Knee wall insulation questions

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  #1  
Old 10-18-15, 07:07 PM
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Knee wall insulation questions

Bear with me a bit, as this is my first house with a knee wall. Ok, we've been in this house for two Michigan winters, and each on we developed frost along parts of our interior walls at the point where it joins with the ceiling. These walls are exterior walls, and under a knee wall. This year I realized that none of the joints on the knee wall...ie, between the base plate and OSB it's sitting on....are sealed and I'm ASSUMING that's how cold air is slipping in causing the frost on the wall below it. All caulked and spray foamed up....however....

Our upstairs is FREEZING in the winter time. After reading a ton about knee walls and dealing with this frost issue, I've concluded that my knee walls are poorly insulated in terms of installation method. I know that you either have to properly insulate the knee walls themselves to block cold from the conditioned room, or block off the area behind the knee wall and insulate that area (allowing air to still flow over top of it). For cost and ease, I want to just super insulate the knee walls themselves and leave my storage space unconditioned air. Now the question:

1. My plan was to just slap hard foam insulation boards over what is pre-existing and seal the crap out of all the seams and cracks and joints. However, per the pictures below, the existing batts are installed with the kraft paper barrier touching the sheet rock and fiberglass exposed to the attic. If I put foam board over top of the insulation, won't I be putting it between two vapor barriers? And isn't that a bad thing? All the videos I see online show the batts installed with the kraft paper on the attic side....so putting foam over it still keeps it with vapor barrier just on one side which I couldn't do if I'm reading it right.

2. So...if I'm right about the double vapor barrier, what do I do? Somehow rip out all the existing batts and start over? Or is there another product besides foam boards I can slap up there that won't violate the vapor barrier rule (if that's really a thing)?

Pictures for reference (sorry for the bad angles...I'm super tall so it was a tough photography session):

Front of house knee wall where I had the frost problem:


Top plate of front side knee wall from above picture. There is a foam baffle up farther you can't see (is that how you do that?):


Rear "non-traditional" knee wall...bump outs are closets and bathroom...heat duct is also exposed to unconditioned air:





Thanks in advance for the help!
 
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  #2  
Old 10-18-15, 11:30 PM
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No pictures came up.
In your area those knee walls should have been 2 X 6's with R-19 insulation.
There should have been Tyvex on the back side of the wall to prevent wind washing.
Almost never was a second floor "bonus room" done right.
The rafters should have been wide enough or built up to fit the needed insulation and and allow for the baffles for proper ventalation.
Recommended Levels of Insulation : ENERGY STAR
 
  #3  
Old 10-19-15, 06:24 AM
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While you work on the pictures, here is a related link.
Two Ways to Insulate Attic Kneewalls - Fine Homebuilding Article

Bud
 
  #4  
Old 10-19-15, 09:18 AM
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Sorry about the bad picture links!!

Front of house knee wall where I had the frost problem:


Top plate of front side knee wall from above picture. There is a foam baffle up farther you can't see (is that how you do that?):


Rear "non-traditional" knee wall...bump outs are closets and bathroom...heat duct is also exposed to unconditioned air:


 
  #5  
Old 10-19-15, 09:26 AM
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Bud9051
While you work on the pictures, here is a related link.
Two Ways to Insulate Attic Kneewalls - Fine Homebuilding Article

Bud
I've read this article a ton...and it's where I originally got the idea to put ridged foam along the back of the knee wall on each side. However, per my original question, I'm still not sure if the foam counts as a vapor barrier or not. If the fiberglass batts currently in my walls were flipped so the kraft paper was on the attic side, I would imagine it wouldn't matter either as I would just be adding to the existing one sided vapor barrier. However since the kraft paper is on the inside conditioned space, I'd be sandwiching the fiberglass between two vapor barriers. Which I thought was a bad thing. So this is what I'm hoping to get cleared up.

Thanks a ton for the replies and sorry again about the bad photo links!
 
  #6  
Old 10-19-15, 10:01 AM
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Quick answer while I do more thinking. Two vapor barriers are indeed bad. However, neither the kraft or 2" of rigid insulation qualify as true vapor barriers. Everything less than a really good vb, like a layer of plastic or foil faced insulation, is classified with some level as a vapor retarder.

From my experience, very rarely would kraft faced insulation qualify as a very good vapor barrier.

So, where to go from here? You have probably seen homes getting the new siding over an added 2" of foil faced rigid foam. That is clearly a true vapor barrier on the wrong side of the insulation and in most cases they ignore what might be existing on the inside. The way they get around this issue is to keep the rigid foam on the cold side thick enough so its inside surface never gets cold enough to form condensation, think 50/50 but I can find the chart if needed. Then, the limited ability to dry to the inside becomes sufficient to slowly remove any moisture that may have gotten in. The other aspect of doing this right would be to address all air leaks as air movement carries the majority of the moisture.

Not sure where in that picture you experienced the frost problem and what was on the inside where that occurred, closet or open living space.

In any case, those slopes are a pain as they are rarely deep enough to provide adequate insulation levels along with the ventilation path above them.

Bud
 
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