Sandwich Panel U-Value vs R-Value

Quick Answer

When comparing supplier data, first confirm the metric, the unit, the material or assembly it describes, and the test or calculation conditions. The same thickness or a numerically larger R-value does not by itself prove that one finished panel performs better than another. A core R-value and a panel U-value describe different things, and they cannot be compared directly without aligning the scope and conditions behind each figure.

What Do Thermal Conductivity, R-Value and U-Value Describe?

Thermal conductivity, thermal resistance and thermal transmittance are related but distinct quantities. The table below summarises what each one describes and its SI unit.

Quantity	Symbol	What it describes	SI unit
Thermal conductivity	λ or k	The heat conduction property of a material under stated conditions.	W/(m·K)
Thermal resistance	R	The temperature difference divided by heat flow rate per unit area for the specified layer or assembly.	m²·K/W
Thermal transmittance	U	The heat transfer coefficient of a specified assembly: heat flow rate per unit area and per unit temperature difference under stated boundary conditions.	W/(m²·K)

Under comparable conditions, a lower λ means lower conductivity for that material, a higher R means greater resistance for that layer or assembly, and a lower U means lower transmittance for that assembly. Compare values with a matching assessment scope and conditions. The products themselves can have different materials or thicknesses.

R can describe a material layer, a panel core or an assembly. U can describe a panel, a roof or wall assembly, or another defined construction. The supporting document defines the scope. Suppliers may use the label “R-value” for either SI or US customary units, so read the accompanying unit.

For an overview of core options, see our guide to sandwich panel materials.

What Does the Thermal Data Cover?

Conceptual examples of assessment scope. The product document defines which parts and boundary conditions each reported value covers. Not to scale.

Check Units Before Comparing Numbers

SI thermal resistance is expressed in m²·K/W. US customary thermal resistance is often expressed in h·ft²·°F/Btu. The two are not numerically interchangeable.

For example, if a source document explicitly states a US R-value of R-20 with units of h·ft²·°F/Btu, the conversion to SI is:

R_SI = R_US × 0.1761102

20 × 0.1761102 ≈ 3.52 m²·K/W

The conversion changes the unit only. It does not change what is being evaluated. If the original figure described a core material, the converted figure still describes that core material. If the original figure described an assembly, the converted figure still describes that assembly.

An “R-20” without a stated unit should not be automatically assumed to be a US R-value. The source document should state the unit system, and the reviewer should confirm it before any comparison.

Why a Core Calculation Is Not Automatically a Panel U-Value

For steady, one-dimensional heat flow through a hypothetical homogeneous layer, thermal resistance can be calculated as R_layer = d / λ. Consider this example:

Layer thickness: 100 mm = 0.100 m.
Assumed thermal conductivity: 0.025 W/(m·K).
R_layer = d / λ = 0.100 / 0.025 = 4.00 m²·K/W.

Here, d is the material layer thickness, not the overall depth of a profiled panel. The figures are hypothetical, not Bingfa product specifications or standard values for a core material category.

This result describes the hypothetical layer only. A finished-panel assessment must also define the relevant surface conditions and the construction included. The relationship U = 1 / R_total requires total resistance and U to refer to the same construction, assessment area and boundary conditions, using compatible units. Taking the reciprocal of a core-layer resistance does not establish a finished-panel U-value.

A Practical Supplier Comparison

Consider a hypothetical procurement scenario. Supplier A provides a core thermal resistance of US R-20. Supplier B provides a U-value for a specific panel type in W/(m²·K), but the document scope and conditions still need to be verified.

Supplier	What is currently known	Key missing information	What to request
Supplier A	Core thermal resistance stated as US R-20	Finished-panel data for the quoted panel type, thickness and assembly	Panel-level thermal data for the quoted configuration, with units, conditions and a document reference
Supplier B	Panel U-value stated in W/(m²·K)	The exact panel model, evaluation method, conditions and included components	The model and configuration, calculation or test basis, boundary conditions, and treatment of joints and fasteners

The available information is not yet sufficient to rank the finished panels. Obtain data for both quoted configurations using a comparable metric, units, assessment scope and conditions, then compare the documented performance against the project requirement. The core materials or thicknesses can differ if each configuration is clearly identified. More complete paperwork alone does not establish better thermal performance.

Have thermal requirements or supplier data for a planned roof or wall? Share them with your proposed panel type and drawings to discuss the available documentation and supply scope.

Discuss Your Panel Insulation Requirements

What to Record Before Comparing Quotations

Before comparing quotations, record the following information for each supplier.

Record item	What to capture
Product and application	Product model, roof or wall application, thickness definition
Metric and value	Metric name, numerical value and complete unit
What is evaluated	Whether the value describes the core, the panel or another defined assembly
Document reference	File name, date, and the stated test or calculation basis
Conditions	Temperature, relevant moisture conditions, declared or design value, and any ageing statement
Included components	Whether joints, fasteners or other construction factors are included

Some product documents include standard joints in the U-value, while certain fasteners or project-specific interfaces may be handled separately. Record what is included and what requires separate consideration; this helps the project team avoid omissions or double-counting. Keep qualifiers such as “declared”, “design” and any ageing statement with the value.

Not all valid U-values must come from the same test method. The calculation or test scope stated in one manufacturer’s document is not automatically a universal industry rule. A U-value alone also does not prove airtightness, absence of condensation, or whole-building energy performance. For broader insulation system context, see steel building insulation systems.

Preparing a Panel Supply Inquiry

To discuss panel supply with Bingfa, send the project location and use, roof or wall application, proposed panel type, available drawings and any specified thermal requirements or existing supplier data. These details help us discuss the available product documentation and define the supply scope. Include the project’s thermal criteria and required supporting documents with the inquiry.

Organise quantities by panel type and length alongside the performance requirements. Our guide to sandwich panel quantity calculation explains how to prepare roof and wall schedules.

Frequently Asked Questions

Can I compare panels by thickness alone?

No. Thickness affects thermal resistance, but the panel’s overall thermal performance also depends on the core material, the facings, the joints and the boundary conditions. A thicker panel is not automatically better in every comparison. Compare the complete documented configuration, not only the thickness.

Is a core R-value the same as a panel-system rating?

A core R-value describes the specified core layer. A reported panel or system value applies to the construction and conditions defined in its supporting document, which may include other components or boundary effects. Check that scope before using it in a supplier comparison.

Can I convert any R-value directly into a U-value?

Not directly. The relationship U = 1 / R_total applies only when the assembly, evaluation scope, units and boundary conditions are compatible. A core R-value cannot be converted directly into a panel U-value without accounting for the complete assembly.

What documents should I request before comparing quotations?

Request the product data sheet for the quoted panel type, including the metric, value, unit, evaluation scope, document date, test or calculation basis, conditions, and whether joints and fasteners are included. Also request confirmation of the model and configuration the data applies to.

For a panel supply inquiry, send the project location, roof or wall application, proposed panel type, drawings and available thermal requirements.