[mjhaston]

Wondering if this helps, hurts or has no effect? I'll have 2x4 knee walls in an upstairs renovation that will have crawl space behind them. Should I just stick with R-13 for 2x4 walls or can I try to go up in R value with something mean for 2x6 walls since I'm not worried about the depth of a close cavity?

Wondering if that might lead to droop? Something else I was thinking was putting 2" of foam board on the back-side and then then R-13 for 2x4 walls.

 

[Windows on Wash]

Let's see what you see. But foam board, with a proper thermal barrier, works well in knee walls many times.

 

[Nealtw]

We usually add a 2x2 or 2x3 to the back of the 2x4 for more insulation and a couple 1x4s to hold batt insulation in place.
I don't see a problem with adding foam board instead.

Many people miss the solid block that should be below this wall between the joist to stop air flow there.