Insulating Knee Walls

Old 10-04-10, 01:35 PM
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Insulating Knee Walls

We purchased a 53 year old Cape Cod home in Louisville a couple of years ago. The insulation behind the kneewalls in the attic was about 80% percent fallen down and useless.

The insulation on the floor of the attic is blown in cellulose. This seemed to be in fine shape to me.

But regarding the kneewall, I removed all of the old decomposing insulation and am nearly ready to replace them with new fiberglass insulation. I know that in my part of the country I am to place the kraft faced barrier of the insulation toward the room, but I have a couple of questions.

First - I have read in some places that you should try to have R-18 or R-19 against the knee wall. The studs we have running up the wall are 2 X 4 studs, and the insulation that I find that fits 2 x 4 areas is only R-13. Should I just go ahead and put the R-19 (usually made for 2 X 6) spaces and have the insulation stick out into the attic 2 inches? It is an attic that won't be used for anything other than storing a couple of seasonal items, so we don't need the space.

Second - I have read that new studies say to wrap the insulation of the kneewall after it installed (wrapping in this case would mean putting a plastic barrier of some kind on the exposed outside of the insulation). but I also have read not to wrap it because the insulation needs to let air escape out of it so it will not build up moisture.

Any opinions would be great.

Old 10-04-10, 04:34 PM
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If you are going to install dry wall over the insulation, use R13.
Old 10-04-10, 06:15 PM
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The best way doesn't involve fiberglass. You do not want a vapor barrier (plastic) on the attic side wall. You could use foam board or a house wrap to let the moisture diffuse out. F.b. under the wall, as well as above- butting the baffles there. This will prevent wind-washing of the insulation and stop air going under the wall from the outside venting. If you go fiberglass, use R-13 or R-15, higher densities to prevent convective looping inside the insulation itself. Install them carefully. Info-501: Installation of Cavity Insulation — Building Science Information

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Old 10-15-10, 10:36 AM
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It is also fine to use the R-19 and have it stick out beyond the studs a couple of inches to the outside. You can just anchor the wrap (don't use plastic - you want an air barrier but not a vapour barrier on the outside) to the studs loosely so it doesn't compress the insulation. If you want to do an even better job, buy some 2 inch styrofoam (just use the cheap open cell stuff) and attach a 1.5 inch strip to the back of each stud, to flush them out equal with the face of the insulation before you put the wrap on. Then you don't need to worry about compressing the fibreglass and you have a thermal break on the studs. I prefer rockwool by the way for an installation like this. It has slightly higher R-value, it is stiffer so it holds it's shape better and it is denser to prevent cenvection, but it is also a little more expensive.

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