Designs That Work
Hot-Humid Climate:
Montgomery Profile
PDF Version
Detailed Cross Section
Construction Recommendations
- Foundation: Conditioned crawl
- Above Grade Wall: Wood frame
- Cladding: Vinyl or aluminum siding
- Attic: Unconditioned
- Roof: Standing seam metal roof
Building Science Details
- Conditioned crawlspace – Conditioning of the crawlspace means
that this space must be constructed much like a living space — it must be
supplied by the HVAC system and have a transfer grille to return air back
to the HVAC system located in the living space. The supply air should be
directed horizontally across the crawlspace with good enough “throw” to
provide some mixing, not directed down at the floor. Sizing of the supply
air flow should be based on about 5% of the conditioned crawlspace floor
area (For example: 0.05 cfm/ft2*1600ft2=80 cfm for a
1,600 ft2 conditioned crawlspace). A single 6-inch diameter
supply duct typically suffices. Transfer air should go back to the central
area of the living space above the crawlspace. Two grilles (10 inches by 4
inches) on opposite sides of the crawlspace will usually be sufficient.
The transfer area should be calculated in the same manner as for closed
bedrooms connecting to hallways, using the 3 Pa pressure difference limit.
Some form of mechanical moisture control for the crawlspace is necessary.
We recommend one of the following approaches:
- Controlled ventilation strategy using the intermittent central
fan-integrated supply — it provides both mixing and moisture removal for
the crawlspace as well as the house.
- A stand-alone dehumidifier installed in the crawlspace.
- A continuously-operating crawlspace exhaust fan with make-up air
extracted from the house.
In the assembly the rigid insulation is applied to the interior face of
the exterior foundation walls. Moisture control is important to proper
performance, in particular the vapor barrier ground cover on the floor of
the crawlspace. The vapor barrier must be continuous and sealed to the
perimeter wall and any supporting piers.
Figure 13 |
- Air sealing details at transitions – Air sealing can be
particularly difficult, but no less important, at assembly transitions
such as band joists, and between attached garages and living spaces. These
are discussed below because they have proven to be a consistent challenge
for builders.
- Band joists – Continuity of an exterior air barrier can be
maintained at the band joist with sealed or taped housewrap or rigid
foam insulation. Continuity of an interior air barrier can be maintained
through a combination of cut foam blocks and sealant/caulk, rigid
draftstopping material (wood blocking) and sealant/caulk, or spray foam.
Note that neither cellulose nor fiberglass (batt or blown) can be used
for the air barrier. The air barrier detail on second-story band joists
is important because it is inaccessible (covered by structural/finish
floor and ceiling finish) after construction. The air barrier/thermal
envelope detail is important on ground floor band joists because of the
thermal bridge that can occur at the top of crawlspace foundation walls
(as the result of the air barrier and thermal envelope moving from the
outside to the inside of the building enclosure and termite inspection
zones located at the top of crawlspace foundation walls). Note that
while fiberglass batts fulfill the requirement for protection from
ignition in the open band joists, fiberglass batt material by itself
cannot maintain the air barrier.
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Attached garages – The building enclosure surfaces shared between
conditioned space and an unconditioned garage must have a continuous air
barrier. See Figure 13 for details in terms of using sealants and rigid
insulation to create a continuous air barrier between the attached
garage and living space. Refer to
Air Sealing Details.
- Drying mechanisms – In any climate, vapor control is based on
the relationships among the following: the permeability of wall
components, the type of cladding (reservoir or non-reservoir), the
presence/lack/nature of an air space, and the magnitude/duration of the
vapor drive (based on the relationship between the exterior and interior
moisture content and temperature differences). The type of sheathing and
housewrap used in any wall assembly must be based on an understanding of
these inter-relationships. See “Insulations,
Sheathings, and Vapor Diffusion Retarders” for more information.
This wall assembly permits drying to both the interior and the exterior
(depending on the selection of exterior sheathing – see Material
Compatibility and Substitutions). The roof assembly is designed to dry
only to the interior (without continuous back-venting of the impermeable
metal roof, little if any drying can take place to the exterior).
