When it comes to the thermal performance of insulated metal panels (IMPs), building designers and construction professionals may sound like they are speaking in secret code as they talk about R-values and U-factors.

It helps to have a decoder ring handy because R-values and U-factors are crucial metrics that guide architects, engineers, and building owners in making informed decisions about insulation and energy efficiency.

If you slept through science class (like most of us!), here’s the “simplest” explanation: R-value measures thermal resistance while U-factor, also known as U-value, measures thermal conductivity.

Let’s take a closer look at both R-value and U-factor and they can help evaluate IMPs thermal performance.

R-Values: Thermal Resistance

To understand R-values, it helps to understand the general principles of insulation and heat flow.

The U.S. Department of Energy says that three basic mechanisms are involved:

• Conduction: This is the way heat moves through materials such as IMPs.
• Convection: This is the way heat circulates through liquids and gases.
• Radiation: This is the way heat travels in a straight line and heats anything solid in its path that absorbs energy.

“Most common insulation materials work by slowing conductive heat flow and convective heat flow. Radiant barriers and reflective insulation systems work by reducing radiant heat gain. To be effective, the reflective surface must be in contact with an air space,” says the U.S. Department of Energy.

We then come to R-value which is how an insulating material’s resistance to conductive heat flow is measured or rated. The higher the R-value, the greater the insulation effectiveness.

For flat objects, such as IMPs, the thickness of the insulation in inches divided by the thermal conductivity of the insulation will yield the R-value.

Green Span Profiles IMPs, for example, typically have an R-8 per inch of thickness (1 inch divided 0.125 thermal conductivity = R-8) so that at two inches of thickness, Green Span Profiles IMPs are approximately R-16.

The U.S. Department of Energy says to keep some of these things in mind regarding R-values:

• Installing more insulation in your building or home increases the R-value and the resistance to heat flow. In general, increased insulation thickness will proportionally increase the R-value.
• The effectiveness of an insulation material’s resistance to heat flow also depends on how and where the insulation is installed. For example, insulation that is compressed will not provide its full rated R-value. The overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because heat flows more readily through studs, joists, and other building materials, in a phenomenon known as thermal bridging.
• The amount of insulation or R-value you'll need depends on your climate, type of heating and cooling system, and the part of the building or house you plan to insulate.

U-Factor: Assessing Overall Heat Transfer

U-factor, also known as U-value, measures the rate of heat transfer through a building material or assembly, such as a wall, window, or roof.

The U-factor is calculated by measuring heat loss in watts per square meter per degree Kelvin in metric units or BTU per hour per square foot per degree Fahrenheit in imperial units.

Here are a couple of key aspects of U-factor:

• Unlike R-value, where higher is better, for U-factor, the lower the number the better the insulation properties.
• U-factor is the inverse of R-values in that to calculate the U-factor you can use the formula: 1/R-value. Example: Green Span Profiles IMPs with a typical R-8 per inch of thickness have a U-factor of 0.125 (1 divided by 8). At two inches, the IMPs R-value increases to 16 and the U-factor lowers to 0.0625. This indicates that at R-16 there is less heat transference (0.0625) than at R-8 (0.125) – highlighting that a lower U-factor is desired.
• U-factors are used for evaluating complete assemblies such as windows or entire wall systems.

Importance in Evaluating Insulated Metal Panels

R-values and U-factors are important in evaluating insulated metal panels (IMPs) because IMPs, which have an insulating foam core sandwiched between two metal faces, typically achieve very low U-factors, high R-values, due to their continuous insulation design.

Unlike traditional wall assemblies, IMPs provide continuous insulation without thermal bridges (except at connection points), which helps maintain their stated U-factor performance across the entire panel.

Here are the benefits of IMPs in terms of thermal performance:

• Energy Efficiency: Thanks to higher R-values and lower U-factors, IMPs indicate better insulation for buildings, leading to reduced energy consumption (and lower bills!) for heating and cooling.
• Code Compliance: IMPs can satisfy stringent building codes that specify minimum R-values or maximum U-factors for various building components.
• Cost-Effectiveness: IMPs insulation efficiency results in significant long-term energy savings, justifying initial investment costs.
• Comfort: Well-insulated buildings maintain more consistent temperatures, enhancing occupant comfort.
• Environmental Impact: The improved insulation provided by IMPs helps reduce a building’s carbon footprint by lowering energy consumption.

Additional Considerations for IMPs and Thermal Performance

Here are some additional considerations for IMPs and thermal performance:

• Testing standards: R-values for IMPs are typically measured using ASTM C518 via heat flow meter, while U-factors are determined through ASTM C1363 hot box testing. ASTM C1363 is more comprehensive than C518 as it tests complete wall, roof, or floor assemblies rather than individual materials.
• Material Comparison: Polyurethane-Polyisocyanurate (PUR-PIR) foam, commonly used in IMPs, offers superior insulation with an R-Value of 7.5 per inch, compared to mineral fiber (R-3) or expanded polystyrene (R-4).
• Thickness Impact: IMPs allow for customization of insulation performance since R-values increase proportionally with material thickness.
• System Performance: Since R-value focuses on individual components, the U-factor provides a more comprehensive view of an assembly’s thermal performance, including air gaps, thermal bridges, and installation factors.
• Balanced Approach: Optimizing insulation should be part of a comprehensive energy efficiency strategy, including considerations for ventilation, air sealing, and HVAC system efficiency.

By understanding and properly applying R-values and U-factors, designers and builders can optimize the thermal performance of insulated metal panels, resulting in more energy-efficient and comfortable buildings.

Contact Green Span Profiles today to learn more about how their American-made, high-quality IMPs can help provide thermal protection to your building.