Designs That Work

Hot-Humid Climate - New Orleans

The Basic House - 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.

Figure 6: Diagram of Drainage

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 1/2 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.

Figure 11: Roof Thermal Resistance

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.

Figure 12: Wall Drainage

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.

Figure 20: Window Pan Flashing

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.

Figure 21: Window Air Barrier Continuity

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 8.5% when compared to the energy consumption of the Building America Benchmark house design.