- Drainage plane, air barrier, and vapor control – This wall
assembly is a screen system—the vinyl siding promotes air movement through
all laps; the drainage plane is the lapped building paper or housewrap
installed over the exterior sheathing. Alternatively, the exterior surface
of the Thermo-ply® sheathing applied “shingle fashion” acts as the
drainage plane. The roofing paper underneath the standing seam metal roof
has two functions. It serves as a slip surface for the roofing for
expansion and contraction of the metal, and it serves as a secondary
drainage plane for moisture that might condense on the underside of the
metal roofing as the result of night-sky radiation and subsequent
condensation. Note that the OSB roof deck has the capacity to act as a
hygric buffer for interior moisture.
- Thermal barrier – In this climate, moisture control does not
require specific levels of insulation. Inside/outside temperature
differences do not require any cavity-warming exterior rigid insulation to
control wintertime condensing surface temperatures. Having said this,
insulating sheathing in general is a good idea. We recommend full cavity
fill in the walls, but the 2x6 framing is more about advanced framing than
the depth of cavity insulation that can be achieved. The R-22 cellulose or
R-30 batts in the conditioned attic has proven to be adequate to protect
HVAC equipment and duct performance when they are located in the attic.
Note that the cellulose netting or fiberglass batt supports create the
insulation “belly” and accommodate cavity fill depth that exceeds the
depth of the truss top chord.
- Window flashing – Window flashing details are wall assembly or
cladding specific. See Figures 14a and 14b and refer to the EEBA Water
Management Guide at the EEBA
Bookstore.
Figure 14a |
Figure 14b |
- Advanced framing – An important element of high performance
wood-frame construction is an advanced framing package, in this case
including cross bracing for shear resistance. For more detailed
information on advanced framing techniques, see
Advanced Framing.
- Framing on slabs – Installing a capillary break between the
sill plate and a concrete slab on all walls—exterior, interior,
partition—is good practice. A closed cell foam sill sealer or gasket works
well. Alternatively, a strip of sheet polyethylene can be used. This
isolates the framing from any source of moisture that may be either in or
on the concrete slab (and using sill sealer on all walls maintains wall
height exactly the same).
- Soil gas ventilation – The crawlspace to roof vent system
handles conditions that are difficult if not impossible to assess prior to
completion of the structure—resultant confined concentrations of air-borne
radon, soil treatments (termiticides, pesticides) methane, etc. The cost
of this "ounce" of prevention is well balanced against the cost of the
"pound" of cure. A conditioned crawlspace only heightens the importance of
soil gas ventilation. Note that this system is a passive system, but it
can easily be converted to an active ventilation system by installing an
in-line fan into the stack in the attic.
- Sub-slab stone bed - The four-inch deep 3/4" stone bed
functions as a granular capillary break, a drainage pad, and a sub-slab
air pressure field extender for the soil gas ventilation system. The
sub-slab stone bed is a practical method for venting soil gas should it be
necessary.
- Crawlspace access – The preferred location for crawlspace
access is through the subfloor; any access through the perimeter wall must
be airsealed and insulated.
Climate Specific Details
- Termite management - In hot-humid climates, termites are best
managed with a three-pronged approach that deals with the three things
termites need - cover from sunlight, moisture, and food (wood or paper):
- Reduced cover - keep plantings 3 feet away from the building
perimeter, thin the ground cover (wood mulch or pea stone) to no more
than two inches depth for the first 18 inches around the building, and
maintain the termite inspection zone on the exterior of the foundation
above grade.
- Control moisture - maintain slope away from building as
shown, carry roof load of water at least three feet away from building,
and make sure that irrigation is directed away from the building.
- Chemical treatment - use an environmentally-appropriate soil
treatment (such as Termidor®) and a building materials treatment (such
as Bora-Care®) for termite-prone near-grade wood materials.
- Inter-relationship of first three points – since a builder
and a homeowner’s ability to employ or stick to each of the three
strategies above will vary, make sure that an inability to fully employ
one strategy is compensated for by complete rigor by the others. For
example, if for some reason, chemical treatment of soil or building
materials is not an option, then complete rigor in controlling moisture
and ground cover must be maintained.
- Mechanical systems - The key elements of a system for this
climate are:
- Sealed combustion gas furnace - for energy efficiency and
health/safety with the unit inside conditioned space.
- Minimum 12 SEER AC unit - for energy efficient management of
sensible load.
