A common industry platform for SIPs design
This document was created by ACME Panel and the Manufacturing members of the Structural Insulated Panel Association (SIPA). It highlights important considerations during the design phase of a Structural Insulated Panel (SIPs) structure. Decades of combined knowledge from SIPA Manufacturers will help reduce the learning curve and leverage SIPs exceptional qualities to achieve the high-performance results owners expect when building with SIPs. ACME and SIPA hope to make your design-role easier and more efficient.
High-performance building envelopes use SIPs
SIPs yield high-performance building envelopes with energy performance well beyond conventional framing. SIPs (walls, floors and roof) provide a core of high R-value solid insulation typically requiring no additional continuous insulation on the building exterior. Elimination of traditional batt or spray insulations eradicates the installation quality challenges these products create and removes the need for insulation subcontractors. The large size of SIPs results in fewer air gaps, reduced thermal bridging from fewer lumber connections (lower framing factor) and elimination of air spaces within the wall cavity. SIPs provide a virtually airtight envelope improving Indoor Air Quality (IAQ) and creating healthier homes and businesses. Airtightness also reduces HVAC sizing and dramatically improves occupant comfort by providing consistent room temperatures, regardless of the number of floors. A SIPs thickness determines its insulation R-value, but the greatest gain comes from the tightness of the engineered design and the other high-performance components (HVAC, windows, etc.) specified by the designer.
SIPs performance is based on more than its stated R-Value
R-values of SIPs are readily available from SIP Manufacturers. These are useful insulation metrics but only one component in the evaluation of a high-performance building envelope. Integration of all system components and airtightness of the envelope assembly are more important considerations for designers seeking performance over individual component metrics. To illustrate this, the Department of Energy’s Oak Ridge National Laboratory (ORNL) tested the performance of large section wall assemblies. The resulting whole-wall R-value data revealed that a 4-inch SIP wall rated at R-14 outperformed a 2×6-inch wall with R-19 fiberglass insulation. The whole-wall R-value was R-21 for a 6-inch SIP wall or 96% higher than the whole-wall R-value of R-11 for the 2×6-inch wall using theoretically R-19 fiberglass insulation. ORNL also tested a SIPs structure side by side with a conventional 2×4 structure to evaluate air leakage. The SIPs structure had only 7% the air leakage of the conventional structure. In short, unlike for SIPs, joining real-world air-leakage rates and thermal bridging for conventionally framed structures lowers performance far below their theoretically calculated effective insulation R-values. For more information relating to key, high-performance building envelope metrics, visit the technical drop-down menu at www.SIPs.org or click here.
SIPs structural capabilities cater well to virtually any design
SIPs structural capacities comply with building codes through evaluation reports from third-party evaluators including ICC NTA, Intertek, and IAPMO. SIPs increased strength over conventional framing enables greater design flexibility. A SIP can span up to 24 feet when incorporating structural connections (splines) eliminating intermediate support structures and creating spectacular vaulted ceilings. Many designs eliminate headers with conscious awareness of where point loads are located, reducing costs, labor and thermal breaks. SIP Manufacturers publish charts to determine load capacities and work with designers ensuring code conformance. SIPA Manufacturing members are an invaluable resource; each provides understanding on how SIPs height, thickness and connection methods affect the structure’s design. Manufacturers can provide lists of structural engineers experienced with SIPs.
SIPs are typically factory cut for accuracy, quality and reduced onsite labor
The cost and scarcity of construction labor is a challenge; the more that can be done by the SIP Manufacturer, the less demand for what has become a scarce resource – jobsite labor. Because SIPs are manufactured in very large sizes (up to 8 x 24 feet), there are fewer connections, resulting in faster installations and a much tighter envelope. Working with SIPA Manufacturers, designers can reduce costs by employing common SIPs dimensional sizes of 4’, 8’, 12’, 16’, etc. in their designs. Factory lumber installations minimize jobsite labor while ensuring joint and boundary connection quality.
