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Community and Q&A

Insulating hot water pipes?

Adam W | Posted in Lakesideca Techniques on

Hi folks –
At some point we’ll get a roof and then be able to close up the walls. While we wait – I have long thought about wrapping our hot water pipes with insulation. At the local big box – I have a couple option: 1. Rubber, Foam, and a fiberglass product.

– What are the thoughts on insulating hot water pipes? Any risk of condensation that would be problematic?
– We have installed a hot water recirculation loop – insulate that?
– Probably too big to insulate – but we also installed a drainwater heat recovery pipe…

We are in northern Virginia.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    In most areas of the country, pipe insulation is no longer optional. It's code-required. Check with your local code official to determine what the requirements are in your area.

    Insulation of hot water lines saves energy, while insulation of cold water lines eliminates condensation.

    I like to use the foam pipe insulation. Remember to tape any seams or corners.

    Drainwater heat recovery units usually aren't insulated, but you can if you want.

  2. Adam W | | #2

    Martin -
    Thanks. I must not be in the part of the country that requires it as we've already passed our plumbing inspection.

    On the drainwater heat recovery - could there be any benefit (if I can find it) in insulating the drain lines (say from the shower to the drainwater heat recovery unit)?

  3. User avatar
    Dana Dorsett | | #3

    The IRC 2013 requires a minimum of R3 on all recirculation loops (including returns) regardless of pipe diameter, and for all hot water distribution plumbing that is 3/4" or larger. IRC 2012 used to specify R3 on half inch or smaller if it was over a certain length. Whatever- go ahead and insulate whatever parts of the plumbing you can easily access, don't sweat any that is already inside framing cavities or behind ductwork etc. that isn't easily reached.

    The cheap foamy stuff is fine for domestic hot water temps- fiberglass is more relevant for 215F steam heating (and pretty expensive) The rubbery stuff is fine (often the best choice for refrigeration lines) but more expensive than the cheap foamed polyethylene.

    The tepid output temperatures of drainwater heat recovery units don't really call for insulation. It's not really not HOT water- not a lot warmer than room temperature unless you have a really big one (like a 4" diameter ten footer). I don't think I've ever measured my 4" x 4' PowerPipe output much north of 80F even at the end of summer when incoming water temps are warmer and the basement temps are in 70F range. In the winter with ~40F incoming water it's delivering water north of 70F, with the plumbing located in a ~65F basement.

  4. Adam W | | #4

    @Dana -
    Good to hear from you. I believe I looked at the PowerPipe because of one of your comments some time ago. I've got an 4" x 8' installed (probably too tall - we had to hammer out some of the floor to install a clean out below the pipe.

    I'll get the foam insulation around the hot water as best I can. Biggest problem I see going forward is just all the places where the pipe is anchored to framing.


  5. Nick Welch | | #5

    I'm sure there could be some benefit in insulating the drain lines that run to the DWHR, but I would imagine it's small. If you have foam-core PVC/ABS drain pipes, they already offer some amount of insulation. Cast iron pipes would benefit more, since they conduct heat so well.

  6. User avatar
    Dana Dorsett | | #6

    Adam: The R4-96 has a 68% return [email protected] 2.5gpm under NRCAN's test protocol, so with a total of 60F rise (45F incoming, 105F at the showerhead) you should be seeing about 2/3 of the total, or about a 40F rise between the incoming water and output, so expect to see ~85F out of the heat exchanger, maybe even 90F when incoming water temps are at their warmest.

    The R4-48 in my house tests at 47.5% efficiency, good for only about half the total temperature rise.

    Very little of the heat energy is given up to the drain plumbing- the surface contact area with the water on horizontal runs is very small compared to the vertical runs, where the water clings to the sides and spreads out in a thin film. The the amount of heat picked up and re-radiated or convected to by the drain plumbing is too little to really matter, since it's such a tiny fraction of the time. Between showers there water in the drains with a stored heat content- any losses that occur are when the water is running. If that adds up to a half hour per day (about 4 showers) that's still less than a 5% duty cycle.

    By contrast a recirculation loop has very high duty cycle, high surface area contact (100% of the inside of the pipe), and a fair amount of heat stored in the thermal mass of the water to lose over time- definitely worth insulating.

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