Designs That Work

Mixed-Humid Climate

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 mixed-humid climates, walls are generally designed to dry to both the interior and the exterior (flow through assemblies) or, they are designed with insulating sheathing in order to control the temperature of the condensing surfaces. The thickness of the insulating sheathing is determined by calculation based on the severity of the climate.

To control thermal transfer, the intention is to maximize 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 lateral load resistance must always be incorporated into the design.

The house is designed to be located in areas where termites are a concern. 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). Keeping plantings away from the building a minimum of 3 feet and thin the ground cover (wood mulch or pea stone) to no more than 2 inches for the first 18 inches around the building will help to dissuade termites due to the lack of cover. A termite inspection gap at the top of the foundation wall should be left as well. To reduce the presence of water near the perimeter of the building, maintain a positive slope away from the building to carry ground water away from the foundation. It is also considered a good idea to make sure that irrigation is directed away from the building. Finally the use of an environmentally appropriate soil treatment (such as Termidor®) and treated wood in near grade applications is often warranted.

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.

Roof Design

The roof is designed with asphalt shingles installed over a predominantly cathedralized ceiling. While the shingles will ensure that the vast majority of the liquid rain water and snow melt sheds off the surface, an SBS roof membrane (similar to a W.R. Grace Ice and Water Shield) fully adhered to the roof sheathing is installed at the eave locations and completely over the low slope roof areas to protect the roof from ice damming and potential water penetration from wind driven rain. A primer may be required to facilitate the adhesion of the membrane to the OSB. The overhangs from the roof are designed to extend a minimum of 12 inches from the exterior wall. This amount of overhang will provide some 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 vented cathedral ceilings and attics are designed with the interior plane of air tightness is located at the plane of the interior gypsum board. All the joints in the gypsum boards must be taped and sealed. In addition, any penetration through the gypsum must be air sealed, and all light fixtures should use air tight electrical boxes. In order to maintain the continuity of the air seal between the roof and the wall, the interior gypsum board is sealed to the wood framing at the top of the wall assembly at band joist locations or to the wall gypsum board. In order to maintain the interior air seal at the band joist location, sealants or gaskets are used to seal between the framing members.

Figure 9: Roof Air Barrier

The vapor control strategy of the assembly is to promote drying primarily to the exterior and to reduce the amount of vapor able to diffuse from the interior environment into the roof structure. This roof assembly has continuous back-venting from eave to ridge of the structural roof deck, providing higher drying potential of the assembly to the exterior. With the low vapor permeability of the rigid insulation on the interior of the catherdralized ceilings and the latex paint finish on the ceiling of the vented attic portions, the assembly will prevent interior moisture from penetrating into the roof assembly, however, this will also reduce the drying potential to the interior during the summer months.

Figure 10: Roof Vapor Management

The thermal resistance of the cathedral assembly is provided by the R-30 blown in cellulose insulation and the 1 inch of rigid XPS insulation installed to the interior of the rafters. With cathedral ceilings, the framing members are thermal bridges through the insulating layer. These thermal bridges will reduce the rated R-value of the insulation approximately 20%. This means that the 2x12 rafters with a span of 24 inches on center, and with rated R-30 blown cellulose will in reality have an effective R-value of around R-24 for the entire assembly. The rigid insulation however spans the rafters. This means that close to the entire rated insulating value of the rigid insulation will be effective in providing thermal resistance. 1 inch of rigid XPS installed to the interior of the structure will have an effective R-value of R-5. Added together, the R-value of the assembly is approximately R-29.

Figure 11: Roof Thermal Resistance

Wall Design

The wall water management system is designed with a shingle lapped vinyl siding. While no intentional ventilation and drainage gap is provided behind the vinyl siding, research has shown that the cladding is still very effective in draining water back out to the exterior and open enough to allow for air flow behind the cladding to help with drying of the cladding and wall assembly. The drainage plane of the assembly is the rigid insulation. For the rigid insulation to be effective as a drainage plane, all the vertical joints in the insulation must be taped and sealed, while at the horizontal joints a through wall flashing of polyethylene is installed. Additional protection at the vertical joints could be provided by using an insulating sheathing material with ship-lapped joints (fiberglass faced rigid insulation boards are not acceptable in this application due to problems of adhering membrane flashings and sheathing tapes to the fiberglass facing. All other flashings such as head flashing and step flashings should be regletted into the face of the rigid insulation (ensure that the cut does not fully penetrate the foam sheathing) and the top edge taped to seal against water penetration. In some cases, such as areas of increased risk of water infiltration due to increased rain and wind exposure, it may be warranted to install a layer of housewrap over the exterior of the insulating sheathing. This housewrap will become the drainage plane for the wall assembly.

Figure 12: Wall Drainage

Figure 13: Drainage plane and Window Flashing

The interior plane of air tightness for the wall assembly is located at the plane of the interior gypsum board. In order to maintain the continuity of the air seal, the interior gypsum board is taped and sealed at all the joints. At the roof to wall connection the air seal is maintained by sealing the gypsum to the top plate of the wall assembly and sealing the framing members at the band joist with sealants or gaskets. For the foundation connection similar strategies are used. In addition, any penetration through the gypsum must be air sealed. All electrical penetrations should use air tight electrical boxes that are sealed to the gypsum. An exterior plane of air tightness is created through the taped and sealed rigid insulated sheathing. This exterior air sealing element will help reduce some of the negative effects of wind washing of the insulation.

