BSP-031: Designs that Work: Hot-Humid Climate (Lake Charles, LA)

Description of the House

The case study house is a 1255 square foot, three bedroom, one-and-a-half-story single-family detached house raised on a pier foundation.  

The ground floor has two entrances: one from the large porch at the front of the house, and the second to a smaller deck at the garage side of the rear of the house.  All of the essential rooms in the house are located on the ground floor making a conversion to a fully accessible home possible.

Figure 2: Hot-humid climate house floor plan (top: second floor; bottom: first floor)

The perspective drawing on the previous page shows the prototype house for Lake Charles raised off of the ground by structural piers.   The pier foundation is intended to comply with the FEMA design guidelines for flood-prone areas.  The design reflects lessons learned about building in hurricane-prone coastal areas, including:

• Elevate structures and mechanical equipment
• Build with materials that are non-water sensitive
• Design assemblies to easily dry once wet

The following sections will explain how these lessons are applied to the building details.

On the second floor, the drawings show a second bathroom off the hallway at the top of the stairs and two bedrooms to the front and back of the house.  Since all of the mechanical systems are located downstairs and all of the insulation is located outside of the framing, the ceilings on this floor may be left open to make the bedrooms feel larger.  At both sides of each bedroom, knee walls can be added to provide large closets for storage.  The stairwell, the bedrooms, and the second floor bathroom are given more floor area by dormers on either side of the roof peak.

A high-performance, energy-efficient house depends on rational and efficient space planning.  The Lake Charles Hot-Humid Climate house plan presented here is organised to simplify construction and reduce the materials and operating costs.  However, it does this while still providing the homeowner with a convenient layout and large, spacious rooms.  Attention to architectural design, it should be noted, is one way of securing a high-quality, affordable and comfortable home.

The following section discusses how the building enclosure and mechanical systems have been designed help this house be durable, healthy and energy-efficient.

Siting and Orientation

The choice of an appropriate building site is an important first step in constructing the affordable, high-performance house described in this package.

In selecting a site, priority should be given to urban lots with existing service infrastructure, access to public transportation and mature neighborhood amenities.  The next best choice would be a lot in a responsibly-planned new development.  In either location, the building site should be chosen for good solar access, with consideration given to orientation, slope, existing or potential overshading, and lot proportions.

Figure 3: Site plan diagram

The site plan above shows an area for installing solar thermal or photovoltaic panels on the south face of the house rooftop that will not be shaded by neighboring buildings.  Large roof overhangs shade the walls and windows of the house to reduce the summertime cooling load. The garage is located towards the rear of the property to provide an additional or alternate location for PV panels.

In a hot-humid climate, landscaping should be arranged to shade the house from early morning and peak afternoon sun further reducing the energy needed to cool the building.  Note that the shading should not extend over any part of the solar panels (PV), as a small amount of shading can significantly reduce their output.  Well placed trees and other planting can create a cool microclimate around the house.

In flood zone or coastal areas, all floor levels need to be built above the flood plain and extra attention should be given to the grading of the site to ensure that water is carried away from the house.

Energy Analysis Overview

An energy analysis was done for the house plan to examine the energy consumption of the building.  With any energy analysis a start point for comparison is required.

The Building America Benchmark Definition Version 2005 along with recent revisions was used as a template for performance evaluation between the advanced building system (Prototype) and the reference building system (Benchmark). The Benchmark Definition requires hourly building energy simulation.

The Building America Benchmark Protocol is generally consistent with mid 1990’s house construction.  Unlike other rating performance systems, the Building America Benchmark includes not only heating, cooling and hot water, (which accounts for roughly 50% of total energy consumption of the home), but also energy consumption from lighting, appliances, and other miscellaneous loads.

The following table highlights the differences between the Building America Benchmark House design characteristics and the Prototype design characteristics that were incorporated into this house design.

The simulation program used to run the energy model was EnergyGaugeUSA version 2.42 from the Florida Solar Energy Center.  

