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  1. #1
    Join Date
    Dec 2011
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    5

    Question Attic Condensation

    We have added a couple of bedrooms and a bath to the attic area of a 80+ year old farmhouse. We built 5 foot knee walls, insullated with R19 and 1/2" foam board on the attic side. Then 6ml vapor barrier and 1/2" sheetrock on the room side. The sloped ceiling is 2x6 rafters so we installed vent chutes (1" air sppace),R21 insulation, 6ml vapor barrier and 1/2" sheetrock. In the open attic area we blocked off the cavities under the new floor and insultaed the open area to R38 and have vent chutes in every rafter cavity to the soffit.

    We have a continous ridge vent and continous soffit vents but have a lot of condensation in the open attic area and between the vent chutes and the insulation in the sloped ceiling.This only happens on the North side of the house where it doesn't get any sun in the winter. We live in NE Kansas.

    Any ideas would be greatly appreicated.

  2. #2
    Join Date
    May 2011
    Posts
    251

    Default Re: Attic Condensation

    Fiberglass and foam board does almost nothing to stop air leaks. SO you're getting moist air form inside leaking into the unconditioned space. This is why sealing the space and using spray foam to stop air movement is often best in these applications.

    You need to find where you have air leaks and seal them up. Most likely it's around light fixtures or any electrical penetrations.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  3. #3
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Attic Condensation

    Quote Originally Posted by Kansas Farmer View Post
    We have added a couple of bedrooms and a bath to the attic area of a 80+ year old farmhouse. We built 5 foot knee walls, insullated with R19 and 1/2" foam board on the attic side. Then 6ml vapor barrier and 1/2" sheetrock on the room side. The sloped ceiling is 2x6 rafters so we installed vent chutes (1" air sppace),R21 insulation, 6ml vapor barrier and 1/2" sheetrock. In the open attic area we blocked off the cavities under the new floor and insultaed the open area to R38 and have vent chutes in every rafter cavity to the soffit.

    We have a continous ridge vent and continous soffit vents but have a lot of condensation in the open attic area and between the vent chutes and the insulation in the sloped ceiling.This only happens on the North side of the house where it doesn't get any sun in the winter. We live in NE Kansas.

    Any ideas would be greatly appreicated.
    Some things come to mind --- while placing the rigid foam on the attic side of the knee wall is a good idea to increaase the insulation performance with a continious thermal break , I'm thinking the 1/2 inch isn't really much benifit. I would recommend at least 1 inch or more.
    The rigid foam itself does have an acceptable air permance value but it's the seam joints that unless are sealed will leak air.

    The insulation in the 2X6 rafters --- R21 is designed to fill that spacing however, you have the vent chutes that set back from the roof sheathing 1 inch which means you have a void in the ceiling insulation. If the vent chutes extend into the attic space this void will have a negative impact of that ceiling insulation by allowing moving air within the insulation --- which is not good.

    The North side is not benifiting from solar heating as the South side does. The solar heat gain creates more active convective air currents which also helps with carrying the moisture out.

    Also. depending on what spaces are below on the North side ( like a bathroom ) --- if there are any penetrations from plumbing stack vents , electrical , etc., that aren't properly air sealed --- these could be causing more moist warm air netering the attic and condensing on that side.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  4. #4
    Join Date
    Dec 2011
    Posts
    5

    Default Re: Attic Condensation

    Thanks Guys,

    We have been checking all sources of air leaks into this area and fixing them as we find them.

    Canuk--We have re-sealed all of the joints in the foam board to make sure they don't leak. I can add more foam board as you suggested if i have to.

    The vent chutes and the insulation in the sloped ceiling do extend into the open attic area stopping right above the knee wall. Should I seal up the insulation at this point so only the vent chute is open into the attic area? If i seal this up should i also seal it up at the ridge vent end?


  5. #5
    Join Date
    May 2011
    Posts
    251

    Default Re: Attic Condensation

    Start also looking for air leaks downstairs as well. Stack effect is driving the warm moist air up and out. It's driven by the difference in air temperature and humidity. It's also accelerated by vent fans on either floor and by having low return ducts near the floor on HVAC systems.

    You could also try keeping the humidity indoors a little lower.

    For sealing leaks while insulating, nothing beats spray foam, but it's not cheap and has some other drawbacks.

    The challenge with vented attic assemblies, is that they are driven by convection currents. In winter, with the low sun, they are not very strong and almost non-existent at night and early morning, which is likely when all the condensation is occurring. so you're not getting a lot of airflow to mix with the warm moist air leaking into the attic.


    Old houses built before WWII, often had sealed attics, but didn't use humidifiers and cracked a window when taking showers or cooking.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  6. #6
    Join Date
    Dec 2011
    Posts
    5

    Default Re: Attic Condensation

    Hi motoguy 128,

    Spent all day in the attic sealing up leaks and reinstalling insulation. We had a strong south wind today 20-25mph and i noticed air coming in the south side soffit vents, going up the sloped ceiling vent chutes and then going back down the sloped ceiling vent chutes into the north attic area. Is this normal for some air to move from one side to the other?

    Any thoughts?

    Thanks

    Confused Kansas Farmer

  7. #7
    Join Date
    May 2011
    Posts
    251

    Default Re: Attic Condensation

    Quote Originally Posted by Kansas Farmer View Post
    Hi motoguy 128,

    Spent all day in the attic sealing up leaks and reinstalling insulation. We had a strong south wind today 20-25mph and i noticed air coming in the south side soffit vents, going up the sloped ceiling vent chutes and then going back down the sloped ceiling vent chutes into the north attic area. Is this normal for some air to move from one side to the other?

    Any thoughts?

    Thanks

    Confused Kansas Farmer
    Its not ideal, but to be expected in high winds. Not really a problem since air is still being pushed through the vented space. Now the bad news... is lighter winds, the airflow on the leeward side will almost completely stall. Meaning that as as many as 25% of the time in the fall, winter and spring when the wind is between maybe 5-10 mph, one side of the roof will get minimal ventilation depending on wind direction. The ventilation works great in the summer with a high sun driving it. IT works poorly in cooler times of the year without that driving force and stagnant air.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  8. #8
    Join Date
    Dec 2011
    Posts
    5

    Default Re: Attic Condensation

    Thanks motoguy 128

    I understand. Could some of this be caused by improper installation of the ridge vent? I dosen't look to me that the roofing contractor cut a wide enough slot at the ridge. We have a ridge board and it is my understanding there should be a 1" slot on each side of the ridge board (3-1/2" total width). I'm only seeing about 1/2" and in some places less than that.

    I have rechecked for leakage in the attics this morning with my thermal imaging thermometer (cheap one) and it looks a lot better than yesterday. Now to get the ventlation correct i hope.

    Thanks

    Kansas Farmer

  9. #9
    Join Date
    May 2011
    Posts
    251

    Default Re: Attic Condensation

    I'm not sure what the width of the slot is supposed to be. But an undersized opening would definitely reduce airflow. Not much you can do to fix it now. Adding vents on the roof or the gables will short circuit the airflow and can actually reduce overall air movement.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  10. #10
    Join Date
    Dec 2011
    Posts
    5

    Lightbulb Re: Attic Condensation

    I'm going to remove the ridge vent, recut the slot tothe manufacutres specifications and reinstall a higher quality ridge vent.

    Any recommendations on a high quality ridge vent? One with internal or external baffles?

    Thanks
    Kansas Farmer

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