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duffman56 11-20-2013 11:16 PM

Knee walls and vapor barrier
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 always much appreciated.

Gary in WA 11-22-2013 02:18 PM

Little short (SPF R-6 per in. on the sloped ceiling code requirement of R-38.... will you be furring down the rafters to add more insulation?

Be sure to keep that daughter safe with window egress per code;

The asphalt coated paper is on the wrong side for your location, should be next to the drywall;

Why Masonite board? for decoration?

Any gaps between vapor retarder paper facing are fine as it is area weighed, not supposed to be air barrier; Page 5;

Push it in so the facing is in full contact (including corners) with the drywall/Mb, then let it expand out slightly. Best to add some rigid foam board on the stud faces (tape seams foil tape, run fb under knee wall to block air flow there, and up to block air-flow to rafters as the upper/lower attics should have their own ventilation) to get minimum required R-20 for your area. Check local AHJ for foam ignition/thermal barrier required. This will stop attic wind-washing of the fibrous cavity insulation.

Insulate the access door, air seal the wiring/plumbing holes in top plates;

OK, had to reread your post-so much info.... lol. Better than not enough.
1" XPS has a perm rating of 1.1 or 1.5 per inch, depends on brand. So R-15 (1-1/2")XPS is; 1.1perms divided by 1.5" = 0.73perms.------ or two inch thick= divided by 2 = 0.55perm. for 2"

Your .73 perms (XPS) is a vapor retarder, same as the asphalt faced paper; 0.3-0.5 perms but it is variable;


AND the previous link showed hardboard- standard= 11perms and hardboard (Masonite)- tempered= 5 perms. Long story short- IMO, use the open cell (EPS) with no worries. BUT, lol, using the XPS/kraft/Masonite it could still dry to attic if not a facer on fb; "double vapor barrier"-

Read pages 5-7 here:

Use this for your min. fb thickness for your Zone to prevent cavity condensation;

OR; tell me the closest large city for your local climate and I'll figure dew-points for you with different thickness foam boards?


Gary in WA 11-22-2013 10:16 PM

Ah, the Masonite is under the drywall.... the Kraft facing is permeable enough to permit drying to the inside and still use fb; Ginger vs. Mary Ann;

AND, using both together;


duffman56 11-23-2013 12:25 PM

Sheetrock is old and original, masonite board I'm guessing was used for added strength?

As for location - we are actually an equal distance from Nebraska and Oklahoma border, and about 60 miles East of Colorado border. I believe this puts us right on the border between zones 4 and 5. Humidity is usually very low here, MUCH lower than central and Eastern KS.

Gary - very informative, thank you. Just want to make sure I understood everything correctly:

Does the old insulation (w/ asphalt coated paper) need removed so I can install insulation with the vapor barrier against masonite board? Since I'll be installing it backwards (with paper away from me instead of facing) is there a way to secure it to hold it in place, at least long enough for me to get the foam panels up?

OR - since the fb acts as a vapor retarder (as does the primer) should I cut out the kraft paper vapor barrier altogether, to avoid trapping the moisture inside? I might point out that I'll be adding at least 3" of fb, not sure what the permeability would be down to - .37? According to the site you listed [] it sounds like I'd be OK without the kraft paper vapor barrier.

The info I found on EPS suggests it's closed cell, but that it does have a higher rate of permeability than XPS. If I used as much as 4" (to attain R-16) would this still be an effective vapor permeable air barrier? Does this affect the answer above, regarding using faced or unfaced fiberglass batts? What is the best way to secure these fb panels? (I'll likely have to use two 2" layers, as that's all that's available at Menard's in smaller quantities) I can see it being difficult to find the stud with a screw after going through 4" of fb.

After reading parts of [] I thought it worth mentioning that this bonus room has a bath tub that will be used.

I know I'm asking a lot of questions, but I really want to do this right. :thumbsup:

Gary in WA 11-24-2013 06:57 PM

1. Do you have a link to the fb for me? The paper facing requires on the heated room side to stop moisture from there.

Best is removing the paper facing, though not required as it is less when you don't need it and more when you need it then. 2. Why so much insulation, are you keeping heat in? Only R-5 for your location, and that is added benefit of being protected from most of the moisture/rain/wind because of in attic; Echo, echo....

There is a balancing point, IMO, ;,d.cGE&cad=rja

I think you should spend time air-sealing attic, outlet holes in existing drywall/Masonite, and caulking/foam under the drywall behind the baseboard;

3. Did you air seal the attic or basement/crawl yet?


duffman56 11-24-2013 10:44 PM

Those are the foam panels I'm planning on using. Two layers, still not sure how to secure them.

I know it's a lot of insulation, but the room can get extremely hot in the summer, the attic holds in very high temperature heats, which is why I'm trying to insulate between room and attic side to such a great extent.

Do I need to remove old insulation (with reversed paper facing), or is it ok as is to add the fb over?

Gary in WA 11-25-2013 05:29 PM

Pull the paper face to be safe. The RayLite boards have a 3-5perm rating. Depending on density, which they don't say. You should be fine, IMHO.


duffman56 11-26-2013 06:57 AM

So no vapor barrier (at least not between Masonite and fiberglass insulation).

What about a method of securing the foam board? 4" is a lot to go through and find the center of a stud.

Gary in WA 11-26-2013 08:51 PM

Facing installed at the drywall is fine, not at the foam. Use 3"nails and fender washers, then bigger nails with washers. Or the cap nails for house wrap (if long enough) online for foam fasteners.... get creative. Pre-measure studs from a fixed location- wall, corner framing, mark on the roof sheathing....


duffman56 11-26-2013 09:10 PM

3" nails wouldn't be long enough. Maybe I can use the fender washers on something longer?

Gary in WA 11-27-2013 01:46 PM

Gutter spikes and washers.


duffman56 12-14-2013 12:25 PM

Just re-reading your post Gary - haven't started on the project yet with things so busy for the holidays. :eek:

You said "facing at the drywall is fine, not at the foam". Does this mean I need to remove all of the insulation that is currently in the knee wall space?

If that is the case, could I get away with unfaced fiberglass insulation? I'm not sure how I'd attach the paper-side, since sheetrock is already up.

Gary in WA 12-14-2013 10:46 PM

IMO, flip it around so the paper facing is against the drywall. This will help stop adding moisture to the wall cavity from the wet room. The paper facing does not require stapling to be effective;


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