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While finishing my attic, I added 1" extruded polystyrene to the backs of the 2x4 knee walls. However, when doing this, I noticed the builder placed R-19 in the 2x4 knee walls. From a chart I found online, the compressed R-19 will provide R-13 insulation value. Therefore, I am considering leaving the R-19 in place since it proved the same R-value as replacing it with R-13. Any suggestions or comments?
 

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You can buy R15 batts for the 2x4 wall and the foam borad. As a head's up, you're supposed to cover the foam with sheetrock as it will burn and give off toxic smoke.
Ron
 

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The R-value is just the insulation value of the stuff between the studs/joists and not a acurate measuement of the wall/ceiling R-value. The wood studs will cut the wall insulation value by 10 to 20%. - Even much more with steel studs. - This is just for short term advertising numbers and do not accurately reflect the longer term properties that also relate to mass and weather cycles. Extruded polystyrene board is good at minimizing the thermal short-circuiting of the studs, since it is a continuous barrier and does not permit infiltration.

Dick
 

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I would leave it be. The compression means it is now denser than before, unable to have convective loops within it, the same as the denser R-15. If you can meet all 5 requirements in 4.2.1.2 here, you may not have to cover the board. Air space above sloped ceiling to upper attic- need to cover it; http://www.hcpdc.com/pdf/Dryer Vent Requirements.pdf

Be sure to close the floor bays and the end of upper insulation between rafter bays to prevent wind-washing; below vents: http://www.simplesavings.coop/simplesavings/SIMPLESAVINGS knee walls.pdf

Gary
 
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