# Kneewall insulation



## Windows on Wash (Aug 30, 2011)

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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## maddog1 (Aug 21, 2012)

Windows on Wash said:


> 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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## Yodaman (Mar 9, 2015)

Windows on Wash said:


> 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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## Windows on Wash (Aug 30, 2011)

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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## maddog1 (Aug 21, 2012)

Yodaman said:


> 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.





Windows on Wash said:


> 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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## Windows on Wash (Aug 30, 2011)

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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## Yodaman (Mar 9, 2015)

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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## Gary in WA (Mar 11, 2009)

Hot, humid climates require any vapor barrier/retarder on exterior of wall, including attic knee-walls; http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/35793.pdf

ADA the drywall to stop the hot, moist air from reaching the cooler inside room; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-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. 

Gary


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