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sub slab water line insulation?

mikeysp | Posted in General Questions on

Hi. I am in zone 4a and building a post frame house/shop.

I want hot water to arrive quickly to fixtures, so I was leaning toward a hot water loop using a circulation pump. 

Since the hot water loop will be under the slab and under the 2″ of recycled XPS, how do I effectively and economically insulate the water lines
so I am not paying to heat the earth?

Thus far, I have seen 6ft lengths of foam at the big box stores, but they do not appear very impressive for this strategy.

I am planning to use PEX for all water supply lines.

Thank you for your advice.


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  1. charlie_sullivan | | #1

    The best thing to do is design your floorplan like people did 5o years ago: all the bathrooms and the kitchen back to back, with the water heater right underneath so the runs are all about 10 feet or less. Then use small diameter pipe for each so there's not much water volume you need to displace each time you use it.

  2. Expert Member


    See if you can run the lines in interior partitions or bulkheads and avoid having them go back under the slab once the main supply comes into the building.

  3. mikeysp | | #3

    Malcolm, I think I can do that. I will look at my plan and make it happen.

    Charlie, I am trying to keep them close together from a efficiency standpoint; however, I do not see how a gallon out of a fixture changes anything in the lines or on the other end, since it will be the same gallon in for each gallon out. Am I missing something?


    1. charlie_sullivan | | #4

      We might be talking about different things, but what I'm talking about is if you have a 25 foot, 1" pipe, you there's about a gallon it in. If the water in the pipe is cold, and you turn on a 1 gallon-per-minute faucet, you have to wait a full minute before the hot water makes it to the faucet. If you cut the length to 12.5 feet, and the diameter to 1/2", you only have 1/8th of a gallon in the pipe, and it takes 7.4 seconds for the hot water to arrive at the faucet at 1 gpm. That's fast enough that you no longer need recirculation.

    2. Expert Member
      MALCOLM TAYLOR | | #8


      I did several houses with the waterlines poking in and out of the slab to service the various rooms. They drove the concrete finishers crazy, were difficult to locate exactly, and then make any future changes difficult. If you can I would avoid them.

  4. Expert Member
    Akos | | #5

    I've used pool noodles for low cost insulation. Stuff at the local dollar store comes in two convenient sizes, one fits 1/2" pex the other 3/4". Slit the pool noodle to install over the pipe and tape the seam. Run the whole thing inside an O-pipe.

    You can also double wrap the pipe with insulation for larger diameter pipe. 1 1/4" pipe insulation is a good fit over 3/4 pipe wrap.

    There are also pre-insulated pex lines you can order. They are commonly used to connect to outdoor wood boilers.

    1. Expert Member
      BILL WICHERS | | #6

      >"There are also pre-insulated pex lines you can order. They are commonly used to connect to outdoor wood boilers."

      It's probably worth mentioned that they outdoor boiler guys will tell you there are two types of pre-insulated PEX for underground use: one is so-called "wrap pipe", which has a material that looks like foil bubble wrap wrapped around the pipe. This stuff does NOT last, and should be avoided. The second type looks like pex with spray foam around it and a corrugated outer jacket. That stuff is much better, and also more expensive.


    2. charlie_sullivan | | #7

      I imagine that the difference between pool noodles and pipe insulation is that the pipe insulation includes fire retardants. Polyethylene foam pipe insulation is cheap enough that unless you are trying to avoid the chemicals, preferring the fire risk to the chemical risk, I think you might as well get the real stuff. It's often stocked locally in pathetic thicknesses, but it can be ordered in at least 1" thickness.

  5. Mark_Nagel | | #9

    A guy in Port Alberni (sp?) Canada (BC province) routed his hot water lines in the slab's EPS (under the concrete but above the ground): there are a bunch of YouTube videos of the build he did for a customer. Cold lines run completely under (to help keep them cold). I think a tip is to also sleeve the lines through the slab so that the concrete isn't bearing directly on them as they pass through the slab.

    I'm pondering which way I'll go. I have a bunch of Pex-A that was to be used for a retrofit of my existing place and that design was to utilize trunk and branch. My current design might utilize this (design and pieces). Pex-A doesn't have the nice large manifolds like Pex-B does, so the advantage of doing all home-run isn't quite there. Further, laying out all those lines in the slab adds to more possible headaches for getting things right: I want to concentrate on getting the drain stuff right!

    If I decide to place my trunks sub-slab then I'll for sure look to do what the guy in Alberni did. I don't, however, recall where exactly he ran the hot water lines: I'd think not having them being in contact with the concrete would be the way to go; if 3 layers of EPS then locate in the top of the second layer.

  6. mikeysp | | #10

    I decided to use a 2" electrical conduit 90 degree super long sweep and use that to rout some 1" pex into the building a couple feet into bathrrom wall. I will put a ball valve outside where I transition form PVC to PEX. This way, I can replace the water line easily at any time. I will stuff some steel wool and spray foam and it is below frost. The internal lines will rout in the framing.

    1. Expert Member
      MALCOLM TAYLOR | | #11

      Very good plan.

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