Cold storage: How smart roof design saves energy

January 3, 2022

[1]By Jennifer Keegan, AAIA and Kristin Westover, P.E., LEED AP O+M

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, lest they be forced to work around unsafe areas in the interior due to ice accumulation and struggle to reduce monthly energy bills.

For property owners who are looking to increase energy savings, a cold storage roof not only needs to keep the weather out but also helps resolve operational and safety issues. Cold storage buildings are designed to maintain cold temperatures—at much lower levels than a typical building. Cold storage facilities, such as blast freezers, may be required to maintain an interior temperature of -45.5 C (-50 F).

Having a structure 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 roofing materials can positively 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 you have a jacket and a hat on to insulate your body and keep you warm. The same idea applies to a cold storage facility—the roof and walls of the structure insulate the products inside. In this case, when it is warm outside, they keep the products inside cold.

Not having enough insulation, in either the walls or the roof system, will make mechanical systems work harder to maintain the interior temperatures, increasing energy use and contributing to 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 varies 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 applications, it makes sense to select an insulation with 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. This reduces thermal bridging, which occurs when insulation is discontinuous between joints, allowing for air and thermal movement between the joints or gaps between boards and fasteners (see Figure 1).

[2]
Lower energy efficiency resulting from air movement between boards and fasteners acting as a thermal bridge.

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.

Adding adequate insulation will prevent uncontrolled loss of the interior conditioned air and assist in maintaining required interior temperatures. Better maintaining the interior conditioned temperatures means the cooling systems are required to run less often, which can lead 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 vary based on climate zone, utility rates, radiant properties of roofing products, insulation levels, HVAC equipment efficiency and other factors.

What about the roof membrane?

While there are many choices for the type of roof membrane specified, the most common discussion regarding energy efficiency revolves around colour 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 colour selection of the membrane has a greater effect when the interior of the building is being cooled.

A highly reflective (light coloured) 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-coloured roof to reflect the heat from the sun AND assist in reducing the high costs of 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?

This article has 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 on page 31.

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 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, with subsequent layers of insulation, membrane, and roof cover board, 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, labelled 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, labelled 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 reduced, maintaining interior temperatures, and aiding in energy savings.

[3]
“Good Scenario” with staggered and offset insulation and a mechanically attached roof membrane.

Devil 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 of occupant safety. In a traditional building, such as an office, 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 of the building, 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 or falls, and can also impact operations if a particular area of the facility must 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 preventing condensation and conserving energy in the facility.

Like 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 in the space below. Condensed air flow can  also collect within the roof assembly (including in the insulation), and freeze.

Frozen insulation is a common side effect of a cold storage roof not functioning properly. Frozen insulation is exactly what it sounds like—insulation that has had moisture accumulate within it and freeze. 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 skyrockets.

[4]
Closed cell spray foam installed at a wall to deck interface to prevent air flow into the roof assembly.

Proper planning reaps benefits

Proper detailing of a cold storage facility begins during the planning stage. The type of interior spaces, the sizes, and overall usage of the facility should be taken into consideration. Once the overall layout of the cold storage facility is decided upon, the construction materials, including the roof assembly, must 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 designed. Roof-to-wall interfaces and penetrations must be sealed to prevent air from entering the roof assembly. Even the smallest gap allowing air flow can have detrimental effects on the roof assembly.

The most common method of ensuring sealed terminations and penetrations of a roof assembly 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 25 mm (1 in.) and extends from the deck level to the top of the insulation.

[5]
Ice formation on the underside of a metal roof deck as a result of an improperly functioning roof assembly.

At wall-to-steel deck intersections, it is also best practice to install spray foam in the deck flutes a minimum of 300 mm (12 in.) from the wall. The closed cell spray foam helps to seal the interface so air cannot enter the roof assembly.

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 roof installation.

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 to 35 years.

Once air leakage occurs into a roof assembly, the damage 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.

[6]
“Better Scenario” with the first layer of insulation mechanically attached and subsequent layers of the roof system adhered, reducing the air flow into the roof assembly.

Remediation of the identified problems generally includes removal of frozen insulation, as well as addressing the identified problem areas, 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.

Author 

[7]Jennifer Keegan, AAIA is the director of building & roofing science for GAF, focusing on overall roof system design and performance. Keegan has more than 20 years of experience as a building enclosure consultant specializing in building forensics, assessment, design, and remediation of building enclosure systems. Keegan also provides technical leadership within the industry as the chair of ASTM D08.22 Roofing and Waterproofing Subcommittee, and the education chair for IIBEC; and as an advocate for women within the industry as an executive board member of National Women in Roofing and a board member of Women in Construction.

[8]Kristin Westover, P.E., LEED AP O+M, is a technical manager of specialty installations for low-slope commercial roofing systems on the Building and Roofing Science team at GAF. She has experience with a wide variety of projects in the civil engineering consulting industry specializing in repair and restoration of existing buildings, primarily for commercial buildings and high-rise residential structures. Westover’s project expertise includes roofing, waterproofing, facades, parking garages, and pavements. She has performed assessments, written specification, and design documents, provided bidding services oversight, and performed construction contract administration. Westover also has experience in arbitration and litigation support, as well as LEED O+M certification for existing buildings.

Endnotes:
  1. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/Roof-opener..jpg
  2. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/Figure-1_Door-cold.jpg
  3. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/Figure-2_Door-cold.jpg
  4. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/Spray-Foam-at-roof-edge.jpg
  5. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/Ice-under-roof-deck.jpg
  6. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/Figure-3_Door-cold.jpg
  7. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/JKeegan-Headshot.jpg
  8. [Image]: https://www.constructioncanada.net/wp-content/uploads/2022/01/KWHeadshot-1.jpg

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