Insulating Knee Wall from the Attic Side


Old 11-21-13, 06:45 PM
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Join Date: Nov 2013
Location: USA
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Insulating Knee Wall from the Attic Side

Hello all, location is NW Kansas. We have a "bonus room" that we are trying to achieve a comfortable temperature in so our daughter can move upstairs to a bigger room. It has knee walls all the way around, about 4' high. We recently tore out the sheet rock on the slanted ceiling portion to re-insulate, did about 3 inches closed-cell spray foam in the 2x4 roof. Now we're doing the knee walls.

They are 2x4 construction. Sheet rock is up, as well as 1/4" Masonite board, then 3.5" rock wool insulation. The rock wool has some strange tar-paper I suppose as the vapor barrier? I should note this is facing the attic side, and is the only thing that keeps the insulation from falling into the attic. Also, the primer we used (Zinnser 1-2-3) is rated as a vapor retarder. The attic side of the insulation has no air barrier. I'd like to get r30, given the limited attic ventilation (which I'll try and improve at some point) and how hot it gets in there. My plan is to add fiberglass r-15 batts (from the attic side) to a couple cavities where the rock wool was damaged (after removing damaged rock wool of course). I would be adding the insulation in a backwards manner (from outside the living space), but would be placing the paper-side against the Masonite board/sheet rock. Can I install kraft-faced insulation this way? The vapor barrier for insulation is usually one solid layer, this would have a tiny gap between vapor barrier and 2x4.

Once cavities are insulated I plan to add polystyrene panels as a vapor permeable air barrier. I've looked into vapor permeability of panels, but only found info on 1". I plan to add 2 layers that will total around r-15. This will make the areas w fiberglass batts r-30 and the other around r-26.

Will 3" of polystyrene (or more, if I use the less expensive white kind) still be vapor permeable? I've been a bit reluctant to add the air barrier, given the current seemingly backwards vapor barrier. There haven't been issues so far but is that because there's been no air barrier, allowing any moisture to easily evaporate from air movement?

I looked at other insulation options, but at around $300 this seems to the most cost-efficient and least labor-intensive.

Also - would it be possible to insulate (I'm thinking blown cellulose) between first floor ceiling and the bonus room, to keep first floor heat from getting up there so easily?

Man this turned out pretty long... Thanks for reading, your guys' expertise is much appreciated.
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Old 11-22-13, 02:40 AM
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Welcome to the forums! Yeah, that's your 1000 words. If you want to do a picture, you can do so anytime.

In the meantime, have you considered Roxul insulation for the walls? If they are 2x4 studs, you won't be able to get much more than R15 with fiberglas, since packing insulation reduces its insulative qualities. Not sure how you are planning on getting R30 without adding to the depth of the walls. Roxul is moisture proof and fire retardant, requiring no vapor barrier.
Old 11-22-13, 10:59 AM
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Join Date: Nov 2013
Location: USA
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Thanks for the info on posting photos. Won't do much good in this case though, since I'm mostly discussing my plans for what I'll do.

My plan is to have R-12 to R-15 from rock wool/fiberglass (in 2x4 walls), and install Polystyrene foam panels on the outside of those as a vapor permeable air barrier to add another R-15 to the exterior. 15+15=30
Old 11-22-13, 06:21 PM
Join Date: Mar 2013
Location: USA
Posts: 457
" Roxul is moisture proof and fire retardant, requiring no vapor barrier."
------------------- sorry, chandler, Roxul requires a vapor barrier per location; Comfortbatt;

What you have planned is fine, it can still dry to the inside; "Ginger vs. Mary Ann"; BSI-024: Vocabulary — Building Science Information

May need an ignition/thermal barrier on the foam, check with local AHJ and the fb manufacturer;Eg;

IMO, use Roxul in floor over cellulose (with moisture from below and plywood retarder above, harder to dry with high RH in room.


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