The areas of consideration fall under three main categories, the Building Enclosure, Mechanical Systems, and Appliances and Lights.  A parametric whole house energy analysis was done for the case study house design to illustrate the relative importance of the upgrade strategies in each of the three main areas.

The case study model design achieved a whole house 39.2% energy reduction when compared to the Building America Benchmark.

Note that the estimated cost of change column is a net change, giving credit back for the replaced components.  For example, the Benchmark mechanical system includes standard duct installation, standard efficiency heat pump, and hot water heater.  Crediting the standard system, the high efficiency system with more air tight ducting and higher efficiency water heater would add $1000 over the cost of the standard equipment.

On the basis of BTU/sf/yr of site energy, the above calculations yield the following:

Building Enclosure

A fundamental part of durable, energy efficient, and sustainable construction is the design of the building enclosure.  Water managed, thermally efficient, and leak free building enclosures, while providing for durable structures and reducing energy consumption, also allow us to maintain better control of our interior environmental conditions.  In order to achieve this, the various components of the building enclosure (roofs, walls, foundations, windows and doors) must be designed to fulfill their individual requirements. However, these components must also be tied together in such a way as to create a complete system to control rain water, air leakage, vapor migration, and thermal transfer.  In addition, the systems should be economical while still being robust enough to handle the various climate loads that are imposed on them.

Rain water infiltration is the largest source of material deterioration in buildings.  The control of rain water is best achieved if some simple principles of drainage are followed.  The fundamental design looks to create a means to drain water off the building, out of the assemblies and components, and away from the building.  The design uses a strategy referred to as an open rain screen approach.  In an open rain screen approach, the exterior primary layer of water shedding (cladding, shingles, metal roofing, etc) is not relied upon to be completely watertight.  A secondary drainage layer (usually a housewrap or taped insulating sheathing) is installed behind the main exterior water shedding surface.  This drainage layer, often referred to as a ‘drainage plane,’ in combination with flashing details allows water that may penetrate through the exterior water shedding layer to drain back out to the exterior.

After liquid water intrusion, air leakage is the second most common mechanism for depositing moisture in wall assemblies.  Air leakage occurs due to air pressure differentials causing air to flow through or within the building assembly.  In order to control air leakage a continuous plane of airtightness should be created.  This plane of airtightness or air seal should be continuous not only for each building assembly, but at the connection between adjoining building assemblies.  Uncontrolled air leakage can also impact the energy efficiency of the building as infiltrating air will need to be conditioned or through the loss of exfiltrating conditioned air.  The Building America goal is to achieve an infiltration rate equivalent to 2.5 square inches per 100 square feet of building enclosure area.  Creating a continuous air seal is possible with special attention at transition details between different assemblies and systems.

Figure 7: Moisture transport comparison

Vapor transport through diffusion can be a benefit or a detriment.  In some circumstances, vapor diffusing into a wall assembly can condense and accumulate resulting in problems with material deterioration.  On the other hand, vapor diffusion can also be used as a drying mechanism that will allow assemblies to dry to either the exterior or the interior or both.  In general, the vapor control strategy used should maximize the drying potential of the assembly while minimizing the potential for wetting.  With vapor diffusion being affected by both permeability of building components and temperature gradients across assemblies, the vapor control strategy is often related to, and integrated in, the insulation system design as well.  For hot humid climates such as this, the assemblies are in general designed to prevent hot humid exterior air from diffusing into the assemblies, while allowing the assemblies to dry to the interior.

To control thermal transfer, the intention is to maximizing the thermal insulating value of all 6 sides of the building enclosure to levels that are suited for the climate zone while not becoming cost prohibitive.  The thermal transfer if primarily managed by the insulation type, thickness, and location; however other aspects such as framing design, and window U-value and Solar Heat Gain Coefficient (SHGC) are important as well.

To keep the cost of the systems down, reducing material use in the assemblies and material waste on the project is important.  This can be done by efficient layout of the house plan and efficient use of materials.  Reducing material use must be done in such a way however so as not to affect the robustness or structural integrity of the building.  Provisions to maintain adequate wind and seismic resistance must always be incorporated into the design.

