Basement Insulation/Vapor Barrier Help

Old 11-15-11, 02:49 PM
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Basement Insulation/Vapor Barrier Help


I have read so much and searched google so much my head is spinning. I would just like some guidance and insight into my project.

Remodeling my basement which consists of a below grade concrete wall 4' high then a 4' high studded wall above grade. The walls were prebuilt before i bought the home so they have stayed.
The lower half (concrete) has a vapor barrier against the concrete and stud walls over that, i have used 1 1/2" foam board insulation in the studs.

The top half i am more confused about. Its a complete wood studded wall, outer wall is cedar siding. Previous owner had kraft faced insulation WITH plastic sheathing over it = BAD BAD BAD. you know the outcome of that.

When insulating this area above grade all wood studs. Is it better to use unfaced insulation with plastic sheathing or a kraft faced insulation alone? The stud walls are attached to a sheet of 3/8 plywood that in turn is attached to the cedar siding.

I dont have the funds for spray foam in that area so any opinions are appreciated.


Old 11-15-11, 03:26 PM
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There are only a few occasions when a true vapor barrier is needed. Most of the time something less will do just fine. To fill the gap between a real VB and none they use a group of materials with varying degrees of permeability. Blue and pink rigid foam board with no plastic or foil covering will actually allow a little drying right through it.

A plastic VB over the concrete that is below grade leaves the concrete no place to dry and the wall behind the plastic will equalize to match the moisture level on the outside surface of the wall. Above grade it will dry to the outside and to some degree the first foot or so below grade will also. Thus, with your short wall, you are probably ok.

As for the above grade wood wall, the most important barrier is your air barrier. Most moisture is carried with warm air. If you air seal every little spot where air can get through to the cold outside surface then either the kraft or plastic will be fine. Personally I would use mineral wool in place of the fiberglass, but it would be more expensive. Just be neat and gently fill every little spot.

Old 11-15-11, 04:46 PM
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No VB on any wall! If you use one your asking for moisture issues
Old 11-18-11, 11:09 AM
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Definitely no plastic. I just did my basement using Roxul, and love it. It's fireproof, seals than fiberglass and has a higher R-value, doesn't absorb or even wick water and cannot rot or mold. More expensive than FG, but no where near as much as spray foam.

I would suggest that you seal the rim joist with spray foam (great stuff or other, whichever, I like the Hilti foam better) to get a tight air seal and then use Roxul. More time consuming and a bit more expensive than FG but the result will be much better.
Old 11-25-11, 07:16 AM
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plastic vapor barriers are good to me when placing a wood floor over a dirt base. I have always placed plastic down, then framed and placed gravel before sheeting. Dirt base has enough pours and surface area to dry out on its own.
As for walls I have used plastic once against brick where i used fiberglass insulation (a rental apt bathroom i remodeled). It was about a 4' x 3.5' area but i regret it and PRAY i wont get mold. The VB is behind the studs and Im hoping the brick is good enough to withstand major moisture (all above grade) and pourous enough to breath back out.
Anyway I wont use plastic on walls anymore.
2 reason. I did in the past and what happens is water will go through any nail and screw hole.
Where water does NOT go through it will simply collect and drip down the plastic to the floor eventually puddling if its enough and you WILL get mildew eventually.
Use a primer/masonry paint (im going through this now and looking for new advice) and then use good foam insulation with a foil backer. walls need to breath whether they are wood or masonry.
Aside insulation the wood is above grade and if your outside siding is fine thats VB enough. Dont put plastic up. I use faced insulation myself but more out of ease.

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