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To Vapor Barrier or Not to Vapor Barrier

corrib | Posted in General Discussion on July 27, 2005 06:46am

Hey guys,

I’m getting ready for drywall and I am debating about whether or not to put up a vapor barrier in my below grade basement. I installed Johns-Manville poly-encapsulated (the stuff with the white plastic film on the outside), however it’s installed so that the front edge of the studs are exposed.

I cannot seem to come to a logical answer about the need for a vapor barrier. I’m in Wisconsin and several people have told me that I should use one because of the climate. Others though, have told me that I shouldn’t because the house needs to breathe. So, I’m not sure.

On a related note, I know the bathroom also needs to breathe, but if I install a vapor barrier should I install in on the two outside facing bathroom walls?

Thanks for taking a look at this. I ALWAYS appreicate the expert advice I get at Breaktime.

Thanks! 

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Replies

  1. Winwood | Jul 27, 2005 07:21am | #1

    I would most definitely use a vapour barrier.  What the stuff does is prevent condensation from happening on the structural members (your studs).  It also stops air from getting through which is less important for your situation as you're dealing with a basement. 

    The most critical area will be the wall area above the ground and as Wisconsin does get cold please use a vapour barrier.

    1. DgH | Jul 27, 2005 07:56am | #2

      And the other side of the argument...The wall should be properly air sealed. No gaps around and between the foam insulation. The insulation provides a warm surface to prevent condensation due to air movement. The insulation also slows diffusion so that that last 2% of moisture movement happens in a controlled manner.This way the wall can dry to the inside when it is necessary during the summer (mainly). The amount of moisture going the other way in the winter will be negligible since the air sealing is done.If the foundation wall is not properly air sealed then this whole idea goes out the window.Find out more at http://www.buildingscience.comIf my house wasn't a 100 years old... there would go a really fine hobby.

  2. User avater
    Dinosaur | Jul 27, 2005 08:04am | #3

    Yes, do use a vapour barrier on all exterior walls. In your climate it would be foolish not to.

    As for letting the house breathe, this is also extremely important, especially in buildings that do not have an air-exchanger installed. So you do not want to use polyethylene film as your VB; instead use the older type, foil-backed kraft paper. It does a good job of keeping condensation out of your walls while still allowing enough air to permeate the building envelope to avoid mould and fungus growth.

    Bathrooms are special cases, however. They generate so much humidity that I often put a poly VB on all four walls and the ceiling to keep that excessive moisture out of the framing. Don't forget the exhaust fan, though....

    Dinosaur

    A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...

    But it is not this day.

  3. RayMoore2G | Jul 27, 2005 02:29pm | #4

    I presume the basement waterproofed with a material that will also act as a vapor barrier on the outside. Adding a vapor barrier to the inside will create a wall with a vapor barrier on both sides that cannot dry out through diffusion. Many many basements in your climate have grown mold and rotted through the practice of using an interior vapor barrier in the basement.

    This is a link to a page on Joe Lstiburek's site that addresses this issue. http://www.buildingscience.com/resources/foundations/basement_insulation_systems.pdf 

    On page 7 he states that an interior vapor barrier should never be used in a basement. He also does not recommend fiberglass insulation in below grade applications. Now I know that some people have successfully done both, but it is a very dangerous practice that has led to mold and moisture issues in a huge number of homes.

    The "expert advice" that is offered here is well intentioned, but is often given by experts that are speaking outside their area of expertise. You should also beware of the line, "we've always done it this way."

    Before building a home, a smart owner will spend a great deal of time learning about the building science issues related to their region. It is not wise to ignore this and assume that their builder knows what they are doing in this area. Most do not have any understanding at all and simply adhere to prescriptive codes and building methods that are both outdated and potentially dangerous. Study every minute you have available. Start on Joe's  site and branch out from there.

    1. corrib | Jul 27, 2005 10:50pm | #5

      I'm not sure if I understand what you mean by interior and exterior walls. I assume by interior that I should not put up a vapor barrier facing into the finished area, but I'm not sure. Again, this seems to be a fairly common debate among contractors. It seems that in my area the vapor barrier is common, but.....

      Anyway, I'm interested to hear any opinions on this topic.

      Thanks for your information!!!

      1. Hubedube | Jul 27, 2005 11:18pm | #6

        Apply the Vapor barrier on all EXTERIOR walls. Staple it on the heated  (next to the drywall) face of the studs. Tape or lap any joints.

        The interior walls DO NOT require vb.

      2. DANL | Jul 28, 2005 02:26am | #10

        I think, and since no one else answered you this will prod and answer if I'm wrong, by "interior walls" they mean walls that have both surfaces on the inside of the house (as in walls between rooms) and by "exterior walls" they mean walls that have one face on the inside and one on the outside (the walls that separate the house from the outside world).

