GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Why is the interior of our exterior sheeting wet?

MuskokiKRC | Posted in General Questions on

Renovation on master bath, took drywall off, then vapor barrier and insulation, house is a very well built ( we built it) 30 yr old home, two story, and noticed inside of exterior sheeting wet? Also whether related or not, have had dark dirty water migrating down outside of vinyl siding, in area of this bathroom. Just had siding replaced 5 yrs ago from aluminum siding. Had typar placed as well, have had many contractors look, seems many suggestions, but no solid reasons why?
Have checked attic, ventilation there, moor venting, etc all good. After one contractors advise, I installed a 12 X 12 stack vent and sealed off large ( 24 x30) gable end vents I had as he said the new theory was to discourage cross venting and encourage soffet venting up Thru to peak. We are confused, frustrated and cannot seem to get anyone to solution?
Can you help?
Trust me, ask 10 people ( contractors) and get 10 different answers :0(
There is more I could say, but will wait to answer verses bore you right now.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    The interior side of your wall sheathing is wet because the sheathing is cold. It is a condensing surface during the winter. If warm, humid indoor air has contact with this cold sheathing, this type of condensation is to be expected.

    Do you have reason to believe that this wall sheathing has been wet for years? Or did you only notice the moisture when the wall was opened up?

    There is no way to figure out the problem of the "dirty dark water migrating down the outside of the vinyl siding" unless you post a photo of what you are talking about.

    I doubt that either of these two problems has anything to do with attic ventilation.

  2. MuskokiKRC | | #2

    Totally agree about the wet sheeting caused by cold on warm, I can try to send pic of siding. I guess the confusing part is that the insulation was not wet, the vapor barrier was good, mind you it was only 4 Mil required back then, now it's 6 Mil. I'm just not understanding how it can be so wet, when the insulation was dry and intact and the vapor barrier didn't seen jeopardized to such a degree to allow this kind of moisture / condensation?
    Not to mention the water migrating out of the siding drain holes? obviously this is not a cut and dry problem to solution. Really appreciate your thoughts on this.

  3. GBA Editor
    Martin Holladay | | #3

    Of course it's always possible that your sheathing is getting wet from the exterior side. This might happen, for example, if you have exterior flashing defects that allow rain to get to your sheathing, or if you have recently had an ice dam on your roof that allowed water to enter your wall assembly from above.

    These days, building experts don't recommend the use of an interior polyethylene vapor barrier in your climate zone. The three most important ways to keep your wall sheathing dry are:

    1. Make sure that you have well installed flashing where required, and make sure that the flashing is integrated with your water-resistive barrier (usually asphalt felt or plastic housewrap).

    2. If possible -- this isn't easy on an older house -- make sure that you include an adequate layer of rigid foam on the exterior side of your wall sheathing.

    3. Pay attention to airtightness at all aspects of construction.

    Keeping your wall airtight is more important than including an interior vapor barrier. For more on this topic, see these two articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |