Strategies for Meeting California s High Performance Attics and Walls Prescriptive requirements in California s 2016 Building Energy Efficiency Standards by Steve Easley, Steve Easley & Associates Inc.
Introduction High Performance Attics (HPA) and High Performance Walls (HPW) became prescriptive requirements in California s 2016 Building Energy Efficiency standards on January 1, 2017. These new standards have created challenges for the state s homebuilding industry, which faces a post-recession shortage of skilled labor, and limited experience with construction techniques necessary for code compliance. Workforce Instruction for Standards and Efficiency (WISE) is a training and education program designed to support the transition of California s new residential buildings towards HPA and HPW construction practices. Funded by the California Energy Commission s Electric Program Investment Charge Program, (EPIC) WISE is designed to help accelerate learning and implementation of high performance building by training workers and providing a platform for the exchange of best practices and solutions from industry experts. There have been significant changes in the energy standards that have reduced energy transfer and as a result reduced the drying potential of building assemblies. Long lasting structures require thoughtful integration of building product selection and best practices for building design and material installation. A significant goal of the WISE program is to educate the building industry to design and construct homes that are: 1. Durable and long lasting 2. Healthy to inhabit 3. Energy efficient and comfortable to live in To achieve these goals and create high performing homes it is important that industry professionals utilize sound principals of building science in each step of the design and construction process. Building science is the understanding of the interworking relationships between climate and the various products, assemblies and systems in a structure and how those relationships impact energy use, comfort, indoor air quality and building longevity. Homes are constructed from thousands of pieces put together by hundreds of people so there are many opportunities for errors and omissions that can negatively impact the durability, energy performance and the comfort of homes. There are also many approaches, processes and technologies that can lead to a highperformance home. 2
It is the goal of this commentary to illustrate and address the various technologies and techniques to meet California s prescriptive requirements for achieving HPAs, ducts in conditioned space and HPWs. This report is not intended to evaluate, recommended or censure specific manufactures or products. It is designed to educate the industry about: 1. Building science assessments regarding the HPA and HPW strategies. 2. Constructability 3. Benefits and challenges for HPA and HPW technologies Prescriptive Requirements: High Performance Attics (HPA) Figure1: Prescriptive Requirements for Roof Ceiling Insulation Rational for High Performance Attics and Ducts in Conditioned Space In most California homes the HVAC systems are in attics. This common practice has a large energy penalty because attics in warm weather can reach temperatures of over 130 degrees. HVAC ductwork insulation has much lower R-values, R-8, than attic insulation. If the attic temperature is 130 degrees and the air flowing through the ductwork is 55 degrees there is a 75-degree temperature difference between the conditioned air in the ductwork and the surrounding attic. This results in significant heat gain from reradiated solar energy in the summer and heat loss in the winter. 3
For example, if the surface area of R-8 ductwork in a 130 degree attic is 650 square feet the energy penalty in BTU s per hour = (Area x Temperature Difference)/Rvalue. BTUH = (650 x 75) /8 = 6,094 BTUH, or (1/2 ton) of cooling energy. (A ton of air conditioning = 12,000 BTUH. If a home has a 3-ton, (36,000) BTUH air conditioning system the energy penalty would be slightly over a 15%. This energy loss is further compounded by duct air leakage, which can draw 130 F or more of air into the system requiring the system to work much harder. The rationale for high performance attics is to reduce the attic temperature to as close as feasible to the ambient outdoor air temperature thus saving significant amounts of energy. The CEC Prescriptive Requirements Offer Three Solution Strategies 1. HPA-A: Option A Insulation Above the Roof Deck Sheathing or Rafters Option A utilizes an insulation system that has been used on flat, commercial roof decks for decades with good success. The procedure is to construct a standard vented attic with an additional layer of continuous insulation above and in contact with the roof deck, or on the bottom side of the roof deck over the rafters/trusses to reduce attic temperature. See Fig. 2-5. When the insulation is installed above the roof rafters it is directly in contact with the roof deck. An air gap between the roofing material and the continuous insulation can help reduce attic temperatures by convective cooling of the roofing materials. The code recognizes this and requires less insulation when an air gap is employed. This air gap can be furring or designed into the roofing material. Requirements: R-6 when an air gap is present R-8 when no air gap is present Radiant Barrier R-38 insulation above the ceiling lid A ventilated attic 4
Figure 2: Option A Continuous Rigid Insulation Above Roof Decking OSB Over and Under Continuous Rigid Insulation, Vented Attic Source: Building Science Corp. Figure 3: Nail Base Rigid insulation One Sided (One Sided Structural Insulated Panel SIP) Vented Attic 5
Figure 4: Asphalt Shingles and Furring Strips Installed With Above Deck Insulation Figure 5: Wedged Insulation Placed below Tile With No Air Gap 6
Climate Zone Requirements for Option A In general, option A is a solution in the climates zones where cooling is a significant use of energy. Climate zones 1-3 and 5-7 are predominately heating climates so this option is not requirred. Figure 6: Climate Zone Requirements for Option A Building Science Commentary HPA-A At first glance it seems illogical to place insulation above an already insulated and vented attic space. In conventional attics, the attic space temperature is significantly warmer on sunny days because the intense energy from the sun is absorbed in the roofing materials. This energy is then conducted through the roofing assembly and reradiated into the attic space which creates attic space temperatures of 130 F. or more. Placing secondary insulation, usually rigid foam panels, above the roof deck sheathing, or above the rafters, reduces the heat transfer of reradiated solar energy into the attic. This reduction of reradiated energy helps reduce attic temperatures to ambient outdoor temperatures, which are far lower than traditionally vented and insulated attics. Condensation at the underside of the roof deck is not a concern because the interior moisture is removed via traditional attic ventilation. The code states that if there is an air gap between the roofing materials and the roof deck (i.e. concrete roof tiles, or furring) the code allows for an R-6 instead of R-8 because the airspace provides ventilation under the roof deck, which enhances convective cooling of the roofing materials. For example, if standard asphalt shingles are applied directly over the roof deck sheathing, in a configuration that has 7
no air space, R-8 rigid insulation is required. An acceptable alternative to rigid insulation is insulated shingles or roofing products, which work using the same building science principles. All HPA-A strategies require a radiant barrier: typically foil covered roof decking facing the attic interior. The radiant barrier s low emissivity properties further reduce the radiation of heat into the attic space because shiny surfaces have difficult time re-radiating (emitting) energy. Figure 7: Example of An Insulated Roofing product HPA-A Constructability The HPA-A strategies do not impose foreign constructability issues. There are a variety of product choices in the market place. Many of these strategies have been used for decades and when properly installed do not pose any durability concerns. Additional labor and materials are required (except the insulating shingle approach) due the addition of rigid foam, potentially additional roof deck sheathing and wider trim boards at fascia and gable ends to cover the additional thickness of the system. Some roofing product manufacturers require a structural sheathing product directly below and in contact with their roofing products for support, which will require additional labor and materials. HPA-A does not impose significant scheduling issues. The roofing or framing contractor can install the insulation. Note: Rigid insulation can be placed above the roof rafters but below the roof deck. See Fig. 3. This approach still requires a radiant barrier which could be foil covered foam rated as a radiant barrier, free from logos that would impact its emissivity. An alternative would be to drape a radiant barrier under the ridged insulation but over the rafters (shinny side facing down). This would require an additional cost for the 8
radiant barrier material and labor. Benefits: Reduces attic temperatures and HVAC cooling loads No change to air handler locations Rigid insulation installation is straight forward and easy to install Concerns: May require 2 layers of sheathing and additional fastener costs & labor Ventilation strategy required for air space with roofing tile in addition to attic ventilation Requires radiant barrier strategy Foam board installed over rafters require a separate radiant barrier product Extra thickness can create fascia and rake trim detail challenges and expense 2. HPA-B: Option B Insulation Below the Roof Deck Sheathing Between Framing Option B is likely to appear counter intuitive as mentioned in the building science commentary HPA-A above. The process is to construct a standard vented attic with an additional layer of insulation on the underside of roof deck sheathing to reduce heat transfer into the attic therefore reducing attic temperatures. An air gap between the roofing material and the roof deck can help reduce attic temperatures by convective cooling of the roofing materials. The code recognizes this and requires less insulation when an air gap is employed. This air gap can be furring or designed into the roofing material. Requirements: R-13 when an air gap is present R-18 when no air gap is present R-38 insulation above the ceiling lid A ventilated attic 9
Figure 8: Vented Attic Below Deck Insulation With Insulation Stopper with air gap below roofing Climate Zone Requirements for Option B In general, Option B is a solutions in the climates zones where cooling is a sinifiicant use of of energy. Climate zones 1-3 and 5-7 are predominately heating climates so these option are not requirred. 10
