Homes of 1200 square feet or smaller make up 8-10% of U.S. housing start. (Census, 1997) Characterized by high occupant density, these homes accrue energy expenses that rival mortgage payments. Responding to consumer demand for lower operating costs, builders have typically reduced infiltration (tightened) and increased insulation. These efforts have netted homeowners both lower utility costs and more comfortable living conditions. However, consumerconfidence in these strategies has been eroded by implications that very tight homes have poor indoor air quality.

To demonstrate both energy efficiency and healthy construction in the entry level housing market, FSEC partnered with a Central Florida contractor to design and build three 1,228 ft2 (Figure 1) homes. All three homes qualify for the Energy Star designation. One has extra energy features and another has indoor air quality features. FSEC conducted testing to evaluate several indoor air quality parameters as well as monitoring the energy use of the homes before occupancy.

After completion in August of 1998, the three houses sold immediately illustrating the high market potential of super efficient entry level housing.

Characteristics of the Homes

Table 1. Entry Level Housing Demonstration Project - House Characteristics

	Material/System	House #1, Block	House #2, AAC	House #3, Frame
Conventional Construction	Exterior Wall System	Concrete Block	Autoclaved Aerated Concrete Blocks (AAC)	2 X 4 Wood Frame
	Wall Insulation (Overall R-value)	R-4.2 Foam Board (6.1)	None (11.5)	R-11 Fiberglass Batts
	Caonditioned Area	1,228 square feet
	Orientation (front door)	North
	Foundation	Slab on Grade
	Ceiling Insulation	R-30
Energy Star Enhancements	Air Conditioning Equipment	High Efficiency Heat Pump, SEER = 12, HSPF = 7.5
	Duct System	R-6 Insulated Ducts Sealed with Mesh and Mastic in Attic
	Air Handler Location	Conditioned Space
	Infiltration Reduction	Penetrations in Thermal Envelope Sealed
Special Features	Reduced VOC Materials	None	Low Emission -Carpet (100% Nylon) -Carpet Pad -Cabinets	None
	Window Type	Single Pane Clear	Double Pane Clear	Double Pane LowE (Shading Coefficient = 0.41)
	Ventilation System	None	Fresh Air System -Outside Air Duct -RanRecycler Control -Transfer Ducts -1" Pleated Filter	None
	Additional Attic Features	None	None	-Enhanced Attic Ventilation -Radiant Barrier

The three neighboring homes, built with identical floor plans (Figure 2) and slightly different roof lines, have similar solar heat gain characteristics and conventional regionalcharacteristics such as slab on grade foundations. Several improvements on conventional practice were incorporated into all three homes to bring them up to Energy Star status. Extensive sealing of both the duct system (Figure 3) and penetrations in the air barrier (Figure 4) reduce cooling loads. The air conditioning are all high-efficiency (SEER 12, HSPF 7.5) heat pumps. To minimize the impact of return side leaks, the air handler is located inside the conditioned space (Figure 5).

Each of the three homes features a different structural system (Figure 6) to illustrate that energy efficiency can be achieved in this market with conventional materials (concrete block and wood frame) as well as with innovative systems such as autoclaved aerated concrete blocks (AAC). Though this dissimilarity demanded different types and levels ofwall insulation, all three homes scored above 86 on the Home Energy Rating System (HERS) scale, the Energy Star Homes threshold.

The wood frame home incorporates an attic radiant barrier (Figure 7) and high performance windows for additional energy saving features. These features reduce two of the largest air conditioning loads in Central Florida homes: radiant heat gain via the roof and windows.

The AAC home showcases a variety of low VOC (volatile organic compound) building materials and a fresh air ventilation system (Table 1). For example, the low emission carpet (100% nylon) carries the Carpet and Rug Institute's Green Seal. The fresh air ventilation system draws outside air into the air handler's return plenum through a dedicated duct. Thus, ventilation air is being introduced from a known source through a designed air flow path. Planned ventilation provides much cleaner air than unplanned infiltration. Fresh air isn't pulled through unintentional cracks in the building envelope where it can pick up small particles of building materials, various gases from combustion appliances or chemicals in building materials. Consequently, building cavities (like walls) aren't exposed to unconditioned air and damaging humidity. Another ventilation feature of the AAC house, the FanRecycler, (Figure 8) circulates indoor air through the duct system by switching the air handler fan on even if the conditioning system isn't operating. This improves indoor air quality by dissipating high concentrations of humidity and providing fresh outdoor air even during hours when neither air conditioning nor heating is called for. During these periods, slow wind speed, lack of cross ventilation, closed interior doors and closed windows (for security) hinder natural ventilation. Closed interior doors can also impede proper conditioning by restricting flow of return air from private rooms. This creates infiltration induced by pressure imbalances subsequently placing greater loads on the conditioning system. To overcome this, through the wall registers above bedroom doors allow free air flow bringing the conditioned space back into pressure balance.

