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pasichnyk 11-27-2010 08:25 PM

Advice for insulating attic kneewall from unfinished side...

I'm a new forum member as of today so please be nice. :)

I recently purchased a 1940 craftsman in the Pacific Northwest (Seattle) and have a very cold finished attic room.

When the attic room was originally finshed, the previous homeowners framed in the room (sloped ceiling, with about 4' tall x 14' long kneewalls) and nailed up 1' thick pine panelling to the studs. The backside was at some point (I believe after the fact) insulated with thin (maybe 2" thick) double faced batt insulation. The insulation was stapled to the unfinished side of the studs. The framing is 2x4 with old rough cut boards, so they are a full 4". I believe they are 24" spaced. The ceiling flattens out on the top for about 3' or so width running the lenght of the room, with a few uninsulated recessed lights installed.

When I was recently doing some rewiring of that room, I pulled the insulation off of the unfinished side to route my wires, etc, knowing that it would need to be upgraded/replaced anyway.

I understand that proper insulation of these walls would require removal of the panneling so that a vapor barrier could be properly installed on the heated side. This is however not an option, as the panelling would never go back up in any sort of decent condition, so I've decided to do what i can to insulate it from the attic side while leaving the panneling up.

I have basically come to the understanding that putting up thick unfaced batt insulation, single layer of (as thick as i can get it) rigid foam board, or spray foam, or a combination of rigid foam and batts are my only real options.
  • Unfaced batt insulation wouldn't have a vapor barrier, which could cause some problems (condensation) and/or inefficiencies (wind washing?), but those seem less than an improperly installed (facing wrong way) vapor barrier.
  • Rigid foam board would only get me <10R worth of insulation, so that probably isn't enough... Or can I stack 2 layers of boards?
  • Spray foam I'd have to hire someone, which i'm trying to avoid.
  • Other option would be to put up some rigid foam (taped/sealed) say 2" thick, and then put some additional unfaced batts over them to get some extra R rating. I understand that you should put some sheathing or something over this though to keep from wind washing? Is there something that can do this without creating a second vapor barrier, or is it just not necessary at all?
One other issue I have is with the attic ceiling above this finished room. Currently it looks as if the previous owners just stuffed some fiberglass batt up in the corners (where the kneewall meet the roof line), but this blocks off attic circulation to upper roof vents. I understand that there needs to be a 2" (ideally) vent space between the roof and the insulation for proper flow, but I only have 4" total to work with here. Would it be best to just pull out that existing insulation and slide in (glue in place?) 2" rigid foam boards so the ceiling of the room has some insulation but the attic can still vent? If i did this i'm not sure i could get anything up there (at least well) for the flat top part of the ceiling though... :( I would have access for a few hands up there via the recessed light holes (need to cut more in anyway) so maybe I could slide precut foam board peices into place without too many swear words. :) Anyway, I haven't thought about this part as much, so any ideas would be great.

Any ideas/advice would be greatly appreciated.


jklingel 11-28-2010 02:20 AM

You are right about the FG batts being a poor choice hanging out in the open like that. Forget fiberglass completely, especially if it is exposed to air. It will not impede air flow for stink. Either get cotton or cellulose batts, treated w/ borates. OR, use your rigid foam, stacked as thick as you want to, and spray foam any inevitable gaps. Yes, the attic should be vented. You probably do NOT want a vapor barrier in Seattle. THAT is what can lead to horrible problems. You DO want to air seal as best you can, given that you don't want to rip up the ceiling and do it properly (I understand; that is a large job). By air seal, I mean between the attic and the conditioned (livable) space. Leaky air brings moisture with it far more than vapor diffuses through a wall. Seal any and all penetrations by lights and wires, pipes, etc. j

pasichnyk 11-28-2010 11:26 AM

Thanks for the prompt reply!

Just to be clear, what is the reasoning that makes a vapor barrier undesirable in my climate?

Just based on the density I can get with foam panels, I think I'll likely to that route. So I can just glue and spray foam seal search panel right on top of eachother without concern? Are there specific types you would recommend over others?. Seems like one of the extruded variants would be best?