This house is designed for coastal hurricane prone areas.  These areas experience some of the highest wind loads as well as greatest flood potential.  To account for this, the building must be designed to transfer wind uplift forces from the roof structure, through the walls, and down to the foundation.  Due to the corrosive nature of coastal climates, it is recommended to use stainless steel fasteners and brackets for locations exposed to the exterior conditions (all material exterior of the housewrap).  Other coated metals such as double dipped galvanized fasteners and connectors can be used with increasing risk of corrosion.  In addition, while it is reasonable to expect a house to be free of rain water leaks during normal storm events, it is not reasonable to expect that a house will not experience some wetting during a hurricane storm event.  Therefore the house is designed to withstand periodic wetting and designed to promote rapid drying of the building materials.

Roof Design

The roof is designed with asphalt shingle installed over a SBS roof membrane (similar to a W.R. Grace Ice and Water Shield) fully adhered to a layer of borate treated OSB.  A primer may be required to facilitate the adhesion of the membrane to the OSB.  While the shingles will ensure that the vast majority of the liquid rain water sheds off the surface, the waterproof membrane below the shingles will provide for added protection against water that may be blown up and under the shingles during high winds, or water that my creep up under the shingles due to capillary suction.  The overhangs from the roof are designed to extend a minimum of 2 _ feet from the exterior wall.  This amount of overhang will provide protection for the wall elements such as windows and doors that are traditionally common sources of water leakage.  With the overhangs preventing the wall systems from getting wet, the risk of water intrusion through these elements is greatly reduced.

Figure 8: Roof Drainage

The attic is designed as an unvented attic.  With unvented attics such as this, the plane of air tightness is located at the plane of roof and not at the ceiling plane as is common with vented attic designs.  Since the attic is not vented to the exterior, soffit and ridge vents are NOT installed, and would in fact be detrimental to the performance of the system.   The air tightness for this assembly is provided by the building paper or housewrap sandwiched between the rigid insulation and the interior layer of roof sheathing.  In order to maintain the continuity of the air seal between the roof and the wall, some spray foam is installed from the underside of the roof deck to the top plate of the wall assembly.

Figure 9: Roof Air Barrier

The fully adhered SBS membrane, while providing a waterproofing layer under the shingles, also has a perm rating of less than 0.1 perms, making it a Class 1 vapor retarder.  This membrane will prevent exterior humidity or water absorbed by the shingles from diffusing into the roof construction from the exterior.  The housewrap (usually considered to be a Class 3 vapor retarder or better) will allow for any moisture that may penetrate down to this plane of the assembly to dry to the interior.

Figure 10: Roof Vapor Management

The thermal resistance of the assembly is provided by the 4 inches of rigid insulation installed to the exterior of the structure.  Due to the high temperatures experienced by the roof, EPS insulation would not be appropriate for this system, the insulation should be either XPS or Polyisocyanurate. With cavity insulation, the framing members (studs, top and bottom plates, window headers, etc) are thermal bridges through the insulating layer.  These thermal bridges can reduce the rated R-value of the insulation upwards of 35% to 40%.  This means that a 2x6 stud wall with a rated R-19 fiberglass batt will in reality have an effective R-value of around R-13 for the entire assembly.  For this design, since the insulation is installed exterior of the structure, concerns with thermal bridging of the framing members are essentially eliminated.  This means that close to the entire rated insulating value of the insulation will be effective in providing thermal resistance.  4 inches of rigid XPS installed to the exterior of the structure will have an effective R-value of R-20.

The spacing of the trusses is on 24 inch centers.  The house is designed for high wind locations such as hurricane prone zones.  Due to this the trusses are connected to a stud below with a hurricane tie to deal with the extremely high wind load potential for this area.  The roof framing and sheathing are borate treated to resist rot and decay in the event the material gets wet.  This treatment also protects the wood from insects such as termites.

