Designs That Work
Hot-Dry / Mixed Dry Climate
Best Practices
High performance Building America homes in
hot-dry/mixed-dry climates are faced with a significant heating season (up
to 4500 heating degree days and monthly average outdoor temperatures
dropping below 45ºF during winter months) as well as a significant cooling
season.
The following Best Practices are based on our
Building America Performance
Targets and are reflected in the three building profiles, the “Albuquerque,”
the “Tucson,” and the “Sacramento.” All climate-specific Best
Practices are identified with a bolded and bracketed [HD-MD].
1. Process – Building Design, Systems Engineering, and
Commissioning
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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).
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Systems Engineering
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Commissioning – Performance Testing
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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.
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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.
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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).
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Mechanical ventilation system airflow should be tested
during commissioning of the building.
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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 a hobby room, etc) should be
considered a separate model type.
2. Site – Drainage, Pest Control, and Landscaping
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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.
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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) [HD-MD]
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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). [HD-MD]
3. Foundation – Moisture Control and Energy Performance
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Moisture Control - The building foundation shall be
designed and constructed to prevent the entry of moisture and other soil
gases (see Building Profiles). [HD-MD]
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Slabs require a 6-ml polyethylene vapor barrier directly
beneath the concrete or an equivalent approach (such as rigid
insulation) that accomplishes vapor and capillary control for the slab.
The vapor barrier must continuously wrap the slab as well as the grade
beam.
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Sub-slab drainage shall consist of a granular capillary
break directly beneath the slab vapor retarder.
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Use radon resistant construction practices as referenced
in the ASTM Standard "Radon Resistant Design and Construction of New Low
Rise Residential Buildings.
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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 in
mixed-dry climates). Consider the use of borate-treated rigid insulation
on the slab perimeter (see Building Profiles and
photo detail).
[HD-MD]
4. Envelope - Moisture Control and Energy Performance
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Moisture Control
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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 Profilesand the
EEBA
Water Management Guide for specific details for various wall assemblies.
[HD-MD]
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Vapor management – Roof and wall assemblies must contain
elements that, individually and in combination, permit drying of
interstitial spaces. See the Building Profiles. [HD-MD]
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Energy Performance
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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.
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Windows - Recommend U-factor 0.35 or lower and SHGC
(solar heat gain coefficient) 0.35 or lower. [HD-MD]
5. Mechanicals/Electrical/Plumbing – Systems Engineering,
Energy Performance, Occupant Health and Safety, and Envelope/Mechanicals
Management
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Systems Engineering
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HVAC system design, both equipment and duct, should be
done as an integral part of the architectural design process.
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HVAC system sizing should follow ACCA Manual J and duct
sizing should follow
Manual D
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Mechanical ventilation should be an integral part of the
HVAC system design; see the Building Profiles and
Advanced Space Conditioning.
[HD-MD]
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Energy Performance
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Occupant health and safety
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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.
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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.
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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.
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Indoor humidity should be maintained in the range of 25
to 60% by controlled mechanical ventilation, mechanical cooling, or
dehumidification. See
Relative
Humidity.
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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.
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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.
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Envelope/Mechanicals Management
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Plumbing - No plumbing in exterior walls. Air seal
around plumbing penetrations in pressure boundary (air barrier) such as
rim (band) joist or ceiling.
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Electrical - Seal around wires penetrating air barrier
or pressure boundary.
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