Figure 9: Climate Zone Requirements for Option B Building Science Commentary HPA-B differs from HPA-A in that the secondary insulation is placed below the roof deck sheathing and between attic framing. At first glance, like HPA-A, HPA-B, it seems illogical to place a secondary layer of insulation above an already insulated and vented attic space. In conventional attics, the attic space temperature is significantly warmer on sunny days because the intense energy from the sun is absorbed in the roofing materials. This energy is then conducted through the roofing assembly and reradiated into the attic space which creates attic space temperatures of 130 F. or more. Placing insulation, batts, netted blown fiberglass, spray fiber or spray foam, directly below the roof deck reduces the heat transfer of radiated solar energy into the attic. This reduction of reradiated energy helps reduce attic temperatures to ambient outdoor temperatures, which are far lower than traditionally vented and insulated attics. Generally, with HPA-B condensation at the underside of the roof deck is not a concern because the interior moisture is removed via traditional attic ventilation. However; to avoid potential condensation issues it should be noted that with HPA-B more careful attention to designing and installation of attic ventilation is important in the colder climate zones like 14 and 16 when using vapor open insulation like batts or netted and blown and open cell spray foam insulation. Note the Compliance Manual states you need a vapor retarder (VR) in 14 and 16 but does not specify if its walls and ceilings. Check with CEC on if they mean attics and on final wording The Tittle 24 code states that if there is an air space between the roofing materials and the roof deck (i.e. concrete roof tiles, or furring) the code allows for an R-13 instead of R-18 because the airspace provides ventilation under the roofing, which enhances convective cooling. For example, if standard flat shingles are applied 11
directly over the roof deck sheathing in a configuration that has no air space R-18 insulation is required. The HPA-B strategies require no radiant barrier. For radiant barriers to be effective the shiny surface of the barrier needs to face an open air space of 1 inch or greater. If insulation is in contact with the radiant barrier it losses its emissive properties and becomes ineffective. Figure 10: HPA-B An Example of Un-faced Batt Insulation Under the Roof Deck Sheathing. 12
Figure 11: Placement of Insulation Below Roof Deck HPA-B Constructability There are a variety of product choices in the market place. All HPA-B strategies can pose material, labor and scheduling changes. Additional costs for labor and materials are likely. The under roof insulation is usually installed after roof deck inspection and underlayment and before mechanical, electrical and interior ceiling gypsum board is installed. HBA-B may require return visits from the insulation contractor to correct insulation defects caused from other trade contractors work. These defects are normally corrected with the insulator returns to install the wall insulation. Spray foam installation requires that installers are protected and that only protected installers be in the building during application. HPA-B requires more attention to designing and installation of attic ventilation in the colder climate zones like 3,11, 14 and 16 when using vapor open insulation like batts or netted and blown and open cell spray foam insulation. Vapor retarders may be required. Benefits: Reduces attic temperatures and HVAC cooling loads No change to air handler locations Installation is straight forward and simple to install May be less expensive than rigid foam board Concerns: Requires one or more job site visits to complete Ventilation strategy required for air gap with roofing tile in addition to attic ventilation No radiant barrier, no radiant barrier benefit 13
Careful attention to insulation installation quality Careful attention to insulation vapor barrier and placement and moisture control detailing Most products do not provide continuous insulation over roof framing Spray foam installation requires that installers are personally protected and that only protected installers be in the building during application Option C Ducts in Conditioned Space Requirements: Air handler and ducts to be in conditioned space Radiant barrier to be installed (climate zone dependent) R-30/38 installed at ceiling (climate zone dependent) A ventilated attic Figure 12: Climate Zone Requirements for Option C Ducts in Conditioned Space Most all building science and energy experts agree that placing HVAC and duct distribution systems in conditioned space is the best practice for energy efficiency, comfort building longevity. Numerous studies have documented average energy savings at 15-20%. Typically, when systems are designed to go into conditioned spaces they are better engineered and deliver greater comfort in addition the energy. Both air handler and ductwork need to be in conditioned spaces. 14
Figure 13: Three Basic Strategies To Achieve Ducts in Conditioned Space: 1. Dropped Ceilings, Soffits. Often for esthetic reasons designers will create dropped or tray ceilings as a design element. These soffits are ideal locations for ductwork. The air handler can be located in an interior closet. It very cost effective to make space for ducts to these soffits. Figure14: Example of dropped ceiling used to house ductwork as design element Source, Steve Easley 15
Figure 15: Detail of dropped ceiling used to house ductwork. 2. Open Web Trusses, Between Floors. This requires preplanning the ducts runs during the design phase and coordination with mechanical system and structural designers to ensure adequate depth of floor trusses and space for ducts. A combination of the truss approach and utilizing open web joists or floor trusses between floors can be a cost-effective option. Figure. 16: I-Joists With Pre-Engineered Cut Outs for Duct Runs. Source Georgia Pacific 16
3. Plenum or Scissors Trusses to House HVAC Equipment. Figure 17: Two truss designs that can accommodate air handler and ductwork. Figure 18: Plenum Trusses, A Rigid Air Barrier Material to Be Installed And Sealed On All Four Sides. 17
