# Knee Wall Insulation



## DaHammer (Nov 9, 2006)

I have some knee walls of a bonus area on the 2nd floor that I need to insulate. They are framed with 2x4s on 16" centers. I need to bring them up to at least R19. What I had planned on doing is using R15 fiberglass batts to fill the 2x4 cavities and then use some of the 4x8 sheets of styrofoam insulation board on the attic side of the walls. Doing that will allow me to achieve R19+ & seal off the room from the attic, depending on the thickness of the styrofoam. But from what I'm reading your not suppose to leave the styrofoam exposed, which it would be exposed in the attic. Is that a problem you think? Another idea would be to just use R19 or better batts in the walls and allow the batts to pretrude out beyond the studs. That's not a problem since it's in the attic, but it would leave a gap directly behind the studs. What would you do and any other ideas?


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## Chris Johnson (Apr 19, 2007)

Use a closed cell spray foam insulation such as Demilac, I'm not 100% sure but I believe the closed cell will give you r-19 on a 2x4 wall. Using the batts and ridgid foam will require drywall to cover if it's living space. I can't figure out from your post if this is usable space or just simply an attic above a ceiling which would generally already have drywall. The code is not specific in saying the drywall must be attached directly against the foam. So my interpretation is if you insulate the attic with the foam board, and the attic is not living space and you have a drywall ceiling you are in compliance.


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## DaHammer (Nov 9, 2006)

There is a bonus room, bathroom and closet upstairs. I want to insulate the walls between these living areas and the attic, which is directly ajacent to them on the same level. The idea was to use R15 batts in the wall cavities and then nail on the styrofoam sheets on the attic side of the wall. So the styrofoam would be exposed in the attic. Otherwise just use R19 batts in the cavities, but I figured the styrofoam would seal off the living spaces from the attic better.

Spray foam is very expensive in the area and not option.


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## Chris Johnson (Apr 19, 2007)

Spray foam is an option, just not one you or the client are willing to entertain, but is by far the best way to insulate this area, second is the batts plus ridgid foam and because of the new description you gave no additional drywall is required.


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## DaHammer (Nov 9, 2006)

So it's ok to leave the ridgid foam exposed in the attic?


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## dlcj (Oct 1, 2007)

foam board in the atic would probably be ok but i would just use r19 or better fiberglass. We use r30 on knee walls cuz it is a VERY HOT attic on the other side. Spray foam is the best though.


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## DaHammer (Nov 9, 2006)

From everything I've read thus far, most all of the foam board products are not approved for use in exposed applications. You are required to cover them with some type of thermal barrier, like drywall. However, Dow does make 1 that is approved called Thermax. I haven't seen it, best I recollect, locally though. I'll have to see how if my suppliers can get it and how expensive it is. I'd rather do that then just use thicker batts that protrude beyond the wall, since the foam would insulate the studs themselves and seal it off.

And yeah, it gets extremely hot in attics here as well, North Mississippi.


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## Chris Johnson (Apr 19, 2007)

The foam board can be exposed in the attic, it is not exposed in livable space. It is the same as how I put ridgid styrofoam on the top of the attic lid as it's insulation, it is exposed in the attic, but not in the house or living portion of the house, it is seperated by drywall


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## neolitic (Apr 20, 2006)

If it's a retro-fit type deal, I like to use a polycyanurate/foil "HI-R" type sheathing. Easier to cut, and what with fishing it between studs and bumping onto the rafters/ roof deck with long pieces, I rip it into 2 courses and foil tape the seam. 5/8 sheathing + R-13 +drywall = R19 I think. Anyway I like the reflective side for the attic/hot side. $0.02


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## thom (Nov 3, 2006)

Local codes may not allow combustibles in the attic. Our inspectors will check and fail us if paper faced insulation scraps are left in the attic. Styrofoam would be a major no-no. It burns waaaaay to fast, and to hot.


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## DaHammer (Nov 9, 2006)

Well I really don't have code issues on this project, as there aren't any codes in effect in the rural area the house is located in. But code or no code, I don't want to create a fire hazard. Most all of my research on it suggests it's a major no-no. I've read that the styrofoam was to blame for the Great White concert fire that killed all those people a few years ago, that it was the fuel that made the fire totally engulf the building in such a short time span. Whether or not that's true, I don't know.

Anyway, I checked my "can get you anything you want" supplier today and he called around and could not find the Dow Thermax anywhere within his immediate reach. Best he could do was order it and said it would take him 2 to 3 weeks to get it, not good.

I did the knee wall of the vaulted ceiling in the family room this afternoon with R30 batts just to see how it well it worked. To my surprise it looked pretty good once done. The 9 1/2" batts pretruded beyond 2 x 4 framing into the attic and then closed back together on the back side, completely surrounding the back side of the studs. It appears almost as though there is a solid R30 blanket on the wall. So I'll probably either just use that or drop back to R19 on the bonus area. Anyway, thanks for the help guys.


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## Danb (Oct 7, 2007)

*insulation*

Actually pollyurithane insulation will give you a R30 in 3 inches. easy way to achieve the required R factor. expensive though.


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