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Old 10-28-2015, 06:41 PM   #1
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Kneewall insulation


I'm located in the upstate area of South Carolina. I am finishing a Bonus room located over the garage. My question is knee wall insulation. My research indicates that I should seal off the backside of the knee wall with either OSB, drywall or solid foam board. The knee wall insulation will be fiberglass with a vapor barrier & applied between the knee wall studs.

My question is what R value should I be using for the knee wall. Right now I'm studded out with 2x4's which gives me R13-15. I think this is not enough insulation in what essentially is a "Exterior wall" if you think about it. I'm thinking I need to increase the studs to either a 2x6 for R19 or go to a 2x8 stud for R30. Because this bonus room is essentially built in the attic, I want to avoid issues with heat build up during the summer months, & a cool room in the winter. The room will be on its own heat pump system & separate from the rest of the house as far as heating & cooling system.

I've been thinking to just install R19 or R30 into the 2x4 studs of the knee wall & leave the backside open with no additional sheathing material since it will be inside the "Devil's Triangle" or attic space & I have no intention of using that area for anything. The bonus room floor is insulated with R30 between the garage ceiling & the underside of the bonus room floor.

Would appreciate hearing from some DIYer's & pros that have don this job & can tell me what I should consider doing down here in the sunny, hot & humid south.
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Old 10-28-2015, 08:33 PM   #2
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Some foil faced Polyiso foam insulation would work great here. I would just make sure that you are doubling up your vapor barrier's in creating a vapor trap with your existing insulation.
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Old 10-28-2015, 09:02 PM   #3
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Some foil faced Polyiso foam insulation would work great here. I would just make sure that you are doubling up your vapor barrier's in creating a vapor trap with your existing insulation.
Thanks- I've thought of foam, but its not in the budget & I need to keep this a true DIY project. The bonus room will not see very much use either, so with reasonably decent conventional insulation & the fact that the room has its very own heat pump system, it should be OK-not great, but ok for how I will be using the space. Thanks
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Old 10-28-2015, 09:19 PM   #4
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Some foil faced Polyiso foam insulation would work great here. I would just make sure that you are doubling up your vapor barrier's in creating a vapor trap with your existing insulation.
pretty sure WOW means NOT to double up your VBs and create a vapor trap.

just one comment on the foil faced, if you use it behind the kneewall, you still should cover it with drywall to meet fire code.
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Old 10-28-2015, 09:55 PM   #5
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Thanks Yoda.

Yes...left out the "not".

That's what I get for trying to type on an iPad with fat fingers.
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Old 10-29-2015, 08:46 AM   #6
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pretty sure WOW means NOT to double up your VBs and create a vapor trap.

just one comment on the foil faced, if you use it behind the kneewall, you still should cover it with drywall to meet fire code.
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Thanks Yoda.

Yes...left out the "not".

That's what I get for trying to type on an iPad with fat fingers.
Hi- So lets say I insulate my knee wall with R13 or R19 fiberglass batts & also seal it off on the backside with ridged foam board-Unfaced, I need to cover that also with drywall? for code. Is that IRC code for residential building. Thanks
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Old 10-29-2015, 09:52 AM   #7
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Check with your local code enforcement. Is is best practice...yes.

The foil facing on many foam boards does qualify as the ignition barrier in many cases depending on the access of the space.
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Old 10-29-2015, 11:23 AM   #8
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When I looked into a fire rated foam board about 1 yr ago (84 lumber) the cost was about 50 to 100% higher than regular foam board. And it was special order.

Not really sure why you would need anything on the back side other than the fiber glass. Its no different than loose fiberglass laying on attic floor. I would use a paper faced insul facing inside, add on the foam if you want the extra R-value, tape all the seams, then drywall over. If you want or need something on the back it should breath
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Old 10-30-2015, 11:59 PM   #9
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Hot, humid climates require any vapor barrier/retarder on exterior of wall, including attic knee-walls; http://apps1.eere.energy.gov/buildin...rica/35793.pdf

ADA the drywall to stop the hot, moist air from reaching the cooler inside room; http://www.buildingscience.com/docum...wall-approach/

R-19 and R-30 are both low-density insulation, cover them both with housewrap, including the attic floor insulation. Use 2 layers of unfaced R-13 or attic side only-faced R-13 for a superior thermal barrier with no inherent convective loops and less attic wind-washing degrading the effectiveness.

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