Building enclosure design guidelines for freezers and cold storage facilities
Specialty Buildings Column Series, Part 5 of 6
August 11, 2010
Cold storage facilities can be thought of as typical, heated buildings turned inside out. Instead of designing to keep heat in during cold weather, they are designed to keep it out.
In refrigerated buildings, the stack effect is reversed. Colder, denser air tends to “drop” towards the floor, creating negative pressure at high levels and positive pressure at lower levels. Moisture-related damage in cold storage buildings can vary from minor exterior or interior staining to complete failure of the building enclosure, including structural failure of concrete slabs-on-grade.
There are two primary types of cold storage facilities—detached and attached. Detached facilities are buildings constructed for the sole purpose of producing or storing goods at low
CONDENSATION ISSUES—DETACHED FACILITIES
As noted in the introduction, designing a detached cold storage building requires the designer to think backwards—the warm side is now the cold side; cold air sinks instead of hot air rises; and a primary goal of the enclosure is to keep warm air out during both the winter and summer. Failure to follow these general guidelines can result in a variety of condensation problems.
Insufficient insulation in exterior walls and roofs can result in cold spots on the exterior of the building. The consequences of insufficient insulation can range from increased energy use to condensation or even frost formation on exterior surfaces. Thermal bridging through refrigerated enclosures is less of a problem in buildings that use continuous insulated panels as either the exterior walls or as a component of those walls.
However, in hot, humid climates, even fastener penetrations through insulation can result in exterior condensation and possibly microbial/algae growth or corrosion/discoloration of exterior finishes. The most effective enclosure system for a building depends heavily on the exterior climate.
A less visible (until failures occur) form of condensation is interstitial condensation within wall and roof assemblies. For a cold storage facility in the Northeast maintained at 20°F year-round, the exterior dew point will be higher than the interior temperature in the facility for more than 80% of the year. This creates a relatively constant condensation risk for the majority of the year, and outside air that enters the building could condense on interior surfaces. Water vapor flow through building materials poses a similar risk, as the direction of vapor “drive” will be towards the interior of the building for the same percentage of the year time.
For refrigerated buildings, the effect of colder, denser air “falling” toward the floor creates a negative pressure toward the top of the building and a positive pressure at the bottom—exactly the opposite of stack effect in heated buildings. This effect tends to draw humid air into the building near the ceiling above the space, typically through roof-to-wall joints.
Humid air infiltration can be problematic in buildings with attic spaces above the main cold spaces, especially if the ceiling insulation below lacks sufficient vapor resistance. This is often the case, as the insulation may not be considered “exterior” since it is inboard of the wall cladding and roof membrane.
Photo 1 shows the result of long-term condensation in ceiling insulation that resulted in its eventual overloading. During our investigation of this building, we found that the insulation board had absorbed 10 to 15 times its weight in water. Humid air infiltration into the attic combined with a discontinuous vapor retarder above the insulation contributed to widespread structural failure of the ceiling. In some areas, the problems accelerated after the refrigeration system was shut down due to melting of the accumulated ice within the panels, where the ice had actually increased the insulation board strength and provided additional “holding power” at fasteners—an unexpected result that necessitated removal of the ceiling with the refrigeration system still running.
Photo 1. Overloading of ceiling panels due to accumulated condensation
Due to the number of different trades and materials that meet at the roof-to-wall intersection, these areas are often a primary source of air leakage into buildings. For buildings with metal roof decking, the deck flutes can create conduits through which both air and moisture can flow and cause apparent “roof leaks” at deck joints and penetrations (Photo 2).
Photo 2. Apparent roof leakage at metal deck near roof hatch was actually caused by humid airflow and condensation within the deck flutes
Creating airtight transitions from the walls to the roofs is difficult and requires coordination between multiple designers and installers, but can often make the difference between a successful facility and an expensive remediation project. The overall strategy of installing continuous air and vapor barriers on the exterior of the building and using continuous, high R-value insulation must be followed in both the field of the wall and roof areas as well as at all transitions.
CONDENSATION ISSUES—ATTACHED FACILITIES
Condensation problems in attached cold storage areas are typically more likely to affect the adjacent spaces. A relatively common problem can occur when refrigerated rooms are inadequately insulated, resulting in condensation and possible microbial growth on their exterior surfaces, as well as on the interior surfaces of the adjacent spaces.
Photo 3 shows the exposed concrete ceiling in a retail store below a supermarket freezer room. The pink and black coloring on the concrete is microbial growth due to constant and heavy condensation. In this case, the minimal insulation in the freezer floor became saturated with water from floor washing operations, further reducing its insulating value.
