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we remodeled a trailer home recently, removed the aluminum siding, put up osb in place of the siding, then used the Lowes brand of house wrap. we installed new vinyl siding over that without using any of the thin fan board insulation that some people use.. on the inside, we are replacing drywall in areas where we took out or modified for new windows. we have waited for a week or so before placing the drywall and we noticed moisture between the insulation and the osb, question is, has this occurred mainly because the drywall was left off for several days while it is cold outside and warm on the inside of the house? we are hoping that the fact we didn't use that thin fan board insulation behind the vinyl is the reason for this problem. can anyone relate to this or know if I have a problem that will continue even with the drywall installed?

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2 Answers 2

You need vapour barrier (the plastic you put up before drywall) and air barrier.

To count as an air barrier you stop air flow by using sheathing tape on:

  • OSB joints and edges
  • Vapour Barrier (this is most common)
  • Tyvek Housewrap seams (becoming more common)

Some people think that you only need a vapour barrier or that the vapour barrier is also an air barrier. This is not true. You can make it an air barrier by sealing the joints and seams with tape and mastic, but otherwise it is only to stop Vapour Diffusion and not Air Flow.

Long story short: you had condensation because you did not have a proper vapour barrier, air barrier, or both. Since you seem to have a VB built in to your insulation, all you need to do is tape the seams and seal the edges at the top and bottom plate to make an air barrier.

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You did the right thing by sheathing the house in OSB and using a good house wrap. That is not the problem. It is more structurally sound and will hold your vinyl siding well for decades assuming it was installed correctly.

First you should rule out any other potential sources of moisture. Is this in a basement? Is there water leaking in through the foundation? Was the siding installed correctly?

A bit of condensation on the outside wall to the insulation can be perfectly normal. Make sure that you placed insulation with the vapor barrier (plastic or paper side) facing the outside wall. This will protect the insulation from soaking up moisture and potentially creating a mold problem. The moisture on the OSB cannot be helped, but is normal when you have an extremely cold outside and a warm inside.

The fact that you haven't placed the drywall yet has little to do with this. 1/2" gypsum board has a very small R value so it doesn't afford you very much more in heat loss to cause a condensation problem.

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thanks for the answers that helps. yes the siding is correctly applied, the insulation I have is what is called Encapsulated which has plastic on both sides with the material within the two sheets. I had the mindset that after I installed the drywall that I prevent warm interior air from being in contact with the insulation against a cold wall, I am hoping this is the issue. no water intrusion in basement or roof, in fact the roof was also replaced. –  user11130 Jan 17 '13 at 17:37
    
@user11130 The reason that double barrier insulation is even better is because during the hot summer, the opposite effect will happen, the cold inside air will cause condensation to form on the barrier facing the drywall. Again, this is also perfectly normal, but can be mitigated by running a dehumidifier in the house to reduce moisture. You can't realistically dehumidify the outside however so unfortunately not much you can do about that. –  maple_shaft Jan 17 '13 at 17:47
    
@maple_shaft: You state in your answer "Make sure that you placed insulation with the vapor barrier (plastic or paper side) facing the outside wall." However, this answer mentions " A vapor barrier of 4 mil plastic with taped seams should always be installed between the insulation and drywall.". Are these two advises contradicting each other, or do I misunderstand them? –  Reinier Torenbeek Apr 4 '13 at 0:57
    
@ReinierTorenbeek We are both correct for different scenarios. Think of a cold glass of water on a summer day, the condensation forms on the outside of the glass. Same principle, if you live in a warm humid climate then it makes sense to put vapor barrier between drywall and insulation as the cold inside will form condensation on the drywall because of the hot outside. If you live farther north however then it is colder outside and warmer in, so the condensation forms on the outside wall. The best situation is putting a vapor barrier on both sides for volatile climates. –  maple_shaft Apr 4 '13 at 1:38
    
@maple_shaft: Thank you, that makes sense. I am looking forward to some more of your answers on my future questions ;-) –  Reinier Torenbeek Apr 4 '13 at 1:42
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