Wall Design

The wall water management system is designed with a ventilated and drained cavity behind the fiber cement siding.  The fiber cement is held off of the rigid insulation with 1x4 furring strips.  These furring strips provide for an air gap that acts both as a drainage gap and ventilation gap.  This allows water that penetrates past the siding to drain to the exterior and allows for air flow behind the cladding to help with drying of the cavity.  The drainage plane for the assembly is the housewrap behind the rigid insulation.  Most water penetrating past the cladding will drain down the exterior face of the rigid insulation, however, some water may still get past at the joints in the rigid insulation boards.  For this reason it is still important that the continuity and integrity of the housewrap drainage plane be maintained.  All flashings should be tied back to this plane and shingle lapped into the housewrap. 

The air tightness for this assembly is provided by the housewrap sandwiched between the rigid insulation and treated OSB sheathing.  The continuity is maintained at the top by sealing the exterior wood or gypsum sheathing to the top plate with a bead of sealant, and through sealing the top plate to the underside of the roof deck with spray foam insulation.  At the connection to the floor, the exterior wood sheathing is sealed to the sill plate, and the sill plate is sealed to the floor structure at the sill gasket or with a continuous bead of sealant or adhesive.

Figure 13: Wall Air Barrier

The primary vapor control element in this assembly is the exterior rigid insulation.  All types of insulating sheathing can be used in this design due to the drying capacity to the interior provided by the gypsum, however insulating sheathing with lower permeability ratings such as XPS and Polyisocyanurate would help to limit the amount of moisture able to diffuse through the assembly.  As an example, two inches of XPS insulation is considered to be a Class 2 vapor retarder (between 1.0 and 0.1 perms).  A Class 2 vapor retarder is considered to be vapor semi-impermeable and limits the amount of exterior moisture able to diffuse through the assembly into the interior.

Figure 14: Wall Vapor Management

The thermal resistance of the assembly is provided by the 2 inches of rigid insulation installed to the exterior of the structure.  As mentioned in the roof design section, with cavity insulation, the framing members can reduce the rated R-value of the insulation upwards of 35% to 40%.  This means that a 2x6 stud wall with a rated R-19 fiberglass batt will in reality have an effective R-value of around R-13 for the entire assembly.  For this design 2 inches of rigid XPS installed to the exterior of the structure will have an effective R-value of R-10.

Figure 15: Wall Thermal Resistance

The layout of the walls on the floor plan follows a 24 inch grid.  This 24 inch grid makes use of standard material dimensions for sheathing and insulation products.  This reduces cutting and material waste on site.  Following this, the walls are designed with the use of advanced framing techniques (advanced framing uses 2x4 studs at 24 inches on center, single top plates, two stud corners, and headers over windows only on load bearing walls).  Where in other locations, the exterior wood sheathing can be removed to further reduce material use, in this case, due to the high potential wind loads of the area, the lateral load resistance is provided by completely sheathing the wall area with exterior treated OSB sheathing.  Uplift forces must be transferred from the roof structure through to the foundation.  At the top of the walls, the rafters are strapped to the studs past the top plate.  At the foundation, the studs are strapped to the floor beam.

Similar to the roof sheathing, the wall framing and sheathing is also borate treated to resist rot and decay in the event the material gets wet, and to protect the wood from insects such as termites.  In the case of a wetting event, the bottom portion of the interior drywall can be removed to facilitate drying of the un-insulated wall cavity.

Foundation Design

The foundation design is specific to areas with high flood probability.  The floor is elevated off the ground on pier footings with panels that will blow out under severe weather conditions.  This allows for the water to drain completely under the building without damaging the home.  The design of the piers should reflect the soil conditions and scour potential of the area.

Figure 16: Foundation Drainage

The main air tightness is maintained by sealing the sub floor to the bottom plate of the stud wall.  In addition to control the potential of condensation due to air leakage, all the joints of the rigid insulation installed to underside of the floor structure, are taped and sealed.  The rigid insulation is also sealed to the beams of the foundation structure.