Photo 3. Microbial growth on underside of poorly insulated freezer slab
Repairs to this type of construction must be designed to address both temperature and water vapor flow, as the freezer slab will be at a low temperature for the entire year. This is in contrast to typical exterior walls, where cold temperatures in the winter generally correspond to low interior moisture levels, and seasonal temperature swings and solar exposure provide warming and drying periods for any condensation that forms in the walls. Assuming a 70°F warm-side temperature, condensation will occur on a 40°F slab at a relative humidity (RH) of approximately 33%. Since the slab temperature is relatively constant, heavy condensation can occur during the spring and summer, when interior RH levels may exceed 60%, even in air-conditioned spaces.
High RH levels in the warm-side spaces create a vapor drive towards the slab. For a typical arrangement of a poorly insulated slab above an occupied space, the direction of water vapor flow will be toward the slab for the entire year. This creates the need for a strong, continuous water vapor retarder, such as continuous aluminum foil, on the warm side of the insulation. Without sufficient vapor resistance, water vapor flow through the insulation can cause concealed condensation on the slab and within the insulation. This can result in problems ranging from concealed microbial growth (Photo 4) or failure of the insulation due the weight of the stored condensation.
Photo 4. Microbial growth on slab shown in Photo 3 below 6 inches of vapor-permeable insulation
THE ‘BUILDING WITHIN A BUILDING’ CONCEPT
For detached, and in some cases attached, refrigerated buildings, constructing the freezer enclosure within a secondary shell can greatly increase the reliability of the overall building. In this scenario, the shell provides the primary weather barrier for the building, protecting the inner enclosure—which provides the insulation, air, and vapor barriers—from degradation due to weathering.
A partially conditioned or unconditioned corridor space between the enclosures can be used for maintenance or inspection of the primary enclosure. If appropriately ventilated, the corridor space can help to reduce the thermal load on the refrigerated enclosure, for example, by reducing the effects of solar heat gain. Although much more reliable than a single enclosure, the building-within-a-building concept is more expensive to design and construct.
FROST HEAVE AND FLOORING FAILURES
The cause of frost heave in refrigerated buildings is essentially the same as in ice rinks (see Moisture Control Tips for Ice Rinks). The primary difference between cold storage buildings is that most are operated year-round, eliminating the chances of subsurface ice melting during the “off season.” This creates a significantly higher risk of frost heave, as sub-surface ice accumulation can become significant enough to cause structural failure of floor slabs (Photos 5 and 6). Due to these constant low temperatures, active solutions such as slab heating pipes or ventilated crawlspaces are typically needed to prevent frost heaving in refrigerated buildings.
Photo 5. Slab damage in a refrigerated warehouse due to frost heaving of soils below
Photo 6. Slab damage in a refrigerated warehouse due to frost heaving of soils below
In addition to structural issues, slab cracks will also damage floor finishes. Even if slab cracks do not create the need for structural repairs, refrigerated food processing facilities must often maintain a smooth, unbroken floor system for hygienic purposes, and will not be permitted (by regulatory agencies) to operate with discontinuous flooring. Minor cracking of slabs that results in flooring failure may still pose a significant problem for such facilities.
A less common type of flooring failure may occur in food processing facilities that regularly wash down flooring with hot water. In a refrigerated building, washing with relatively hot (150°F) water can produce a sudden temperature rise in the flooring material, as well as a temperature differential between the center and the top surface of the flooring.
Figure 1 shows the calculated thermal effects of floor washing on an epoxy floor coating. In this case, a 90-second wash creates a 30°F rise in surface temperature and a 20°F temperature differential between the surface and the core. The resulting thermal shock can lead to debonding of the epoxy flooring and necessitate its partial or complete replacement.
Figure 1. Calculated temperatures in epoxy flooring under thermal shock loading
When designing flooring systems, it is critical to take into account the specific use of the building and the expected loads on the floor. In addition to resisting shock loads, floor systems for refrigerated buildings must be capable of handling prolonged periods of low temperature without embrittlement that could make them more susceptible to mechanical damage.
Damage to either the slab itself or the overlying floor system can result in expensive and disruptive repairs to refrigerated buildings. For a simple food storage warehouse, concrete repairs or reinstallation will require above-freezing temperatures, making it necessary to temporarily relocate the contents of the building. This can be extremely expensive due to the need for refrigeration during both relocation and storage of the contents. For refrigerated food processing facilities, whose profits are often directly proportional to their production rates, a month-long shutdown for flooring repairs could cost millions or even tens of millions of dollars in lost revenue. The difficulties in repairing slabs and floor systems in refrigerated buildings highlight the need for more thorough design to avoid these problems.
Sean O’Brien is a Senior Project Manager in the New York City office of Boston-based Simpson Gumpertz & Heger Inc. O’Brien specializes in building science and building envelope performance, including computer simulation of heat, air, and moisture migration issues. He has investigated and designed repairs for a variety of buildings types, from condominiums to art museums, and has published papers on topics including moisture migration in masonry wall systems and condensation resistance of windows and curtain walls. He can be reached at email@example.com.