December 8, 2007
Hot-Humid

Abstract: 

As part of the U.S. Department of Energy’s Building America Program, a partnership was developed with a builder who had learned from experience that just building to code left a lot to be desired from an overall performance perspective. He came to believe that real value lies in implementing building science principals within a systems engineering approach to high-performance housing. A high-performance home will also reduce a builder’s risk of warranty/service call-back and customer complaint, leaving more room for profitability. The whole-house performance approach described here builds a framework of understanding that starts with principals that lead to evaluation of options, that leads to a coherent plan, that leads to quality execution of producing high-performance homes.

1. Introduction

Primarily as a resort location, Hilton Head Island has a somewhat different residential housing market than other more traditional areas. The market is made up of both year-round occupancy and seasonal occupancy, and the size and cost of new homes range from the very large and expensive to those of more normal size but still higher cost. Most houses are simply "built to code." High-performance and "green" housing practices have not begun to penetrate the market.

Figure 1: Entrance to community from the garage area; all hard surfaces are pervious concrete

As part of the U.S. Department of Energy’s Building America Program, a partnership was developed with a builder who had learned from experience that just building to code left a lot to be desired from an overall performance perspective. He came to believe that real value lies in implementing building science principals within a systems engineering approach to high-performance housing.

The whole-house performance approach described here builds a framework of understanding that starts with principals that lead to evaluation of options, that leads to a coherent plan, that leads to quality execution of producing high-performance homes. High-performance homes are comfortable, healthy, safe, durable, energy efficient, and respecting of the environment. A high-performance home will also reduce a builder’s risk of warranty/service call-back and customer complaint, leaving more room for profitability.

2. Systems principals, options evaluation, and as-built characteristics

A logical construction sequence approach is used here to describe the systems principals and to evaluate reasonable construction options. As one progresses through the construction sequence you can see the value of looking both forward and behind to see the linkages in the systems engineering approach.

2.1 Sitework

Historically, Hilton Head Island receives about 50 inches of rain per year. The site is a four-acre parcel with nearly two acres of freshwater wetlands that will remain undeveloped. In addition to site drainage requirement required by code, pervious pavement was used throughout the project for the driveway and parking spaces near the detached garages, and for all sidewalks (Ruiz 2007). Low maintenance grasses and shrubby were purposefully designed into the landscape. The site plan is shown in Figure 2.

Figure 2: Project site plan

Mature trees around the homes were left in place as much as possible to help shade the homes in summer, keeping it cooler and more comfortable indoors and out. A line of trees buffers the north and west sides of the property, providing a windscreen. Topsoil was moved to a safe location on the site, covered to prevent erosion, and then re-used in the project. These measures minimize the amount of excavation and trucking associated with importing new soil.

2.2 Foundation

A slab-on-grade foundation is the most cost effective and trouble-free foundation in most parts of the U.S. where the frost depth is above about three feet. Sealed crawlspaces with insulated walls and a small amount of conditioned supply air could have been used if the grade sloped, or if the first floor needed to be elevated, but that was not the case. A 4-inch monolithic slab with turned-down edges was poured over a 6-mil polyethylene vapor barrier over compacted sand/stone. Soil under the foundation was treated for termites. Slab-edge insulation was not used due to the long-term risk of termite infestation through or behind the insulation.

2.3 Exterior Walls

Detailing of exterior walls can be the most complex of the building enclosure elements. The negative effects of water intrusion, air infiltration, thermal conduction, water vapor diffusion, and solar radiation must all be considered along with the realities of many wall penetrations, architectural detailing, wind and weight loading requirements, security requirements, component attachment, interior and exterior finishing aesthetics, etc. Having a high priority for comfort at low cost in hot-humid climates requires, in order of priority for walls:

  • glazing with low solar heat gain;
  • air sealing;
  • opaque areas with moderately high thermal insulation; and
  • glazing with thermal resistance at least high enough to avoid wintertime condensation.