Post Construction Evaluation

After carefully monitoring the construction process, FSEC conducted a standard battery of testes to evaluate several energy and indoor air quality performance indicators. Two measurements, whole house and duct air tightness, are used in the Energy Star rating process. Results from these tests, the final Energy Star ratings, measured natural ventilation rates (SF₆ tracer gas decay method) and concentrations of volatile organic compounds (VOCs) including formaldehyde, are summarized in Table 2.

Table 2. Measured Ait Tightness, Ventilation, and VOC Levels

Diagnostic Test	House #1 - Block	House #2 - AAC	House #3 - Frame
Whole House Air Tightness	7.6	9.4	7.5
Duct System Air Tightness, CFM25total	166	188	148
Duct System Air Tightness, CFM25out	84	128	65
CFM25out as a % of Floor Area	7.10%	10.80%	5.40%
SF6 Ventilation Rate (Natural ACH)	0.13	0.32 Fresh Air Damper Open	0.25
		0.14 Fresh Air Damper Closed
TVOC, mg/cu.m.	3.8	1.7	5
Formaldehyde, ppb	56	30	58
Final Energy Star Rating	86.8	87.1	88.5

Air Conditioning Energy Use

FSEC requested and received permission from the new homeowners to monitor the energy use in all three homes. Since the homes were not occupied immediately, FSEC researchers were able to monitor air conditioning energy use for one month under carefully controlled operation. During this period, the Frame House consumed about 20% less energy than the AAC house and the Block House (Figure 9). This supports the higher rating, or predicted energy performance, of the Frame House with it's important extra energy features. In the AAC house, the energy used by the mechanical ventilation system offset some of the energy savings from the double pane windows and higher R-value wall. Note that if the Frame and AAC houses were compared to a conventional block house with a lower, standard efficiency air conditioner, they would likely have saved 40% and 20% respectively. These figures bear great potential for the entry level housing market.

Monitoring of energy use under occupant controlled conditioned commenced on October 1, 1998 in the Frame and Block Houses and on November 1, 1998 in the AAC House and continued until June of 1999 for a total of ten months of data. The occupant of the Block House and the three occupants of the Frame House were usually away from home during the day. While at least one of the six AAC House occupants was usually home.

During the Winter portion of the occupied monitoring period (Figure 10), the Frame house continued to consume less energy than the Block house, even though the Frame home was kept warmer.

During the Summer portion of the occupied monitoring period (Figure 11), the differing internal heat gain load (6 occupants) results in higher consumption in the AAC house. Note that, compared to the Block house, the frame house continued to consume less energy despite a higher occupancy load.

In summary, the Frame house consumed 19.7% less energy than the AAC house and 20.8% less energy than the Block house during the unoccupied monitoring period of September 1998. During the occupied period of June 1999, the Frame house consumed 30.1% less energy than the AAC house and 22.5% less energy then the Block house.

Economics

The additional cost of the high efficiency air conditioners (20% better than standard efficiency) was about $300. This element has very attractive, highly marketable appeal and payback. Actual costs for the upgrades in the Frame House exceeded $2,000. Maximum possible savings due to these items is estimated to be about $72/year, assuming an electric rate of $0.08/kWh, resulting in a payback period of close to 35 years. Research is needed to develop more cost- effective envelope improvement strategies.

The indoor air quality improvements in the AAC House totaled about $2,000. While the qualitative nature of these improvements makes calculating a payback impossible, medical savings are a possible avenue for recouping this type of investment. Though a larger sample of families would be needed to assess potential savings, the homeowner in the AAC House reports that her son requires much less allergy medication since moving into the house.

Anecdotal evidence suggests this would be a valid avenue for further research and one in tune with home buyer interest. While a survey of 80,000 households by Contracting Business Magazine found that 46.6% of respondents cited energy cost as the first concern when purchasing a conditioning system (ACCA, 1999.) 33.8% cited indoor air quality as the improvement they most wanted. 54.8% of the group said that if they were purchasing a new home, that air [quality] features, such as those in the AAC House, would be purchased.