RoofViews

Building Science

Is your Cold Storage energy use through the roof?

By Kristin Westover

April 21, 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.

About the Author

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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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

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Defending Against Very Severe Hail

Think that your roof doesn't need protection against hail? Think again.Severe hail events are increasing in geographic footprint and are no longer just in hail alley. The geographic region that experiences 1 inch or larger hailstones has expanded to be nearly two-thirds of the United States. Nearly 10 percent more U.S. properties, more than 6.8 million, were affected by hail in 2021 than in 2020. Coinciding with the increase in properties affected by a damaging hail event in 2021, there was also an increase in insurance claims, which rose to $16.5 billion from $14.2 billion in 2020.Figure 1: The estimated number of properties affected by one or more damaging hail events. Source: NOAA, graphed by VeriskAccording to data from Factory Mutual Insurance Company (FM Global), a leader in establishing best practices to protect buildings, the review of client losses between 2016-20, showed that the average wind/hail losses averaged $931,000 per event. 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Loss Prevention Data Sheets provide FM's best advice for new construction and for Data Sheet 1-34, this includes new or reroofing projects on existing buildings. Data Sheet 1-34 provides guidelines to minimize the potential for hail damage to buildings and roof-mounted equipment. FM Global intends that the data sheets apply to its insured buildings; however, some designers use data sheets as design guidelines for buildings other than those insured by FM Global.FM's LPDS 1-34 identifies the hail hazard areas across the United States: Moderate Hail hazard area, Severe Hail hazard area, and Very Severe Hail (VSH) hazard area which are defined by hail size. Note that the VSH area roughly correlates to Hail Alley. Hail Alley receives more hailstorms, and more severe ones, compared to other parts of the country.Figure 2: FM's LPDS 1-34 map outlining the different hail categories: moderate, severe, and very severe. The Very Severe area is most commonly referred to as "Hail alley".The hail hazard areas are divided by hail size, with the Very Severe hail hazard area being the largest hail size of greater than 2 inches. As a result, roofing assemblies have to meet the most stringent hail testing for designation in the Very Severe hazard area.Figure 3: Description of FM Approval hail regions.Even if you are not in hail alley, or one of the states in FM's Very Severe Hail area, hail larger than 2 inches still has the potential to occur throughout the contiguous United States. The National Oceanic and Atmospheric Administration (NOAA) tracks weather events throughout the United States, including hail. NOAA's hail database includes information such as location, date, and magnitude (size) of the hail stone for each event. A sampling of typical data is provided below; note that several states that are outside of FM's VSH zone, had hail events that would qualify as VSH, where hail stones were recorded to be larger than 2-inches in size.Figure 4: Hail events in states that are outside of the VSH area, but qualify as VSH by size.How Do I Design For Very Severe Hail?In order for a roof assembly to achieve a hail rating, the assembly must pass a hail test. FM Approvals designs the hail tests including a different test for each hail hazard area. Hail testing generally includes the use of steel or ice balls that are dropped or launched at roof assemblies in a laboratory setting. Pass criteria vary by the test, but generally visual damage cannot be present to either the membrane or components below. Roof assemblies that pass each individual hail test are FM approved to be installed in each hail hazard area.There are thousands of FM rated assemblies and it can be difficult to choose just one. To start, it is important to note that selection consists of an entire assembly, however consideration of all roof components including the membrane, coverboard, and attachment method each play an important role in how the assembly defends against hail.Membrane selection is critical for Very Severe Hail prone regions. Thicker roof membranes, as well as higher performance grades that will remain pliable under heat and UV exposure over time and will outperform standard grade materials. Fleeceback membranes also provide an added cushion layer that buffers hail impact.Coverboard selection is a critical component of the roof system design. High compressive strength coverboards are an effective means to enhance the performance of the roof system when exposed to hail events. A coverboard will enhance the roof's long term performance by fortifying the membrane when hail strikes as well as providing a firm surface to help resist damage from typical foot traffic. It will also help the roof insulation below withstand damage from hail. While conventional gypsum coverboards and high-density polyiso coverboards provide excellent protection against foot traffic and smaller hail, they are not effective for VSH. Coverboards for VSH systems were originally limited to plywood or oriented strand board (OSB). The use of plywood and OSB is very labor intensive to install as compared to traditional gypsum coverboards, increasing the cost of the installation. Recently, coverboard manufacturers have developed glass mat roof boards which are a reinforced gypsum core with a heavy-duty coated glass mat facer. Not only do these boards provide protection against 2-inch hail and are an important part of VSH assemblies, they are also a FM Class 1 and UL Class A thermal barrier for fire rated assemblies. These boards are 5/8" thick and are 92-96 pounds per 4'x8' board; about 30 percent heavier compared to plywood yet easier to install as they can be scored and cut like a traditional gypsum board.Consideration of roof attachment method is critical for selection of VSH assemblies. Historically, mechanically attached systems were not able to pass the VSH tests; when an ice ball hit the head of the fastener or plate, the result was a laceration in the membrane. To avoid failures of the membrane at the fasteners and plates, the fasteners were traditionally buried in the system; the insulation was mechanically attached and the coverboard and membrane were adhered. This is still a common installation method and as a result, there are a large number of assemblies where the membrane and coverboard are adhered. Additionally, burying the fasteners allows for the installation of a smooth backed membrane. With the development of glass mat coverboards, there are VSH rated assemblies that can be simultaneously fastened (mechanically attached coverboard and insulation) that utilize an adhered fleece-back membrane.Figure 5: VSH systems. Left is simultaneously fastened 60 mil Fleeceback TPO over glass mat VSH roof board and Polyiso Insulation. Right is 60 mil Fleeceback TPO over glass mat VSH roof board adhered in low rise foam ribbons to mechanically attached Polyiso Insulation.Figure 6: A sample of available VSH assemblies.SummaryWhy Should We Design for VSH?Severe hail events are increasing in geographic footprint and storms with hailstones that meet Very Severe Hail criteria are occurring throughout the country. While designing for VSH is a requirement if a building falls within the VSH area and is ensured by FM Global, many owners and designers are opting for roof assemblies that can withstand VSH storms even if they are not insured by FM Global. Material selection, such as coverboard and membrane, are key components to managing this risk. Glass mat coverboards and thicker, higher grade single-ply membranes, such as fleece-back, increase the roof assembly's resistance to damage. Choosing the right roof assembly could be the difference between weathering the storm or significant damage from hail.What are the next steps?Learn about GAF's Hail Storm System Resources, and as always, feel free to reach out to the Building & Roofing Science team with questions.

By Authors Kristin Westover

January 30, 2023

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