Depth of insulation question

[Easy Livin’ 3000]

Need to decide the dimensions of my knee wall that will be lined with plastic sheeting to form an air barrier then filled with blown in cellulose. Have a maximum of 18" of width from the wall to use. The 18" represents the distance to the attic window opening.

Two related questions: How deep should I make it; and do I need the full 18" width out from the wall?

Some additional information: The attic floor is tongue and groove over blown in fiberglass insulation that has settled.

The main problem, I believe, is the free flow of air at the intersection of the stone wall and the framing.

 

[semipro]

I think we need more info.
What is the intended use of the space?
What are you going to use to contain the cellulose against the walls? Are you building a 2nd knee wall inside the 1st?
Why the plastic and where exactly is it going?
If this is an attic why not add sealing/insulation to the floor rather than the knee walls? If you used foam board with plywood above you could then use the space for storage.

 

[Hogwildz]

Semi got most of the needed questions.
My questions is, why not use the space you're going to wall off? If anything, why not frame out & make storage space or drawers to use the low space you're going to close off?

 

[BeerChemistWhiskey]

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