How do you recommend improving knee wall insulation???

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Permalink Reply by Ed Voytovich on December 6, 2010 at 10:01am

A number of people are talking about how they treat the back side of the kneewall itself, but I'm not seeing much about treatment of the critical junction point where the kneewall rests on the joists (and usually the floor on top of it).

If the room with the kneewall is over conditioned space, the interstitial space between the ceiling below and the floor of the kneewall attic must be air sealed or you might as well go home and have a beer . . . the rest of your effort goes pretty much down the drain. We see this almost always in houses built or retrofitted before contractors fully understood that it's about the convection; the conduction comes in second every time.

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Permalink Reply by Adam Zielinski on December 6, 2010 at 10:13am

If you are in a cold climate, one thing you want to avoid is having a vapor barrier on the cold side of the insulation. So I would use Tyvek or some other vapor permeable air barrier material to cover the insulation, rather than an FSK paper. You want the vapor barrier on the warm side. Sometimes if the knee wall is something really hard to air seal, like tounge and groove, I would put up a 4mil poly vapor barrier to help create a good air and vapor barrier before installing the batts. Then put Tyvek up on the cold side after installing the batts.

And don't forget to seal off any floor joist cavities under the knee wall with foamboard to provide a continuous air barrier.

If the side attic is currently unvented, I would seriously consider bringing it inside as Ed Voytovich recommends. If the side attic is already vented, I usually keep it that way and insulate the knee walls. It just depends on how easy or difficult either option is depending on the home, and whether the homeowner is interested in using the area for storage or not.

Usually it will be more work and cost more to bring the side attics in. In some jurisdictions, you can't leave spray foam or polyiso foamboard uncovered - you have to install an ignition barrier over it, and that means either drywall or intumescent paint. Drywall can be hard to carry upstairs and fit through access hatches.

If you live in a really cold climate with lots of snow, and you bring a side attic inside by just installing 4" fiberglass batts on the sloped ceiling, and covering that with some type of vapor/air barrier, you could end up with ice dams due to the roof slope insulation not providing enough of an R value.
See Buildingscience.com for more papers on ice dams and attic ventilation.

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Permalink Reply by Arthur Jones on February 2, 2011 at 1:37pm

A NEW question about knee wall insulation.

This is in regard to a 1907 house with no insulation in the knee wall areas. The floors of the knee wall space are finished. The triangle spaces on top of the knee wall are stuffed with rock wool. There is minimal rock wool in the attic with minimal attic ventilation. 

There are 5 knee wall areas. Two will be used for storage.

1. For those knee wall areas where want to save the floor space for storage the plan for spray foam insulation under the rafters is too expensive. Are baffles + faced fiberglass under the rafters reasonable? There will be no ventilation. 

2. For the other knee wall spaces the plan is for fiberglass on the walls and the floor with a vapor barrier on the warm side.  

Would appreciate advice. Thanks.

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Permalink Reply by Ed Voytovich on February 2, 2011 at 3:01pm

The rafters in a house build in 1907 are unlikely to be on 16" centers, so there may be a problem with getting the batts installed tightly.  If you use Kraft-faced material there will be possible issues with getting the batts fully enclosed on all six sides to optimize their performance and minimize the potential for condensation on the underside of the roof.  If you use unfaced batts and cover them with Tyvek it's hard to get a good seal at the perimeter.  If you don't air seal the space between the bottom of the knee wall and the ceiling below, warm air will move to the outside through what is probably balloon framing unless you seal that floor cavity at the intersection with the exterior wall.  There is no reason I can think of to install baffles unless there's an attempt at ventilation (never mind the fact that it's not at all clear to me why ventilation in such a situation is ever added). 

In the areas that will not be used for storage, the fiberglass on the floor will only work if it's in contact with the ceiling below and if the space under the wall is air sealed.

 

Unfortunately the methods you propose do not have a very good track record of success.  How do I know this?  Because I tried them over and over again and they worked somewhere between poorly and marginally over and over again.

 

The sad truth is that knee wall attics are stinkers to air seal/insulate, and knee wall attics in pretty old houses are really stinkers.  It's a sad truth, but it's a truth none the less.

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Permalink Reply by Adam Zielinski on February 7, 2011 at 9:46am

Arthur,

 

In knee wall areas with floor boards, if you are going to keep the area unconditioned and ventilate the area, you need to rip one up the floor boards next to the knee wall to gain access to air seal under the knee wall. 

If you are going to seal the area off and bring it inside, making it conditioned space, then you need to rip up the floor board towards the eave to air seal it out on that end. 

 

Probably one of the better options for insulating side attic rafters is polyiso foam board air sealed in place and then covered with a fire resistant rated vapor barrier like FSK.  But depending on the jurisdiction, you might have to paint intumescent paint over it or install some kind of ignition barrier. 

 

A cheaper option might be to install tyvek first up on the underside of the rafters, then install fiberglass batts, and then install a fire resistant vapor barrier material like FSK over that.  Tape up all the seams with Tyvek tape or a compatible tape.  You will get a decent air seal out of that although it won't be as good as regular drywall.  But if you are in a really cold climate, 4" of fiberglass might not be enough R value to prevent ice daming. 

 

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Permalink Reply by Arthur Jones on February 10, 2011 at 3:32am

Ed is right about the spacing between the rafters. They vary from but most are 18+ inches. 

Someone offered to blow cellulose into the interstitial space between the knee wall floors and the ceiling below but I may have live knob and tub wiring there so that was probably ruled out (also gave a very high quote). 

Thanks for the advice guys.

 

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Permalink Reply by dale conner on February 5, 2011 at 9:07pm

using foam insulation is a good option for knee walls as well

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Permalink Reply by Joseph (Joe) LoConte on February 10, 2011 at 9:18am

I suggest they use thermo-ply or foam board to get six sided contact on the exposed side of the kneewall after the insulation is instaled and inspected.

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Permalink Reply by A. Tamasin Sterner on February 10, 2011 at 7:37pm

Just like everything else... it depends on the budget!

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