How to add a vapor barrier to existing building?

fourthsiblingSeptember 1, 2008

I am in East Texas. I plan on converting an old woodworking room into a studio. It is 30 years old and was finished rough plywood sheathing directly over the studs. the interior was paneled in the cheapest rough particleboard the owner could find (looks like a pile or large wood ships and shreds were simply saturated with resin, then pressed into shape), but the paneling is in good shape and I don't really need to remove it. We though about just adding a vapor barrier, then covering the entire thing with a new layer of paneling or drywall. The questions are: If we add a vapor barrier, where does it go - on the outside of the building with a new layer of siding over that or on the inside with a new layer of paneling over that? And where would the insulation go? Is this kind of add-on advisable? Has anyone done this or something like this?


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Vapor barriers should be on the warm side of your insulation under whatever your interior wall finish is. So if you planned on putting a new layer of sheet rock or plaster, they would go under that. If you absolutely can't do that, then you can paint with vapor retardant paint, which helps but isn't as good. The purpose of that vapor barrier layer is to keep interior moisture (cooking, bathing, laundry, plants and humans' breath) from migrating into the insulation cavity and condensing when it reaches a colder thermocline. Once condensed it can saturate the insulation making it less effective and also allowing for mold growth within the wall. Not a pretty thought.

Now, there are other water/moisture barriers in many wall assemblies. The most common one you may have seen is Tyvek or Owens-Corning pink wrap plastered on the walls of houses under construction. These are really intended more as during-construction protection moisture barriers and air infiltration barriers along with water penetration blocks to prevent wind driven rain from getting inside. Theoretically they should actually be vapor permeable while being water impermeable.

Where does the insulation go? Well, basically in between the interior and exterior portions, in the wall cavities.

You may also read about vapor barrier/retarders on the soil underneath structures, These are to prevent moisture in the soil from rising into the living spaces and causing problems.

Now, all of this info is related to the Northeast's weather conditions, not East Texas, so you should check local standards. I expect you have more heat reflector/barrier issues, along with more extreme exterior water barrier codes ( i.e. using the Tyvek-type product) in order tyo cope with hurricane-driven water.



    Bookmark   September 2, 2008 at 1:17AM
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I know nothing about the climate in East Texas but the first thing to do is decide if the wall should dry to the outside or the inside and how much insulation is needed. The Building Science Corp. web site is a good place to start.

Here is a link that might be useful: wall info

    Bookmark   September 2, 2008 at 1:14PM
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Department of Energy region hot and humid. No vapour barrier or exterior only. See the map in the link.

Here is a link that might be useful: Vapour barrier placement

    Bookmark   September 6, 2008 at 12:44PM
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Look into Icynene insulation, a blown-in expanding foam product. I think it provides both insulation and a vapor barrier without disturbing interior or exterior wall surfaces.

    Bookmark   September 7, 2008 at 4:51PM
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If you choose to install a housewrap it is best to use to the non-woven polyolefin products like Tyvwk and Typar instead of the woven kind like PinkWrap, etc.

    Bookmark   September 8, 2008 at 7:42AM
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