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Seeking Insulation/Vapor Barrier Clarification


doruho's Avatar
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Join Date: Jan 2005
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10-20-07, 12:09 PM   #1  
Seeking Insulation/Vapor Barrier Clarification

Hello All:

I am about ready to insulate my framed basement walls. Code requires minimum R10 down to 48 inches below grade.

I was planning on using kraft-faced batts, but leaving some uninsulated space (maybe 6 inches or a foot) at the bottom of the walls. This would be to avoid complications of wet insulation in the (unlikely but not impossible) event I get water in the basement.

I also plan to have several vents in the walls, just to allow air to circulate behind the walls in order to mitigate moisture. These vents could be closed to prevent heat loss in the winter.

My questions are: If I leave this uninsulated space at the bottom of the walls, should I apply a plastic vapor barrier over this space and seal it over the paper facing on the batts above it?

If I have these air vents as I mentioned, do they in fact render the vapor barrier useless because they allow moisture behind the walls, or are they more of a benefit because they allow air circulation and give the moisture a way out?

Thank you.

 
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Concretemasonry's Avatar
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10-20-07, 12:36 PM   #2  
Seeking Insulation/Vapor Barrier Clarification

Obviously, your vents would render a vapor retarder useless, especially since they are not tight, even when closed.

Vapor barriers (and the need or effectiveness) are very questionable in most finished basement areas.

If you are insulating 100% of the exterior walls or creating an island that is 100% insulated and vapor barriered, the value of a vapor retarder or vapor barrier can be effective.

If you are doing a partial finished basement AREA, why insulate a wall when moisture or heat can flow around or over the insulated walls while the other walls that separate you from the semi-conditioned (unfinished) space may not be insulated? Do you have to insulate the interior walls that separate the finished from the unfinished, uninsulated areas?Check you local codes.

Since you have the magic line drawn around your basement, you mat be in Canada or have a local prescriptive code.

Dick

 
airman.1994's Avatar
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10-20-07, 01:44 PM   #3  
Id go 2in polystiren then you don't have to worry about the moisture on the insulation.

 
doruho's Avatar
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10-20-07, 07:30 PM   #4  
Actually, I'm in a suburb of Chicago. I suppose they want to see *some* insulation, but they don't want to make you go all the way to the floor. There is no requirement to insulate interior walls.

I plan to close the vents by covering them with sheet magnets. Should seal them pretty good, but I see your point about unfinished areas. I will have about 15 ft of wall with an unfinished space on the other side - the furnace room. It should be no big deal to insulate this wall too.

I'm still wondering whether having the vents to create good air circulation behind the walls will counteract any moisture.

Airman.1994, I'm not familiar with the 2" polystyrene. How does it install on a wall with 16" o.c. studs? Between the studs or over them?

Thanks

 
d00bs's Avatar
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10-20-07, 08:16 PM   #5  
Posted By: doruho I'm still wondering whether having the vents to create good air circulation behind the walls will counteract any moisture.
Yes, they will create good air circulation but No, they will not counteract any moisture.
Check out the insulation section at any big box store like HD lowes etc. you will see the big 4X8 sheets of it stacked up. You can attach them directly to the walls behind the studs or between them.

 
doruho's Avatar
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10-20-07, 08:32 PM   #6  
Now I know what it is. I don't have that much room behind most of my walls. Do you just cut it to size to fit between the studs? How do you attach it? Does it not require additional vapor barrier?

Also, if I just decide to use batts, and not bother with the venting, then is this whole conversation moot?

 
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