Cold storage facilities are specialized buildings that store perishable goods at low temperatures, typically below 50°F (10°C). The design of these buildings poses significant challenges for architects and engineers, as they have to ensure the thermal efficiency, moisture control, air control, and durability of the roof system. The roof system is a critical component of the building envelope, as it protects the stored goods from external weather conditions, prevents heat loss and gain, and maintains the desired indoor climate. In this article, we will provide a comprehensive guide on the key considerations and best practices for designing a roof system for a cold storage facility, based on the latest industry standards and research.

The first consideration is the type of roof system to use. There are two main types of roof systems for cold storage buildings: adhered and mechanically attached. Adhered roof systems use adhesives to bond the membrane to the insulation and the insulation to the deck, creating a continuous layer of insulation and vapor retarder. Mechanically attached roof systems use fasteners to secure the membrane and the insulation to the deck, leaving gaps between the insulation boards. The choice of the roof system depends on several factors, such as the design load, the wind uplift resistance, the fire rating, the cost, and the availability of the materials.

Adhered roof systems are generally preferred for cold storage buildings, as they provide better thermal performance, vapor control, and air control than mechanically attached systems. According to a study by the National Roofing Contractors Association (NRCA), adhered roof systems have a higher R-value per inch of insulation, a lower air leakage rate, and a lower vapor transmission rate than mechanically attached systems. Adhered roof systems also reduce the risk of condensation, which can cause corrosion, mold growth, and structural damage. Condensation occurs when warm, moist air comes in contact with a cold surface, such as the underside of the roof deck. Adhered roof systems prevent condensation by creating a tight seal between the membrane and the insulation, and by using a vapor retarder with a low perm rating. A perm rating is a measure of the vapor permeability of a material, and it indicates how easily water vapor can pass through it. For cold storage buildings, the vapor retarder should have a maximum perm rating of 0.05, according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

Mechanically attached roof systems can also be used for cold storage buildings, as long as they are properly designed and installed to minimize air and vapor infiltration. Air and vapor infiltration can occur through the gaps between the insulation boards, the fastener holes, and the membrane seams. Air and vapor infiltration can reduce the R-value of the insulation, increase the vapor pressure, and cause condensation. To avoid these problems, mechanically attached roof systems should use a high-density insulation board with a low perm rating, such as polyisocyanurate (PIR), extruded polystyrene (XPS), or expanded polystyrene (EPS). The insulation boards should be tightly fitted and staggered to reduce the gaps, and the fasteners should be sealed with washers or gaskets. The membrane seams should be heat-welded or taped to create a continuous vapor retarder. Additionally, mechanically attached roof systems should use a vented base sheet or a vented nailer to allow the excess moisture to escape from the roof assembly.

The second consideration is how the roof connects to the walls. The roof-to-wall interface is a critical area for preventing air and vapor leakage, as well as thermal bridging. Thermal bridging occurs when heat or cold is transferred through a material or assembly that has a higher thermal conductivity than the surrounding materials. Thermal bridging can reduce the effectiveness of the insulation and increase the energy consumption of the building. To avoid thermal bridging, the roof insulation should extend beyond the wall insulation and be sealed with a compatible material. The roof membrane should also be terminated at the wall with a metal coping cap or a counterflashing, and the wall should be insulated with a continuous layer of insulation and vapor retarder. The roof-to-wall interface should be designed according to the NRCA’s guidelines for cold storage roof systems.

The third consideration is the insulation material. The insulation material should have a high R-value, a low perm rating, and a high compressive strength. The R-value measures the thermal resistance of the material, the perm rating measures the vapor permeability of the material, and the compressive strength measures the ability of the material to withstand loads. For cold storage buildings, the insulation material should have a minimum R-value of 25, a maximum perm rating of 0.05, and a minimum compressive strength of 25 psi, according to the ASHRAE. Some of the common insulation materials for cold storage roofs are polyisocyanurate (PIR), extruded polystyrene (XPS), and expanded polystyrene (EPS).

PIR is a rigid foam insulation that has a high R-value, a low perm rating, and a high compressive strength. PIR is also fire-resistant and compatible with most roof membranes. PIR is one of the most widely used insulation materials for cold storage roofs, as it offers excellent thermal performance and moisture control. However, PIR is also susceptible to thermal drift, which is a gradual decrease in the R-value over time due to the diffusion of the blowing agent from the foam cells. Thermal drift can reduce the R-value of PIR by up to 20% over 10 years, according to the NRCA. To account for thermal drift, the initial R-value of PIR should be higher than the design R-value, and the insulation thickness should be increased accordingly.

XPS is another rigid foam insulation that has a high R-value, a low perm rating, and a high compressive strength. XPS is also resistant to water absorption and freeze-thaw cycles, which makes it suitable for cold storage roofs. However, XPS is also prone to thermal drift, and it has a higher environmental impact than PIR, as it uses hydrofluorocarbons (HFCs) as the blowing agent. HFCs are potent greenhouse gases that contribute to global warming. To reduce the environmental impact of XPS, some manufacturers have started to use hydrofluoroolefins (HFOs) as the blowing agent, which have a lower global warming potential than HFCs.

EPS is a rigid foam insulation that has a lower R-value, a higher perm rating, and a lower compressive strength than PIR and XPS. EPS is also more susceptible to water absorption and dimensional changes than PIR and XPS. However, EPS is also cheaper and more environmentally friendly than PIR and XPS, as it uses pentane as the blowing agent, which has a negligible global warming potential. EPS is also less affected by thermal drift than PIR and XPS, as it has a lower initial R-value and a higher long-term R-value.

The fourth consideration is the installation of the roof system. The installation of the roof system should follow the manufacturer’s recommendations and the industry standards. The installation should also account for the expansion and contraction of the roof system due to temperature changes. Expansion and contraction can cause stress and deformation of the roof system, leading to cracks, leaks, and failures. To accommodate the expansion and contraction, the roof system should have adequate expansion joints, flashings, and fasteners. The expansion joints should be located at regular intervals and at changes in direction, and they should be sealed with a flexible material. The flashings should be installed at penetrations, curbs, and edges, and they should be secured with compatible fasteners. The fasteners should be spaced according to the design load and the wind uplift resistance of the roof system.

The fifth and final consideration is the maintenance of the roof system. The maintenance of the roof system is essential for ensuring its longevity and performance. The maintenance should include regular inspections, repairs, and cleaning of the roof system. The inspections should check for any signs of damage, deterioration, or leakage, and they should be performed at least twice a year, preferably before and after the winter season. The repairs should fix any defects or problems found during the inspections, and they should be done by qualified professionals. The cleaning should remove any dirt, debris, or snow from the roof surface, and it should be done as needed or as recommended by the manufacturer.

By following these considerations and best practices, you can design a roof system for a cold storage facility that meets the requirements of thermal efficiency, moisture control, air control, and durability. A well-designed roof system can help you protect your stored goods, safeguard your operations, and reduce your energy use.

Darren Harrington is Vice President Commercial Business Real Estate, M&A Advisor of Creative Business Services/CBS-Global.

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