Improving Thermal Transmittance Estimation Accuracy in CFS Residential Envelopes using Response Surface Methodology: A Numerical-Based Approach

Earn 1 PDH

The global shift toward sustainable, energy-efficient construction has resulted in increasingly stringent insulation requirements in building codes, including recent changes in New Zealand. These changes present challenges for cold-formed steel construction. While CFS offers many advantages—such as high strength-to-weight ratio, non-flammability, and ease of prefabrication—it is susceptible to thermal bridging due to steel’s high conductivity. This effect significantly reduces the overall thermal resistance (R-value) of building envelopes and complicates efforts to meet modern energy performance standards. Accurately evaluating thermal transmittance in CFS walls is therefore critical.

However, code-prescribed calculation methods such as the parallel path or isothermal planes approach often diverge substantially from experimental or validated numerical results, with errors reported as high as 50%. Numerical simulations, particularly finite element analysis (FEA), provide much greater accuracy, but they are resource-intensive and impractical for widespread application in large-scale housing or commercial projects. This creates a gap between the reliability demanded by codes and the practicality required by industry.

This study addresses that gap by comparing current code-compliant methods with validated FEA models, highlighting their limitations in capturing the complex heat transfer pathways within CFS assemblies. To provide a more practical alternative, the study introduces a simplified calculation method based on Response Surface Methodology (RSM). Using a database of validated FEA simulations, RSM generates predictive equations that capture the nonlinear effects of steel profile geometry, insulation placement, and wall thickness on overall thermal resistance. Results show that RSM achieves close agreement with FEA outcomes, while remaining significantly more efficient than full thermal modelling. The proposed method offers a practical balance of accuracy and usability, equipping designers and industry with a reliable tool for predicting thermal resistance in CFS building envelopes and supporting the broader push toward energy-efficient, sustainable construction.

Learning Objectives

• Understand the limitations of current code-compliant methods for calculating thermal transmittance in CFS assemblies. 
• Learn how thermal bridging affects R-values in steel-framed building envelopes and why accurate evaluation is critical. 
• Explore the role of finite element modelling (FEA) in assessing CFS thermal performance and its practical limitations. 
• Gain insight into the application of Response Surface Methodology (RSM) as a simplified, accurate predictive tool. 
• Identify how the proposed RSM-based method can improve design efficiency, regulatory compliance, and sustainability in CFS construction.

Nader Elhajj
FRAMECAD America

Nader Elhajj is a licensed structural engineer with a Master of Science degree in Structural Engineering and an MBA. He has over 35 years of experience in the design, construction and analysis of building methods and materials. Nader currently holds a Director of Engineering position with FRAMECAD America. Prior to FRAMECAD. Mr. Elhajj was the Director of the Research Centre for NAHB. He managed research and development projects for alternative building materials and emerging technologies. He also managed tasks related to design, construction, analysis, testing and field evaluation of construction materials in housing. Mr. Elhajj developed standardized design and other prescriptive specifications for cold-formed steel framing. Mr. Elhajj is active in writing engineering standards, codes, and specifications for the building industry. Mr. Elhajj authored and co-authored several publications about housing design and cold-formed steel framing. Mr. Elhajj serves on several industry and code-related cold-formed steel committees.

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