Figure 17: Foundation Air Barrier

As with the wall assembly, the vapor control is provided by the 2 inches of rigid insulation installed to the underside of the floor structure.  However, unlike the wall assemblies, control of exterior water vapor diffusing into the assembly is more critical as the subfloor and floor finishes may limit the drying capacity of the assembly to the interior.  Due to this only low permeability rigid insulation should be used.  Both XPS and Polyisocyanurate insulation would be acceptable for this design.  As an example, 2 inches of XPS insulation is considered to be a Class 2 vapor retarder (between 1.0 and 0.1 perms).  A Class 2 vapor retarder is considered to be vapor semi-impermeable and would limit the amount of exterior moisture able to diffuse through the assembly into the interior.

Figure 18: Foundation Vapor Management

Similar to the wall assembly, the thermal resistance of the assembly is provided by the 2 inches of rigid insulation installed to the underside of the structure.  For this design 2 inches of rigid XPS installed to the underside of the structure will have an effective R-value of R-10.

Figure 19: Foundation Thermal Resistance

The wind and lateral loads are transferred from the studs of the wall above to the floor beams of the floor framing.  The total uplift forces are then transferred from the floor beams to the pier foundation.  The pier foundation will need to be designed based on specific site conditions and elevated above the height of the FEMA Base Flood Elevation (BFE).

At the top of the piers a termite shield will not be required.  The solid poured concrete piers allow for a means to inspect for termite activity.

The floor structure is also borate treated to resist rot and decay in the event the material gets wet, and to protect the wood from insects such as termites In the case of a wetting event, drying of the floor structure can be facilitated by removing a portion of the insulating sheathing from the underside of the structure.  This will allow for air flow through the framing which will help with drying of the materials.  After the floor structure is dry, the insulating can be reinstalled and taped once more.

Windows and Doors

The window and door installations are designed to be drained systems.  A pan flashing is installed below every window and door to direct any water that may leak through or around the window back out to the exterior.  The nailing flanges of the window are sealed with a membrane flashing on the jambs and head of the window.  The sill is left open to allow the water to drain out.  At the head, the housewrap should be lapped over the membrane flashing to prevent a reverse flashing from being created.

The continuity of the air barrier is maintained by installing a bead of non-expanding urethane foam between the window frame and the rough opening on all four sides of the window.  The foam is installed from the interior prior to the installation of the interior trim.  The foam should also be closer to the interior so as not to block drainage of the pan flashing at the sill of the window.

The thermal resistance of the window is provided by the overall U-value of the window assembly as well as the Solar Heat Gain Coefficient.  For hot humid climates, it is generally recommended to minimize both these values.  Therefore having a low U-value and a low SHGC will provide for a more thermally efficient window.  The values used for this home were a U-value of 0.33 and an SHGC of 0.30 and are representative of what is available on the market.  The value combination will vary from window manufacturer to window manufacturer and even from different windows and sizes by the same manufacturer.  Choosing windows close to these performance values is recommended.

Other Penetrations

There are many other penetrations that are often overlooked in the design of houses.  These are from dryer vents, bathroom exhaust fans, exterior electrical outlets, exterior lights, gas lines, etc.  These penetrations must be designed into the water management system.  Pipe penetrations such as bathroom exhaust vents or dryer vents should be stripped into the drainage plane with membrane flashing.  Where the electrical box are installed flush with or penetrates through the drainage plane, the box should be stripped in with a membrane flashing to create a flanged seal to the drainage plane.  Alternately there are products available on the market that have flanges as part of the electrical box or mechanical vent.  With these products the flanges can be then integrated into the drainage plane.

All penetrations through the plane of air tightness should be sealed with caulking or spray foam in order to maintain the continuity of the air barrier.

These penetrations are thermal bridges.  In order to minimize the effect of the thermal bridging, the insulation should be installed as close as possible to the penetration to minimize the impact of the disruption of the insulating layer.

Energy Model Results

The results of the building enclosure upgrades represented a reduction in energy consumption of 19.6% when compared to the energy consumption of the Building America Benchmark house design.

