Thermal Resistance Archives

Proper Test Methods to Determine Thermal Resistance of Metal Panels

Posted on November 30, 2017 by MBCI

For a given assembly, if the right information is not specified in conjunction with the desired R-value, the designer will likely not achieve the results he or she expects. This can lead to code compliance issues as well as poor performance of the finished building. Therefore, a more thorough approach must be considered to ensure the specified assembly will be building energy efficiency code compliant. Where to begin? When looking at proper test methods to determine thermal resistance of metal panels, the place to start is ASHRAE 90.1 Chapter 5 (Building Envelope) and Appendix A.

ASHRAE 90.1 Section 5 specifies requirements for the building envelope.

Code Compliance for Thermal Resistance

The most widely accepted energy efficiency standard for commercial construction in North America is ASHRAE Standard 90.1. This standard provides both a prescriptive and a performance path to be chosen at the designer’s option. The prescriptive path is most commonly used. It also provides the baseline performance level that is used to determine compliance for the performance path, so understanding this set of requirements is critical. Within the prescriptive path, two possible methods of compliance are available to determine the minimum thermal performance of opaque areas on the building envelope. Section 5.5.3 is the pertinent passage and it reads:

Minimum rated R-values of insulation for the thermal resistance of the added insulation in framing cavities and continuous insulation only. Specifications listed in Normative Appendix A for each class of construction shall be used to determine compliance.
Maximum U-factor, C-factor, or F-factor for the entire assembly. The values for typical construction assemblies listed in Normative Appendix A shall be used to determine compliance.

Exceptions: For assemblies significantly different than those in Appendix A, calculations shall be performed in accordance with the procedures required in Appendix A.

What does this mean? Basically, there are standard types of construction that ASHRAE recognizes and if you have a wall that fits the description in Appendix A, you don’t have to test or do anything special to determine its thermal resistance. Appendix A provides tables based on calculation methods that have been derived on the basis of previous tests and general experience. What is perhaps less obvious is that if your assembly is adequately described by one of the standard assemblies in the Appendix, you may NOT use a tested or modeled value in place of the values in the table, even if that value has better performance! (i.e., lower U-factor) This is explained in Section A1.2.

The reason the code is set up this way is to prevent people from building unrepresentative assemblies that achieve high performance in the lab but are likely not built to the same specifications in the actual building.

Conversely, if the assembly you want to use is NOT adequately described in Appendix A, the appendix goes on to specify which methods are acceptable to determine the U-factor based on the assembly to which it is most similar. This is covered in Section A9. Two and three-dimensional finite element models are always acceptable and in some cases, simplified calculation alternatives are also available. Note that hot box testing is not always allowed.

Conclusion

To summarize, whether using a prescriptive or a performance path, the first and last stop when determining thermal resistance for metal panels is ASHRAE Standard 90.1 Chapter 5 and Appendix A. Designers would be well advised to familiarize themselves with the Standard and the specific set of requirements for their particular scenario in order to utilize proper testing methods for high-performance results.

Understanding R-Values and K-Factors in Considering Thermal Resistance

Posted on July 19, 2017 by MBCI

Described in their most basic terms, R-value is a measure of heat resistance, while U-factor (also know as U-value) is a measure of heat transfer (heat gain or loss). The lesser known K-factor is simply the reciprocal of the R-value of the insulation divided by the thickness. What they all have in common is a relationship to the effectiveness of insulation material in resisting heat flow through a roof or wall element. There are different ways that this would be spec’d from a manufacturer to an architect or engineer. While the terminology might be familiar, the specifics are not always as clear cut as they seem. Understanding the differences will allow architects to make smart and effective choices to suit a given project’s needs.

Let’s consider some of the variables that might have an impact on what to look for and which metric to spec. As means of illustration, put yourself in the shows of a fiberglass or insulation supplier. You have a product, you know what it’s rated to, you know what the performance capability is, it’s been spec’d out to you—and you submit the bid based on those factors. But at that point you inevitably lose control over how the specs would actually get implemented. For instance, the architect may take that spec and incorporate it into a wall where it’s not used the most efficient way. This may not even be the result of a mistake; it could just be that other project elements have taken over.

Choosing the right insulation for the project can provide the building significant energy savings.

A good example would be stud walls. The fiberglass insulation supplier might indicate a given R-value, such as R-19. This would be the heat resistance value. The architect might spec and submit that bid to supply x number of square feet of that insulation based on that R-value. However, it could be cut or delivered in rolls and designed to fit between the metal studs. Metal studs are much more conductive than insulation and they provide an alternate path for the heat to flow through the assembly, almost irrespective of what the R-value and insulation is. Given these factors, the architect might have to make tradeoffs.