- Central-fan-integrated supply ventilation - this system is
simple, effective, and economical. It provides fresh, filtered, outside
air in a controlled amount using the existing HVAC delivery system for
even distribution and mixing. Set-up intermittent central-fan-integrated
supply, designed to ASHRAE 62.2P rate, with fan cycling control set to
operate the central air handler as much as 33% of the time, but not less
than 25% of the time, occurring within at least every three hours to
provide ventilation air distribution and whole-house averaging of air
quality and comfort conditions ($125 to $150). Include a normally closed
motorized damper in the outside air duct with the AirCycler™ FRV control
(+$50 to $60). See “Air
Distribution Fan And Outside Air Damper Recycling Control" for more
detailed information.
- Supplemental dehumidification - all homes in this climate
call for supplemental dehumidification; the reduced sensible load of
high performance homes reduces the dehumidification the AC unit
provides, extends shoulder seasons, and raises the impact of
occupant-generated moisture. There are a number of different ways to
accomplish supplemental dehumidification with varying costs and
performance advantages (For a detailed discussion of
supplemental dehumidification see
Conditioning
Air in the Humid South.). Described below is one low-cost yet
effective approach and one more costly but higher
performance/systems-engineered approach:
1. Ducted stand-alone dehumidifier: This
system is a "site-constructed" and consists of an off-the-shelf standard
dehumidifier ducted in the attic and controlled by a dehumidistat
located in the living space. This arrangement of individual components
has proven to be an effective and economical system for the production
home building setting. The installed cost ranges from approximately $350
to $550. The system is comprised of any Energy Star dehumidifier that
uses a blower wheel instead of a paddle fan to move air past the coil
(dehumidifier located in attic in an insulated enclosure and ducted to
living space), Honeywell dehumidistat model H8808C located in living
space and Honeywell switching relay (with transformer) model RA89A 1074.
See Figure 15d.
Note: The following manufacturers make Energy Star-qualified
blower wheel stand-alone dehumidifiers:
2. Aprilaire 1700: This is a truly engineered,
coherently manufactured, supplemental dehumidification system with
built-in air filtration, ducted design, and a controls package that
integrates central blower cycling for distribution, dehumidification and
intermittent introduction of outside air ventilation. The system is also
designed for flexibility-it can be connected to the conditioned space
directly or to the central air distribution system in a number of
configurations. It's also compact and lightweight enough to be set on or
hung from most framing. The installed cost for this system is currently
about $950 to $1,050. For more information, see:
http://www.aprilaire.com/category.asp?id=F63D255EB0054BBF811DBB024BF068FA.
For more information on other high performance supplemental
dehumidification systems, see:
http://www.thermastor.com/DesktopDefault.aspx.
- Ducts in conditioned space - The preferred method for keeping
HVAC ducts and mechanical equipment inside conditioned space is keeping
them in the living space. Moving the conditioned boundary down so that
the crawlspace is conditioned works as well. The crawlspace must be
conditioned (with the air barrier and thermal barrier following the
foundation) so that the crawlspace does not become a potential (large)
moisture trap. In no case should HVAC ducts be placed within exterior
wall assemblies-this is not part of what is meant by ducts in
conditioned space. See Figures 15a-c.
Alternatively, moving the conditioned boundary up to the underside of
the roof sheathing so that the attic is also conditioned works as shown
below.
A vented attic assembly may be used in this climate as long as the
ceiling plane is air tight and no ductwork or air handling equipment is
located in the attic.
Figures 15a, 15b, 15c
Figure 15d
- Transfer grilles and jump ducts – Single air returns require
transfer grilles to provide return pathways that prevent pressurization
of bedrooms. For appropriate sizing for ducts, including these pressure
relief methods, see
Cooling Sizing and see Figures 16a-d.
Figures 16a, 16b
16c, & 16d
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- Water heater - any type of gas water heater (in terms of
venting) works if the water heater is located in the garage. If the
water heater is located inside conditioned space, then it must be a gas
power vented or power-direct vented unit, or an electric water heater.
Field Experience Notes
- Air sealing – Unvented assemblies—walls or roofs—are robust
when the air sealing is robust. The hardest spots are not the "fields" but
the "margins" of assemblies. Spray foam applied at the margins
(truss/rafter end blocking) may seem like an expensive element of the
assembly, but the labor savings and air sealing quality in comparison to
the alternatives are clear.
- Roofing - Light-colored standing seam metal roofs yield the
best cooling load reduction of any roof claddings, important in this
climate. We recommend a metal roof that meets the
EPA/DOE Energy Star qualifications.