Figure 19: Example of a scissors truss utilized to house mechanical equipment and ductwork. Note radiant barrier sheathing used to separate conditioned space from the attic above. Source Chitwood Energy Services Building Science Commentary for Ducts in Conditioned Space From a building science perspective, there are no negatives, when designed and installed correctly, for putting ducts in conditioned space. In fact, it makes a home better not only for energy efficiency and occupant comfort but also for moisture management and indoor air quality. Traditionally, supply ducts located in unconditioned spaces that leak can create negative pressures on the inside of the home that can cause combustion appliances like fireplaces or water heaters to back draft combustion by-products into the home, negatively impacting indoor air quality. These negative pressures can also draw in outdoor moisture. Return air duct leakage located outside of conditioned space can cause the envelope to be under a positive pressure that can force interior, moisture laden air to exfiltrate through cracks and gaps in the envelope. In winter this air leakage can dampen the inside surfaces of exterior sheathing and framing potentially leading to mold and decay. Keeping ducts in conditioned spaces reduces the potential for 18
these pressure differentials. In general, ducts in soffits are subject to tighter turns, which creates greater static pressure thus reducing airflow. Ducts located in soffits usually need to fit in smaller spaces so careful design and installation is critical. Ducts that are placed in dropped ceilings and soffits work best with centrally located equipment. This produces shorter runs and reduced friction thus lower static pressure. Use ACCA Manual T for grill selection for proper airflow. It is unlikely that traditional stamped metal grills will be the best choice. The placement of supply and return register location is important as well face velocity for adequate mixing of room air. Good design and pre-planning are important. Even though ductwork is in conditioned spaces it is still important to seal and test for duct leakage and proper airflow. This will ensure the proper amount of heated or cooled air will be effectively delivered to each room helping to provide the intended comfort. Constructability for Ducts in Conditioned Space Placing ducts to conditioned spaces requires up front planning and coordination to be successful. The builder, architect, and mechanical contractor all need to collaborate in the design and construction approaches at the design stage. The designer/builder needs to know the size of the ductwork and the HVAC designer needs to know the space limitations of the structure and framing to design and install a system that is cost effective to build and yet delivers the desired performance and comfort. It is logical that builders will select an approach using a combination of the above approaches that best work for a given home design. With thoughtful planning and design engineering, ducts in conditioned space, can be a more cost effective and higher performing, than the other high performance attic options. When utilizing the boxed or scissors truss approach be aware that service access to replace or repair equipment is required. This may cause aesthetic design issues. Most of the strategies presented work best with a centrally located furnace. This allows easier access to dropped ceilings and vertical chases etc. In addition, shorter duct runs result in more efficient and quieter airflow. 19
Select direct vent furnace or heat pumps to negate the need for combustion air. Never locate the air handler in the garage or outside conditioned space. Sound control is a concern for interior located air handlers. Both noise and vibration noise should be addressed. Select quiet equipment and installation techniques to control noise and vibration. Consider sound insulation and solid core doors. Benefits and Challenges Dropped Ceiling and Ducts between Floors Strategy Benefits: Simple to implement, nothing new to learn, no code issues Builders are already using soffits for architectural features for kitchens, baths and bedrooms Allows for centrally located air handler, shorter duct runs, better performance Framing and drywall are relatively inexpensive Easy to integrate with furnace closets for servicing equipment Concerns: Requires up front planning and coordination with designer HVAC contractor and trades. If used with a centrally located furnace room additional cost for direct vent furnace and sound control measures Needs minimum ceiling height of 9 feet Requires careful duct inspection before drywall is installed Scheduling of trades can be complicated Supplying air to areas with vaulted ceilings is complicated Benefits and Challenges of plenum truss and scissor truss strategy Benefits: Trusses can be designed to easily accommodate equipment and duct work No code issues Does not increase building height, can work with 8-foot ceilings Builders are already using trusses in a variety of configurations Truss framing is relatively inexpensive compared to other HPA options Trusses plenums can be designed to specific size requirements of the HVAC system Allows for centrally located air handler, shorter duct runs, better performance 20
