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Community and Q&A

Condensation prevention for houseboat insulation

Living_Alfoat | Posted in GBA Pro Help on

Hi, this is a revisit of the question about trying to prevent condensation and water damage while insulating, but in this case for a marine houseboat. We are talking a 3/8″ steel hull, 1″ air gap, with wooden frame and “rockwool” insulation bats just under 6 inches deep, then vapor barrier, wood veneered plywood sheeting interior walls. Vessel will have hydronic radiant floor heat using grooved plywood with aluminum face for the PEX tubing, and engineered wood flooring, just under 8′ deck to deck height. A 15 ft beam, and a LOA of 92 ft.. This vessel is in climate similar to Chicago, IL with the lake moisture. This pretty well sums up the situation, any advice to add or change to help avoid condensation issues?

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  1. GBA Editor
    Martin Holladay | | #1

    You may want to consult an engineer who specializes in boat design. My first guess is that you probably want to use closed-cell spray foam insulation, not rock wool.

    The problem is that it is very difficult to get a perfect air barrier and vapor barrier on the interior side of the rock wool insulation. Any small air leaks allow the circulation of warm interior air in the insulated cavity; the moisture in this air would then condense on the cold steel of the boat hull.

  2. charlie_sullivan | | #2

    This does seem like a different problem than one encounters in land-based structures, and we may be reinventing what marine engineers do routinely.

    But on the chance that this helps, here's what I'd think, based on the concepts we use on land. In cold weather, that 1" air gap could be used to circulate a little bit of outside air (forced actively, if necessary), and as long as the dew point of that air was below the water temperature, that would help keep everything dry. In warm weather, you would expect condensation on the inside of the hull, and actively venting would only increase the rate of condensation there. Perhaps that is OK, and there is a way for it to run down to collect at the bottom and get pumped out. If so, that condensation process would be a fine way to keep the insulation dry--it would be like having a dehumidifier running in that space. If it's not OK to let that surface be wet, the only ideas I have would be Martin's spray foam, sealing all the vents to that 1" space and running a small dehumidifier with its inlet and outlet ducted to that space to keep it dry.

    Is there anything used between the wood frame and the steel hull? That wood being wet from being in contact with the wet hull surface during humid weather might be a problem. Or is all the wood treated and finished such that it can survive that kind of moisture routinely?

  3. user-2890856 | | #3


    Could we see a picture of this vessel ? What make / model is it unless it's custom ?

  4. Living_Alfoat | | #4

    Gentlemen, thank you for your solutions, they are very welcome. Spray closed cell foam great idea, but, as a marine vessel, and most of these barge hulls are close to 100 years old, and they have to have new sheets of plate steel welded to them from time to time to stop leaks or collision damage. So, sprayed foam straight to the hull is a bit of a problem when it comes to repairs.

    I am including some links to a Belgium barge that was converted in 2008 through 2009 and has some very good photos of how the insulation and finish out is traditionally done. Maybe after reviewing some of the finish out photos and explanations you will have a better idea of what is currently done and maybe have some innovative ideas for better efficiency or follow traditional method.

  5. wjrobinson | | #5

    John, first I am not a boat conversion expert, not even close. Some thoughts.

    Standard 2x4 construction inside the steel hull; Will rot real nice. No boats are built with such. You mention hull repair with welding, and torches. With an inner 2x4 frame? I don't think so. Not logical to me anyway.

    I would say your friends are planning on their boats having a limit to life equal to the time they expect to own them. I would agree with that premise and do as they do with the improper materials. But I would at least upgrade to pressure treated wood. I think your conversion buddies try to get the hull in long term shape prior to the interior redo. I would. Anyhoot, you do what you want.

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