- Flashing details – Since the vinyl siding is a screened wall
cladding system, flashing details should be accomplished in the plane of
the drainage plane (building paper or house wrap), not the cladding. Do
not caulk siding and do not rely on "J"-channel as part of the drainage
plane. (In other words, never consider vinyl siding, aluminum siding or
any siding, for that matter--as the weather barrier).
- Termite inspection gap - Our experience has shown that building
inspectors will accept a 1-to-2" rather than a 6" termite inspection gap.
This reduces the extent of uninsulated concrete and therefore reduces the
associated heat loss.
Figure 17 |
-
Advanced framing – For a technical resource that may help with
resistance to advanced framing methods from local code officials, see the
Building Safety Journal article written by Peter Yost of BSC.
- Energy trusses – There are a number of different truss
configurations that yield greater depth at the heel, but they vary quite a
bit in cost. The truss shown in Figure 17 (sometimes called a “slider”
truss) has proven to be among the most cost-competitive. And of course,
the pitch of the roof affects just how much insulation you can get at this
location, regardless of the type of truss.
- Conditioned crawlspaces – This assembly may require some local
building code official "building science persuasion." See the
Building Safety Journal article written by Nathan Yost of BSC.
- HVAC commissioning – The most efficient equipment means little
if the system is not set up and started up properly. Follow high
performance start-up procedures such as the following:
Air Conditioning Equipment Efficiency (Startup Procedures).
Material compatibility and substitutions
- Exterior sheathing/building “paper” – A variety of building
papers or housewraps installed shingle fashion can act as the drainage
plane in this assembly. While drying to the exterior is not the primary
mechanism in this climate, the combinations of building papers and
exterior sheathings in this assembly (and their relative permeabilities)
promote intermittent drying in this direction. Note that OSB is more vapor
permeable than XPS insulation, which is more vapor permeable then
Thermo-Ply® (see the Building Material
Properties Table).
- Cavity insulation - Any type of cavity insulation would be
acceptable in this application—spray foam (open cell and closed cell),
cellulose, fiberglass, (as long as air sealing is accomplished by a
separate component or system when cellulose or fiberglass is used). The
only exception is with high permeability sheathings (such as fiberboard
and gypsum), where a vapor permeable cavity insulation should be used.
Since this wall assembly is designed to dry exclusively to the interior,
do not use any layers at the interior surface that have a low vapor
permeability (polyethylene or vinyl wall covering). Note that when foam
insulation is left exposed in an assembly, a "thermal barrier" or
"protection against ignition" may be required. Code
implementation/interpretation have proven to be particularly troublesome
for "gray" areas, such as spaces that are conditioned but not occupied
(conditioned attics and crawlspaces).
- Cast concrete foundation walls – If block is used instead of
cast concrete for foundation walls, the bond beam becomes essential to
maintain air barrier continuity at the top of the block wall.
- Crawlspace floor – Ideally, the crawlspace floor would be a
four-inch gravel (free-draining, no fines) bed, polyethylene sheeting
layer and "rat" (2-inch low strength cast concrete) slab, making this
space more amenable to light storage and housing of HVAC equipment. The
cost of this approach may outweigh the benefits for builders and buyers.
If a concrete slab is cast, it should be placed directly on top of the
vapor barrier.
- Wall sheathing – Moisture drive in a hot-humid climate is
predominantly from the outside in. Select the wall sheathing and building
paper combination to maintain functions of drainage plane and shear
resistance and relatively low vapor permeability. Consult the Building Material
Properties Table.
- Latex paint – The substitution of low permeability finishes
(vinyl wall paper, oil-based paints) for latex paint is strongly
discouraged because of reduced drying potential. The substitution of low
permeability finishes (vinyl wall paper, oil-based paints) for latex paint
is strongly discouraged because of reduced drying potential to the
interior.
- Gypsum wallboard – Areas of potentially high moisture, such as
bathrooms, laundry rooms, kitchens, are excellent candidates for non-paper
faced wallboard systems (e.g. James Hardie’s Hardibacker®, GP’s DensArmor®,
USG’s Fiberock®). In addition, paper-faced gypsum board should never be
used as interior sheathing or backer for tub or shower surrounds where
ceramic tile or marble (any material with joints or grout lines) is used
as the finish.
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