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Community and Q&A

Detailing a Kneewall for Insulation Installation

earthbuilt | Posted in Energy Efficiency and Durability on

Happy holidays, folks!

Updated Images Included!

We prototyped a bay of the attic insulation in our vented attic. Still nervous! Worried most about any condensation and moisture issues in Midwestern mixed climate.

 Vertical mineral wool is r30 in stud bay, an air baffle and top plate from foam (yet to be sealed) leaves 2 inch air flow gap, and more insulation and sloped ceiling and flat ceiling coming. Ceiling will be r60 mineral wool. The devil’s triangle in the back is loaded with cellulose. We buried the mineral wool down to the ceiling drywall below, and include foam blockers between floor joists.

Next up is tyvek on back of insulation? (Yes, we should have done that much sooner, but didn’t. We’re going to have to figure out how to reach over mineral wool.) So it will be tyvek on unconditioned vented side, mineral wool, intello membrane, service cavity, drywall. I suppose I like that it sounds vapor permeable for drying.

Does that sound okay? Can we staple the tyvek in? Is it necessary to tape over those staples in this situation where we really just need an air barrier? Does it have to be air tight for tyvek, or we can just block air flow as much as possible? I suppose I’m looking for a can’t hurt might help kind of possibility before we climb around all the trusses like acrobats. Around the trusses will be tough regardless. We don’t want to try moisture. We’re thinking about tyvek down to the cellulose, where the foam would start. But….there’s nothing down there to attach the tyvek to on the bottom.

Lastly, because it’s an attic, it has some weird spots. Does the intello HAVE to have a service cavity? Can it be right up against drywall in some spots where we don’t plan to puncture it?

Thanks so much for the confidence to proceed or feedback. 🙂 

Take care out there.


Hello Folks, (I’m learning a lot lately, and have posted before but have a better understanding now of my issue.  I would really appreciate any updated ideas as I may not have written it well previously.) We have a VERY tricky devil’s triangle in our 1.5-story earth-built home. The second story is our attic, it’s at almost ground level, a very steep roof, and 7-foot tall knee walls.  The unconditioned triangle behind the knee walls has about 12-15 inch thick cellulose between the floor joists. Soffit vents are at floor level, we have site-built air chutes/baffles up to the upper attic above the ceiling where there are gable vents and an attic fan. (I can’t make the devil’s triangle a conditioned space right now. Maybe one day, but it’s out of my budget and ability currently.) The ceiling will have r60 Rockwool that is 14 inches thick, and the kneewalls will have r30 Rockwool that is 7 inches thick.  We were going to use Intello from 475 on the whole space behind the drywall with a service cavity. We are in climate zone 4a, I believe.  We get freezing winters with snow and humid, sweltering summers.  This has led to a lot of worry about where to put an air/vapor barrier, and a wide variety of local contractors providing different advice.  We’re hoping the Intello is a good choice behind the drywall. My big question that I hope doesn’t make me foolish: do the kneewalls need some sort of backing like OSB or foam board on the unconditioned side in our zone?  Right now, we just have planned the unconditioned triangle soffit area, double studs and r30 Rockwool, then the Intello, service cavity, then the drywall. If the ceiling of an attic doesn’t need osb, does the kneewall?  The ceiling of our first floor, which is insulated with the cellulose in the devil’s triangle, does not have osb or foam top or sealed barrier.  There is cellulose completely filling the area between the first and second floor through every bay. Can I leave the kneewall insulation exposed on the back? I’ve been worried that trying to seal it off, and seal between the floor joists which are wildly interrupted with vents and ducts and electrical, will lead to leaks and trapped moisture issues during one season or the other.  One insulation contractor told me we’re the only region that is wrong half the year whether we choose to put a barrier inside or outside because of the wild variety in weather. I can’t choose cold or warm climate, it’s both.  I was hoping Intello would solve a lot of this problem for me. I would love to be more energy efficient, but I just really fear mold and moisture. We’re already earth built and doing our best to make green choices.  We have planned mini splits to condition this space, it’s about 17×50 feet in size. What do I do about these kneewalls?  I read several articles on here, but I didn’t see anything that felt like it works for this climate area.  Is this an okay plan? Thanks so much, Sarah

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  1. Expert Member
    PETER Engle | | #1


    Your contractors are right about your climate being both hot and cold. The absolute best location for the vapor retarder is in the middle of the thermal envelope, but that's not easy to do with most wall systems. Given that, using Intello behind the service cavity is probably your best bet. The whole purpose of a "smart" vapor retarder is that it is less permeable when it is dry than when it is wet. If it becomes wet (or damp) due to condensation, the pores open up to allow for faster drying. Your interior combination of service cavity and drywall is relative vapor-open, so little risk of condensation there.

    Re: covering insulation on the back of the Devil's Triangle walls. Yes, you should cover the back side of the insulation to prevent wind-washing, convection currents and simply to help hold the insulation in place. This is less necessary on the attic floor because there is less vertical space for convection currents to set up and zero potential for the insulation to fall out because gravity hold it in place. There are many choices for this covering. A simple housewrap like Tyvek can work. XPS insulation is rigid and strong enough, but does come with a carbon cost unless it is recycled. OSB or plywood would be fine as both have variable permeability and are naturally "smart."

    You also mentioned that nasty space under the kneewalls where the ceiling joists cross from the floor of the triangle to the floor of the space inside. As is usual with this sort of construction, those spaces contain ducts, wires, plumbing, etc. It is very important to seal the joist bays directly below the kneewalls to prevent hot/cold air from the triangle from blowing under the interior floors and heating/cooling the floors directly. This subtle air pathway can cause substantial comfort problems and unexpected increases in energy use.