Concerns: Requires up front planning and coordination with truss designer, HVAC contractor and trades If used with a centrally located furnace room additional cost for direct vent furnace and sound control measures Plenum needs air barrier containment and sealing before equipment is installed Service access needs to be provided may create aesthetic issues Requires careful duct inspection before drywall is installed Scheduling of trades can be complicated Supplying air to areas with vaulted ceilings can be complicated 3. High Performance Sealed Attic HPSA High performance sealed attics is another approach to reduce attic temperatures. They are not covered in the prescriptive approach and therefore requires performance approach modeling. It is important to know that HPSA s do not qualify for ducts in conditioned space. Requirements: Complete air sealing between attic space and exterior Code minimum insulation levels underside of roof deck Could use rigid insulation above roof deck, foam board or structural insulated panels (SIPS) Sealed combustion furnace or a heat pump Gable end walls to be insulated Attic ventilation not needed or allowed 21
Figure 20: An example of an un-vented, sealed attic spray foam installation applied to the roof deck sheathing Building Science Commentary for HPSA Sealed or unvented attics have been recognized by various building codes since 2006. Sometimes they are referred to as conditioned attics. California HPSA s do not qualify for ducts in conditioned space. Unvented attics offer significant energy savings particularly when HVAC systems and ductwork are located in the attic. In addition, they eliminate the need to air seal the all ceiling penetrations from can lights, electrical and mechanical systems that lead to the attic. Cautionary note: some fire codes my require air sealing between the attic and the living spaces. How non-vented, sealed attics work: Codes require traditional attics to be ventilated to remove moisture from the interior of homes as well as vent excess attic heat in warm weather. In cold weather, when moisture vapor piggybacks air exfiltration into the attics from the interior living spaces, it can rise and condense on the cold, un-insulated attic surfaces. Traditional attic ventilation removes this moisture. In unvented, sealed attics, moisture, heat gain and losses are controlled in three ways: insulation, insulation location and air sealing. 22
The insulation is relocated from the traditional attic floor to the bottom of the roof deck sheathing and gable walls. In winter the roof sheathing, gable end walls, now insulated, are warmer and therefore above the dew point where condensation is unlikely to occur. Typically, the attic space in homes with unvented, sealed attics are within 2-5 F of the conditioned living space temperature. The result is the space is essentially the same temperature as the conditioned living spaces. Therefore; the moisture stays in a vaporous state thus little chance for condensation or excessive surface relative humidity. Warmer surfaces have lower surface relative humidity. When surface relative humidity is at or above 70% that surface is more prone to support fungal growth. This assumes the interior relative humidity is with in normal range of 30-55%. Popular insulation materials for unvented sealed attics are: Closed cell spray foam, can be used in any climate Open cell spray foam, typically warmer climates Netted and blown and spray fiberglass, vapor retarder per manufacturer specifications SIPS, Any Climate Code minimum insulation levels required regardless of insulation type 23
Figure 21: HPSA. An example of a sealed attic with netted and blown fiberglass installation applied to the roof deck sheathing and gable end walls. Fig. 22: HPSA. Section view of a sealed attic with netted and blown fiberglass installation applied to the roof deck sheathing and gable end walls. 24
Figure 23: Structural Insulated Panel HPSA Constructability Insulating and air sealing unvented attics can impact construction schedule and costs. Unvented attics require thoughtful selection of insulation materials to match the right materials to the severity of the climate. In colder climates vapor open insulations can allow moisture vapor to permeate through the insulation where it can condense on roof decking and lead to moisture problems. Follow manufactures recommendations for the vapor retarders and their location. When selecting materials and systems for unvented attics it is important to understand how moisture impacts building materials and the mechanisms of moisture movement. Wood has hygrometric properties, meaning it can absorb and release water. This absorption and drying cycle is not a problem if the wetting rate of the wood does not exceed the drying rate. If the wood gains moisture and contains enough moisture to support fungal growth and if the surface relative humidity is in the 70% range and 25
air temperature, approximately 30 F to 120 F, decay fungi can grow and deteriorate the wood. When wood gets wet it expands and when it dry s it shrinks. This expansion and contractions can also impact the longevity and the dimensional stability of construction materials. Moisture moves as a liquid and vapor. Moisture vapor is transported on air currents such as air leaks in the building envelope. This is why air sealing is so important. Stopping the airflow stops the vast majority of moisture flow. All manufacturers of unvented attic insulation products stress the importance of diligent attention to air sealing at the roof plane. Why vapor retarders? Moisture also moves by diffusion. Diffusion is the process of moisture vapor moving from wetter areas to dryer areas by permeating through building materials. Moisture movement by diffusion is caused from differences in humidity and temperature between conditioned and unconditioned spaces. Vapor retarders prevent or reduce this moisture movement to protect the building materials from moisture flow by diffusion. Some manufactures have vapor barrier product recommendations by climate. In colder climates zones that experience cold or freezing temperatures (and humid climates) moisture vapor can permeate though vapor open insulation and wet and/or condense on the roof deck sheathing above and the insulation. The insulation reduces the inward drying potential. Moisture