Insulating Attic Knee Walls

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  #1  
Old 12-03-08, 07:34 AM
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Insulating Attic Knee Walls

Folks I have a question. I am in the process of finishing the attic space with is actually above my master bedroom. 2 knee walls and 1 of the other exterior walls will have unheated attic space behind it. The 4th wall is part of the exterior wall of the home.

I have heard every way to insulate this space, but is their any benefit of installing a higher R value like R19 in the 3 walls with attic space behind them? The walls are framed with 2x4 lumber but the thicker insulation would simply spill over the studs into the attic space and not be compressed.

Help is appreciated!
 
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  #2  
Old 12-03-08, 09:00 AM
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Hi jbr, there are several recent threads that may cover some of the issues you need, just try searching "knee wall" and see what you get. When you insulate the knee wall, you also have to think about the floor in that attic space. It should be insulated as well, as venting will keep the temp at outside temp and your floor and ceiling below suffer. Many consider the knee wall the same as the ceiling which needs about R-50, so more is better. Before you put up the drywall, you have the option to add some rigid foam in adition the the fg. Be sure to insulate and air seal the access door and any electrical or other air leaks.
good luck,
Bud
 
  #3  
Old 12-18-08, 08:17 AM
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Thanks for the reply Bud, you would be amazed how many scenarios I have been supplied with on insulating this attic! I already have the floor taking care of, just trying to figure out the knee walls that back up to the attic. They are framed with 2x4 lumber but my inspector told me to install R19 that would simply run past the stud another 3 inches. However, there would be gaps behind the stud b/c no insulation would be present I didn't know if this would be anymore effective than installing R13 in the stud bays.

I was going to use rigid foam on outside back side of studs for more R and air barrier, but inspector told me not to b/c of the potential of 2 vapor barriers. I have seen faced R13 installed in stud bays then another layer of unfaced R13 stapled in horizontally across the stud bays which sounds promising. In this scenario, housewrap was used as an air barrier and support.

Any thoughts?
 
  #4  
Old 12-18-08, 09:25 AM
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Hi again, one of my first dilemmas when I entered this business was where to put the VB. The answer they gave me was “on the warm side of the insulation”. OK, then up north that would be on the inside. Down south that would be on the outside. What the people in the middle do hasn’t been determined as yet. Now, when you use rigid foam, it insulates and acts as a VB. The trick is to make it thick enough. I would certainly think 2” of rigid plus 3 ˝” fiberglass would be more than enough in Alabama. But the inspector is correct, you don’t want two, so use un-faced in the wall cavity or slit the kraft. Sheetrock is an air barrier, not a VB. Now I don’t know how the inspector is going to feel about this, as from what you said it sounds like he wants the VB on the inside. You’ll probably need to talk.

Here's a link I should have given you before, I find it helpful.
Welcome To Home Energy Magazine Online

If you go with the R-19, just cover it with house wrap to keep it in place. As for the width of the wood, stuff it. That is if you can get in there.

As for the number of opinions, I not surprised at all. Old habits are hard to break, new innovations aren’t always correct, and sales people will say anything to make a sale. The only salvation, in my opinion, is the internet and access to an unlimited number of opinions. Then I get to decide for myself .

Digest that link and let me know if you have more questions.
Bud
 
  #5  
Old 12-21-08, 05:16 AM
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covering with house wrap

Bud,

Is not house wrap a vapor barrier. I am asking because I had been considering using it over the insulation on my top floor ceiling...

Dave
 
  #6  
Old 12-21-08, 07:23 AM
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House wrap will shed the water that finds it's way behind your siding, but it is designed to breathe and will allow moisture to evaporate through it. Remember, you are in air conditioning country and the choice of where to put the VB alternates between heating and cooling, thus those who both heat and cool are caught in the middle.

You can get away with it behind a knee wall just holding your insulation in place, but I would hesitate to cover your ceiling insulation for fear it will trap rising warm moist air and condense out in heating season. Vertically behind the knee wall it won't trap much air. Another opinion for you to sort out.

good luck
Bud
 
  #7  
Old 12-28-08, 04:23 PM
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knee walls

Hey first time here on the forum just thought I would tell my story . Right or wrong I framed in like 1000 sq. ft. attic room at my home here in illinois I used house wrap staple on the back of the studs/knee wall then A company in my area applies a netting on the inside of the studs and used blowin celluse insulation. Don't know the exact R view but I am sure its pretty good it was blew pretty tight and bulging on the back side also did the roof rafters that way I was concerned there about mosture there but the insulater told me it would be alright so far 14 mouths later no problems. I f knew how I would try to post pics here. Any way good luck.
 
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