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-   -   Is it ok to put vapor barrier first, when insulating basement? (http://www.diychatroom.com/f19/ok-put-vapor-barrier-first-when-insulating-basement-92213/)

Red Squirrel 01-13-2011 04:46 PM

Is it ok to put vapor barrier first, when insulating basement?
 
It seems to me this would make most sense, unless there is something I'm missing.

Basically, could I just staple/seal the vapor barrier on the top plate? I'd let it go all the way to the ground, leave about a foot extra and seal all intersections with acoustic sealent, then put rigid foam on the VB with glue, then put a bottom plate over the excess vb at the bottom, put the studs right up against the foam as to mechanicly hold it (glue would more or less be to keep it in place for the work to be done), then add some fiberglass, then the drywall. Could this method of insulating the basement work? My basement is hollow cinder block walls with some minor moisture issues. A bit of efflorescence in some spots, and some bricks are a bit darkened. No water leakage though. There are some holes in the cinder blocks in some areas due to nails that the previous owners put, and no water has ever come out so I know it's fairly dry, but there is a slight sign of moisture after a huge rainfall, but they arn't actually wet.

When I bought the house the basement was finished, but they used tar paper, 2x2 studs, faced fiberglass, vapor barrier, then drywall. There was some spots with rot and mold. I want to ensure that I don't have this happen.

Do I need to allow the wall to breathe or do I have to seal it completely?

Jackofall1 01-13-2011 04:51 PM

Red, you don't want VB on a concrete wall, it needs to release its moisture, just put the rigid foam right on the concrete. Followed by your framing complete with pressure treated bottom plate.

Red Squirrel 01-13-2011 06:35 PM

So I'd put the VB between the studs and drywall? Or do I actually leave it out so the wall can get proper moisture release? The way I see it if I put one then the moisture will be trapped betweeen the VB and the wall, and rot out the wood/insulation etc right? Or does it get released through the top plate of the wall?

Jackofall1 01-13-2011 06:51 PM

No VB on studs either, you can stuff fiberglass bat between studs, but the recommendations are no vapor barrier. The wall has to breath

Red Squirrel 01-14-2011 03:02 PM

Ahh ok, so none at all. I always see vapor barrier installed on unfinished basements, though most people don't do the foam either.

Trainer019 01-14-2011 05:21 PM

Check local building codes. The building code for my area, here in Canada also, specifies that in bacements plastic must be installed from the ground level down, wrapped under bottom plate. Vapor barrier then installed on inside wall before drywall. My personal opinion is to not do this as I agree concrete needs to breath, however building code specifies it must be there so I had no choice when I did my bacement. Always make a quick call to check what code is in your area before doing work. It is always cheeper to do it to code the first time then doing it twice.

SPS-1 01-14-2011 05:37 PM

The moisture in your basement is in the warm inside air. If the basement wall is cold, and the face of the insulation up against the wall is cold, moisture can condense on the insulation. That is why they suggest putting the vapor barrier on the "warm in winter" side. ( in northern areas anyways. It gets more complicated in the southern US) You suggested putting down foam board first. Since extruded polystyrene is both an insulator and a vapor barrier, what you suggested might work if the foam board is thick enough. But as trainer suggested, your local building department has already made the decision for you on how you should do it.

Red Squirrel 01-14-2011 07:16 PM

Yeah guess best bet is to contact them first and see. I find in this department I tend to get lot of mixed suggestions (local or internet) so it's tough to decide what is the best way to go about it, but number one is going by the code given they've probably tested all this and 99% of houses are doing it that way and are fine.

Jackofall1 01-14-2011 10:31 PM

Red Squirrel check this link out for your basement renovation

http://www.buildingscience.com/docum...g-your-basment

ccarlisle 01-15-2011 09:25 AM

But remember that Building Codes are a minimum standard i.e the next best thing to having nothing at all there. Just because the BC says to put a vapour barrier, doesn't mean that's the best nor the most modern of technologies nor what is the most effective.

Read up on insulating basement in cold climates and see what the prevailing state-of-the-art actually is...ie get the Canadian perspective.

jklingel 01-15-2011 03:33 PM

jack, et al, have it. no vb, regardless of what "code" says. ask to READ the code, as even an inspector up here, of all places, said "all we accept is 6 mil poly" (which has a perm of 0.6, ie, damn near a perfect vapor barrier) but the code, when I asked him to read it, says "one perm or less". That allows more breathable vapor RETARDERS, like vrpaint (ben moore makes one, and i am sure others do). you want a basement wall to dry to the inside, and a vb will prevent that. in mn, "the standard is a vb of poly..." which may be true, but the mn code says "one perm or less". do not trust anyone on what the code says, as people extrapolate from it too much; read it. i'll bet it says "one perm or less". good luck. j


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