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Enclosures That Work

Building Profile: Hot-Humid Climate: Montgomery

By Building Science Corporation
Created: 2009/04/27

  • Foundation: Conditioned crawl
  • Above Grade Wall: Wood frame
  • Cladding: Vinyl or aluminum siding
  • Attic: Unconditioned
  • Roof: Standing seam metal roof

Enclosure Design

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:

    1. 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.
    2. A stand-alone dehumidifier installed in the crawlspace.
    3. 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.

  • 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.
    • 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.

 

Figure 13

  • 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 Water Management Guide.

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:
      • LG Electronics (all models):
      • Haier America (all models):
      • Heat Controller (all BHD models use a turbo-impeller with turned blades):
      • Or you can check the EPA Energy Star website for dehumidifiers from these manufacturers

      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.

      For more information on other high performance supplemental dehumidification systems, see: http://www.thermastor.com.

    • 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
       

    • 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.

  • 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.

  • 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.

 

Figure 17

  • Conditioned crawlspaces – This assembly may require some local building code official "building science persuasion." See the Building Safety Journal article written by Peter Yost.

  • 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.