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Community and Q&A

Insulate PEX?

David McNeely | Posted in Energy Efficiency and Durability on

If all plumbing lines are within the envelope, and a recirculating pump is installed, what might the payback period be for insulating PEX hot water lines?

Because the advantage of PEX is that the pipes are easily curved around obstacles etc., I bet insulation coverage wouldn’t be more than 90% in even careful installations.

In any case, insulation would serve no purpose whenever the wait between uses exceeded a couple of hours, no?

Is this one of those cases where the money would be better spent on more insulation elsewhere?

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  1. GBA Editor
    Martin Holladay | | #1

    I would guess that most pipe insulation only helps when the second hot water draw is within one hour of the first. So it is of limited use.

    If you have a tank-style water heater, however, it always makes sense to insulate the first 4 or 6 feet (at least) of the hot water pipes near the water heater. Beyond that, I would still recommend it -- but I haven't tried to perform a cost/benefit analysis.

    What you really want is a good plumbing layout, with the water heater close to the faucets. That way you won't need a circulation pump.

  2. David Meiland | | #2

    To some extent, your answer depends on whether you're in a heating or cooling climate, or in between. Your HVAC equipment efficiency also matters. We have a long (although mild) heating season, so I don't get concerned when I see water heaters and uninsulated piping inside the house--it's usually as efficient as the electric heat that is likely to be in the same house. In houses heated with propane, standby loss from water heaters and pipes is actually cheaper than the primary heating system. If you're in a cooling climate, you have the opposite issue.

    FWIW, PEX is very easy to insulate, easier than copper.

  3. Expert Member
    Dana Dorsett | | #3

    R3 pipe insulation is now code min for all hot water distribution plumbing under IRC2012:

    Are you really thinking going sub-code might be the right thing?

    At R4 (typical 5/8"-3/4" wall closed cell foam) it gives you about 30-40 minutes of hang time before the water is too tepid to be useful, so on successive short draws it's quite useful. See:

    Whether it "pays back" in a reasonable amount of time relative to some other measure depends on your use patterns, water heating fuel/efficiency, heating system fuel/efficiency, and the lengths of the runs, since that determines just how much heat is abandoned in the plumbing, eventually showing up as heat in the conditioned space, but in most cases R4 will have payback in well under 10 years, less than 5 if done as a DIY.

    David: "In houses heated with propane, standby loss from water heaters and pipes is actually cheaper than the primary heating system." Electricity is NOT universally cheaper per BTU than propane. Both are highly variable with location. Houses heated with propane usually have propane-fired water heaters too, and the net efficiency of a condensing propane furnace/boiler is far higher than a propane-fired tank. It pays to do the napkin math on the real utility-rates and fuel costs/efficiencies- there is very little basis for this type of sweeping assertion, even if it's true in many locations.

  4. David Meiland | | #4

    Dana, my answer only refers to my area. Maybe it's not clear that I am describing what I see here. It's intended to be a framework, not an analysis of his particulars, and my assumption is that he can apply it accurately to whatever his fuels and costs are.

  5. David McNeely | | #5

    Martin, thank you for your response; as always, helpful and pragmatic. I did indeed consider the plumbing layout as I designed the house, but in this case, other priorities took precedence (e.g. living space on south side, utilities on north side).

    David, please inform me how PEX can be easier to insulate than copper! Bending the foam insulation seems to open the seam, and with PEX there is a lot of bending! Also, do you drill holes big enough for the insulation through studs and joists? BTW, I am in zone 4, mixed humid—both heating and cooling.

    Dana, my house as designed is HERS 40; clearly, I am going well beyond what code requires (and my locale has yet to adopt 2012). But in this particular, according to your own example, insulation seems to offer value for a wait time of only 10–30 minutes, depending on how hot one wants the water to be. If the goal is conservation of resources, the recirculation pump seems far more efficacious than insulation.

  6. David Meiland | | #6

    David, have you personally put pipe insulation on both copper and PEX? I have, quite a bit, and PEX tends to have longer, sweeping sections than copper, which is straighter but has a fitting any time it needs to change direction. If the PEX has a tight bend with a bend support, that section can be hard to close the insulation around, but you can wrap it with a few strategically placed zip ties or possibly upsize the insulation a notch and simply insulate the bend support too. Sorry, I just have not found it difficult, certainly not moreso that copper. You don't drill the framing for the insulation, you install rattle plugs where the pipe goes through framing, and the insulation is interrupted.

  7. David McNeely | | #7

    David, I absolutely confess to being an idiot in all things generally, and specifically with regard to insulating PEX. Asking you to inform me was a sincere question, and I am sorry if it sounded snarky. I wonder whether this might even be a suitable topic for an article in Fine Homebuilding.
    Thank you for sharing your expertise.

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