Pathways to Professionalism Proceedings of the RCI 20th International Convention & Trade Show Miami Beach, Florida March 31 – April 5, 2005 © Roof Consultants Institute 1500 Sunday Drive, Suite 204 • Raleigh, NC 27607 Phone: 919-859-0742 • Fax: 919-859-1328 • http://www.rci-online.org Design Considerations for Cold Interior Environments Ray Wetherholt, PE, RRC, CPWC Wetherholt and Associates, Inc. Kirkland, WA ABSTRACT This paper will discuss the uses of roofing as the vapor barriers and air barriers on cold storage buildings. Insulation types and thermal translation used in the installation will also be discussed. When designing a roof system on a cold storage building, the normally used principles of insulated roofing design are reversed. The vapor barrier has to be on the outside of the insulation, and has to be continuous with the vapor barrier from the inside of the building, at least in some cases. The materials used in roofing may have permeability that allows vapor transmission that causes icing on the interior of the building. The amount of icing may vary as a function of exterior air moisture content and the interior temperature of the building. SPEAKER RAY WETHERHOLT, PE, RRC, CPWC, is president of Wetherholt and Associates, Inc., and has been providing roofing, waterproofing, and building envelope consulting for over 20 years. Since 1984, Wetherholt and Associates, Inc. provides services in western Washington and to select clients throughout the West. Wetherholt – 131 Wetherholt – 133 Freezer buildings may contain many food staples such as fish and seafood, frozen fruit juice, dairy products, meat products, and others. According to the USDA, the United States has 3.16 billion gross cubic feet of general refrigerated storage capacity, including both freezer and cooler space. The five states with the largest gross capacity are California with 449 million cubic feet; Florida, 253; Washington, 189; Wisconsin, 167; and Texas, 159. Freezer space is defined as space that maintains temperatures at 0ºF. Cooler space is defined as space that maintains temperatures between 0 and 50ºF. Similar to an upside-down boat, a freezer building and its “exterior envelope” have to keep the water out. But it gets a bit more complicated. The building must also keep out uncontrolled exterior air. Design Considerations for Cold Interior Environments Photo 1 – Frozen and refrigerated seafood processing facility. Wetherholt – 134 The outside air may be warmer or cooler than the inside of the freezer, and usually contains more moisture. Cold air and air below freezing contain significantly less water than air above freezing. The colder it gets, the less likely the air will contain any significant amount of water. Think about the dry snow in the cold Rockies. Snowfall usually occurs between 25ºF and 35ºF, when a warmer and wetter air mass meets a cold air mass. Remember the deep fluffy powder skiing days? Or for you eastern skiers and boarders, the “ice ages?” Once the air gets cold enough, the water or moisture vapor gets wrung out of the air mass. The same sort of thing needs to be thought about when working with a freezer or cold storage building. The cold air within the storage building wants to wring the water out of the air entering from the outside. Problem is…this is condensation, which becomes ice in a freezer. Defrosting a freezer warehouse is considerably harder than defrosting your mother’s home freezer. Freezers and cold storage are usually cooled with ammonia, as it is the cheapest and most readily available refrigerant. Other refrigerants include CO2, CFC, HCFC, and similar products. The problem with ammonia is that it is both toxic and can be explosive in a volume of 16-25%. During the 1930s, fluorocarbon refrigerants were developed and became common in smaller package units and the residential marketplace (see Photo 2). Walls of freezer buildings may be steel, tilt-up concrete, CMU, or brick. The trick is to have a solid wall and roof deck onto which the continuous vapor/air barrier/ retarder may be applied. The vapor barrier should have a vapor permeance less than 0.01 perms. Note, however, that the laboratory perm rating of 0.01 may be 0.05 in the field. The principle is that as warm air meets the vapor barrier, the temperature at the vapor barrier is above the condensation point or dew point. The moist air is not allowed past, just as rain would not go past, and therefore the interior side of the vapor barrier stays dry. However, the interior face of the vapor barrier has to be protected and insulated to maintain that warmer temperature. Various insulations can be used, Photo 2 – Ammonia or freon compressor units, rack mounted. Photo 3 – Fiberglass