Photo courtesy of GAF

High-performing cold storage roof assemblies often begin with mechanically attached polyiso (shown here) under an adhered coverboard and an adhered single-ply membrane.

Introduction

Cold storage buildings are often thought of as a typical building turned inside out. These buildings are designed to keep heat out instead of keeping heat in during cold weather. But is it that simple? This course will review the importance of the unique construction assemblies required for cold storage buildings, the primary control layers and their continuity, and how we need to think about the use of the primary control layers to reduce the risk of condensation in this inside-out scenario.

The intent of this course is to provide information that promotes long-term performance and durability, as well as the long-term energy efficiency of roofs and the overall building enclosures that are part of cold storage buildings. Much of this course focuses on transitions from roofs to walls and rooftop penetrations.

Let’s Start with a Definition

A “cold storage building” is a building or a portion of a building or structure designed to promote the extended shelf life of perishable products or commodities. It’s important to recognize that there are varying levels of cold storage, such as coolers, chill coolers, holding freezers and blast freezers. Coolers range from approximately 25–50 degrees Fahrenheit (minus 3.8- 10 degrees Celsius), while blast freezers can have interior temperatures from minus 40–50F (minus 40–45.5C).¹ The biggest difference from a roofing perspective is the amount of insulation for the varying levels of cold storage applications.

Note: For the purposes of this article, all of these varying degrees of cold storage and freezer facilities will collectively be referred to as “cold storage buildings.”

What Is the Risk?

The primary concern for proper roof design of cold storage buildings is the significant vapor drive that occurs predominantly from the relatively warmer exterior toward the colder interior. That directly leads to two critical aspects of the roof design: 1) proper placement of a vapor retarder to manage the vapor drive, and 2) proper detailing to prevent air infiltration or exfiltration at enclosure transitions and penetrations. Additionally, the reduction or elimination of thermal bridges (i.e., locations such as fasteners where heat flow is increased in comparison to the insulation) is important because of the critical need for a highly effective thermal boundary—which, of course, keeps the items within the cold storage building at the proper temperature all while using the least amount of energy.

Examples of Risk

There are many different examples that indicate there is some type of an air/moisture issue at roof penetrations and/or the roof-to-wall intersection of cold storage buildings. Many of these are substantial issues serious enough to call them enclosure failures. Typical scenarios that constitute failures include snowflakes (literally frozen water floating down in the interior) and icicles at perimeters and penetrations. These types of issues manifest themselves on the interior and are not only energy efficiency issues but also potential safety issues for occupants.

Ice can form within the joints of roof insulation and within the insulation itself. Gaps between insulation boards can fill with water vapor and become ice (see Photo 1). The R-value of the insulation is effectively zero when the boards and board joints are frozen. Insulation boards can freeze and heave up. Remember: water expands when it freezes. Non- or under-insulated expansion joints are probable locations for condensation to occur, possibly resulting in ice development due to excessive and continued formation of condensation. Roof details that do not include an air barrier are also potential locations for condensation to occur. A final example, the joints between insulated metal wall panels (IMWPs) used as the wall (and parapet) are susceptible to air movement and freezing. These examples represent a loss of performance, durability, and energy efficiency, as well as the potential of premature failure and the need to replace a roof system on a cold storage building.

Photo courtesy of Hutchinson Design Group Ltd.

PHOTO 1: Shown is ice buildup between rigid insulation boards in the roof of a cold storage building

Energy Efficiency and Air Leakage

There is a direct relationship between energy efficiency and air leakage. Therefore, a discussion about cold storage buildings would not be complete without some background about energy efficiency and air leakage. Energy standards have improved over the past 20 years, and these improvements have increased energy efficiency of buildings overall. Roofs can be a large portion of the building enclosure for one and two-story buildings. It’s important to manage the energy use of buildings, especially cold storage buildings that are users of large amounts of electricity.

Managing energy use starts with the design of the building enclosure. The building enclosure encompasses all six sides of a building, but oftentimes the interior of a cold storage building will have multiple interior space conditions, such as a loading dock or the office portion of the building. A cold storage unit within a larger storage facility with open truck docks, office space, and semi-conditioned storage can create a network of separate interior conditions that can lead to difficult enclosure tie-ins and connections between spaces with varying working environments.

The keys to managing energy include properly insulating the building enclosure and interior separations and reducing air leakage for a cold storage building. Every roof penetration is a potential air leakage location and should include air-sealing components. Because a roof can be such a large part of the building enclosure, it plays a large role in the overall energy efficiency of a building. In fact, energy efficiency has been shown to be improved in all climate zones when air barriers are used.²

For air to move into or through the building enclosure, two things are needed. First, there needs to be a pressure differential. This pressure differential can come from wind, stack effect or mechanical ventilation. Second, airflow needs a pathway into or through the building enclosure. Architects and roof designers—through the design of proper construction details—can significantly affect the long-term control of airflow into and through the building enclosure. It’s prudent to specify the use of air barriers that are constructible, compatible and properly located to reduce or eliminate paths for airflow.

There are three types of air movement that need to be considered when designing the building enclosure: infiltration, exfiltration and intrusion. Figure 1 shows each graphically. Essentially, infiltration and exfiltration are types of air movement through the entire building enclosure. Air infiltration is air from the exterior that enters the building through the enclosure, and air exfiltration is air from the interior that exits the building across the building enclosure. Air moving through the enclosure (both infiltration and exfiltration) is also called air leakage. Intrusion is air that enters a roof or wall system but does not exit to the exterior; an air barrier prevents the passage of air through the entire building enclosure.

FIGURE 1: Shown is air intrusion into a roof system and air exfiltration and infiltration across the building enclosure.

While this article is focused on the moisture that air carries, it’s important to point out that air also carries heat and contaminants. The unwanted loss or gain of heat energy via airflow increases the need (and expense) for heating and cooling equipment to operate and maintain a comfort level. Therefore, appreciably reducing the unwanted loss or increase of heat energy by incorporating air barriers reduces the burden on heating and cooling equipment. Also, air contains particulates, some of which can be contaminants for users and occupants of a building. For example, an idling delivery truck’s exhaust can be brought into a building when the air leakage is from the exterior to the interior.

The Building Science Perspective

Heat and moisture flow through the walls and roof of a cold storage building is quite different than a conventional building type, such as an office building. The second law of thermodynamics helps explain how heat, air and moisture move (see Figure 2), and a cold storage building is an excellent example of the need to understand these fundamentals. These principles include:

• Heat flow is from warm to cold.
• Moisture flow is from warm to cold.
• Moisture flow is from more to less (wet to dry).
• Air flows from higher pressure to lower pressure.
• Gravity acts down.

FIGURE 2: Shown is a graphic example of the second law of thermodynamics as it relates to heat and moisture.

Importantly, heat, moisture, and pressure always want to equalize across a boundary. For a cold storage building, this boundary is the “six sides” of the building enclosure—the roof, walls and floor.