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condensation on roof

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DJF
DJF Member Posts: 2
in THE MAIN WALL
I designed a project about 10 years ago which included a series of small guest rooms, each served by it's own split system heat pump with auxiliary heat. The roof is a standing seam metal cathedral roof with no air space between the insulation and the roof, and no vapor barrier. Condensation has been forming on the bottom of the roof assembly (at the underside of the vaulted ceiling of the space). One theory which has been mentioned is that the moisture is occuring because a heat pump was used, rather than a gas furnace. Is this possible/likely? Could the roof assembly be contributing or causing the problem? The project is in a climate which is quite dry, with high extremes in temperature.

Any thoughts would be appreciated.

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  • DJF
    DJF Member Posts: 2
    condensation on roof

    I designed a project about 10 years ago which included a series of small guest rooms, each served by it's own split system heat pump with auxiliary heat. The roof is a standing seam metal cathedral roof with no air space between the insulation and the roof, and no vapor barrier. Condensation has been forming on the bottom of the roof assembly (at the underside of the vaulted ceiling of the space). One theory which has been mentioned is that the moisture is occuring because a heat pump was used, rather than a gas furnace. Is this possible/likely? Could the roof assembly be contributing or causing the problem? The project is in a climate which is quite dry, with high extremes in temperature.

    Any thoughts would be appreciated.
  • Brad White_145
    Brad White_145 Member Posts: 17
    Only two things cause condensation

    Moisture in the air and a surface cold enough for it to condense upon. Sorry if that was a "duh" explanation, but it is really that simple.

    I would take a sling psychrometer or a hygrometer (humidity meter) and an infra-red temperature gun and check the space RH, drybulb temperature and surface temperatures. With a psychrometric chart absent better instruments you can determine the dewpoint. From there, seek surface temperatures colder than that dewpoint at differnt times of the day. An HVAC engineer familiar with psychrometrics might be able to help.

    Where is this located and at what altitude? What time of day is this observed?

    It does not matter if a heat pump or gas furnace is used to heat the space.

    (If you had gas and condensation, that could mean a flue gas leak which contains a lot of moisture. Not the case here we all hope and assume.)

    The means of heating neither add nor subtract actual moisture as part of the heating process. Makes it lower "relative humidity" but does not remove it.

    Absent a timeline of occupancy and conditions, I will surmise that the temperature swings are cooling the roof overnight (or the AC mode of operation is at any time) and internal moisture is coming to rest there. People respirating are worth between a cup and a pint an hour. Showers, cooking, coffee makers also contribute.

    If you have data loggers (www.onsetcomp.com) you can trend and see what is revealed for Temperature, RH, dewpoint over time. A place to start. But it gets down to the basics.
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