Figure 14: Wall Air Barrier

The 1 inch of rigid insulating sheathing is designed to elevate the condensing surface in the wall assembly to reduce the risk of condensation occurring within the assembly. During the winter months, the interior humidity levels should be kept lower to limit the amount of moisture able to diffuse into the wall assembly. During the summer months the vapor drive will primarily be from the exterior to the interior. To accommodate this, the assembly is designed to be able to dry to the interior through the use of semi-permeable latex paint on the interior gypsum. Drying to the interior is important, therefore, interior vapor barriers should not be installed.

Figure 15: Wall Vapor Management

The thermal resistance of the assembly is provided by the R-19 blown in cellulose cavity insulation and the 1 inches of rigid insulation installed to the exterior of the structure. Similar to the cathedral ceiling discussion in the roof design section, the wall framing members reduce the rated R-value of the wall assembly upwards of 35% to 40%. This means that a 2x6 stud wall with a rated R-19 cavity insulation will in reality have an effective R-value of around R-13 for the entire assembly. In order to limit the amount of thermal bridging that occurs, the house is designed with advanced framing techniques (advanced framing uses 2x6 studs at 24 inches on center, single top plates, two stud corners, and headers over windows only on load bearing walls). This can reduce the framing fraction of the wall from approximately 23% down as low as approximately 16%.

Figure 16: Wall Thermal Resistance

In order to reduce material use and construction waste, 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 and insulating sheathing as the primary sheathing material to reduce the overall material used in the design. Though the primary sheathing is the rigid insulating sheathing, OSB sheathing is still required to be placed at the corners to provide for lateral load resistance for the house. At these locations, the insulating sheathing thickness is reduced to 1/2 inch to accommodate the thickness of the OSB.

Foundation Design

The foundation is designed with a condition crawlspace. The exterior foundation walls are cast-in-place concrete with a gravel floor covered with polyethylene. At grade, a layer of impermeable soil (such as compacted clay) sloped away from the foundation, should be installed to direct rain water away from the foundation and prevent water from absorbing into the soil in the immediate area around the foundation. Below grade, the exterior of the concrete is coated with a dampproofing to prevent liquid water from penetrating through the concrete. In addition, the back fill material around the foundation should be free draining to allow ground or rain water to drain down to the perimeter drain installed at the base of the footing. The perimeter drain is also connected to the gravel bed below the polyethylene on the crawlspace floor through pipes cast into the footing. This allows for any water in the gravel bed to be drained away as well.

To prevent moisture migration between the concrete foundation and the floor structure above, a capillary break (a closed cell foam sill sealer or gasket) is installed between the top of the concrete and the sill plate. This isolates the framing from any source of moisture that may be either in or on the concrete foundation. Using sill sealer on all walls maintains the same wall height. Similarly, to limit the amount of ground water absorbed through the footing, a capillary break (polyethylene) is installed between the footing and the concrete wall. The four-inch deep, 3/4-inch stone bed functions as a granular capillary break below the polyethylene, a drainage pad, and a sub-polyethylene air pressure field extender for the soil gas ventilation system. Without it, a soil gas ventilation system is not practically possible.

Figure 17: Foundation Drainage

The interior plane of air tightness is maintained through the taped and sealed foil faced polyisocyanurate (polyiso) rigid insulation and the polyethylene ground cover. The polyethylene ground cover should be returned up the walls and sealed the wall foil faced polyiso. At the rim joist the concrete wall is sealed to the sill plate through the use of a sill gasket and sealant.

Figure 18: Foundation Air Barrier

The concrete foundation wall is able to dry to the exterior through the exposed above grade portion of the wall. On the interior, the 1 1/2 inches of foil faced polyiso insulation (foil faced polyiso is a Class 1 vapor retarder - at 0.03 perms) considered to be vapor impermeable, would eliminate interior moisture from being able to diffuse through the assembly and condense on the interior surface of the concrete wall, however this also prevents the assembly from being able to dry to the interior. Due to this, it is recommended to wait to install the foil faced polyiso until near the end of construction to allow for as much drying of the concrete as possible. After installation, the concrete will still be able to dry through the exposed portion above grade. For exterior soil moisture, the dampproofing on the foundation walls is used to control the migration of moisture from the exterior soil into the foundation walls. For the ground, the vapor control layer is provided by the polyethylene ground cover over the gravel bed.

Figure 19: Foundation Vapor Management

The thermal resistance for the crawlspace is provided through the 1 1/2 inches (R-9.75) of foil faced polyiso insulation installed on the walls. 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 in a continuous layer, concerns with thermal bridging of the insulation are essentially eliminated. This means that close to the entire rated insulating value of the insulation will be effective in providing thermal resistance for the crawlspace walls.

Figure 20: Foundation Thermal Resistance

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, a head flashing should be regletted into the XPS sheathing and the top edge taped (Please refer to window installation sequence details on drawing A-6).

Figure 21: 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 22: 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. The values used for this home were a U-value of 0.33 and an SHGC of 0.28 and are representative of what is available on the market. For cold, it is recommended to minimize the overall U-value of the windows for all orientations, however having a higher SHGC on the South elevation can be of some benefit through increased solar gain in the winter months offsetting the heating loads for the house. While this is a good idea in theory, finding a window that has a low U-value and a high SHGC can be difficult. In general windows with lower U-values also have lower SHGC's.

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 6.0% when compared to the energy consumption of the Building America Benchmark house design.