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  1. #1
    Join Date
    Dec 2009

    Default How do i insulate my slanted ceilings? (knee wall?)

    Ok, I dont really know what these are called, but I'll try to be as detailed as possible. My house was built circa 1840 here in Ontario and has had several renovations to it since then, most before i was alive. The second story is really a half story, in that the wall (which is an outside wall, formed of stackboard construction) goes up about 4 ft vertically, then slants upward toward the peak. At the 7.5'-8' level the horizontal ceiling begins, and there is an attic above that. The attic has cellulose blown in.

    Second story rooms look kind of like this in cross-section... (see sketch below)

    Its a knee wall, essentially, though every time i see 'knee wall' mentioned anywhere it is in the attic with a crawl space behind it. That is NOT what i have. No crawl space, just outside vertical wall, with outside slanted roof, and a bit of ceiling in the middle with a low attic above that.

    There are no soffit vents, since this was well before soffit vents, and i believe the cellulose is at least partially blown in between the roof boards and the plaster lathes that make up the slanted ceiling.

    My question is this: How the heck do i insulate this?

    The walls are not insulated, since they are essentially solid wood. (Stackboard is a lumber-town technique of construction that is basically taking 2x4s and laying them on top of each other on the broad sides, offset by about an inch either way, making a stack of 2x4s about 6" wide. The stuff they came up with when wood was cheaper than dirt...) My plan there was to build a 2x3 stud wall inside to put in some styrofoam board insulation, then drywall over it. This is only for the vertical surfaces.

    For the slanted surface i was thinking that to do this properly i will need to rip off all the plaster, rip off all the lathes (all hand split, up to 8' in length - crazy how they used to build stuff, eh?), then when the roof boards are exposed I fill the space between the rafters with more styrofoam boards and finish it with drywall.

    Do i need plastic sheeting to make an air pocket?

    is that even possible when going up against nail-filled roof boards?

    Do i need to do something with soffits or those waffle vents or any of that stuff, even though i dont have it now, nor any issues with moisture? I really dont have the space for both that vent stuff AND insulation, so i imagine the answer to that is going to have to be no.

    Would spray foam be a better option?

    Money is not really a major issue here, I just want my bedroom to be as warm as possible. Space and workability is the problem. I dont really think i'll be doing it myself either, but i want to know what to look for if i get a contractor to do it (and do it right).

    I'm also pretty leery of the moisture issues. I have never had any, but that goes hand in hand with having thick walls and no insulation to speak of - the merry little breezes carry everything away. If i start putting modern techniques into my old home will that suddenly change?

    I was going to give the horizontal ceiling another layer of blown insulation, but its up to the top of the joists and I'll have to lay new ones cross-wise to have something to blow up to (otherwise theres nothing to walk on and i'll fall through the plaster). Of course, I cant really blow more insulation in until i know exactly what I'm doing with the slanted ceilings.

    Anyway, thanks in advance. Hopefully you guys have some ideas.
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  2. #2
    Join Date
    Mar 2009

    Default Re: How do i insulate my slanted ceilings? (knee wall?)

    I recommend spray foam in the sloped parts, you wont need any soffit vents or vapor barrier, you could also spray foam the rest of your ceilings, do you have any vents on the top of your roof?

    Oklahoma City, OK

  3. #3
    Join Date
    Dec 2009

    Default Re: How do i insulate my slanted ceilings? (knee wall?)

    thanks for the quick reply cjsand. Yes the roof has basic vents.

    Regarding the spray foam insulation, do you recommend closed cell or open cell?

    Do i rip all the plaster off and put in high expansion stuff, or just pop holes in it and put in low expansion stuff? Im not sure i trust that the slow expansion stuff would cover everything it needs to...

    Any environmentally friendly types of spray foam that you know of that are preferable? ie no CFC filled bubbles in the foam.

    I found these that look appealing... (though these arent local to me, unfortunately)

    Just to doublecheck, this technique definitely isnt a moisture hazard? Call me paranoid, but i dont want mold.

    as for doing the rest of the ceilings, would i just go over top of the cellulose, or remove the cellulose first? I suppose that would eliminate the dust issues in the attic, but i have a hard time trusting it for moisture. At least if i leave part of the ceiling the way it was it seems less likely to have issues.
    Last edited by Swirlingchaos; 12-15-2009 at 12:43 PM.

  4. #4
    Join Date
    Aug 2009

    Default Re: How do i insulate my slanted ceilings? (knee wall?)

    Well, closed cell foam is not water permeable and it will act as a vapor barrier. Closed cell foam also tends to have a higher R-value then open cell foam. So for those reasons that I what I would lean toward.

    This technique, if using closed cell foam, should not present a moisture hazard. The closed cell foam will function as a vapor barrier so you don't need to add one.

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