Having a high priority for durability in hot-humid climates requires (Lstiburek 2005):

  • a continuous water drainage layer behind the cladding, integrated with window, door, roof, and other penetration flashings, to protect water sensitive materials located deeper in the assembly;
  • a capillary suction break between foundation materials in soil contact and walls above;
  • water vapor diffusion resistance between water absorptive claddings and wall sheathing to retard moisture movement driven by solar heat; and
  • interior finish materials that do not retard water vapor movement to allow drying to the inside air.

To accomplish all of these goals, and having started with an initial builder criteria of conventional wood stud framing, the wall system chosen was as follows:

  • 2x6 16” o.c. wood studs filled with open cell spray foam insulation in the cavity;
  • Insulated headers were used in conjunction with other advanced framing techniques at corners and partition intersections to reduce unnecessary wood and increase insulation;
  • Foam gasket and capillary break between the pressure treated bottom plate and the foundation;
  • Gypsum wallboard and latex paint interior finish;
  • OSB sheathing covered by corrugated house wrap to increase drainage, covered by 1/2” XPS foam sheathing to retard solar driven water vapor, covered by fiber cement siding and paint;
  • Windows with NFRC rated solar heat gain coefficient of 0.33 and U-value of 0.32;
    and
  • Butyl flashing membranes integrated with the house wrap drainage layer for all windows, doors, roof-to-wall flashings (including kick-out flashings, see Figure 3), and through-wall penetrations for plumbing, venting, and wiring.

Figure 3: Kick-out flashing at roof-wall intersection

2.4 Roof

Primarily as a strategy to lower costs, traditional attics leave a lot of potentially useful sheltered space unused. Prefabricated trusses have reduced construction cost and construction time in trade for more sprawling plans to make up for the loss of living space. In many high-performance homes, cathedralized attics make better use of that space than traditional attics by moving the insulation directly under the roof plane to enclose all air distribution system components within the thermal and air pressure boundary of the building enclosure. Illustrated in Figure 4, that construction method is specifically allowed by the International Residential Code (section R806.4), and the Florida Building Code.

Figure 4: Cathedralized attic enclosing the central space conditioning air distribution system within thermal and air pressure boudnary of the building enclosure

Going further than cathedralized attics, cathedral ceilings generally make the most use of sheltered space by finishing the underside of the roof framing to convert all of what would have been attic space to conditioned living space. A 1” to 2” soffit-to-ridge vent space may or may not be left under the roof sheathing, but in either case, application of open cell or closed cell spray foam insulation is the best way to insulate and air seal the roof/ceiling assembly.

All of the houses in this project had either cathedralized attics or cathedral ceilings, and all were insulated to R-30 with open cell spray foam. Gypsum board and latex paint provided the interior finish. Wood sheathing, roofing paper underlayment, copper flashings, and 40 year fiberglass/asphalt shingles provided the water and structural protection.

2.5 Central Forced Air Heating and Cooling

Proper comfort conditioning, filtration, and distribution of indoor air are critical to the success of high-performance homes. These systems should be designed and properly sized, but in order for that to be successful, the builder’s commitment to the following
criteria items must be in place:

  • Building enclosure leakage not more than 0.25 cfm50 per ft2 of enclosure surface area
  • Duct leakage to outside not more than 5% of the high speed flow rate, and total duct leakage not more than 10% of the high speed flow rate
  • Provision for return air transfer from rooms with doors to assure less than 3 Pascal pressure difference between rooms and the common area
  • High-performance glazing with U-value and SHGC in the range of 0.35
  • Uniform and properly installed insulation

By a commitment to quality control practices backed up by testing, the builder should provide assurance of these things to the HVAC contractor in order to expect the contractor to have confidence that right-sizing the equipment will reduce his risk of comfort complaints.