SIPs are manufactured using 3D modeled drawings for accuracy
SIPA Manufacturers convert architectural drawings into SIP shop drawings. Shop drawings specify SIPs sizes, layout, assembly details and installation specifications. They are also used for factory fabrication purposes. Shop drawings are provided to the client, or their authorized representative, for review and approval. Commitment to the drawing review process is crucial. Attention to detail during review ensures fabricated panel accuracy, installation ease and meeting SIPs performance expectations. Early interaction with the SIPs Manufacturer helps design optimization resulting in material cost savings and installation speed. SIPA Manufacturers can share samples of SIP shop drawings for your review and understanding.
SIPs are customized to varying levels depending on client needs
SIPA Manufacturers offer differing levels of SIPs fabrication. They include blank SIPs, prefabricated SIPs and ready-to-assemble (RTA) packages.
- Blank SIPs are the least expensive, but limit your installation speed advantage, create additional waste, and require a higher degree of installation skill as all cutting is performed onsite.
- Prefabricated SIPs are designed and cut in the factory increasing site installation speed, improving fit and finish.
- RTA packages include internal lumber and headers provided and pre-installed in the factory to maximize speed and efficiency onsite.
Prefabricated is less typical although, as labor becomes scarce, RTA packages are gaining in popularity. SIPA Manufacturers can provide detail concerning benefits of each option.
Roof and wall assemblies
SIPs are compatible with conventional roof and wall claddings. Unlike traditional framing, there is no internal air cavity within the SIP. The foam core of SIPs is solid and continuous throughout the wall and roof, eliminating convection and condensation issues occurring in conventional cavities. It is important to ensure a code-approved weather-resistive barrier is specified under wall claddings and approved underlayments for roof covering. Designing for the appropriate climate zone will help maximize durability. In some climate zones, a back-ventilated assembly may be appropriate. Because SIPs use very little solid lumber, an increased fastener schedule could be required when attaching exterior cladding. Application of fully adhered products to SIPs roofs is not recommended. A separation layer should be added between the SIPs roof and underlayment to avoid damage to the exterior SIP facing should there be a need to remove and replace the underlayment in the future.
HVAC system rightsizing reduces costs and enhances comfort and performance
SIP envelopes deliver a high-performance, virtually airtight shell. Because of this, the building must have mechanical make-up air. Several options introduce fresh air with varying degrees of complexity and cost. To ensure a balanced HVAC system, consider specifying a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV). Superior SIPs energy efficiency and airtightness reduce HVAC equipment load and often provide savings via smaller unit requirements. Design should also include HVAC installed inside the conditioned SIPs envelope. Conventional practice cannot be used in sizing HVAC equipment for SIP structures. Oversized equipment is not only more costly but short cycles inhibiting the building’s ability to dehumidify causing comfort and mold concerns. SIPs building designers, HERS raters and HVAC professionals can accurately calculate thermal performance of SIPs envelopes using energy modeling with either ASHRAE Manual J or REM/Rate design software. Actual air leakage performance is best determined by a pre-drywall blower door test. SIPs structures typically achieve less than 2 ACH@50pa at this stage, but values less than 1 ACH50 are often achieved with proper design and installation. While SIPs provide the basis for an airtight structure, overall performance can be compromised if proper consideration for energy load calculation inputs or installation is not given to the other critical system components (e.g. windows, HVAC, plumbing, etc.).
Factory cut electrical chases reduce electrician time in the field
Electrical chases are typically provided by SIPA Manufacturers simplifying electrical rough-in and saving electrician labor by eliminating time spent drilling holes in studs. Vertical and horizontal chases are provided in SIPs walls to assist with wiring. Standard practice is to cut chases horizontally at outlet and switch heights. Chases can be added to SIPs roofs upon request. Use of surface mounted LED lighting is recommended in place of recessed lighting as they don’t penetrate the envelope and jeopardize airtightness. Determining electrical requirements prior to SIPs manufacturing reduces the inefficiency of field installation. Plan a shop drawing review with the electrician prior to final approvals to verify chase locations, accommodate electrical layout and ensure code compliance.
To better understand the science of building with SIPs
Review “Builder’s Guide to Structural Insulated Panels for all Climates” by Joseph Lstiburek. This resource provides invaluable assistance in Building Science related details and can be purchased online at Amazon or www.SIPs.org/products.
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