All images courtesy of GAF, unless otherwise noted

Cold storage facilities are exceptional structures that experience exceptional extremes. Due to their distinctive nature, cold storage buildings require unique construction assemblies, including roof assemblies.

Section 1: Defining Cold Storage Buildings

Cold storage refers to a building or portion of a building designed to promote extended shelf life for products or commodities. Cold storage buildings can house various industries including food processing plants, pharmaceutical manufacturing facilities, and holding facilities for dry, refrigerated, and frozen goods. Typically, these structures have year-round temperatures below 50 degrees Fahrenheit (10 degrees Celsius). There are varying levels of cold storage, such as coolers, chill coolers, holding freezers, and blast freezers. Coolers range from approximately 32–55F (0–13C), while, at the other extreme, blast freezers can have interior temperatures from minus 20–50F (minus 29–46C).

Cold storage buildings are exceptional structures because they experience extremes in both internal temperature and humidity compared to typical buildings. Due to their distinctive nature, cold storage buildings require unique construction assemblies, including roof assemblies.

The marked difference in vapor pressure between the building interior and the exterior can cause a pronounced vapor drive through the roof assembly. When warm humid air that infiltrates into the roof assembly there can be extreme consequences including significant build-up of condensation within the roof assembly, or even inside the building, if the roofing system is not properly designed and installed. These factors can lead to severe deterioration of both the roof assembly itself and interior components, as well as damage to stored goods and health and safety concerns for employees.

It is both the challenge and the responsibility of the design professional to correctly specify the cold storage building type and its roofing system. Careful selection of primary control layers of water, air, thermal, and vapor, and design of their continuity will aid the roof in its role of reducing the risk of condensation. High-performance roof design is essential to operations efficiency and decreased energy use of the cold storage facility.

Section 2: Cold Storage and Building & Roofing Science

Temperature and thermodynamics. Relative moisture risks. Vapor control materials. Air transport and air leakage. All of these elements are inputs into the equation of a high performance roof. Failing to plan for even one of these, or planning incorrectly, frequently results in energy loss and inefficient operations. This is especially true for a cold storage facility.

Why is a cold storage building so different? Cold storage is not a typical building simply because of its temperature, but due to the temperature differential between the interior and the exterior, and the resulting vapor drive from the exterior to the interior is significant. Because cold storage buildings are maintained at temperatures that are often much lower than the exterior temperature, the warm, moist, outside air wants to move to the interior of the building to achieve equilibrium. The direction of the vapor drive is predominantly from the exterior to the interior for most times of the year since in most climates the interior will be colder than the exterior.

While a concern is the vapor drive that occurs from the warmer exterior toward the colder interior, air infiltration into the interior of the cold storage building is a greater concern and can have detrimental effects to the roof assembly. Combating this requires two critical measures in the roof design: first, proper placement of a vapor retarding material to manage the vapor drive and second, proper detailing to prevent air infiltration at enclosure transitions and penetrations.

Condensation control

A common way to control or minimize condensation problems in a roofing assembly is to use a vapor retarder. Vapor retarders help reduce vapor diffusion into a roof system. The most common uses of vapor retarders occur in buildings located in colder climates, over concrete decks, and for buildings with high interior humidity levels, such as swimming pools, museums, and data centers. In these scenarios, the most effective location for a vapor retarder is directly above the roof deck and below the roof insulation layer, or in some cases, directly above a rigid board, like gypsum board, that is secured to the roof deck. The vapor retarder restricts moisture from the building interior into the roof assembly. This location of the vapor retarder is installed on the warm side (in winter) of the insulation.

Although the same principle applies for cold storage buildings, because the vapor drive is generally from the warm to cold, the vapor retarder is located on the exterior side of the insulation. This is generally true for most geographical locations in the U.S. for most months of the year; the interior is most often colder than the exterior of the building. Most commonly, the roof membrane serves as the vapor retarder; using the roof membrane as the vapor retarder places unique needs on any membrane selected.

In a typical cold storage configuration, the roofing membrane will act as both the vapor retarder and air barrier, keeping both vapor and air from getting into the roof system. However, successful installation and detailing are key to prevent air and vapor intrusion into the building interior and roof assembly. If heat and moisture reach a cold surface, this can create condensation. When condensation reaches the insulation, the insulation’s performance is reduced, which includes a reduction of R-value and equates to energy loss within the building. Condensation can also occur at building interiors such as at the underside of roof decks or on storage floors, which can impede interior operations.

Ice formation on the underside of the roof deck.

Fundamentals of moisture control

One of the primary purposes of a building envelope is to keep moisture out of a building. What makes this seemingly simple goal a challenge is that moisture comes in many forms and can take many paths into a building. Building designers need to account for bulk water, air-transported moisture, and water vapor, and defend against each of these in different ways.

Bulk water

Bulk water, such as rain and snow, is kept out of buildings with roof membranes and wall cladding systems. Roofs were first created for this purpose, and keeping out bulk water consists of watertight detailing that is typical for any roof assembly. Cold storage detailing requires augmenting of typical roof details to include air tightness in conjunction with water tightness.

Air-transported moisture

Air-transported moisture, as the name implies, is carried into or out of a building by air that infiltrates or passes through the building envelope. In a warm climate, air transports up to 10 times more water than diffusion, and in a cold climate, air transports up to 100 times more water than diffusion. To complicate this, there are several types of air transportation that can occur through the building envelope. Air infiltration is where exterior air enters into the building through gaps in the exterior enclosure. In cold storage, this can be problematic since most often the exterior air is warmer and more humid than the interior, which can lead to condensation when the warm air meets a cold surface.

Exfiltration is where interior air exits from the building, also through gaps and inconsistencies in the building exterior. Exfiltration can lead to the cooling systems having to work harder to maintain interior temperatures, which ultimately leads to higher energy bills. 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. Since air that enters from the exterior often contains moisture, condensation is likely to form as a result, often forming within the roof assembly. Condensation within the roofing assembly is not likely to evaporate, and instead this can lead to frozen insulation and roofing components. This is why air-transported moisture is much more critical to prevent than water vapor that enters a building by diffusion; condensation is likely to occur at any time there is uncontrolled air movement.

Air transportation.

Vapor drive

Cold storage buildings are commonly maintained year-round at temperatures that are, in most geographical areas and for most of the year, considerably lower than the exterior temperature. For cold storage buildings, the warm, moist outside air wants to move inward. This means the direction of the vapor drive experienced by a cold storage structure is opposite that of a conventional building—from the exterior to the interior. Therefore, the importance of a continuous vapor retarder from on the exterior of the building, such as the roof membrane, is critical to inhibit vapor diffusion.