# Rim Joist vapor barrier?



## wescoaster (Nov 17, 2010)

Hi, 

In the process of renovating my kitchen (second floor of slab on grade construction) I had to lift a portion of flooring to reroute a drain and water supply. 

I was quite surprised to see that the rim joist cavity was packed with fiberglass insulation and no vapor barrier over it. I can see the edge of the vapor barrier of the lower floor wall and I know the upper floor is vapor barriered. I pulled out the insulation at the rim joist and as expected it is quite soiled on the exterior wall side. 

The house is 24 yr old 2x6 exterior walls with building paper/vinyl siding over OSB sheathing.

I am leaning towards a DIY two part spray foam kit and replacing the glass batt insulation in the rim joist cavity only. This will be a slow progression around the entire house, as the remainder of the upper floor is destined for flooring upgrades over the next couple of years. Most of the lower floor is finished with textured ceilings that I would prefer not to disrupt, but will in a couple of areas where access from above is impossible.

The question I have is how to satisfactorily seal the existing vapor barriers where the edge is exposed?


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## jklingel (Dec 21, 2008)

wescoaster said:


> Hi,
> 
> 
> The question I have is how to satisfactorily seal the existing vapor barriers where the edge is exposed?


 •• Tremco acoustical sealant, then staple the vb if you can, just for kicks. Tremco sticks to poly real well.


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## wescoaster (Nov 17, 2010)

Thanks, there is a very narrow edge of the vapor barrier exposed, if I run a bead between that and the framing, that will seal that edge. Should i also be trying to seal the exposed top plate/rim joist with poly or just feather out the foam to cover the surface of the top plate all the way to the existing poly edge?


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## jklingel (Dec 21, 2008)

To cover the boards on the edges or not, I would have to say it depends, and then admit that I am not an expert on that part. Here are some thoughts: I would be inclined to leave them exposed so they can dry, unless they can dry to the outside; then I'd foam over them. If the foam were outside, I'd surely cover the boards completely and with enough foam to keep them warm. However, exposed edges will be cooler and may condense, like in a crawl space, if you have high humidity inside. Juggle those variables, and perhaps someone will add others, into your decision equation and go.


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