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Vaulted open-beam ceiling in Hawaii — concern for trapping moisture?

Andrew Wolcott | Posted in General Questions on

Hello GBA,

We have an old Hawaii home that is open air –windows open 24/7. The inside and outside are warm with a pretty high humidity.

The main room is a high open beam vaulted ceiling with a plywood ceiling. The plywood is also the roof deck. For looks we want to install cedar planks between the open beams.

 

The Plan (Sketch attached) : install 1×2 (or similar) furring strips inside the beams against the ceiling. Attach the cedar planks to the furring strips.

This will leave a 1 ½” gap between the cedar planking and the ceiling.

Optional – we may install radiant barrier on top of the cedar planks with the shiny side up toward the air gap.

 

Question – is there any concern for trapping moisture in this space?

 

Thank you for your time and comments,

Andrew

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Andrew,

    Do you now have AC? Do you ever envision adding AC?

  2. Andrew Wolcott | | #2

    Steve,
    We do not have AC, but may had mini-split AC in the future. Our home is very "leaky" with single-pane windows. I would envision running the AC in mostly a "dry mode" function with temperature no lower than 75. We have a pretty consistent large temp/dewpoint spread, and I would be concerned about creating condensation if we lowered the temperature too low. I think even 80 degrees interior with a lower RH (80degrees with <55-60%) would feel pleasant with ceiling fans. We could then lower it for sleeping.
    Thank you for any thoughts.
    Regards, Andrew

  3. Jon R | | #3

    Moist air rises, so yes, it would tend to accumulate there and then quite possibly drop to the dew point (there are many reports of this). Some insulation (which means less cooling) and no enclosed gap would be better.

  4. Andrew Wolcott | | #4

    Jon R,
    Could you please elaborate?
    We do plan to insulate on top of the roof.
    "Enclosed Gap" -- how tight is T&G? If just fit in between beams and nailed in place, I assumed this cedar T&G layer would still be a bit breathable.
    I suppose I could leave a small gap (not sure how this would look) at both ends to permit air flow?
    The dewpoint spread here is usually quite wide. I'm not sure how the rising air could cool enough to reach dewpoint. The ceiling should remain warm, so not create condensation?
    Any further explanation is appreciated!
    Thank you,
    Andrew

    1. Jon R | | #6

      You can search for summer ceiling condensation on GBA for more info. IMO, it usually occurs because there is a source of saturated warm air (eg, inward solar vapor drive) that rises up the walls and to the ceiling . There it cools to room temp, which is below the dew point. Blowing on it to mix/disperse it is one solution - but wouldn't do a good job on the 1.5" enclosed area you have drawn.

      In some cases, night sky radiant cooling can cool ceiling air to the dew point. Insulation (on top is fine) greatly reduces this effect.

      Your plan may be fine - just consider the potential issues.

  5. Steve Knapp CZ 3A Georgia | | #5

    Andrew,

    See this article on building an insulated cathedral ceiling (https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling).

    I asked about AC because it increases the risk of moisture accumulating in the roof sheathing. How big a risk you might have with occasional AC in a tropical climate I'm not sure.

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