Designs That Work
Mixed-Humid Climate
Best Practices
High performance Building America homes in mixed-humid climates are
faced with a significant heating season (up to 4500 heating degree days
and monthly average outdoor temperatures dropping below 45°F during winter
months) as well as a cooling season with significant latent load. 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. Correct sizing of
air conditioning systems is important in the control of interior humidity. Oversizing air conditioning systems in this climate typically leads to
moisture problems.
The following Best Practices are based on our
Building America Performance
Targets and are reflected in the three Mixed-Humid building profiles,
the "Charlotte," the "Atlanta,"
and the "Louisville." All
climate-specific Best Practices are identified with a bolded and bracketed
[MH].
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
- 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 percent 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 Formis
available for download as a convenient way to record the testing
information.
Instructionsfor
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 a hobby room, etc) should be considered a separate model type.
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) [MH]
- 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). [MH]
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). [MH]
- Slabs require a 6-ml polyethylene vapor barrier directly beneath the
concrete or an equivalent approach (such as rigid insulation) that
accomplishes vapor control for the slab. The vapor retarder must
continuously wrap the slab as well as the grade beam.
- Sub-slab drainage shall consist of a granular capillary break
directly beneath the slab vapor retarder.
- 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 shall be insulated
at the perimeter (energy modeling reveals significant heat loss at this
portion of the building enclosure during the heating season). Consider the
use of borate-treated rigid insulation on the slab perimeter (see
Building Profiles and
photo detail).
[MH]
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 Profiles
and the EEBA Water Management Guide for specific details for
various wall assemblies. [MH]
- Vapor management - Roof and wall assemblies must contain elements
that, individually and in combination, permit drying of interstitial
spaces. See the Building Profiles.
[MH]
- 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
Air Sealing / Air Drywall Approach Details.
- 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. [MH]
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
Manual D
- Mechanical ventilation should be an integral part of the HVAC system
design; see the Building Profiles
and
Advanced Space Conditioning. [MH]
- Energy Performance
- 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 6 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.
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