Mechanical Systems

As with the building enclosure design, working towards energy efficient mechanical systems is also very important in reducing the overall building energy consumption.  Creating efficient mechanical systems is not just a matter of using high efficiency units; the overall system strategy, the location of the equipment and ducts, and the design of the distribution systems all impact the efficiency of the design.  This section examines the impacts of efficient mechanical systems through examining the design of the cooling, heating, ventilation, dehumidification, and domestic hot water systems.  

Prior to deciding on the specific system design for a house, a calculation should be made as to the maximum heat loss and heat gain of the house to determine how much energy the mechanical system needs to transfer to provide indoor comfort.  The Air Conditioning Contractors of America has developed a methodology titled Manual J, which calculates the heating and cooling loads by taking into account the characteristics of the building enclosure.  With this information, the system type and size can be determined depending on other constraints. 

There are numerous methods for creating and distributing heating and cooling energy within homes, each with their own set of benefits and compromises.  The primary decisions about mechanical systems tend to be controlled by available fuels, and by programmatic considerations.  In general, there are two types of distribution systems – air based systems and water based systems.  While heating can be accomplished with either system, cooling has thus far primarily been provided by air based systems due to the considerations with humidity. 

With a tight building enclosure, mechanical ventilation and pollutant source control is also required to ensure that there is reasonable indoor air quality inside the house.  A further consideration with the space conditioning system is how it might inter-relate with the mechanical ventilation system.  Ventilation air flows are relatively small, and could be accomplished with smaller ducting, but there are certain advantages to coupling the space conditioning and ventilation systems.  Exhaust fans located at potential pollutant sources can minimize the need for ventilation, but make-up air must also be considered for the air exhaust fans remove from the house. 

In order to ensure good indoor air quality, all combustion appliances are recommended to be sealed combustion and directly vented to the outdoors.  These systems are completely decoupled from the interior environment through the use of dedicated outdoor air intake and exhaust ducts connected directly to the unit.  Not only are the combustion products decoupled from the interior environment and concerns of back-drafting of the unit removed, but the usual make up air ducts soft connected to an area near the combustion appliance are eliminated.  These make up air ducts (required for naturally aspirated units) are a source of uncontrolled air leakage through the building enclosure, and therefore increase utility use.   Finally, the sealed combustion appliances tend to be more efficient than the naturally aspirated units. 

Forced air systems can integrate the heating and cooling requirements as well as the ventilation requirements into one system, and therefore are often more cost effective than other specialized heating systems.  Intermittent central-fan-integrated supply, designed to ASHRAE 62.2 ventilation requirements, with fan cycling control set to operate the central air handler is recommended to provide ventilation air, distribution, and whole-house averaging of air quality and comfort conditions.

Also, an integrated space conditioning and ventilation system is more likely to be serviced, and provides whole house mixing of indoor air.  However, if a cooling system is not being installed, then a water based distribution system can be used instead, with smaller ventilation system ducting, and potentially a Heat Recovery Ventilator (HRV) to economize on heat used for ventilation air. 

Typically, cooling requires a ducted air conditioning system, and the use of electricity.  Depending on the climate, it may also make sense to use electricity and the ducted system to provide heating, in the form of an air source heat pump (ASHP), or ground source heat pump (GSHP).  Where there is significant heating required, and natural gas is readily available, the performance of an ASHP or cost of a GSHP may prove to have a higher life-cycle cost than a condensing furnace.  In the case where a cooling system is not desired, the duct system can either be downsized, or deleted and a hot water or radiant system can be used instead. 

The location of the duct system can have a significant impact on the overall performance of the system, both the utility use and the ability to provide comfort.  The energy loss from the ducts for forced air heating and cooling systems can be significant depending on the location of the ducts, and how well the ducts are sealed against air leakage.  Though it is conceptually easy to imagine sealed duct systems, it is uncommon to find tight duct systems - duct leakage values of 20% of system flow are common.  In many houses, the distribution duct work is located either in a vented crawl space or in a vented attic – effectively outdoors.  With the ducts located exterior of the thermal envelope of the home, any leakage and conductive losses from the duct work is lost directly to the outside.

