RoofViews

Building Science

Is your Cold Storage energy use through the roof?

By Kristin Westover

April 19, 2021

Cold storage facility for fresh produce

This piece is co-written by Jennifer Keegan, AAIA.


The headaches of Cold Storage facility operations extend beyond making sure the ice cream doesn't melt. Owners and Operators are regularly challenged with:

  • Selecting a cost-effective roof system that is going to be long-lasting
  • Working around unsafe areas in the interior due to ice accumulation
  • Struggling to reduce monthly energy bills

For Owners who are looking to increase energy savings and safety records, your roof not only keeps the weather out, but can help resolve these operational issues.

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Cold Storage buildings are designed to maintain cold temperatures, much colder temperatures than a typical building. Cold storage facilities, such as blast freezers, may be required to maintain an interior temperature of minus 50 degrees Fahrenheit. Having a structure that is properly insulated and sealed to maintain the required temperature and minimize ice build-up is important not only for the products being stored inside, but also for potential energy savings over the life of the facility.

How can roofing materials impact energy savings?

Think of the walls of the Cold Storage facility as a jacket, and the roof as a hat. When it is cold outside, you want to make sure that you have a jacket and a hat to insulate and keep you warm. The same idea applies to a Cold Storage facility — the roof and walls of the structure insulate the products inside. But in this case, when it's warm outside, they keep the products inside cold. Not having enough insulation, on either the walls or the roof, will make your mechanical systems work harder to maintain the interior temperatures, which increases energy use, and can result in higher energy bills.

The effectiveness of roof insulation is determined by its R-value. According to Energy Star, R-value is a measure of an insulation's ability to resist heat traveling through it. The higher the R-value, the better the thermal performance of the insulation and its effectiveness at maintaining interior temperatures. R-value is typically expressed as a value per inch of insulation, and the recommended R-value of Cold Storage spaces will vary based on the interior temperature, although they are much higher than typically recommended for a traditional building. For comparison, a traditional office building may require an R-value of 30. In the 2018 edition of the American Society of Heating, Refrigeration and Air Conditioning Engineers' ASHRAE Handbook – Refrigeration, there are suggested minimum R-values for Roof Insulation between 30 and 60, depending on the cold storage type.

R-values will vary by product, including factors such as thickness and density. When calculating the total R-value of a multilayered installation, adding the R-values of the individual layers will provide the total R-value in the system. Particularly in Cold Storage, it makes sense to select an insulation that provides a higher R-value per inch, such as Polyisocyanurate (Polyiso, R-5.6 per inch), Extruded Polystyrene (XPS R-5.0 per inch), or Expanded Polystyrene (EPS R-3.8 per inch).

While insulations come in many thicknesses, it is a best practice to install several layers of thinner insulation rather than one or two layers of thicker insulation in order to reduce thermal bridging. Thermal bridging occurs when insulation is discontinuous between joints, allowing for air and thermal movement between the joints or gaps between boards. During installation, the use of several layers of insulation allows for staggering and offsetting the insulation joints, and blocks the passages that allow for air to bypass the insulation. Limiting thermal bridging can increase energy efficiency as it limits air movement between insulation boards.

Figure 1: Lower energy efficiency resulting from air movement between boards and fasteners acting as a thermal bridge.

Adding the adequate amount of insulation will prevent uncontrolled loss of the interior conditioned air, as well as assist in maintaining the required interior temperatures. Better maintaining the interior conditioned temperatures means that the cooling systems are required to run less often, which can equate to energy savings. While there may be an additional upfront cost to install an additional layer of insulation to increase the overall R-value of the roof, the cost should be minimal compared to the long-term savings of the added insulation. Of course, energy cost savings are not guaranteed and the amount of savings may vary based on climate zone, utility rates, radiative properties of roofing products, insulation levels, HVAC equipment efficiency and other factors.

What about the roof membrane? While there are many choices when it comes to the type of membrane, the most common discussion revolves around the color of the membrane. For a typical building, maintaining a comfortable space involves both heating and cooling, depending on the season. For the typical building, the color selection of the membrane has a greater effect when the interior of the building is being cooled. A highly reflective (light colored) roof membrane offers extra benefits when the interior is being cooled, because it will reflect heat from the sun. Similarly, for a Cold Storage building, it is beneficial to select a lighter-colored roof in order to reflect the heat from the sun to assist in reducing the already high costs related to cooling the building. Reflecting heat from the sun will decrease the heat radiating into the interior, which means the cooling equipment will not have to work as hard to maintain interior temperatures, and will ultimately work more efficiently.

What about roof attachment? We discussed the concept of thermal bridging and how energy loss occurs at discontinuities between the joints of the insulation, but thermal bridging can also occur where there are fastener penetrations through the roof system, as seen in Figure 1. Fasteners are used to attach the insulation and the membrane to the roof deck, which is referred to as a mechanically attached system. A way to reduce the thermal bridging that occurs at fastener penetrations is to bury them in the system or eliminate them altogether and install an adhered roof system. An adhered roof system typically fastens the bottom layer of insulation to the deck level and then subsequent layers of insulation, membrane and coverboard, are adhered. By eliminating the fasteners, the path for air to travel into the roof system is also reduced.