build up can lead to decay. In cold climate zones, vapor retarders or vapor closed insulation, like closed cell spray foam, would be recommended. Generally closed cell foam greater than 2 inches thick is considered a Class II vapor retarder, which is less than 1 perm. (Check for the code and manufacturer specifications for vapor retarder requirements by climate zone). Vapor retarders and low permeability insulation products reduce moisture flow by diffusion through the insulation thus can reduce the potential for moisture damage. Open cell spray foam is not recommended in cold climates. Closed cell foam products can be used in all climate zones. Fiberglass insulation products are also an option but pay diligent attention to the manufacturer s instructions regarding installation and the use of vapor retarders by climate zone and make sure those recommendations are in sync with local codes. When using netted blown fiberglass insulation follow the manufacturers recommendations for the use of vapor retarders and any ventilation air requirements, as some require 50 cfm per 1000 sq. ft. of floor area, air from the supply air ducting. 26
In warmer climate zones, vapor open products like open cell spray foam and netted, or sprayed fiberglass, can be used because temperatures of attic building materials are usually warm enough to prevent condensation from moisture diffusion through the insulation. When using Structural Insulated Panels (SIPS) be sure to follow the manufactures recommendation regarding vapor retarders. In all sealed, unvented attics careful attention to manufacturers recommendations and inspection is required. Complete air sealing between the attic and the exterior is essential for building durability and energy savings. Do not air seal at the attic floor as the attic floor is not the thermal boundary. *This may conflict with fire codes so be sure to investigate local code requirements. Spray foam installation requires that installers are personally protected and that only protected installers be in the building during application. Use only certified applicators. Benefits: Reduces attic temperatures and HVAC cooling loads Provides continuous insulation over roof framing No change to air handler locations No need to air seal attic floor* No attic ventilation required Experienced contractor base available Concerns: Could be more expensive than other options No radiant barrier, no radiant barrier benefit Vapor open products, requires careful attention to insulation vapor barrier and placement and moisture control detailing Some products require supply air into the attic space Requires sealed combustion furnace or a heat pump Spray foam installation requires that installers are personally protected and that only protected installers be in the building during application Temperatures below 40 F can impact scheduling 27
Prescriptive Requirements for High Performance Walls Figure 24: Prescriptive Wall U-Factor Requirements A high-performance wall for Package A climate zones 1-5 and 8-16 requires an overall U- factor of 0.051 and 0.065 in climate 6-7. U- Factor is a measure of how much energy a material or an assembly conducts. R-value is the resistance to heat flow. A U-factor of.25 equals an R-value of 4, a U-factor of.5 equals an R-value of 2 and so on. U-factor is for the entire wall assembly not just the cavity. U factor is the amount of energy conducted in BTUH per sq. ft. per degree of temperature difference between indoors and outdoors. Designers can choose a variety of assemblies that meet or exceed these requirements. This report outlines and accesses a variety of assemblies that meet the 2016 Package A requirements. Rational for High Performance Walls Most residential buildings in California, and the United States, have framing factors of 25% for 16-inch on center framing and 22% for 24-inch on center framing. This means that per thousand square feet of window less wall surface area 250 to 220 square feet of that wall is only insulated to R-value of the wall framing. 2x4 and 2x6 28
wood studs have R-values of 4.38 and 6.88. This is well below the R-value of the stud cavity. To remedy these thermal bridging energy losses, Title-24 Package A prescriptive requirements, have instituted requirements for walls to have exterior insulation. This practice is not new and in fact 1 inch thick foam panels are currently being used under 3/8-inch-thick stucco by a number of builders in California. When greater than one inch thick exterior insulation is specified this creates conditions where, window manufacturers and some cladding manufactures, are concerned about the structural integrity of the system. For example, many window manufacturers set the maximum nailing distance from the window flange to the sheathing of one inch. Their concern is that windows floating on foam insulation greater than 1 inch thick will have too much weight and distance from the nail fin and the fasteners can deflect nails and the nailing fins deform resulting in a window failure. Exterior insulation approaches must accommodate these installation requirements. Building Durability and Building Science Related Issues for High Performance Walls Compared to High Performance Attic approaches High Performance walls have fewer building science related concerns. There are two major issues that need to be addressed when selecting an exterior insulation. 1. Managing moisture a. Weather barrier system and location b. Window and penetrations flashing 2. Structural attachment of windows for adequate support a. Providing structural support b. Exterior insulation location Managing Moisture: Careful water and moisture management is critical to reduce the potential for moisture damage and building enclosure failures. Many of the popular exterior insulation products are not vapor permeable. As a result there is reduced drying capacity of the assembly. Water that infiltrates the exterior cladding can get trapped behind these products. If the wetting rate exceeds the drying rate, structural decay can occur. All cladding systems leak. It s not a matter of if the cladding will leak but when the cladding will leak. Preventing the leaks and proper management of water intrusion is vital for long-term performance and durability. 29
