Designs That Work

Hot Humid Climate

Best Practices

The primary consideration for high performance Building America homes in hot-humid climates is maintaining moisture control both inside the home and within building assemblies, particularly as energy conservation shifts the relationship between sensible and latent loads. Reducing solar gain, using energy conserving appliances and compact fluorescent lighting, reduces the sensible load. This affects the ability of the air conditioner to remove moisture or dehumidify the air.

The following Best Practices are based on our Building America Performance Targets and are reflected in the three Hot-Humid building profiles, the "Houston," the "Maitland," the "Orlando," and the "Montgomery." All climate-specific Best Practices are identified with a bolded and bracketed [HH].

1. Process - Building Design, Systems Engineering, and Commissioning

• Design for Energy Performance - Energy performance 40% better than the 1995 Model Energy Code base case house (i.e. equal to 10% better than Energy Star performance requirements).
• Systems Engineering
  • Design structure using advanced framing methods (see Advanced Framing Details or the Advanced Framing Presentation from EEBA Conference 2001).
  • Design structure to accommodate the most efficient duct distribution system that places all ducts and air handling equipment within conditioned space (see Using Wood Efficiently; specifically Figure 3 and page 5).
  • Design and detail structure for durability, in terms of wall and roof assembly drying potential, continuous drainage plane, and continuous thermal barrier that clearly defines the conditioned space (see Building Profiles). [HH]
     
• Commissioning - Performance Testing
  • Air leakage (determined by blower door depressurization testing) should be less than 2.5 square inches/100 square feet surface area leakage ratio (CGSB, calculated at a 10 Pa pressure differential); or 1.25 square inches/100 square feet leakage ratio (ASTM, calculated at a 4 Pa pressure differential); or 0.25 CFM/square foot of building enclosure surface area at a 50 Pa air pressure differential. If the house is divided into multiple conditioned zones, such as a conditioned attic or conditioned crawl space, the blower door requirement must be met with the access to the space open, connecting the zones.
  • Ductwork leakage to the exterior for ducts distributing conditioned air should be limited to 5.0% of the total air handling system rated air flow at high speed (nominal 400 CFM per ton) determined by pressurization testing at 25 Pa. Two acceptable compliance mechanisms are (1) test duct leakage to outside at finish stage, or (2) test total duct leakage at duct rough-in stage.
  • Forced air systems that distribute air for heating and cooling should be designed to supply airflow to all conditioned spaces and zones (bedrooms, hallways, basements) as well as to provide a return path from all conditioned spaces or zones. Interzonal air pressure differences, when doors are closed, should be limited to 3 Pa. This is typically achieved by installing properly sized transfer grilles or jump ducts (see Transfer Grille Detail and the Transfer Grille Sizing Table).
  • Mechanical ventilation system airflow should be tested during commissioning of the building.
  • Testing of the house should be completed as part of the commissioning process. The SNAPSHOT Form is available for download as a convenient way to record the testing information. Instructions for completing the form are also available. Unique or custom house plans should each be tested. In a production setting, each model type (i.e., floor plan) should be tested until two consecutive houses of this model type meet testing requirements. At this point, testing on this model type can be reduced to a sampling rate of 1 in 7 (i.e., 1 test, with 6 "referenced" houses). Small additions to a floor plan (e.g., bay window, conversion of den to bedroom) should be considered the same model type; major changes (e.g., bonus room over the garage, conversion of garage into casita or hobby room, etc) should be considered a separate model type.

2. Site - Drainage, Pest Control, and Landscaping

• Drainage - Grading and landscaping shall be planned for movement of building run-off away from the home and its foundation, with roof drainage directed at least 3 feet beyond the building, and a surface grade of at least 5% maintained for at least 10 feet around and away from the entire structure.
• Pest Control - Based on local code and Termite Infestation Probability (TIP) maps, use environmentally-appropriate termite treatments, bait systems, and treated building materials that are near or have ground contact (See http://www.uky.edu/Agriculture/Entomology/entfacts.htm). [HH]
• Landscaping - Plantings should be held back as much as 3 feet and no less than 18 inches from the finished structure, with any supporting irrigation directed away from the finished structure. Decorative ground cover - mulch or pea stone, for example - should be thinned to no more than 2 inches for the first 18 inches from the finished structure (See Building Profiles). [HH]

