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Dew point in a summer wall Zone 4A

rhl_ | Posted in General Questions on


I’m doing a chilled water system. According to this e pipe heat loss calculator here, I can work out what the skin temperature of a pipe would be at various insulation thicknesses.

According to this thing, if it’s 80 degrees ambient, and the chilled water is 45 degrees, then the jacket/skin temperature will be:


71.2 with 1/2″ inch insulation
75.6 with 1″ inch insulation
77.3 with 1.5″ inch insulation

This piping will be inside walls of the house. My question is, what could the dewpoint possibly be, so I can select the appropriate insulation thickness.

Could the dewpoint be higher than outdoor air dewpoints? Will it likely be lower?

The reason I am asking is I am using the uponor preinsulated pipe and I want to avoid buying more insulation than I need, since this stuff is extremely expensive.
I was planning to go 1″ but if I don’t need it, and 1/2″ will suffice, i’d like to understand that.

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  1. rhl_ | | #1

    This is what weatherspark has to say about humidity in my area, on the muggiest day of the year.

    Which suggest 1" insulation will keep the pipe above the _outdoor_ dewpoint on the hottest day of the year.

    It seems like if dewpoints inside the house were naturally lower than exterior dewpoints than I could easily get away with 1/2" insulation.

  2. Jon_R | | #2

    Interior dew point shouldn't go to 71.2F - your dehumidifier should ensure this.

    On the other hand, inside a windward exterior wall with wet siding and solar vapor drive - who knows? You can get condensation even with no pipe. Routing in interior walls would be safer with 1/2" being fine (except for any gaps in it). Would spray foam be a viable alternative?

    Pipe size depends on delta-T, pressure and the BTU you need to move.

  3. CramerSilkworth | | #3

    Typical worst-case dew points in Westchester are around 70-75F, with about 5 days' worth in that range. Assuming this chilled water system is for AC and/or dehum., it's highly unlikely indoor dew point will be anywhere near that - 60ish, worst case, for a properly functioning system keeping you comfortable. Unless you leave the AC/dehum off for a while in summer. Even then, it typically won't be any worse than outdoor dew point unless you have a major ventilation problem and/or indoor water/moisture source.

  4. rhl_ | | #4


    Of course the thing I am installing is the air conditioner.

    The wall in question is quasi-interior: I've made this little ascii diagram to illustrate whats going on:
    interior plaster wall | pipe | <- pipe chaise is ~6" wide.
    vvvv. | chaise |
    ---------------------------------------------| here |
    | 2 x 4 | exterior wall | studs 16" O.C. |

    The distance (inside the chaise) from the pipe to the wall is 3'. So it's not exactly "against" the exterior wall, but it is inside it's air space.

    Uponor sells preinsulated pex with the insulation bonded to the pipe (so no gaps!). This means when drilling holes you have to accomodate for the jacket of the insulation. So a 3/4" ID pipe, with 7/8 OD, plus 1/2" of insulation means a 2" hole. 1" of insulation is a 3" hole. And I need two of them.

    So from what you guys are saying if we assume inside wall dewpoint == exterior dewpoint then likely 1" of insulation is necessary, if inside wall dewpoint < exterior dewpoint by a at least 4 degrees then I can get away with 1/2" insulation always.

    So whats the consensus opinion on this?

  5. Expert Member
    Akos | | #5

    The temperature you are running at pretty close to mini split liquid line temperature. Most pre-insulated line sets will have 3/8" of insulation on it and I've never had issue with them sweating either outside or inside. Any exposed line though makes a mess, so make sure you insulate and air seal well near the ends/valves/manifolds/pumps.

    Running pre-insulated lines through interior walls is a pain, drill as big of a hole (even better notch if non load bearing) as you can. It is very easy to damage the insulation when puling through studs.

  6. rhl_ | | #6

    Thanks Akos.

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