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Old 01-11-2012, 07:07 PM   #1
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Insulation below knee wall


My house has an attic that was finished before I moved in. I had a quote to have insulation improved and part of it was to remove some of the finished floor and install rigid foam below the knee walls.

I understand that if the attic was properly finished, you would want to block the joist under the knee wall. I took a look and there is kraft R-22 spanning the entire room (from before the attic was finished). This insulation fills most of the 2x8 joist. I know that this isn't ideal, but it doesn't seem cost effective to correct this problem. Am I losing a lot of heat by not having the joist blocked off?

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Old 01-12-2012, 07:16 AM   #2
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Insulation below knee wall


Depends on the leakiness of the home.

The only way to get and idea of the air loss would be to actually have a blower door test of some sort.

Is there not kneewall access right now? Cutting into the finished floor seems a bit invasive and not the preferred access methodology for those type locations.

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Old 01-12-2012, 10:05 AM   #3
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Insulation below knee wall


I had an audit but not a blower door test. There isnt enough space to work behind the knee wall. However, when i took a look.at the insulation it was clean. So i dont think there is much air moving over it.
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Old 01-12-2012, 02:43 PM   #4
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Insulation below knee wall


Oh and the floor isnt like finished hardwood. Its roof deck, plywood, carpet.
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Old 01-12-2012, 08:45 PM   #5
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Insulation below knee wall


In the air moving through the insulation may not be moving into the living space but strips the R-Value from the insulation. That is often times the larger benefit.

If there is not much air movement, I would leave the floors alone.
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Old 01-15-2012, 08:58 AM   #6
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Insulation below knee wall


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Originally Posted by yaemish View Post
My house has an attic that was finished before I moved in. I had a quote to have insulation improved and part of it was to remove some of the finished floor and install rigid foam below the knee walls.

I understand that if the attic was properly finished, you would want to block the joist under the knee wall. I took a look and there is kraft R-22 spanning the entire room (from before the attic was finished). This insulation fills most of the 2x8 joist. I know that this isn't ideal, but it doesn't seem cost effective to correct this problem. Am I losing a lot of heat by not having the joist blocked off?
Simply put...YES! The cold air entering from soffit vents is creating a convective flow through your insulation reducing the effective R-Value drastically. Hot moves to Cold so even when no wind exists air is moved due to the delta T.

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