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Thread: moisture in attic.... help!

  1. #1

    Default moisture in attic.... help!

    Well, I've been slowly insulating my attic space (R30fiberglass unfaced on top of existing R19 kraft faced which is over some felt paper filled with vermiculate or similar) over the past few weeks. Noticed today as I went up for another round of installation that there's mold on the sheathing surface and a large wet area on the sheathing about 16'wide by 2-3' high. Lots of moisture on the nail heads as well. this is all on the north side of the house. South side surface is perfectly fine. It's been fairly dry even though it was raining for the first time in a week today.

    I had a couple other threads where Dana had helped me with some ideas on insulating near the soffits. This is a brick house with no soffits. The only venting comes from the gable ends. I was going to add the rigid foam and seal around at those locations but it's impossible to reach and do a thorough job. Now I'm just using baffles (not sure why at this point) and sending insulation (fiberglass) underneath along the whole ceiling.

    I'm really confused as what to do now. I'm 90% certain this is a moisture issue related to water vapor and not a roof leak because the area is so large. Not sure why its just affecting an area about 1/3 of the way up.(roof is 3 tab installed in 98) Originally insulation was shoved all the way to the ends of the roof line with no baffles. As I pulled the insulation back it looked like the sheathing was dry below but immediately above it was when the moisture would begin. It actually feels soaked to the touch and the knots (tongue and groove pine) are really wet making me believe its definitely an issue of condensation. I'm going to keep insulating and installing baffles. My hope is there is some air leakage at the ends of the eaves where some air may roll up into the attic cavity.

    Part of my concern is due to the construction (brick, air cavity, stud wall, plaster) in a really wet ground (high ground water table) that the cavity between the brick and stud wall is sending moist vapor up into the attic. Not sure if this is the case as I'm not certain how the construction is done in a case like this.

    If anyone has any idea what might be going on please help. I've also gone through the attic and sealed off all the locations I can find of wires or vent stacks/light fixtures coming through the living area. Will eventually insulate the basement with rigid foam and spray foam in the area above the bottom plate hoping this will help reduce the migration of heat loss up to the attics space. Just not sure if humidity is coming from brick or somewhere else.

    I've been in this house for 4 years now. I've noticed on previous observations some water staining in this same area and assumed it was ice damn related but not too certain now. I'm guessing this has been going on for some time but more worried this is more as a result of my recent activity in pulling the insulation blockage away from the roof.

    Hope you can help. Not liking the way it is up there right now.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Before the added insulation, you could have been leaking enough heat to keep the dew point at bay...now, the roof deck is colder since there's less heat leaking, and whatever moisture is there is condensing. The southern half gets more sun, thus heat during the day, and that may be why it is drier. Having a vapor barrier burried in the middle can be problematic as well. This is just a guess, see what others come up with.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    ventilation problem

  4. #4
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    jimbo wins the Kewie doll!

    This is an air-leakage/ventilation problem and NOT a vapor diffusion problem. Jim has it mostly right: when you simply add more insulation without air-sealing the ceiling (and sealing any leakage coming up through studwalls) the attic runs cooler, which makes the problem more obvious. The south side roof deck stays drier because of the greater solar-heat input.

    If the brick veneer cavity is vented into the attic rather than to the exterior, that's also a contributing factor. With the uninsulated walls the condensation doesn't happen below the roof insulation because it's all warmer there, but since it's not air-sealed air from the cavity can relatively freely convect through the insulation to the roof deck above. Depending on the particulars it may be possible to core in some exterior side vents to the brick cavity and air-seal the top to limit the amount of wall-leakage from entering the attic.

    If the sheathing on the exterior of the wall assembly is in good shape you can probably get huge reduction in air volume moving through the walls by blowing cellulose into the wall cavities, but if there are any big holes where the cellulose might end up in the cavity behind the brick that wouldn't be a good idea. Non-expanding injection foam would be safer (but more expensive). Find an insulation contractor who specializes in air-sealing to make that call. But don't be surprised if there are also big air leaks from the ceiling into the attic (which are best fixed BEFORE you insulate), or even from the basement to the attic via flue/plumbing/electrical chases. The first couple grand of air sealing in a house like that is almost always cost effective on fuel use alone, let alone the long term maintenance issues of condensation in the attic.

  5. #5


    Thanks for the feedback. Dana, I've gone through the attic per your earlier advice and sealed all the holes for wiring or can lights. I can't make it to the vent stacks. (not sure how anyone ever can fit into that crevice to reach where the stack comes up through the exterior wall. The stack takes a 90 then another 90 to go up the roof so its right in my way as I try to reach past it to the hole.) I'm going to seal the stacks that go through the basement up into the wall cavity to at least stop what's coming from the basement. All electrical outlets were sealed as well. I know you mentioned there's a lot of loss through the top plate and sill area. I plan on doing the sill as per your instructions once the attic is complete and then refinish but now that I'm in this jam I may just start moving with the basement. I don't have the will power to try and install those blocks up at the eaves in the attic. I can't get my arm far enough in there to do it properly.

    I'll contact some contractors. My neighbor up the street just had his done last year. I might bug him tonight to take a look up there and see if his is now doing the same as mine. (brick construction as well.)

    Not sure how to verify if this is coming from the brick air gap as opposed to the living space. I'm afraid if I insulate the walls as you suggest then the moisture coming up from the ground will instead condense on the now colder surface of the wall sheathing and start problems there....

  6. #6
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Ground moisture will wick into the brick and dry to the cavity and to the exterior. It's the LEAST of your problems relative to attic condensation. The wintertime outdoor air dew points are quite low compared to what your warm interior experiences. slowing down the transfer of that moisture-laden interior air into the attic is key, and with empty wall cavities the size of the relative pathway is huge. Caulking/foaming every electrical & plumbing pentration into the walls, every windo & door casing, every crown molding & base board on those walls can reduce, but not fully eliminate the air leakage into the walls, but insulating the cavities with sufficient density insulation impedes the flow to where it doesn't matter as much.

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