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Moisture problems at the soffet vents

| Posted in Energy, Heating & Insulation on January 28, 2004 07:57am

We have a new home, well built, well insulated.  We are experiencing moisture problems below the soffet vents.  We have exposed rafter ends, and the venting is in the blocking between rafter tails.

There is aprox. 22 sq.ft of soffet vent area, and about 9.25sq. ft. of roof vent surface area.

There is no visible moisture problem on the interior of the house, although I have not tried to get above the ceilings to check the top surface of the batt insulation.

The moisture was at its most dramatic with sub freezing temps, and got to the point where there was visible moisture on the cedar siding, as well as ice on the metal screening of the soffet vents.  It is clear from discoloration on the siding that this problem has been present for some time.

I’ve attached a couple pictures taken while the condition was dramatic.

I want to think that the genesis of the problem lies in the imbalance of the surface areas of the roof/soffet vents.

Any help would be greatly appreciated.

Thanks

Frank Duff

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Replies

  1. gdavis62 | Jan 28, 2004 08:07pm | #1

    If you have had periods when the ridge vent is blocked by snow, this can happen.

    1. fduff | Jan 28, 2004 08:21pm | #3

      Thanks for taking the time to reply.  We have 'through the roof vents', not ridge, and they were not blocked.

  2. User avater
    johnnyd | Jan 28, 2004 08:08pm | #2

    What, if any, kind of vents do you have up on the roof?  Continuous ridge?  Through the roof?  Are they blocked? Is this a vaulted or cathedral ceiling?  Looks like you have warm , moist air getting into the attic or joist spaces and exiting through the soffit.  It shouldn't be there in the first place, and any that is there should have a better way to get out. 

    Are any bathroom exhaust, range, or clothes dryer exhaust fans venting into the attic instead of out through the rook?

    1. fduff | Jan 28, 2004 08:23pm | #4

      Hi

      Thanks for taking the time to reply.

      We have through the roof vents which were not blocked,  10' flat ceilings,  and to the best of my knowledge all of the exhaust venting goes to its on exit ports.

  3. xMikeSmith | Jan 28, 2004 08:33pm | #5

    fd... that's a huge file and hard to view... some ca't view it at all..

     here it is reduced to about 100  which is still big for dial-up modems

    Mike Smith   Rhode Island : Design / Build / Repair / Restore



    Edited 1/28/2004 12:36:54 PM ET by Mike Smith



    Edited 1/28/2004 12:37:31 PM ET by Mike Smith

    1. xMikeSmith | Jan 28, 2004 08:38pm | #6

      i brought it into Irfan view and reduced it some moreMike Smith   Rhode Island : Design / Build / Repair / Restore

      1. xMikeSmith | Jan 28, 2004 08:43pm | #7

        now that i can see it.. it looks like you have a short-circuit

        warm moist air may be leaking from inside your house and getting into your vent system , where it is below the dew-point.. the vapor condenses faster than your vents can carry it away to the ridge

        i would bet you have a weak point in your ceiling insulation scheme, right where the rafter sits on the plate..

        what is your insualtion at that point ?Mike Smith   Rhode Island : Design / Build / Repair / Restore

  4. dIrishInMe | Jan 29, 2004 05:41pm | #8

    I agree with most all of the above; warm air is flowing out of your soffit vents, and the moisture is condensing on the screens - but I also agree with your initial diagnosis that the imbalance of the roof to soffit vents is a root caues.  I think you know that air is supposed to flow in the soffits, and out the ridge, via the "stack effect".  Generally, using "through the roof vents" it is difficult to get enough ventilation, and it is not well distributed.  Ridge vents work better - hands down, and it's not a big deal to install "after the fact".  The brand I like supplies 18 sq in per liniar foot.  The ventilation should be roughly balanced between the ridge and the soffits, and if it is somewhat unbalanced, the ridge should have more.  This is supported by some model building codes - don't know about yours.  So, you have 2 options: 1) increase ridge ventilation or 2) decrease the soffit ventilation. 

    To further explore option #2, you would have to tell us the approximate square footage of the attic "floor" for the area that is being ventilated.

    BTW - batt insulation for an attic is a poor choice.  The inherient air leaks around pipes, wires, light fixtures, bath fans and whatever coupled with thermal bridging of the framing members makes it a marginal thermal barrier.  I think that after correcting the ventilation, you should consider getting proper baffles installed at the soffit vents and having loose fill insulation blown on top of the batts.
     
    Matt

    1. fduff | Jan 29, 2004 06:11pm | #9

      Thanks for taking the time to reply.  It's always nice to find some support for your position.  I think that you are right about the ridge vent being the end solution. 

      Adding more insulation at this time is probably not realistic, but now that the idea is planted, who knows.

      Again thanks to you, and everyone else for helping me.

      Frank Duff

  5. User avater
    rjw | Jan 29, 2004 06:19pm | #10

    I disagree with the view that simply improving venting at the ridge is "the" answer.

    That will probably treat the symptoms, but I believe the problem is that you're getting way too much moisture up there in the first place.

    I think you need top track down the source of the moisture and the route by which it is getting into the attic.

    Possible sources can include (but are not limited to: very wet crawl space (in my area, probably the leading cause of excessive moisture in the attic!) Or A bath or kitchen vent terminating in the attic. Possibly a waste vent stack - an unhealthy condition - and yes, they can move a lot of moisture up and out.

    Possibly a gas appliance flue exhausting into the attic or use of an unvented gas space heater. Either of these two can be deadly!

    Or simple failure to use you bath fans.

    _______________________


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