Moving the duct work and air handlers inside the thermal enclosure or extending the thermal enclosure to include areas such as crawl spaces and attic as part of the conditioned space of the house can be used to help prevent this energy loss to the exterior. 

In general, the placement of the mechanical equipment will depend on the design of the house.  For houses with conditioned crawlspaces and basements, it is often logical to place the air handler or furnace in those locations.  For slab on grade designs or elevated floors, space can become a concern, in which case unvented conditioned and semi-conditioned attics provide for a convenient location for the mechanical equipment and ducts.  Otherwise, placement of the equipment and / or ducts in a dropped ceiling or in closets is sometimes necessary.  Consideration for space requirements for the mechanical equipment should be made early in the design.  The case study house was designed with an unvented conditioned attic, so that all of the duct work and mechanical equipment was able to be located inside the conditioned space of the attic.

Cooling System

The cooling system is designed with a 14 SEER air source heat pump unit (similar to a Carrier Infinity 17 or an American Standard Heritage 16), which is a high efficiency unit.  Higher efficiency units are available and will further reduce the energy consumption of the house, however the 14 SEER equipment strikes a good balance between efficiency and cost.  Since this is a cooling dominated climate, the efficiency of the cooling system is significant in the overall energy consumption of the house, and any upgrades to the system provide good payback terms.  In addition, proper sizing (right sizing) of equipment through Manual J calculations is done in order to prevent over sizing of equipment.  Over sized equipment increases cost and creates other performance concerns (such as lack of proper dehumidification through short cycling of the system). 

Heating System

The heating system is an air source heat pump rated at 8.5 HSPF (again similar to a Carrier Infinity 17 or an American Standard Heritage 16).  The seasonal efficiency of air source heat pumps increases as one moves into warmer climate zones, since the outside temperature is higher for a larger portion of the year, and rarely drops to freezing.

While the standard ARI rated efficiency is 8.5 HSPF, the Air Conditioning and Refrigeration Institute has a climate zone map that shows adjusted efficiencies for different areas of the country.  Hot humid climates are in either Zone 1 or Zone 2, which increases the HSPF rating by 0.8 to 1.3, meaning that the actual seasonal efficiency will be between 9.3 to 9.8 HSPF when the unit is used in climates such as Lake Charles.

Duct Distribution System

A ductwork distribution system is designed to supply air to rooms in the house with the return being through a central return grill.  The Manual J calculations typically yield the duct sizing and flow requirements to the various rooms to satisfy the loads therein.  These flow volumes are used in the duct layout strategy.  For the Prototype house, the air handler is located in the living space for ease of access with filter changes and maintenance with the duct work running in the unvented attic.  The distribution is from ceiling registers in each of the rooms.

As with any distribution system, there must be a return path for the energy distributing fluid.  In the case of an air-based duct system, there is a central return that is open to the primary living space, with transfer means from bedrooms to the main space.  The return path from the bedrooms needs to be able to allow sufficient return flow to prevent room pressurization and allow supply flow.  While door undercuts can account for some of the return air path, wall transfer grilles or jump ducts should be installed to provide acceptable means for return air.  The flow rates for the Prototype house in the Lake Charles, LA climate are shown in the duct layout strategy shown in the drawing set.  

Figure 27: “Jump duct” over interior partition wall

Ventilation

The ventilation system for this house is designed as a central fan integrated system, which is made up of a 6 inch outdoor air intake duct connected to the return side of the air handler.  This duct draws outdoor air in to the air distribution system and distributes it to the various rooms in the house when the air handling unit is running.  The intake duct has a motorized damper controlled by a fan cycler to close the damper to prevent over ventilation of the house during times of significant space conditioning demands.  Below is schematic example of the central fan ventilation system with 6” electronically operated damper.

Filtration

It is generally considered good practice to provide for some filtration of the distributed air in the house.  It is common to place a filter on the return side of the air handler flow.  Standard furnace filters will provide some amount of air cleaning; however in some instances it may be warranted to install a high efficiency 3 to 5 inch filter instead.  Even if the high efficiency filter is not added originally, leaving enough room at the return side of the air handler (approximately 12 inches) would allow for the filters to be added to the design at a later date.

