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Knee wall insulation on ends


Tony P.'s Avatar
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02-06-15, 02:34 AM   #1  
Knee wall insulation on ends

I'm in the process of adding insulation to a knee wall floor and noticed the ends of the knee wall have insulation. This means there's insulation between two unconditioned spaces, knee wall area and the exterior. I can just leave it but am wondering if there's a detriment to having it, perhaps in the summer. I live in zone 5.

BTW, I have another knee wall question in a separate thread.

 
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02-06-15, 02:46 AM   #2  
Insulation between knee wall and attic

I'm in the process of adding insulation to a knee wall floor. As usual, there is a space between the knee wall and the attic but in this case the space is short because the knee wall is 7' tall. I've seen diagrams where this space is left uninsulated to allow air to move between the knee wall area and the attic. Is this correct? If so, should I use baffle vents?

 
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02-06-15, 02:54 AM   #3  
I merged your threads to keep all thoughts together. You'll get better results that way.

 
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02-06-15, 03:35 AM   #4  
There are two general approaches, kneewall floor and the kneewall itself or insulate the rafters and end walls. At the top of the kneewall, does the insulation and venting continue up the slope to an upper attic space.

If you will have access to that space and plan to utilize it for storage or built-in shelves or other, then the rafter approach is probably the best. But, whichever you choose, a primary objective is to air seal that space (or the rafter space) to prevent warm moist air from reaching a colder surface. Your insulation, vapor control, and air barrier must always be in contact and in the same plane.

This warm air and cold surface also becomes a problem when using the rafter approach as the kneewall cavity may get so cold that the drywall covering the insulation becomes a condensing surface.

Also, note that the insulation on the slopes should be double or more of what fits into a 5.5" rafter that has a ventilation gap above it. At 7' tall it doesn't sound like you have a lot of sloped ceiling beyond that wall??

Bud
http://www.finehomebuilding.com/how-...kneewalls.aspx


Last edited by Bud9051; 02-06-15 at 03:59 AM. Reason: add link
 
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02-06-15, 05:21 AM   #5  
Bud, thanks for the quick response and for the link, which was also helpful. I already have R13 on the wall and R32 on the floor so insulating at the rafters won't be a worthwhile option. And yes, the sloped space is under 2' in distance. I'd appreciate your thoughts on 2 things: 1) need I use baffle vents for the short sloped space 2) what do you suggest as a cover for the back side of the knee wall as my opening is under 2X2 so something like rigid insulation would be difficult to bring in., perhaps an air seal product?

 
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02-06-15, 05:51 AM   #6  
Where there is air flow it is good to protect the fiberglass insulation with baffles. FG performs poorly with air moving through it.

Before I forget, between your r-32 and the joist cavities under the conditioned space inside you need to install an air barrier. That is mentioned and illustrated in the above link and very important. Plus, air sealing those kneewall stud cavities from warm air seeping in is important as well.

A product I haven't used as yet is mineral wool board. It comes in 2x4 pieces and I'm guessing is a bit flexible. but it is very vapor open so would not act as an additional vapor barrier, yet dense enough to act as an air barrier to protect the cold side of the existing insulation. Adding the extra r-value is also good as r-13 is only half of what should be there.

If that is outside your budget, then a house wrap would provide the air barrier, but no added r-value.

Bud

 
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02-06-15, 06:43 AM   #7  
Bud, thanks again. Where exactly is the area "between your R32 and the joist cavities under the conditioned space"? I assume you referring to the area directly beneath the knee wall but want to make certain.

 
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02-06-15, 09:07 AM   #8  
That's correct. If that isn't blocked then air circulates right through the fiberglass insulation under the floors that you would prefer to be warm.

There are different ways that the roof rafters attach to the walls below so I'm not sure if the insulation is exposed to the incoming air or blocked by a perimeter joist. If exposed, then you want a baffle with a flap on it that hangs down to block the incoming air.

Fiberglass isn't totally terrible as an insulation (it is just inexpensive), but it needs to be protected on all 6 sides where possible.

Bud

 
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02-06-15, 03:08 PM   #9  
Bud, thanks for everything. One more question from your comments. Given that fiberglass "needs to be protected on all 6 sides", should I add an air barrier to the floor insulation? I've opted for house wrap as an air barrier and can add it as a final top layer. FYI, I'm also adding insulation to the attic so I would include house wrap as a top layer there, as well.

 
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02-06-15, 03:24 PM   #10  
Unfortunately that may not be best. A wall cavity is usually described as enclosing the fiberglass on all 6 sides, but in a ceiling they will want to avoid the final cold condensing surface. Just my guess, but the extra r-value required in ceilings is probably an attempt to compensate for the exposed nature of the insulation. I can't say for sure that house wrap over the top of that ceiling insulation would be bad, but it is not a common practice. On the back of a kneewall, the insulation is more exposed to convective air flow (vertical) so protection is used.

Bud

 
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