3. Foundation - Moisture Control and Energy Performance

• Moisture Control - The building foundation shall be designed and constructed to prevent the entry of moisture and other soil gases (See Building Profiles). [HH]
  • Slabs require a 6 ml polyethylene sheeting directly beneath the concrete or an equivalent approach (such as rigid insulation) that accomplishes vapor control and capillary control for the slab. The vapor barrier must continuously wrap the slab as well as the grade beam.
  • Sub-slab drainage where necessary shall consist of a granular capillary break directly beneath the slab vapor barrier.
  • Use radon resistant construction practices as referenced in the ASTM Standard "Radon Resistant Design and Construction of New Low Rise Residential Buildings".
• Energy Performance - Slabs in this climate are generally not insulated, even at the perimeter, because of the low overall heating load (less significant delta-T overall) and code restrictions on below-grade use of rigid foam insulation. Local codes and economics permitting, consider the use of borate-treated rigid insulation on the slab perimeter (see the Building Profiles and photo detail). [HH]

4. Envelope - Moisture Control and Energy Performance

• Moisture Control
  • Water management - Roof and wall assemblies must contain elements that provide drainage in a continuous manner over the entire surface area of the building enclosure, including lapped flashing systems at all penetrations. See the Building Profilesor the EEBA Water Management Guide for specific details for various wall assemblies. [HH]
  • Vapor management - Roof and wall assemblies must contain elements that, individually and in combination, permit drying of interstitial spaces. See Building Profiles. [HH]
• Energy Performance
  • Air leakage - Exterior air flow retarder - foam sheathing; interior air flow retarder - gypsum board is sealed to the slab and frame walls (Airtight Drywall Approach). For details of the ADA for interior air flow retarder, see the Building Profiles. [HH]
  • Windows - Recommend one of two approaches:
    • U-factor 0.35 or lower and SHGC (solar heat gain coefficient) 0.35 or less, regardless of climate.
    • Climate-specific glazing properties if passive solar orientation and design can be employed by the builder and occupants employ proper window treatments and their use. [HH]

5. Mechanicals / Electrical / Plumbing

• Systems Engineering
  • HVAC system design, both equipment and duct, should be done as an integral part of the architectural design process.
  • HVAC system sizing should follow ACCA Manual J and duct sizing should follow ACCA Manual D.
  • Mechanical ventilation should be an integral part of the HVAC system design; see the Building Profiles or Advanced Space Conditioning.
  • A whole house dehumidification system integral to the air handling system is recommended for this climate (see the Building Profiles and Air Conditioning. [HH]
• Energy Performance
  • Air conditioner or heat pump shall be installed according to best practices; see Air Conditioning Equipment Efficiency.
  • Major appliances meet high-energy efficiency standards using current appliance ratings. Only those appliances in the top one-third of the DOE Energy Guide rating scale should be selected.
• Occupant health and safety
  • Base rate ventilation: controlled mechanical ventilation at a minimum base rate of 15 CFM per master bedroom and 7.5 CFM for each additional bedroom should be provided when the building is occupied.
  • Spot ventilation: intermittent spot ventilation of 100 CFM should be provided for each kitchen; all kitchen range hoods must be vented to the outside (no recirculating hoods). Intermittent spot ventilation of 50 CFM, or continuous ventilation of 20 CFM when the building is occupied, should be provided for each washroom/bathroom.
  • All combustion appliances in the conditioned space must be sealed combustion or power vented. Specifically, any furnace inside conditioned space should be a sealed-combustion 90%+ unit. Any water heater inside conditioned space should be power vented or power-direct vented. Designs that incorporate passive combustion air supply openings or outdoor supply air ducts not directly connected to the appliance should be avoided.
  • Provide filtration systems for forced air systems that provide a minimum atmospheric dust spot efficiency of 30% or MERV of 10 or higher.
  • Indoor humidity should be maintained in the range of 25 to 60% by controlled mechanical ventilation, mechanical cooling, or dehumidification. See Relative Humidity.
  • Carbon monoxide detectors (hard-wired units) shall be installed (at one per every approximate 1000 square feet) in any house containing combustion appliances and/or an attached garage.
  • Information relating to the safe, healthy, comfortable operation and maintenance of the building and systems that provide control over space conditioning, hot water or lighting energy use shall be provided to occupants.
• Envelope/Mechanicals Management
  • Plumbing - No plumbing in exterior walls. Air seal around plumbing penetrations in pressure boundary (air barrier) such as rim (band) joist or ceiling.
  • Electrical - Seal around wires penetrating air barrier or pressure boundary.