tensioned, concrete slab-on-grade. Diagonal bracing is of metal strapping and the exterior is sheathed in 2-foot by 8-foot sheets of 1-inch expanded polystyrene insulation board. Joints between the polystyrene are taped with 3M Y808 sealing tape to improve the air tightness of the shell. The exterior is covered with building paper, metal lath, and 3/8-inch thick conventional stucco.

Part of the efficiency of this solution is the simplicity of the framing. The building footprint is nearly square. There are few framing headers and no dropped beams to complicate loading. This simplifies air flow for heating and cooling and is an efficient way to create space with the least amount of material.

Building this house involves a few extra steps. To ensure non-toxicity of materials, the builder had the client place samples of the construction materials in her bedroom for a few days to monitor her exposure to them before giving the go ahead to install. This provided reasonable assurance that the client would be able to live with these new materials. Materials that could not be excluded from the building finishes (such as OSB, composite lumber, plywoods etc.) are sprayed with Crystal Aire I by Pace Industries. This paint-like coating of polyvinyl solution effectively seals wood surfaces from out-gassing. The material cost for interior sealing was about $500.00. Standard paints and construction adhesives were replaced with non-VOC substitutes. In addition, recycled products were used wherever possible.

Sealing the exterior envelope is critical to the air tightness of this house. In addition to 3M tape, GE Silicone caulk and Foam Plus expanding foam sealant were used. Taping joints and caulking and sealing the skin is time consuming and adds expense to the job. But the exceptional performance of the Healthy House is largely due to its tight envelope. However, there is no vapor barrier in the wall cavity because heating and cooling are relatively balanced in importance. Because water vapor is generated inside during the winter and outside during summer, trapping it on either side would invite condensation to form somewhere in the wall. Instead, the walls are built to breathe. The paint specified for the wallboard is "permeable" to ensure that the painter does not apply an unintended vapor barrier.

Benefits/Costs

This 1,470 sf home cost about $65/sf for a completed price of $95,000 including land. The selling cost is about 25% below the median price for homes selling in Dallas and would, of course, be lower without the need for multiple chemical allergy mitigation. But the benefit to the owner is clear. Lower utility bills are an ongoing benefit both to the homeowner and to the utility.

Code/Regulatory

Because the builder, BBH Inc. is not non-profit, permit fees could not be waived or reduced. Nor could a requirement to provide 20'-wide alleys be negotiated down despite the lack of need. The health department did not approve a greywater system for the Healthy House despite licensed engineering design. The building department did not understand the HVAC system but permitted its installation.

Feedback

The homeowner is happy with her new house and wrote a testimonial to the builder praising the excellence and quality of the home and describing how she was healed of her asthma and chemical reactions because the house air is so pure. She also wondered why more houses aren't built this way. The project mechanical engineer, Ken Muse of Total Air HVAC of Dallas admitted that he was "pretty skeptical" using the floor plenum design. After installation the system performance eased his concerns -- it worked exactly as it was intended. The house has received the Edison Institute's E Seal Award and will receive Energy Star recognition in the coming year. The builder, Barbara Harwood, has produced nine variations on the house, geared to the affordable market. Her new focus will be on the up-scale custom market with its healthier profit margins. The lack of municipal support for lower income development is also a factor in her new approach.

Contact(s)

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

NuTech Energy Systems Inc.
511 McCormick Blvd.
London, Ontario N5W 4C8
519-457-1904

WaterFurnace International
9000 Conservation Way
Fort Wayne, IN 46809
219-478-5667

BBH Inc.
2718 Wagonwheel Drive
Carrollton, Texas 75006
972-418-7772
energyxpert@aol.com

Total Air Mechanical Systems
Dallas, Texas
972-881-0020