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Thread: Hydronic heat problem?

  1. #1

    Default Hydronic heat problem?

    My close friend recently finished building a new house with hydronic heat. The house is properly insulated and the heating system works perfectly. The basement (which is only about 1/3 enclosed) is as yet unfinished, making it possible to look behind the insulation (both r-13 and r-19). When it's cold outside these walls are all wet. So much so that it runs onto the bottom stud and plate. In areas where there is no insulation whatever, the walls are completely dry. The house was built with a very tight house wrap to block wind. So what is causing the wet walls (osb)? The upper floor may be the same but as the walls are finished we haven't yet open one up to check their condition. Could the hydronic heat be part of the problem? The basement loop was put in 6" of concrete. The upper story hung from the 1st floor. Any ideas? All replies will be appreciated.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Get a hygrometer and see what the relative humidity level is in the house. If there are any uninsulated walls, just like a cold glass in the summer, it will sweat. You need to have enough insulation to move the dew point out further or lower the moisture levels. A really tight house, with numerous people taking showers, washing clothes, cooking, etc. (and even breathing) will add a fair amount of moisture into the house. You need to run the exhaust fans when showering and cooking. They also may want to consider an air-to-air heat exchanger to bring fresh air and exhaust the old from the house to prevent the sick house syndrom. There's more to making an efficient house than buttoning it up tight...people make pollution, too, and you need to dilute that with some hopefully fresh air from outside.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Master Plumber nhmaster's Avatar
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    Most likley it's the moisture in the curing concrete causing the situation. As the concrete finishes curing the moisture should abate also. Common thing with in slab radiant and something that often get's overlooked. A De-humidifyer will take care of it untill the curing is over.

  4. #4

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    Thanks guys, you both make sense. Will pass advice along. More replies would be welcome.

  5. #5
    Like an engineer alternety's Avatar
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    It does take a while for construction water to move out of the building. Concrete, drywall, paint, wood, etc. From what you have described, the basement is humid. Circulating air (wrong time of the year) or a dehumidifier will reduce this. You probably should do this in any case.

    The reason for the wet is that although your have insulation, it is not air tight at this point. So what is happening is you are keeping the insulated walls at a lower temperature (from the insulation) but then letting warm humid air behind the insulation. Result - condensation.

    The fix is to put in a vapor barrier. However it is not good practice to just cover an exterior poured concrete wall without a vapor barrier on the wall (concrete block, never never). You may also be getting more water diffusion through the concrete than you want. The barrier should extend under the headers and footers to channel water out of the wall structure if stray condensation does occur.

    Put the vapor barrier on the concrete, then reinstall the insulation. Closed cell spray foam would be the best solution (it is insulation and vapor barrier) but it is expensive. With a vapor barrier on the exterior wall you should not put a vapor barrier under the finished wall surface. It will trap any moisture that leaks in. If the condensation is ONLY interior humidity you could seal the finished wall. But you would really have to seal it perfectly. I would seal the outside wall myself. If you can remove or get behind the framing that would be best. Seal joints in the vapor barrier by folding over and sealing with a pliable caulk rated for the material inside the folded over joint. Taping is used but I don't trust it.

    Generally be sure you do not generate any water pressure on the foundation. Don't dump rain spouts at the foundation. Everything else you need to do, it is too late if the foundation is already back filled.

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