Figures 2 and 3 illustrate good and better scenarios, in terms of limiting thermal bridging and reducing air flow into the roof assembly. In Figure 2, labeled as the 'good' scenario, there are multiple layers of insulation, staggered and offset, but they are mechanically attached to the deck. While the staggered insulation layers limit some of the air flow into the roof assembly, air is still able to travel throughout the roof. In Figure 3, labeled as the 'better' scenario, only the first layer of insulation is mechanically attached and subsequent layers are adhered. By adhering the subsequent layers, air flow into the roof assembly is greatly reduced. Reducing air flow assists in maintaining interior temperatures, which can result in energy savings for the facility.

Figure 2: "Good Scenario" with staggered and offset insulation and a mechanically attached roof membrane.

Figure 3: "Better Scenario" with the first layer of insulation mechanically attached and subsequent layers of the roof system adhered, greatly reducing the air flow into the roof assembly.

The Devil is in the Details

The result of limiting air flow through the roof assembly of a Cold Storage facility is not a matter of occupant comfort, but a matter of occupant safety. In a traditional building, such as an office building, a poorly detailed roof termination could result in drafty offices or temperature complaints. In a Cold Storage facility, those same drafts condense due to the large temperature differential between the interior and exterior and the condensation can turn into ice. The ice can form on various surfaces including locations where air leakage is occurring, such as at roof-to-wall interfaces, but also on the Cold Storage floors where the surface of the floor is cooler than the air above it. When ice forms on the floors, it can cause slips, trips, or falls, and can also impact operations if a particular area of the facility has to be avoided.

Ice formation inside a Cold Storage facility is the result of improperly designed or executed details. Details, such as those at the wall-to-roof interface, or sealing around penetrations, are crucial to not only keep out rain, but to conserve energy within the facility. Similar to the loss of energy created by thermal bridging, air flow through the roof created by poor detailing results in considerable loss of the cooled temperatures required in the space below. Additionally, air flow that condenses can collect within the roof assembly, including within the insulation, and freeze. Frozen insulation is a common side effect of a Cold Storage roof that is not functioning properly. Frozen insulation is exactly what it sounds like — insulation that has had moisture accumulate within it and then freezes. Frozen insulation has properties similar to wet insulation and is ineffective, since it provides virtually no insulating properties. A frozen roof is almost like having no insulation at all, and the energy used to maintain the interior temperatures goes through the roof!

Proper detailing of a Cold Storage facility begins during the planning stage. Determining the type of interior spaces, the sizes, and the overall usage of the facility should be taken into consideration. Once the overall layout of the Cold Storage facility is decided, the construction materials, including the roof assembly, will need to be determined. Once the roof assembly is selected, design of the roof details is crucial. Typical details, including roof-to-wall interface and penetrations, must be meticulously thought out and designed.

Roof-to-wall interfaces and penetrations must be sealed to prevent air from entering into the roof assembly. Even the smallest gap that allows air flow can have detrimental effects on the roof assembly. The most common method of ensuring sealed terminations and penetrations is the use of a closed-cell spray foam. Closed-cell spray foam is typically installed at the intersection of the exterior walls and the roof insulation at a width of one inch and extends from the deck level to the top of the insulation. At wall-to-steel deck intersections, it is also best practice to install spray foam in the deck flutes a minimum of 12 inches from the wall. The closed cell spray foam helps to seal the interface so air cannot enter into the roof assembly.

Figure 4: GAF Detail 201C Coated Metal Roof Edge at Insulated Wall Panel

Proper execution of the roof installation is critical and requires a contractor with Cold Storage construction experience. Having the right partner who understands the importance of their role in the project and collaborates with the team can make or break the project. A future article will dive into these details. In the meantime, for information on GAF-certified contractors, talk to GAF first.

The benefits outweigh the risks. Seemingly insignificant decisions made during the design and construction of the roof of a Cold Storage facility can impact the functionality and energy usage of the building for the lifetime of the roof system, which is typically 25-35 years. Once air leakage occurs into a roof assembly, the damage that occurs is often irreversible. Ice accumulation on the floor can be a serious hazard for occupants and workers. The challenge of identifying where the breaches in the roof assembly occur, let alone remediation, can be difficult and costly. Remediation of the identified problems generally includes removal of frozen insulation as well as addressing the identified problem areas which are often attributed to detailing and air leakage. The associated consequence of a poorly designed and installed roof is the cost of the energy loss. Mechanical equipment having to work harder to maintain temperatures will result in higher costs due to an increase in energy use, and the effect of the equipment working harder often leads to premature mechanical failures. The benefits associated with designing and installing a proper Cold Storage roof far outweigh the risks. A properly designed and constructed roof will save energy, prolong the life of mechanical equipment, and protect both the building's occupants and the goods being stored inside the facility.

Need to talk to an expert regarding Cold Storage roof design? Talk to GAF first. Email us at coldstorage.assistance@gaf.com for design questions, detailing assistance, and expert advice.

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About the Author

kristen westover headshot
Kristin Westover
Kristin Westover

Kristin Westover, P.E., LEED AP O+M, is a Technical Manager of Specialty Installations for low-slope commercial roofing systems at GAF. She specializes in cold storage roofing assemblies where she provides insight, education, and best practices as it relates to cold storage roofing. Kristin is part of the Building and Roofing Science Team where she works with designers on all types of low-slope roofing projects to review project design considerations so designers can make informed roof assembly decisions.

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We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these:Roof fasteners have a measurable impact on the R-value of roof insulation.High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners.Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches:Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes.Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study.Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners.Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

By Authors Elizabeth Grant

November 17, 2023

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