Insulating PEX hot water pipes

art_sMarch 30, 2012


My home was built with PEX water pipes. The water heater is in the crawl space and I would like to insulate the lines to help reduce the heat loss from the hot water lines. I have not been able to find much information on how this should be done. All of the poly and fiber glass insulation tubing that I found says its usage is for copper or iron pipe. The PEX manufacturers say to use the poly type insulation. I also have not been able to find what to do with "Ts" and elbows. While some products show them, others don't

I am confused and would like to hear from anyone out there that has any experience with this. Any and all help would be gratefully appreciated.



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Both fiberglass and poly pipe insulation can be used all all the types of pipe commonly found in residential construction. The designation that it is for Iron Pipe or Copper Tubing is simply telling you the internal diameter of the insultion.

Pipe is defined by its "inside diameter" commonly expressed as ID and the size is officially listed as "IPS" (iron Pipe Standard), while "Tubing" is defined by its "outside diameter" commonly expressed as OD and officially listed as "CTS" for the copper tube standard.

By example, if you have a 1/2 galvanized iron pipe the internal diameter of the pipe is 1/2" but the outside diameter is equal to the inside diameter and two times the wall thickness, thus a 1/2" iron pipe actually has an outside diameter which is slightly larger than 3/4" so the internal opening in the insulation for 1/2" iron pipe has to be slightly larger than 3/4" to reach around the pipe.

The IPS standard is a standard dimensions for iron pipe that is published by the ASTM ( American Society for Testing & Materials) and it is fundamentally the same for Galvanized Iron Pipe, Brass Pipes, PVC pipe & ABS pipe.

CTS is the ASTM standard for Copper Tubing, but it is also used as the standard for CPVC & PEX tubing so if you want to insulate PEX you would select insulation that is sized for copper tubing, although even if you were to use the IPS size, which is larger and would fit rather sloppy, none the less it would insulate just as well. (In fact, a purist could probably make an argument that the larger insulation might insulate slightly better because the additional trapped air space is also insulating.)

Pay very close attention to the insulation when you buy it. Some insulation is formed as a continuos tube, while some is a continuous tube but it has be cut lengthwise and a glue and a thin plastic separator has been put in the cut.

The continuous length type is often used when the insulation is being installed during initial construction because the pipe can be slipped through the insulation before being fitted into place. If you were to try to use that type during a retrofit you would not be able to get the pipe inside the insulation unless you either disconnect the pipe or cut the insulation.

For retrofit The poly type is much more user friendly because it is flexible and easy to use in tight quarters. It is also the best for insulating PEX because often the PEX is bent around a corner in a continuos run rather than using an elbow, however when doing so the PEX has a long radius bend which is very difficult to cover with rigid lengths of fiberglass insulation, whereas the poly will just bend with it.

For retrofit such as your project when you get the insulation make sure you get the type that has the opening on one side. You then open the insulation, slip it over the pipe, then pull the protective strip and the glue will seal the opening on contact. When you come to a tee you slip the insulation of from the opposite side of the tee side opening and just continue running the insulation in a straight line. You then go back and slip another length of insulation on the line connecting to the tee and slide it up tight against the insulation covering the tee. You need not worry about gluing it because slight gaps here and there will have nearly a negligible effect on the overall run.

    Bookmark   March 30, 2012 at 5:20PM
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Thank you for the great, detailed information. It will be a big help when I go shopping for the insulation.

I am not quite sure I understand the procedure for insulating around tees. I am trying to picture what you described in your message but I'm not certain that I get it. What I think you are saying is that I skip the tee and pick up the straight run following the tee. If that is the case, it seems very easy. I was also considering covering the exposed pipe under the first floor sinks. Does this make any sense or just stick with the crawl space pipes?

Once again, thank you very much for the information you shared.



    Bookmark   March 30, 2012 at 8:40PM
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Rather than get you more confused with a long winded explanation I took a few minutes and made up a simple illustration which may help you.

Or as you mentioned,when you come to a fitting, hanger or a joist you can simply cut the insulation and start again on the opposite side. (Lets face it, this ain't rocket science..LOL).

I would insulate all the hot water lines that are accessible and if you live in a cold climate where those lines in a crawlspace are subject to freezing I would insulate both the hot & cold lines.

Occassionally when doing rework over a slab we have to run lines through the attic space. In that case I insulate both the hot & cold lines. The hot to help deliver hot water to the load quicker and the cold to reduce the likelyhood of condensate forming on a line in summer and dripping on the ceiling below.

    Bookmark   March 30, 2012 at 11:19PM
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Thank you for the clear drawing. I now understand what you meant about how to cover the Tees. Thank you again for all of your terrific advice. Now all that's left is to buy the insulation and break my back installing it.

    Bookmark   March 31, 2012 at 11:14AM
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Let me add another tip:

Pick up some cable ties (zip ties). They come in really handy to secure the ends of the insulation where the adhesive doesn't really hold. Also good at tees.

If you have insulation that's slightly too large, the zip tie will close down the air gap at the ends to keep in the heat.

I miter the insulation at an elbow - it only takes a second, and it looks much better. That said, a little bit of pipe exposed isn't going to make much difference in terms of heat loss.

    Bookmark   April 2, 2012 at 3:10PM
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Thank you for the additional tips about using the zip ties and mitering the elbows.


    Bookmark   April 3, 2012 at 9:37PM
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