insulation held in place with wood frame/grid in frozen juice plant. Wetherholt – 135 depending on the designer’s preference. In modern construction, these commonly include fiberglass, extruded and expanded polystyrene, and polyisocyanurate. Historically, ground cork and sawdust have been used. Think about how thick those walls were! The insulation can be installed with adhesive and/or stick-pinned to the inside wall vapor barrier, and then protected by a grid of wood framing or other similar method. The additional framing helps hold the insulation in place, as well as provides protection (see Photo 3). Obviously, the laps or seams in the vapor barrier need to be sealed, and the insulation joints offset to avoid a thermal short circuit. The attachment methods for any framing need to be isolated and minimized to avoid thermal bridging. Problems with freezers most commonly occur (at least in this writer’s experience) at roof-to-wall transitions and at penetrations. The roof-to-wall transition usually forces the vapor barrier from the inside surface of the wall to the outside top edge of the wall. This transition forces the vapor barrier to deal with the anchor points for the roof structure in an attempt to keep the vapor retarder continuous (see Detail 1). While Detail 1 with the embedded metal is not a great roof edge detail, it is pretty simple. One can see that the vapor barrier (pointed to by thick arrows) is relatively continuous. Sometimes, during the construction process, the vapor barrier becomes damaged as it wraps over the top of the wall (Photo 4). In this case, the warm, moist air entered the top of the wall and showed up as ice balls on the wall below, after saturating and freez- Detail 1 – Edge detail of frozen juice freezer shown in Photo 3. Wetherholt – 136 ing in the fiberglass insulation. Many times, the cooling and refrigeration equipment is outside the building or the freezer unit (Photo 5). Where the piping for the refrigerant goes through, the wall needs to be sealed in a manner that does not let air penetrate, yet allows for movement of the piping. The piping may move due to thermal expansion and contraction or vibration. So flashing boots, just like roofing, are required. Sometimes the designer relies a bit too heavily on the materials manufacturer who wants to sell a product that may not be appropriate for the application. The design in Detail 2 shows a lack of knowledge on the part of the designer (or at best, the drafter). Note that the roof system is a mechanically fastened single ply. In this case, the manufacturer convinced the owner that the thermal bridge of the roofing and insulation screws would not present a problem. Another source of air through the wall can be joints at wall panels. While in theory, air vapor transmission should be blocked on the inside of the wall by the vapor barrier, open joints in a tiltup wall panel or masonry wall may allow degradation of the vapor barrier, and, subsequently, vapor (and water) entry. Obviously, the roof membrane on the outside of the building serves as an excellent vapor barrier. However, some materials are more vapor permeable than others, and should be reviewed prior to use. The roofing system should be compatible and integral with the materials used for the vapor barrier from the interior. Photo 4 – Torn vapor barrier at the perimeter allowed moist air into the building in design shown in Detail 1. Photo 5 – Insulated refrigeration line penetrating exterior wall of freezer building. Wetherholt – 137 In summary, while the design can be complicated, the concept is simple. The vapor barrier is just like a roof over insulation, except that it is applied to both the roof and the walls. We have not discussed how the slab or floor is insulated and the vapor barrier installed, but the concept is the same. The consequences of not considering the effects of moisture vapor and vapor barrier can be expensive. Detail 2 – Shows mechanical fasteners (air fasteners) creating thermal bridging if membrane is mechanically attached to the metal deck below, as approved by the materials manufacturer. FOOTNOTES 1. USDA, Capacity of Refrigerated Warehouses, 2003 Summary, page 3. 2. USDA, Capacity of Refrigerated Warehouses, 2003 Summary, page 13. 3. When this paper was peer reviewed, the reviewer was concerned with the use of the term “barrier” instead of “retarder” as we use it on insulated roof assemblies. In this writer’s opinion, the term barrier is more appropriate. The intent is the same – but – a barrier should be just that, not letting air or vapor through. The barrier better be a very good, almost perfect, retarder! So much for semantics. 4. Fortifiber, Suggested Application Specifications.