So as for the ceiling, just pull out the batts and try to retrofit in some 2" thick panels and do my very nest to seam up all the seams and lighting holes?

Thanks again,

pasichnyk 12-05-2010 01:31 AM

Would you use Blue/Pink rigid foam or would the foil backed polyiso (higher R value) be ok too? I know you said no vapor barrier, but aren't all these foam boards essentially a vapor barrier?

I plan on buying a bunch of this stuff tomorrow to get started on the project, so please let me know the best foam board type for the project...


pasichnyk 12-05-2010 12:19 PM

Ok, I'm going to assume blue/pink is the way to go here.

New question is around instalation. I have recently installed some new wiring in this room and have some horizontal runs through the cavities I'll be insulating, about the height of the recepticals and through the center of the studs... What is the best way to get the foam installed around this obstacle ? cut two peices sandwitching the wire and then spray foam it in? Does this meet code? What about the actual receptacles? And finally , what's the best way to secure it to the studs and face of the wall? Do I need to glue it with pl300 or can j just use a bead of spray foam around the edge?

Is it better to put the insulation on the backside of the studs creating a continuous sheet of insulation, or fit it into the cavities?

I also came across this: It basically says that you can install batts in the wall cavities, and then install foam board in the attic side as a draft stop. Wouldn't this be the best for me? Say, unfaced batts in the cavities (easy to get around hte wiring and receptacle boxes) and then a continuous sheet of foam over the top of everything (studs included) to seal off any drafts? Or is that installation assuming I had an interior vapor barrier on the conditioned side of the batts?

pasichnyk 12-05-2010 05:30 PM

Ok, I purchased a bunch of 2" xps today to start this project. Any recommendation on the best way to apply this (especially with regard to the wiring and receptacles) would be greatly appreciated.

jklingel 12-05-2010 08:04 PM

MHO... avoid fiberglass batts, period. Ok to put whatever in walls, but they should be dense material (not fg, imo, if possible) and foam over them if you want to. Put foam in the bays, if you want to. Whatever works best to stop the air. Split foam as little as possible, seal as best you can; spray foam, caulk compatible w/ the foam, etc. Those are the principles.

pasichnyk 12-19-2010 11:24 AM


Originally Posted by jklingel (Post 546218)
MHO... avoid fiberglass batts, period. Ok to put whatever in walls, but they should be dense material (not fg, imo, if possible) and foam over them if you want to. Put foam in the bays, if you want to. Whatever works best to stop the air. Split foam as little as possible, seal as best you can; spray foam, caulk compatible w/ the foam, etc. Those are the principles.

Just to be clear on your last post... It would be ok to put cotton batts into the framed cavities (easier to get around obstacles like wiring), and then create a continuous sheet of xps glued/taped/sprayed on the unconditioned attic side of the studs forming a draft stop and adding more R value to the kneewall? this seems like it would be much easier to install and would result in more insulation for the walls. Just want to make sure putting a semi-impermeable xps on the unconditioned side won't be an issue. If this is ok, what is the maximum xps (or eps?) thickness that would be ok and not cause issues?

jklingel 12-19-2010 12:57 PM


Originally Posted by pasichnyk (Post 553651)
If this is ok, what is the maximum xps (or eps?) thickness that would be ok and not cause issues?

The xps over the filled bays should be fine. As to how thick, I am not sure. Some folks put 6" on up here (google REMOTE wall). Batts plus 2" is pretty typical, and should give you about R22 or so. Check w/ a local code official for recommended R in your area.

pasichnyk 12-19-2010 02:08 PM

Ok, uta pretty moderate here, so hopefully r23 (3.5" cotton batts + 2" xps) would be ok. Much better than my current R0. :)

If read a bunch of times that xps must be covered due to fire safety. Does this just mean on the interior side or does it need to be covered in an unconditioned attic too?

jklingel 12-19-2010 07:38 PM

Covering the plastic board is a code question: I don't know. Call locally. You don't have to cover it on the outside of a house, so my GUESS is that it is ok here. only a guess. j

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