In addition to the central fan integrated ventilation system, provision is also made for point source pollutant control.  Exhaust fans located in the bathrooms and kitchen are used to remove the localized odors and higher humidity levels created in these areas.

Dehumidification

In more energy efficient building enclosures the air conditioning loads are reduced, especially the “sensible” (non-humidity) heat gains.  Since there is less of a sensible air conditioning load, there also tends to be a reduction in the amount of dehumidification from regular air conditioning operation.  Therefore, for hot-humid climates, it is recommended to provide supplemental dehumidification to avoid times of uncomfortably high indoor humidity.  

The house is designed with a stand alone dehumidifier located in the base of a mechanical closet behind a louvered door and the air handler located directly above the dehumidifier.  This configuration places the dehumidifier directly in the return air path so that the air will be drawn past the dehumidifier during the fan cycling periods.  This system has been shown to provide reliable dehumidification while still maintaining affordable installation costs.  With this system it is important that the house have proper mixing and redistribution of interior air, therefore central fan cycling is required for distribution of the dehumidified air.

Domestic Hot Water

The Domestic Hot Water system is designed with a high efficiency electric tankless hot water heater with an efficiency rating of 0.98 EF.  As well as having lower energy use, an electric tankless hot water heater is much smaller in size, and can be located under a sink or similar area, leaving more storage space elsewhere in the house – valuable with smaller floorplans.  Note that an electric tankless water heater draws a significant amount of power when the water is turned on, and the electric panel and utility service size must be considered based on this load.  

Energy Model Results

The results of the mechanical systems upgrades represented a reduction in energy consumption of 8.6% when compared to the energy consumption of the Building America Benchmark house design.

Appliances and Lighting

Efficient appliances and lights are readily available on the market.  Many new appliances are Energy Star rated indicating that the appliance consumes less energy then compared to the current federal standards.  The amount of energy consumption reduction will vary from appliance to appliance.

Compact Fluorescent Lighting

Compact fluorescent lights (CFL) consume on average 70% less energy than regular incandescent lights.  In addition they will last around 10 times as long.  Even with these benefits, there has been resistance to incorporate CFL’s into common use, due to the light quality and the length of time that it took for the bulbs to warm up.  Advances in technology have made great improvements in both the quality of light provided by the bulbs and the response time to turning on the switch. However, this does not mean that all the lights are the same.  CFLs are available in a range of color temperatures and intensities to suit different lighting requirements in any part of the house.

The ENERGY STAR Advanced Lighting Package recommends that 50% of the lights in high-use rooms and outdoors, and 25% in other rooms be CFLs.  However, the energy-use model done for the basic house assumes that all 90% of the lights are compact fluorescents to achieve the maximum energy savings.  

While using efficient lights and lighting design can reduce the energy consumption, responsible use of the lights is also factor.  The energy model assumes a certain usage amount based on reported lifestyle averages; however actual use will vary dramatically from household to household.  Turning off lights in unoccupied rooms or when natural daylight is adequate can be an even more effective energy reduction strategy.

Energy Star Appliance Package

Clothes washers and dryers, refrigerators, chest freezers, and dishwashers, are significant energy-users in a typical home.  ENERGY STAR-rated appliances use 10-50% less energy and water than standard models.  The case study house was designed and modeled using Energy Star Appliances.

As with lighting, savings are calculated based on reported lifestyle averages and actual use will vary from household to household.  Further reductions in overall energy consumption are possible through the wise use of appliances.  

Homeowner choices like hanging laundry outside to dry at the right time of year, running washers with full loads only, and turning off and unplugging appliances that are not in use will save energy and lower the operating costs of the house.  These lifestyle changes can be encouraged by the builder.

Energy Model Results

The results of the appliances and lighting upgrades represented a reduction in energy consumption of 10.9% when compared to the energy consumption of the Building America Benchmark house design.