The design and construction of building envelopes for refrigerated spaces are critical to the long-term performance and function of that space. There are a lot of specific details that could be discussed related to this topic, but in general, a well-designed building envelope and assembly includes the following components — insulation, air barrier, vapour retarder, doors, and general construction systems.
We will explore the first three and go into more detail on general construction systems and more complex assemblies in the future.
Fundamentally, the purpose of insulation is to slow down heat transfer from the exterior into the refrigerated space. Insulation in refrigerated space construction assemblies serves two interconnected goals. First, the insulation needs to be thick enough to achieve any desired energy efficiency criteria. Second, it needs to be sufficient to prevent condensation on the exterior surface of the building assembly. Insufficient insulation thickness increases energy consumption and increases the size of the refrigeration system. For refrigerated spaces less than 3,000 sq. ft. that are constructed using prefabricated panels, there are energy code minimum R-values for coolers and freezers. Because of the additional energy costs associated with larger spaces, it is somewhat common that they are built with insulation values that exceed these energy code requirements.
There are many different insulation materials available, and while it is not possible to get into great detail here, it is important to understand that different materials have different insulating values and are not equivalent for the same thicknesses. The most common materials are polyurethane and polystyrene. In general, polyurethane is the most common material used for walk-in coolers and freezers; polyurethane foam used in insulated panels for refrigeration has an insulating value of approximately R8/inch, while comparable polystyrene products have an insulating value of approximately R5/inch. This means that for the same performance of insulation, polystyrene panels are thicker.
However, polystyrene is less expensive and certain dense cell polystyrene products have a lower moisture accumulation over time and therefore can be more effective over the life cycle of a freezer.
Other insulating materials can be used and it’s not really uncommon to see more generic building envelope construction techniques in “homemade” style walk-in boxes or when large areas of an existing building are refrigerated. In these cases, it’s extremely important to understand the vapour barrier because non-foam insulation is not a closed cell and can be damaged by water very quickly if the vapour barrier is incorrectly installed.
In most cases for refrigerated spaces, the air barrier and the vapour barrier are the same components, and this is because, unlike typical house construction in Canada, they both need to be on the same side of the insulation.
It is obvious that air infiltrating into a refrigerated space is not desirable as it will add humidity and heat to the space. To prevent infiltration, the building envelope and all penetrations need to be adequately sealed by some type of barrier that prevents airflow. It’s hard to overstate how important effective sealing is as air continuously passing through even a small opening is a very significant source of refrigeration load and can cause serious frost problems in a freezer.
While many materials can stop or slow air movement, it is more difficult to stop water vapour from diffusing through building materials. This is critical to understand and can sometimes be confusing. First, we need to recall that water vapour will always try to move from areas with higher vapour pressure to those with lower vapour pressure, even through many types of materials. The vapour pressure of water in the air is determined by the actual amount of water in the air. A material’s ability to prevent or slow this diffusion is measured in “perms.” Metals and glass are usually non-permeable but drywall, for example, has a permeance of around 30 perms, while a poly “vapour barrier” has a permanence of less than 0.1.
Warm and humid
The confusing part sometimes comes because people think in percentage-RH instead of the content of water in the air.
Room temperature 70 F air at 50 per cent RH has 55 grains of water per pound of air, which results in a vapour pressure of 0.3 in Hg vapour pressure (there are 7,000 grains in a pound), and a refrigerated space that is at 35 F and 95 per cent RH has 27 grains/lb air, which results in a vapour pressure of 0.18 in Hg. This means that even though the RH is much higher in the cooler, the moisture is going to drive from the outside air through the building materials and into the cooler unless an effective vapour retarder is installed. In Northern climates, this is the same thing that happens in the winter when the inside of your house is warm and humid, and the outside is cold and dry.
Moisture diffusing through the walls of a cooler or freezer is problematic for two reasons. The first reason is because it adds moisture to the space which causes additional refrigeration load. The second and more important reason is because it allows for water and frost accumulation in the wall, which can lead to serious damage to the building and/or structure and bacteria/mould growth.
Figure 1 shows a simplified representation of this. Imagine that there is no vapour retarder in place; the temperature through the wall will gradually decrease from the exterior temperature to the interior temperature. The moisture will also migrate through the wall because the vapour pressure on the exterior is higher than the interior. At the point where the moisture hits the temperature that corresponds with its dew point, it will condense, leaving water (and possibly ice if it’s a freezer) in the wall.
The discussion about proper vapour retarders comes up frequently and I often rely on the same story to highlight how critical it is to get this right. Consider Figure 3, which represents a simplified version of a medium- size 30 F cooler that I was involved with. To save costs, the owner elected to build typical two-by-six walls with a poly vapour barrier and rock wool insulation. They also increased the insulation amount in the existing attic.
You can see the problem immediately; it would be close to impossible to properly install a vapour barrier on the outside of the ceiling insulation. Instead, the contractors installed the typical interior wall vapour barrier. I wish I had a video of this, but about one week after this cooler was turned on, the owner complained of some dripping water in the cooler. When we responded and poked a hole through the ceiling and vapour barrier, dozens of gallons of water poured out. The entire cooler ceiling was completely destroyed.