When selecting HPW (high performance wall) solutions always employ these key strategies in design and construction to reduce moisture related problems: Deflect: Create designs that deflect water away from the enclosure. The better your designs deflect water the less water you have to drain and dry. Drain: Design walls systems that have the capacity to drain water away from the wall assembly and not trap water in the wall system. The better the wall system drains the less you have to dry. Use drain wraps and rain screens. Figure25: Ancient Reference to the Value of Rain Screen Wall Systems Drying: Design a wall system that has the capacity to dry to reduce moisture build up and potential for decay. The better the system dries the less impact moisture has on the life of the system. Durable materials: specify materials that can deal with moisture cycles without premature decay. Proper interior humidity control and ventilation is also required to prevent excessive moisture vapor drive from the interior to the exterior. The reduced drying capacity of the HPA assemblies can increase moisture levels of wood sheathing and framing if interior moisture is not controlled. It s important to follow ASHRAE 62.2 for mechanical fresh air ventilation as well as utilizing adequate spot ventilation in 30
kitchen and baths for moisture control. Exterior Insulation can also reduce the potential for decay and create a more durable wall system. Increasing the exterior insulation outboard the framing and sheathing creates a warmer wall cavities and framing thus lower surface relative humidity, which in turn, reduces mold and decay potential. The window and window flashing manufactures trade organizations, FMA/AAMA/ WDMA have developed window flashing standards when using insulating sheathing that instruct on flashing procedures and how to integrate window, door, and other penetrations with weather resistive barrier for sound water management. Most window manufactures require builders to follow these installation standards to comply with window and door warrantees. These organizations have based these flashing standards on the ASTM E2112 window and door flashing standards which have been in place for over two decades and are considered the standard of care for the industry. These standards are based on an experienced building science foundation and are key to building structures that are durable and long lasting. These standards are our guide for selecting approaches to HPW s. Figure 26: FMA/AAMA/WDMA 500-16 Summary of Installation Methods 31
Approaches to Structural Attachment and Flashing Of Windows Windows mounted on Rough Opening Extension Support Elements, (ROESE)also known as bucks that act as spacers to accommodate the exterior insulation thickness and provide a solid nailing surface for window attachment Windows mounted directly to the wood sheathing, behind exterior insulation, before or after the weather resistive barrier (WRB) Windows mounted to structural or composite sheathing or with exterior insulation behind the and adhered to structural sheathing or structural Insulated panels (SIPs) Benefits and Concerns for Selecting HPW Assemblies Approach 1. Exterior Insulation Installed Over Wood Sheathing FMA/AAMA/WDMA 500-16 Method A Figure 27: FMA/AAMA/WDMA 500-16 Method A 32
Benefits: Simple concept, insulating sheathing can be installed directly over wood sheathing Window is in line with exterior insulating sheathing plane so no significant change in window flashing sequence except buck installation Window in line with exterior sheathing plane so no changes in siding integration or sequence Concerns: Potential for water to get sandwiched between insulating sheathing and wood sheathing with little drying potential increasing the risk of decay Thermal bridging at window buck Additional materials and labor for bucks. There are insulating bucks available to reduce thermal bridging at bucks. Bucks need to be sealed to wood sheathing Approach 2. Exterior insulation installed over WRB which is installed over wood sheathing, FMA/AAMA/WDMA 500-16 Method B Figure 28: FMA/AAMA/WDMA 500-16 Method B. Method B describes a window that will be installed into a buck, and the FPIS is exterior to the WRB. 33
Benefits: WRB over the wood sheathing provides protection. Drainage wrap type products are recommended to facilitate drainage. Window in line with exterior sheathing plane so minor changes in siding integration or sequence This method employs liquid applied and/or self-adhered flashing, either flashing system is acceptable. Typically, the buck is protected with fluid applied flashings in additional to window flashings if the bucks are made of wood. Concerns: All water management takes place behind the exterior insulation requiring careful attention to flashings integrate provide proper shingling of water adequate drainage paths out of the wall. Requires additional labor to apply fluid applied flashing in over wood bucks and window rough opening. Perforated building wraps may allow water to dampen wood sheathing. Foam plastic insulation (FPI) can inhibit drying increasing the risk of decay Additional costs for flashings and drainage wrap products Bucks need to sealed to wood sheathing Additional materials and labor for bucks. There are insulating bucks available to reduce thermal bridging at bucks. Approach 3. Exterior Insulation Installed Over WRB Which Is Installed Over Wood Sheathing, FMA/AAMA/WDMA 500-16 Method C Bucks This system relies on deeper window/door profiles or trim to cover the exterior insulation. Essentially the windows and doors are recessed into the all system. 34
