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Musings of an Energy Nerd

Insulating Behind Kneewalls

Those cramped attics behind kneewalls should be included in your home's conditioned space

The right way to insulate the cramped attic behind a kneewall. Ideally, this type of attic is included in a home's conditioned space. That means that the insulation will follow the sloped roofline. Drawing: Fine Homebuilding

Although I wrote a 2012 article for Fine Homebuilding on ways to insulate the cramped attic behind a kneewall, I’ve never written such an article for GBA. So it’s clearly time for a comprehensive article on the topic.

A kneewall is sometimes defined as “a short wall under a sloped ceiling.” A typical kneewall is between 3 and 5 feet high. In most cases, a room with a kneewall has a sloped ceiling that extends from the top of the kneewall to normal ceiling height—about 8 feet off the floor. At that point the sloped ceiling usually transitions to a horizontal ceiling.

This description applies to the top floor of many one-and-a-half-story homes (like Cape Cod homes) or to a typical bonus room above a garage. Kneewalls can also be found on the top floor of some three-story or four-story buildings.

The illustration below shows the various components I’m talking about. (Note that the word “kneewall” refers to the short wall—not to the cramped attic behind the kneewall.)

Insulation challenges

If you are insulating a house with kneewalls, you face an important decision: Should the triangular attics behind the kneewalls be included in the home’s conditioned space, or should they be excluded?

For a variety of reasons, the best approach is to include these triangular attics within the home’s conditioned space. Bringing the triangular attic into the home’s conditioned space makes the task of air sealing much simpler, and is the only acceptable method to use if the triangular attic includes ducts or plumbing pipes.

If you go this route, you’ll need to insulate along the sloped roof assembly, all the way from the area above the top plate of the exterior wall (at the floor of the attic) up to the “top attic” above the…

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  1. Ira Broussard | | #1

    What if the area below the devil's triangle is not a conditioned space? My new home will have a covered porch, and the devil's triangle is above it. Using your first diagram as an example, move the exterior wall under the joists so that it is directly below the knee wall, and the left side of the roof/joists is supported by columns. The underside of the covered porch has a ceiling, i.e., joists/rafters are not exposed.
    I'm being told to spray foam between the rafters all the way down to the end of them, and to spray foam above the covered porch ceiling between the end of the rafters and the exterior wall under the knee wall, then spray foam the exterior wall. Do you agree with that approach? How much open cell foam should be used in the area above the covered porch ceilings?
    There are actually two covered porches. The front porch is 50' wide and 8' deep. The back screened porch is 50' wide and 12' deep, so the total area is 1000 sqft. Both porches have 10' ceilings. The house architecture is "southern Louisiana".


    1. User avater GBA Editor
      Martin Holladay | | #6

      The basic principle of insulation is that the insulation boundary must be continuous around the building, with an air barrier adjacent to the insulation layer. In some cases, a builder can choose between two ways to proceed.

      If you don't need your "devil's triangle" to be conditioned -- in other words, if there are no plumbing pipes, ducts, or vulnerable items in storage there -- it's OK to design your house so that the devil's triangle is outside of the home's thermal envelope. In most cases, this is tricky, though -- because it's hard to make the thermal boundary continuous and airtight.

      In your case, it's your choice. If the roof assembly above the devil's triangle is insulated, you'll also need to insulate the floor of the devil's triangle (since you have a porch below). On the other hand, if you want to keep the devil's triangle unconditioned, you'll need to insulate the kneewall, and you'll need to make sure that the air barrier is uninterrupted -- especially between the floor joists directly under the kneewall.

  2. User avater GBA Editor
    Brian Pontolilo | | #2

    Hey Ira,

    Where is your new house going to be located? In Louisiana?

  3. David Bainbridge | | #3

    I'm to the point of considering converting a 1940's cape code into a "normal" two story with large overhangs. Mostly to get rid of all the issues with inability to properly insulate the areas mentioned in this article. The "back" half of roof is 4/12 sloped and has only 4" space between ceiling near end of rafter leaving no room for proper amount of insulation. It has all the typical problems shown here in GBA with moisture condensing on underside of roof sheathing and massive ice dams. The thought of being able to just dump 2' of cellulose across an entire vented attic floor sounds so nice compared to the nightmare of fixing all the intricacies of a cape cod style.

    1. User avater GBA Editor
      Martin Holladay | | #7

      You're right. A Cape Cod style house is a nightmare to detail right. In most cases, establishing a continuous air barrier is extremely difficult, and the sloped ceilings aren't deep enough for adequate insulation.

      Compared to a Cape, an ordinary two-story house with an attic above the second story is a pleasure to insulate. If the house has raised-heel trusses, you just blow 16 inches or 24 inches of cellulose on the air-sealed ceiling, and you're done.

  4. Mark Hays | | #4

    Dear Martin: Great article on a major problem -- particularly with remodels and upgrades for homes in New England built in the mid-1900s. Our home in Massachusetts has classic knee walls, and when we hired a company to install poly spray foam I wondered "How can we also solve the knee wall problem?" Cold winter air flows directly into the joist bays -- cooling the entire floor / ceiling. I did not want to tear out all of the interior knee walls, however, and pay for all of the plaster and paint repairs. More than half were also inaccessible.

    We found a good solution: remove the soffit trim board on the exterior, which opened the end of all of the knee walls. Our foam installer simply filled half of the knee wall 'triangle' (including the end of the joist bay) insulating and air-sealing these pockets. We also replaced the soffit trim with cellular PVC -- which will never rot and never needs to be painted.

    I hope this tip is helpful to other readers with similar homes. Thanks again for all of your excellent work!

    Mark Hays

    1. User avater GBA Editor
      Martin Holladay | | #5

      Thanks for your comment. Indeed, it's often easier to access this tight area from the exterior of the house (by temporarily removing the soffit) than from the interior. There are a lot of variables, though, so this approach won't work on every house.

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