    1. earthbuilt | | #2

      Hello! Thanks so much for your thoughtful and helpful reply. I attached a photo, it's older- before we doubled up depth of studs to hold Rockwool and make room for air baffles. It was like this when we got here. These bays are relatively free, the other side is wild with vents and plumbing and cords.

      I will keep thinking about how to do a backing on the insulation on the unconditioned devil's triangle size. There are diagonal trusses in the triangle behind insulation making the install very difficult in every bay, and there are 60 bays! The bays didn't have a header top horizontal piece either, and we're doing that to prevent windwashing up the angled part of the wall to ceiling.

      I'm worried if I try to use tyvek or foam board (the air barrier?) on the back that moisture will be trapped or leak in with the humidity or moist air because I honestly might not have the skills to do it well. How tight of a seal should it be? Do I foam or tape all cracks? Wrap around each angled truss near the floor and tape it? While leaving it open is probably the least energy efficient, is it even possible or just a huge no no?

      My selfish hope was that by some miracle that the dense 7 inch Rockwool on back would suffice or withstand windwashing so I could be done. :) I don't know about convection though. Alas, that might all be wishful thinking. Rockwool does fit tightly into the stud bays without sagging so that's a good thing. I buried the rockwool into the cellulose down to the ceiling drywall below which is how I know how much is in each bay below the floor. It was easier to cut the Rockwool to fit, but getting foam would be tough.

      It has been about 25 here this week, a bummer week to be up there but looking forward to wrapping this project up. Hope you are somewhere warm!


  2. Expert Member
    PETER Engle | | #3

    You've discovered one of the real challenges when working with attic trusses and one of the reasons this area is called the Devils Triangle. I wish I had an easy solution for you, but I don't. The good news is that Rockwool fits more snugly and is denser than fiberglass. This lowers the risk of falling out, windwashing and convection. But the risk is still not zero. For the air barrier on the back, I would fall back on the maxim that "more is better." The more airtight the barrier on the backside of the wall insulation, the better its performance will be. However it is very hard to quantify how much better. insulation can be weird, where sometimes a 98% solution is only 50% effective (or less) and sometimes a 98% solution is actually 98% better. If you can get an air dam at either the top or the bottom of the stud cavities, that would go a long way toward addressing the risks. I'm thinking about folding down a flap of the air chutes (if they're long enough) and taping it to the air barrier membrane. Tyvek might be your best bet for the membrane, being reasonably lightweight and easy to work with. Don't worry about Tyvek trapping moisture - it is very vapor-open.

    1. earthbuilt | | #4

      Thanks so much for your help! We got some tyvek.
      I was looking at the tyvek today, I can't imagine how it would be really sealed in this space at every crack and seam. It seems if like the tyvek is just for blocking most air or even some air that it doesn't need to be sealed as much as if it were blocking rain and such, right? (Or am I just talking myself into a shortcut here.)

      I updated the original post with some new photos and thoughts. We're making progress. I'm always nervous, and just taking time to make sure things are as good as they can be with a diy job.

      Mold and moisture makes me worry, it feels like such a strange space. I think a lot stems from not being experienced or totally understanding building science and such.

      Thanks again, and I hope you are having a nice week. :) Take care out there.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Peter has given you great advice.

        The only thing I differ with him on is the necessity for any covering on the backside of the knee-walls. As Peter said, because of their density, mineral wool batts only lose a negligible amount of R-value to wind-washing or convection. It isn't zero, but it's close enough not to matter.

        The batts look tightly fitted, but if you are worried about them falling out over time, I would staple several runs of string on the backside of the studs and be done.

  3. Expert Member
    BILL WICHERS | | #6

    If you can work from the inside, and not on the attic side where access is difficult, I would try to use polyiso as an attic side air barrier to both gain you some R value, and get a radiant barrier (from the foil facer on the polyiso) "for free". Polyiso is also very lightweight, so relatively easy to work with in awkward areas. The thicker the better for the polyiso in terms of R value, but 1/2" would work fine if you want it to be cheap, or easier to install. I would glue the polyiso to the studs, which eliminates the need to get into the triangle to install fasteners. Once the polyiso is up, you can install the mineral wool from the interior side. The goal is to minimize how much you have to do from the difficult to access attic side.

    Mineral wool is less prone to convection currents degrading performance, but a vertical wall is something of a worst case scenario for that paricular problem, so I would try to put up an air barrier here. Using rigid foam here, like the polyiso I recommended, also helps with thermal bridging to help improve overall R value performance of the entire wall.

    Note that I have exactly this situation in my own home, with the notable exception of the "triangle" being much larger, so much easier to work in. My first step was to install a catwalk in the triangle so that I could work more safely and easily. My next step (I had to work from the attic side only), was to install mineral wool batts, replacing the original fiberglass batts, then polyiso (I used 2-1/2" polyiso for about R19). I did do an experiment several years ago where I put up mineral wool with 1-1/2" XPS (R7.5) as an attic-side air barrier on some of the wall, mineral wool only on some of the wall, and the original fiberglass at the end of the wall (before I built the catwalk to access that part of the wall). There is a noticeable difference (compared to the original fiberglass) in the winter where the XPS is installed, and much less of a difference where the mineral wool only is installed (which is likely due more to the extra R value of the XPS than the air barrier). The really big difference I notice is in the summer though, when the attic gets HOT. The difference during the summer is HUGE! I highly recommend the extra continuous insulation the rigid foam provides for this reason. Note that if you install rigid foam, be sure to cover the entire back of the framing, which means all the way up the attic side of the top plate, ideally tying in with your air baffles above. The same goes for the bottom plate if it's accessible.


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