        As someone (Maverick, I think) pointed out, there is good arguement for not putting vapor barrier or insulation that acts as a VB on the inside of a basement wall because it is almost always damper on the outside of the walls than on the inside and you will only trap the moisture coming through the walls in the walls where it won't be able to get back out. Yet people constantly do this. Every basement remodeling job I've worked on the boss puts up a stud wall against the block wall, then we install fiberglass insulation with the vapor barrier facing inward. Not the way I'd do it on my house, but I am just following orders, mein herr!

        Vapor barriers on walls between rooms are unnecessary because the walls are warm and no dewpoint conditions exist where the vapor will condense on the insides of the walls.

  4. maverick | Jul 28, 2005 12:35am | #7

    I guess my answer will be unpopular but in my opinion a vapor barrier is counter productive in a basement. Before you erect any walls place a large sheet of plastic against a section of wall and leave it for a week or so. You'll see how much moisture wicks through concrete

    Moisture is going to wick through a concrete wall with or without it. Without it moisture will eventually make its way into the surrounding atmospere (like it does now) where it will be conditioned to acceptable humidity levels by your HVAC system or a dehumidifier.

    If you use a vapor barrier where is the moisture supposed to go if it is not allowed into the conditioned space? It will be trapped inside the walls.

    Which scenario is going to support mold?

    1. RayMoore2G | Jul 28, 2005 01:24am | #8

      ding ding ding

      Ladies and gentlemen, we have a winner!

    2. Hubedube | Jul 28, 2005 02:08am | #9

        But there will be NO moisture trapped if you use vapor barrier on the interior side (heated) of the exterior walls as there will be no moisture coming from the interior (liveable)side. A poly type vapor barrier will stop it from entering.

      So if the only moisture that is available is created on the weather (outside) side, and the 'tyvex' breathable wrap is on  the sheathing then it will halt any vapor /moisture from ever entering the stud space.

      1. User avater
        BillHartmann | Jul 28, 2005 03:47am | #12

        How many foundations have you seen that are wrapped in Tyvec?

      2. maverick | Jul 28, 2005 03:38pm | #15

        I dont mean this to be sarcastic but you're not getting it.

        Moisture will permeate a concrete or masonry wall. Period. Thats why we use a pressure treated mud sill even though the siding is flashed over the sill and no rain "should" get it wet, Moisture wicking up from the ground will rot an untreated sill it in a short period of time

        If you put vapor barrier either front side or back side of a wall erected next to that source of moisture you are trapping water between it and the source.

        The best way to counter the effects of wicking water is to get all the water you can away from the foundation, coat the exterior with waterproofing, and coat the inside with a masonry sealer. Then condition the air the way you did before there were any interior walls.

      3. maverick | Jul 28, 2005 03:46pm | #16

         

        >>So if the only moisture that is available is created on the weather (outside) side, and the 'tyvex' breathable wrap is on  the sheathing then it will halt any vapor /moisture from ever entering the stud space.

        Tyvek is NOT a vapor barrier. Tyvek is designed as an air infiltration barrier that allows moisture to pass through. In other words it stops the wind from blowing through the seams in your sheathing

  5. Gabe | Jul 28, 2005 03:30am | #11

    A lot of advice from different people who unfortunately are not informed as to your home or it's construction.

    The perm rate of masonry is ten times that of concrete but means little if the exterior is protected by either a platon system or other waterproofing membrane.

    You're less likely to have moisture problems if you're home is build on and backfilled with sand or other material with excellent drainage as opposed to clay for example.

    Regardless of the above, in most cases, the VB should be on the outside of the foundation to really work at optimum. Yes, I said on the outside of the foundation. You want to stop the moisture from permeating into your home. Dry concrete or masonry is the objective. You go to great lenghts to protect your upper house from moisture permeating into your studs and behind your walls so why would you not take the same steps to protect your lower house's walls from moisture as well.

    If you have poor dampproofing on the outside of your foundation then the next place to attach a VB would be on inside face of the concrete or masonry. Preferably a spray on VB.

    Under no circumstances should you put together a system that would trap moisture inside your finished wall.

    But again, it would be great to see your house construction before making too many comments.

    Gabe

  6. corrib | Jul 28, 2005 07:06am | #13

    Thanks for all the responses so far. 

     When I framed the walls, I snapped chalkline line to use as a guide for the plates, as the block walls were not plumb.  This gives me a little space between the block wall and the studs for air to circulate. My estimation is that this would allow most of the moisture to evaporate and not trap it in the stud cavity.  Would I be correct in the assumption?

    Thanks!

     

    1. RayMoore2G | Jul 28, 2005 02:48pm | #14

      Where would that cavity ventilate to? If it vents to the interior, then the effectiveness of the insulation is negated and the point of a vapor barrier to the interior would be more than moot. Way way way more moisture can be deposited on a cold surface by an air current than by vapor diffusion. If you put a vapor barrier to the interior side of the stud and let interior air freely enter that space, you will likely collect water in a puddle at the base of that concrete basement wall in the winter; although, it may absorb into the concrete for a long time before this happens. There are so many factors to consider in this equation, such as interior humidity levels, construction details, and winter surface temperatures of the basement concrete walls.

      Edited 7/28/2005 7:57 am ET by RayMoore

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