Choosing U-Factor

Because of all the variables encountered with R-value, U-factor is actually more recommended and reliable, and it more appropriately meets code requirements.* The concept of U-factor relates to the heat transfer coefficient but is described in the code as total heat flow per unit area through the assembly inclusive of all the short circuits as it is planned out to be built. So, an architect or engineer would know the stud spacing, the cladding material, the interior finish material and the R-value of the insulation. With that information in hand, one can go to a textbook, ASHRAE 90.1 or the ASHRAE Book of Fundamentals and find the U-factor for the assembly. It is this U-factor that is actually compared against the code requirements. It’s a better way to spec because it already takes into consideration all those things that come into play and encourages the use of suppliers (such as MBCI) that staff people who can help do those calculations or give assistance as opposed to saying, “I need R-19” and then wind up with a building that’s bridged or has more short circuits than anticipated—and having the building not perform as needed. This, in essence, is the key difference between R-value and U-factor.

A Word About K-Factor

As for K-factor, as noted this is the thickness of the insulation divided by the R-value. Its intention is to spec out an insulation when you’re not entirely sure what thickness it will be at the time you spec it out. This is fine for design-build scenarios but not a good practice for a hard bid. Bottom line: U-factor is most often the most reliable choice.

*Note: The code defines U-factor as discussed but underlying heat transfer theory may describe U-factor as 1/R-value. Insualtion suppliers might invert it and make it an R-value (but doesn’t take all the variables into consideration). Therefore, an architect would be advised to specify a “U-factor in compliance with ASHRAE, ” which includes thermal bridges, joints, etc.

Wellness and Envelopes: Four Ways Single Skin & Insulated Metal Panels Keep Us Healthy

Posted on November 17, 2015August 8, 2022 by Brad Johnson

Is there a connection between building design and human health?

We know the answer must be yes, but figuring out how the connection works is the job of experts like the team behind the WELL Building Standard^®, a new certification that takes on the question. Among the solutions that can help make a building better? Metal roofing and siding, according to many healthy building experts.

First, let’s learn about WELL. According to the International WELL Building Institute, the WELL Building Standard “takes a holistic approach to health in the built environment addressing behavior, operations and design.” Their performance-based system measures and monitors such building features as air, water, nourishment, light, fitness, comfort, and mind. Two ratings have been offered: WELL Certified™ spaces and WELL Core and Shell Compliant™ developments. Done properly, these “improve the nutrition, fitness, mood, sleep patterns, and performance of occupants.”

Pilot programs are currently available for retail, multifamily residential, educational, restaurants and commercial kitchens projects. In many of these projects, the use of metal claddings and insulated metal panels (IMPs) is recommended by many health-focused professionals. Why?

1. Occupant comfort

IMPs tend to have excellent R-values and very good thermal efficiency – including long-term thermal resistance, or LTTR, a key measure of how the building will perform over time. For the wellness factor from pure thermal comfort, IMPs are highly effective over conventional construction.

2. Nourishment of people and earth

IMPs are often made with recycled metals and improve the energy performance of the building. With energy cost savings ranging from 5 percent to 30 percent, they cut the carbon footprint of the facility. Plus the interior and exterior skins include up to 35 percent recycled content – and they are 100 percent recyclable – reducing impact on the global carbon load.

3. Daylight for all.

Using metal roofs with skylights or light-transmitting panels in conjunction with integrated dimming lighting is a highly cost-effective strategy, and IMP systems also have integrated window systems that increase available sunlight within building interiors. Light is essential for healthy buildings, and daylight is the best kind of all.

In addition, because rigid insulation per inch offers more R-value than per inch of fiberglass insulation and IMPs have metal liner skins, day-lighting fixtures such as light tubes can be integrated more easily with these roofs.

4. Proper moisture and air control.

Issues such as leaky walls and wet, moldy construction materials are anathema to wellness, and must be controlled for healthy building certifications. Mold has a negative impact on indoor air quality and indoor environmental quality, and one of the main culprits is trapped moisture. This can also corrode the metal studs and furring members, even if they are galvanized, leading to structural issues such as reduced fastener pullout resistance and leaks.

How Does a Building Become WELL Certified?

IMPs used as either rainscreens or as sealed barrier walls backing up a rainscreen are shown to protect against moisture issues and mold over time. They also serve as a continuous layer of insulation and air barrier. In this way, the single-component system can eliminate the need “for air barriers, gypsum sheathing, fiberglass insulation, vapor barriers, and other elements of a traditional multicomponent wall system,” says one industry executive. In fact, many masonry buildings are being upgraded with IMP retrofits on the exterior, directly over the old concrete, brick or stone.

All of these traits of IMPs certainly contribute to more healthy buildings, but do they add up to WELL Building certification levels, such as Silver, Gold or Platinum?

To get there, building teams must undergo an on-site WELL Commissioning process with rigorous post-occupancy performance testing of all the features. If it meets the “preconditions” — the WELL features necessary for baseline certification — WELL Certification is given. If the team pursues “optimization features,” the higher levels of achievement are granted.