Figure 29: FMA/AAMA/WDMA 500-16 Method C Benefits: WRB over the wood sheathing provides protection. Drainage wrap type products are recommended to facilitate drainage. Window in line with wood sheathing plane so no change in window flashing sequence No additional materials and labor for bucks. Insulating sheathing covers all opaque wall area thus less thermal bridging. Concerns: Likely that the window will be recessed in the opening and the insulating sheathing will protrude proud of the window resulting in cladding installation challenges and additional potentially costly trim details. (Not an issue with stucco cladding) All water management takes place behind the exterior insulation requiring careful attention to drainage paths. Additional costs for drainage wrap products 35
Approach 4. Composite Structural Insulating Sheathing Composite structural sheathing uses a fiberglass reinforced composite skin over closed cell ridged foam insulation. Thickness ranges form 1-1/8 to 2-1/8 R- values 6-12 can be applied over wood or metal studs. Figure 30: Example of Composite Structural Sheathing Benefits: Easy installation with screw gun, one insulated structural sheathing product. No change in window/door, flashing installation procedures Hi R-value, may negate the need for 2x6 wall framing Durable, skin does not support fungal growth or decay 8-2, 9-2 and 10-2 sheathing sizes add significantly to wind loading capabilities by fastening to the top plate framing member Often can be installed and left unfinished for up to 180 days No thermal bucks required Concerns: Weather barrier system relies on multi-step process of sealants and tapes that require attention to providing clean dry surfaces. All joints require sealant and flashing tape over the sealant. Components costly and labor intensive compared to traditional WRB s and flashings. Cost may be prohibitive 36
Approach 5. Exterior Insulation Installed/Adhered Behind Wood Sheathings This approach uses ridged foam, some products laminated to the back of OSB sheathing, to create an insulated nail base sheathing. Sheathing thickness with insulation ranges from 1 to 2. R-values 3.6-7.8 Benefits: Easy installation with one insulated structural sheathing product May include a protective skin, more durable than standard OSB Often can be installed and left unfinished for up to 180 days No change windows, WRB, flashing and cladding installation procedures No thermal bucks required Concerns: May not be approved for all seismic zones Weather barrier system relies tape like flashing that requires careful attention during installation and providing clean dry surfaces. Overdriven fasteners can compromise water hold out. If flashing tape fails water issues likely as there is no redundancy to the system. Figure 31: Example of Nail Base Insulating Sheathing. 37
Approach 6. Structural Insulated Panels (SIPS) Sips have been in use for residential buildings for over 50 years with great success. SIPS panel systems are very fast to construct, Are energy efficient, very air tight and almost far less thermal bridging. Figure 32: Example of a SIPS Wall Panel Benefits: Almost eliminates thermal bridging Reduced construction times Simple to install, no special tools High air tightness levels No change windows, WRB, flashing and cladding installation procedures Concerns: Moisture resistance, if the OSB decays so does structural integrity Requires accurate foundations Requires pre-planning 38
Requires pest protection May be more costly Approach 7. High-Performance Structural Foam Wall System The high-performance structural foam wall assembly is a wall system combines medium density closed cell spray foam (ccspf) for cavity insulation and rigid foam panels installed over the framing. This system utilizes the adhesive characteristics of spray ccspf to bond the foam cavity insulation to the framing and the exterior insulating sheathing, creating a near monolithic structural and insulation system. Windows, doors, weather barrier wraps, flashings and cladding are installed with traditional industry practices. The bond that occurs between the ccspf, the framing, and the continuous rigid insulation can provide code-compliant wind and seismic structural load resistance without the use of OSB sheathing. Benefits: Hi R-value, can negate the need for 2x6 wall framing Very durable, not impacted by moisture or decay No change windows, WRB, flashing and cladding installation procedures Can add OSB sheathing over the foam panels for greater structural and Seismic performance Concerns: Likely more costly than traditional systems Requires a certified spray foam contractor Workers other than ccspf cannot be in the structure while ccspf is applied. 39
Approach 8: Insulated Siding Products There are currently no products exist that have a high enough R-value to meet the prescriptive requirements of the code unless combined with ccspf. Figure 33: Example of Cement Board Siding with Backside Insulation. Approach 9: Insulated House Wrap This product consists of a fibrous insulation laminated to a breathable house wrap. This provides an R-5 insulation to be installed in at the same time as the WRB. The product is compressible so additional insulating battens are required to achieve a continuous insulation rating. 40
Figure 34: Example of Insulating Housewrap Benefits: Breathable Water, air barrier, and exterior insulation in one product Concerns: Additional Labor and materials battens Battens are expensive 41