Right-sizing the equipment starts with using software adhering to ACCA Manual J version 8 to calculate system loads and room air flow requirements for cooling and heating systems. There are many inputs to such software, and different ways to somewhat subjectively inflate the system size. Good information is given in the Manual J text, but here are some of the most important factors to get right for the highperformance homes discussed here:

  • All ducts and air handlers should be input as located within the conditioned space (while the actual location may be directly or indirectly conditioned, the important factor is that the entire air distribution system is within the thermal and air pressure boundary of the enclosure). The resulting duct heat gain/loss load reported by the program should be zero.
  • Infiltration should be set at 0.1 ach for both winter and summer. Mechanical ventilation should be set at 65 cfm.
  • The glazing U-value and SHGC should be set at the values shown on the NRFC rating label attached to the glazing (U=0.32, SHGC=0.33). A “custom window” should be created to do this. For most glazing, the interior shading should be set for drapes-medium, 50% drawn, no insect or external shade screens, ground reflectance equal to 0.20.
  • Outdoor design conditions should not exceed the ASHRAE 0.4% design for cooling, which for Savanah, GA is 95.4 F dry bulb and 77.1 F wet bulb. Indoor conditions should be set at 75 F dry bulb and 63 F wet bulb (50% RH).
  • For production built communities, the building design load for each plan should be calculated for the orientation that creates the highest system load, but the individual room duct sizing should be done based on the average flow of all four cardinal orientations. For rooms with more than 18% glass to floor area ratio, the duct size can be increased to the maximum of all four orientations.
  • Equipment selection should be based on indoor and outdoor sections that are matched in capacity and listed in the ARI directory. The equipment will be selected based on the manufacturers extended performance ratings to meet the design sensible load at the actual (not nominal) outdoor and indoor design conditions. Use the ACCA Manual S provision that allows one-half of the unused latent capacity to count as sensible capacity. If the total load (sensible+latent) exceeds the total capacity of the system by more than 900 Btu/h then go to the next bigger size, otherwise, stay at the smaller size. A thermal expansion valve refrigerant metering device should be used and the refrigerant charge checked using the sub-cooling method.

The air distribution system needs to be designed as well, to make sure that the right amount of air gets to where it is supposed to go to meet the space conditioning loads and provide comfort. Use of supply registers that allow adjustment of the airflow rate and airflow direction is also important for final balancing and room air circulation. In high-performance homes, the duct systems can be more compact because the air flow requirements are lower, and there is no need to have extended duct runs to “wash” outside walls and windows with cool or warm air. A more compact duct system makes it easier to locate the ducts inside conditioned space (in floor systems, soffits/chases/furdowns, and inside walls). Air handlers were centrally located within the conditioned building envelope to minimize duct lengths. Additional important practical principles to apply are listed here:

  • Supply air run-outs should be sized for the room airflow requirement at 500 ft/min. Supply air trunks or plenums should be sized for not more than 750 ft/min. Return air ducts should be sized at not more than 350 ft/min, return grilles should be sized for no more than 300 ft/min.
  • Total external static pressure (defined as the pressure differential between the return side ‘and the supply side of the air handler cabinet) should not exceed the manufacturer’s specification, usually 125 Pa (0.5 inch water column).
  • Ducts feeding supply registers in bedrooms should not be larger than 6” diameter (100 cfm maximum) to avoid blowing too much air too fast on sedentary people. Reasonable care should be taken to avoid blowing air directly on beds. Most master bedrooms will require at least two supply registers rather than one large supply register.
  • Provision for return air transfer from closable rooms is required (jump ducts or transfer grilles). Pressurization or depressurization of rooms or the common area should not exceed 3 Pa.

These design guidelines were followed for each of the five house plans used in the project. An example system and duct sizing output for one of the plans is shown in Figure 5. All of the houses were fitted with either 1.5-ton or 2-ton (nominal) air source heat pumps with variable speed indoor fan and thermal expansion valve. Published efficiencies were 15.1 SEER and 8.05 HSPF as shown in Figure 6.

Figure 5: System and duct sizing output for the Maggie plan

Figure 6: ARI Directory listing for the matched equipment

2.6 Supplemental Whole-house Dehumidification

High efficiency houses in warm-humid climates have low sensible heat gain. Low sensible heat gain is good for reducing cooling costs, but contributes to part load moisture control challenges. . .

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