Humidity, part deux
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  1. #1
    Join Date
    Feb 2007
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    999

    Humidity, part deux

    I had posted earlier regarding high humidity and noted that I was pulling a lot of water out of the air (about 9 lbs per hour of running time), but household humidity ran in the low 50s most of the time.

    The basement humidity was found to be on the high side (about 70%), and since the (tight) windows are kept closed, this was assumed to be the source.

    Thanks to Dash's 'trick', my attention was drawn to the return system and found some large, hidden, leaks.

    I've sealed the leaks and my 'telltale' indicates approximately neutral leakage. Since then, the humidity in the basement has risen considerably, while the house is about the same.

    Would my assumption be correct that 'stack effect' is pulling moisture into the basement, and since it is no longer being drawn into the return ducts, it is 'collecting' in the basement (% RH rising) and into the house, since the RH is not lowering and I'm still pulling a lot of water out of the air.

    My basement has a full perimeter french drain which feeds a sump pump. The drain is always bone dry and the sump never runs, except when I test it.

    Will be examining foundation and sill plate.

    AM

  2. #2
    Join Date
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    What is the temperature in the basement when the relative humidity is 70%. It is possible that the absolute humidity is the same as in the house, but the relative humidity is very different due to temperature differences.

  3. #3
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    As Paul said.
    54 grains of moisture at 70* may be 50%RH, but 54 grains of moisture at 60* is 70%RH.
    Same moisture content, but relation to amount teh air can hold at that temp is what changes.
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  4. #4
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    Basements are damp because they are below grade, and are not often sealed well to prevent water vapor and moisture from wicking through the walls/floor and into the basement interior.

    The humidity levels rising in the basement (assuming temperature is about the same) would indicate you met with success on sealing the return side of your ductwork. Why you're not seeing lower humidity in the house proper may indicate there's not an adequate barrier between basement and house. Moisture is able to diffuse through gaps in the flooring over the basement...if a door is left open between basement and house, that's an obvious route for water vapor to travel to a drier area.

    You might consider getting a dehumidifier for the basement, and see how that affects humidity levels in the house. I would also want a good seal between the basement of the house, provided the basement is not finished, nor conditioned.

  5. #5
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    Quote Originally Posted by beenthere View Post
    As Paul said.
    54 grains of moisture at 70* may be 50%RH, but 54 grains of moisture at 60* is 70%RH.
    Same moisture content, but relation to amount teh air can hold at that temp is what changes.
    Paul/Been:

    The basement usually ran 10* +/- lower than house, as I recall.

    AM

  6. #6
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    So it probably has the same amount of moisture per pound of air. but the air temp can't hold as much.

    A dehumidifier would help.
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  7. #7
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    So it probably has the same amount of moisture per pound of air. but the air temp can't hold as much.

    A dehumidifier would help.
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  8. #8
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    Quote Originally Posted by shophound View Post
    Basements are damp because they are below grade, and are not often sealed well to prevent water vapor and moisture from wicking through the walls/floor and into the basement interior.
    The walls (below grade) were sealed with the usual stuff (black, tar-like).

    The humidity levels rising in the basement (assuming temperature is about the same) would indicate you met with success on sealing the return side of your ductwork.
    Roger, that.

    Why you're not seeing lower humidity in the house proper may indicate there's not an adequate barrier between basement and house. Moisture is able to diffuse through gaps in the flooring over the basement...if a door is left open between basement and house, that's an obvious route for water vapor to travel to a drier area.
    Disregarding stack effect, will moist (heavier) air 'diffuse' upwards to an area of lesser concentration? If so, I need to seal off all of the first floor boots and other penetrations. I guess a wiper on the basement door wouldn't hurt.

    You might consider getting a dehumidifier for the basement, and see how that affects humidity levels in the house. I would also want a good seal between the basement of the house, provided the basement is not finished, nor conditioned.
    I will probably end up getting a dehum for the basement, but I want to hold off and see what if any incremental improvements can be made.

    Any other suggestions gratefully accepted.

    AM

  9. #9
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    Quote Originally Posted by ampulman View Post
    I had posted earlier regarding high humidity and noted that I was pulling a lot of water out of the air (about 9 lbs per hour of running time), but household humidity ran in the low 50s most of the time.

    The basement humidity was found to be on the high side (about 70%), and since the (tight) windows are kept closed, this was assumed to be the source.

    Thanks to Dash's 'trick', my attention was drawn to the return system and found some large, hidden, leaks.

    I've sealed the leaks and my 'telltale' indicates approximately neutral leakage. Since then, the humidity in the basement has risen considerably, while the house is about the same.

    Would my assumption be correct that 'stack effect' is pulling moisture into the basement, and since it is no longer being drawn into the return ducts, it is 'collecting' in the basement (% RH rising) and into the house, since the RH is not lowering and I'm still pulling a lot of water out of the air.

    My basement has a full perimeter french drain which feeds a sump pump. The drain is always bone dry and the sump never runs, except when I test it.

    Will be examining foundation and sill plate.

    AM
    Very little stack effect during warm weather compare to winter. After trying all tactics, consider a good, 90 pint dehu. They are able to provide 50%RH throughout your home. I am maintaining near 50%RH with very little a/c operation. If your a/c was removing 50-70 pints (lbs.)/day, your home should be 50%RH. It is not hot enough in most parts of the green grass climate right now. Posting my last week data. If you seen it before, sorry. This house works well with minimum cooling and max dehumidification.
    Attached Images Attached Images  
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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