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During February 2021, 69 percent of Texans lost electricity during Winter Storm Uri, and almost half lost access to running water for an average of more than two days, according to a report released by the Hobby School of Public Affairs at the University of Houston. Texas was just one of several states battered by sub-freezing temperatures, snow and ice, but suffered the most from widespread power outages. This extreme weather was unprecedented.
In the immediate aftermath, as buildings warmed up and frozen pipes thawed, it was revealed that thousands of pipes had burst due to freezing, resulting in extensive property damage. Nearly one-third of residents reported water damage in their homes.
Plumbers from across the nation raced to affected areas with supplies, tools and expertise to help repair plumbing systems of all types, but some homeowners and building owners had to wait three weeks or more to have their pipe repaired so that water could be restored.
It was not just plumbing pipe that was affected — fire protection and hydronic heating/cooling systems were also frozen and sometimes broken by the deep freeze unless they were protected by adequate antifreeze solutions.
This extraordinary weather event and catastrophic damage in homes and businesses across the southern United States has placed a greater focus on several issues related to plumbing pressure pipe installations, such as:
• Should plumbing pipe be insulated? If so, by how much?
• What plumbing pipe materials are the most freeze-break-resistant?
The Plastics Pipe Institute (PPI), a nonprofit trade association based in Irving, Texas, had already been working to answer these questions for the plastic pressure pipe materials that its members manufacture (CPVC, HDPE, PEX, PEX/AL/PEX, PE-RT, PP-R, and PP-RCT). Thanks to the collaborative work of its members, two recent PPI publications help to address these questions.
Should Plastic Plumbing Pipe Be Insulated?
Even without the consideration of power outages, there are several situations where plastic pressure pipe materials should be insulated for protection against freezing. Plastic piping materials are inherently better insulators of heat energy and, therefore, worse conductors of heat as compared with traditional metal pipe materials.
However, in the situation of a prolonged power outage when entire buildings freeze up, pipe insulation buys time to hopefully prevent water in piping systems from freezing until heat is restored.
Insulation also reduces heat transfer through the walls of piping materials, improving system efficiency and performance in a wide range of applications.
The new document, PPI Technical Note (TN) 65, “Insulation Recommendations for Plastic Pressure Piping Materials in Residential Applications,” focuses on insulating plastic pressure piping materials from the perspective of freeze protection.
Requirements and guidance for insulating all types of piping materials for energy conservation are provided in plumbing, mechanical and energy codes, as well as industry standards such as ASHRAE 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and handbooks such as those published by the American Society of Plumbing Engineers (ASPE).
PPI TN-65 provides recommendations to prevent freezing in a variety of applications (e.g., plumbing, hydronics and fire protection). It also addresses the use of heat trace cable and shares techniques for thawing frozen plastic pipe. It’s intended for plumbers, builders, designers, inspectors and others.
Model plumbing and mechanical codes address the need for piping insulation and freeze protection in a variety of ways. To help readers identify the freeze protection requirements that exist in codes, TN-65 also includes excerpts from IAPMO’s Uniform Plumbing Code, Uniform Mechanical Code and National Standard Plumbing Code; ICC’s International Plumbing Code; Canada’s National Plumbing Code; and other model codes.
Neither the model codes nor PPI, however, can instruct anyone about exactly how much insulation to apply in general terms. The authors of TN-65 ultimately realized that the exact amount of insulation that is needed to protect pipe depends upon so many variables that it is impossible to provide accurate requirements in a prescriptive format, such as a chart or table.
For example, considerations include:
• What is the expected coldest ambient temperature for a given building?
• What is the tightness of the building to prevent infiltration of cold air?
• What is the exact position of every pipe within a structure and its surrounding air temperature?
• How to rely on historical weather data for a geographical location when extreme weather events are breaking records?
There is an old saying that “rules of thumb only work if everyone has the same size thumb.” The only safe way to provide specific R-values for insulation in a code would be to recommend levels of insulation that could be excessive for many projects and possibly insufficient for others. In the end, builders, plumbers, engineers and inspectors should collaborate to discuss the expected exposures for each pipe and the most appropriate insulation protection on a case-by-case basis.
What Plumbing Pipe Materials Are the Most Freeze-Break-Resistant?
Contrary to common belief, pipe bursting isn’t normally the result of water expanding in volume by 9.1 percent as it freezes into ice. Instead, pipe bursting occurs when freezing temperatures create ice blockages or “ice dams” at places within the pipe, and then additional ice growth applies extremely high pressures to the confined water volume.
Water is not compressible and acts as a hydraulic fluid when compressed. Experience has shown that even the strongest pipe materials can burst when liquid inside is frozen.
