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Understanding Fiberglass Pipe Insulation Sizing

Understanding the correct size of fiberglass pipe insulation you need to order can be confusing at times, especially when distinguishing between Iron Pipe Size (IPS) and Copper Tube Size (CTS). Often times fiberglass pipe insulation will have two sizes on the box: one for IPS and one for CTS. The most common misconception is that you need a bigger pipe insulation size than pipe size. Intuitively, this makes a lot of since but it is important to know that the pipe insulation size has already been adjusted to fit the pipe. Therefore, if you are looking to purchase fiberglass pipe insulation for a 2" Iron Pipe then you would simply order 2" insulation size. The actual inside diameter of the pipe insulation will be about 2-3/8" I.D. which perfectly fits the O.D. of a 2" Iron Pipe.

The insulation size is most commonly sized to fit Iron Pipe Size. If you have a copper pipe you will have to convert to the correct IPS size. There is a copper tubing size conversation chart available on the fiberglass pipe insulation page but a general rule of thumb is to drop down one size from your copper tube size and order that size pipe insulation. For example, if you have a 1.5" CTS (1-5/8" Outside Diameter) you would order 1.25" Pipe Insulation Size (IPS size). If you have a 3/4" CTS you would drop down to 1/2" Pipe Insulation Size (IPS size). Make sense? Hopefully! But there are a few wrinkles. Any copper tube size 2" or higher has its own pipe insulation size. Over 2" CTS you take the outside diameter of your pipe and order that exact size. For example, a 2" CTS pipe measures 2-1/8" outside diameter so you want to order 2-1/8" pipe insulation. A 3" CTS pipe measure 3-1/8" outside diameter so you order 3-1/8" pipe insulation size. 

The last wrinkle in copper tube sizing is that there is one size below 2" CTS that has it's own pipe insulation size and that is 1/2" CTS which has an outside diameter of 5/8" so you want to order 5/8" fiberglass pipe insulation. 

We hope this help clarify some sizing confusion. Please feel free to drop us a line if you have questions: contact us


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