This situation can occur when uninsulated pipes are exposed to slightly different air temperatures or exposures, such as portions of pipe being installed in a colder area than other portions. The most exposed sections of pipe freeze first, creating ice blockages that pressurize the unfrozen water. As more ice forms, that water becomes more highly compressed.
Figure 1 shows the end result with the release of water pressure through a pipe wall. A flexible pipe can do a better job at resisting bursting than a rigid pipe.
PPI’s other new publication, Technical Report 52, “Resistance of PEX Pipe and Tubing to Breakage When Frozen (Freeze-Break Resistance),” addresses research that has been conducted on PEX tubing, regarded as a highly freeze-break-resistant piping material, and provides several types of data.
1. Thermal conductivity. According to PPI Technical Report 48, “R-Value and Thermal Conductivity of PEX and PE-RT,” the normalized thermal conductivity (K-factor) for PEX is 2.86 BTU·in/ft2·hr·°F, and for PE-RT it is 3.15. This compares with a thermal conductivity for copper tubing of 196 BTU·in/ft2·hr·°F.
Another way of expressing the relatively low thermal conductivity of PEX is to covert this to an R-value of 0.38 ft2·°F·hr/BTU, expressed as per inch of material thickness. The higher the resistance value, the slower the rate of heat transfer through the insulating material (see Table 1).
This means that for similar material thickness, PEX is 68 times less conductive than copper, which will significantly delay heat transfer through the wall of PEX tubing. This property can delay the freezing of fluids and potentially prevent many freeze events.
2. Material elasticity. “Elasticity” has been defined as “a measure of material stiffness or the ability of the material to stretch or deform temporarily under a load.” We use the modulus of elasticity to quantify the strength and flexibility of a material. A higher modulus of elasticity means the material is more rigid.
According to the Copper Development Association, the modulus for annealed copper tubing is 17,000,000 psi. The modulus of elasticity for PEX (see Figure 2) or PE-RT tubing is typically less than 150,000 psi, more than 100 times more flexible than copper. So, while copper tubing is many times stiffer than PEX and PE-RT, these plastics have elastic properties that allow them to expand somewhat and then return to their original diameter.
Since water expands when frozen, this elastic property is beneficial during a freeze event, as the pipe can expand with the water. CPVC, PP-R and PP-RCT pipes and fittings have some ability to withstand the freezing of fluids without breaking.
3. Cold weather behavior. Even at temperatures below -40 F (-40 C), both PEX and PE-RT tubing retain their flexibility. In fact, the so-called “glass transition temperature” for PEX materials, meaning the temperature below which the material becomes brittle and can shatter, has been published as below -148 F (-100 C).
This flexibility means that if water-filled PEX or PE-RT tubing freezes solid, the elasticity of the material typically allows them to expand without cracking or splitting.
4. Research. The topic of PEX tubing’s freeze-break resistance has been studied by several institutions and research centers during the past decades. PPI TR-52 references specific studies that have analyzed the behavior of PEX when subjected to repeated freeze/thaw cycles.
One of these reports notes that “PEX pipe was conclusively shown to be freeze tolerant up to 400=plus cycles” and “PEX piping materials are experimentally and analytically shown to be reliable under repeated freezing conditions.”
Based on the research, it appears that PEX tubing is one of the most highly freeze-break-resistant plumbing materials. Although a similar study has not yet been performed on PE-RT tubing, the results could be expected to be similar, based on its material properties.
Despite the ability of PEX or PE-RT to withstand freezing in certain situations, the freezing of water or other fluids within any pressure pipe should be prevented because a piping system with frozen fluid cannot perform as expected. For example, a frozen plumbing system cannot deliver water; a frozen fire protection system will not activate to extinguish a blaze; a frozen hydronic system will not provide heat to occupied areas.
Finally, fluid-filled pipe that freezes inside a concrete slab may not be able to expand evenly and may suffer localized damage, such as splitting. Even if the embedded pipe doesn’t burst, it could be damaged, and the concrete surrounding it could crack due to the expansion forces of the ice.
The information found in the new PPI publications TN-65 and TR-52 provide insight into how to install and protect plastic piping materials safely. Installers, builders, inspectors, engineers and designers should always refer to local regulations to determine the most appropriate requirements for protecting all types of plastic pipe and tubing against freezing.
PPI TN-65 and TR-52 can be downloaded from www.plasticpipe.org/buildingconstruction.
Lance MacNevin, P. Eng., is director of engineering, Building & Construction Division (BCD) of the Plastics Pipe Institute Inc. During his nearly three decades of experience in the plumbing and hydronics industries, he has authored numerous articles and has been called on frequently to give keynote presentations and educational seminars.