What Is an Attic Knee Wall?

best way to insulate knee walls

If you’ve been in your attic recently, you’ve probably noticed the triangular slope of the roof on either side that makes it difficult or impossible to utilize the space where the rafters and the floor connect. If you’re only using your attic to store boxes and holiday decorations, this probably isn’t of much concern to you. However, if you’re planning to use your attic to increase the living space in your home, you may want to consider adding attic knee walls.

With attic knee walls, you can transform your attic space into a cozy and relaxing environment. Whether you’re creating a space to entertain guests, a quiet work environment, or an additional bedroom for your home, installing attic knee walls is the perfect way to transform a bland, uncomfortable storage space into tasteful home addition. Read on to learn about building knee walls in attics, how to insulate knee walls, and the pros and cons of installing them.

What Is an Attic Knee Wall?

A knee wall is a short wall that covers the area where the rafters and the attic floor meet. Knee walls are usually two to three feet high and create a 90-degree angle from the floor to the wall as opposed to the sharp acute angle the roof’s slope makes. Depending on the size and shape of the roof, a builder may have to customize the knee walls to match it.

Your home’s attic may already have knee walls or two-by-four planks in place for them. If your attic still has the triangular shape made by the roof, you do not have knee walls.

The Need for Attic Knee Walls

Knee walls are necessary if you plan to use the attic as an additional living space. Knee walls allow you to create a comfortable setting for you and your loved ones to enjoy without the temperature issues associated with attics during the hot and cold seasons. Knee walls provide a certain level of insulation that keeps exterior air from infiltrating your living space.

Knee walls also provide additional support for the rafters. It’s common for roofers to build temporary knee walls while working on a roof for maximum stability. Having permanent knee walls in your attic lightens the workload for the rafters, increasing the roof’s overall lifespan.

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How to Build a Knee Wall

In this section, we’ll go over the step-by-step instructions for building a knee wall. If you don’t consider yourself particularly “handy,” you can hire a team of professionals to do the installation for you.

For this example, we’ll be building an eight-foot-long attic knee wall that’s two and a half feet high.

1. Find the Roof Angle

The best way to build a knee wall is to assemble the frame on the attic floor and slide it into place. Before you can do that, you’ll have to determine the angle of the rafters. Get a piece of two-by-four lumber that’s two and a half feet long and hold it vertically against the floor while lining it up with one of the rafters.

2. Create a Template

Use a pencil to mark the angle on the two-by-four created by the rafter. Cut the wood along the pencil line using a circular saw or miter saw. This piece of lumber will be the template for the other two-by-fours you’ll use to build your knee wall.

3. Cut the Two-by-Fours

You’ll need three eight-foot-long two-by-fours for this wall. One of them will remain uncut and act as the base for the wall. Use your saw to cut the other two pieces of lumber into six two-and-a-half-foot lengths.

Do not attempt to cut them at an angle: Get them to the proper length first.

4. Mark the Two-by-Fours

Line up each of your six two-and-a-half-foot two-by-fours with the template and mark the angle on each one with a pencil.

5. Angle the Two-by-Fours

Use your saw to cut the six two-by-fours along the pencil line to create the proper angle. Precision is crucial in this step. If one of the two-by-fours is longer or shorter than the others, it will affect the spacing.

6. Connect the Two-by-Fours

Grab your uncut eight-foot two-by-four and use a tape measure and pencil to measure and mark it every 16 inches. Nail each of the six short two-by-fours to the long one at the 16-inch marks so that the angled ends are in the air when you stand the assembly up. There should be a two-by-four at each end of the long, uncut piece.

7. Complete the Frame

An eight-foot length of one-by-four will serve as the top of the attic knee wall frame. Line it up with the angled ends of the standing two-by-fours and nail it into place.

8. Put the Knee Wall in Position

Put the knee wall in place against the rafters firmly. Nail the base down into the attic floor. Be sure to nail the wall to the floor in several spots along the length of the base.

9. Fasten the Knee Wall

Nail the one-by-four at the top of the knee wall to the rafters. Like the base, you’ll need to nail the top of the wall in several spots along its length to keep it in place.

10. Attach Drywall to the Knee Wall

Use your tape measure to find the true length and height of the knee wall. Using a drywall square and a utility knife, cut out the dimensions from a four-foot-by-eight-foot sheet of drywall. Place the drywall against the knee wall and screw it into place with drywall screws.

If you need your knee wall to be longer than eight feet, you’ll need to add additional drywall sheets and use drywall tape and drywall mud to join them.

How to Air Seal Knee Walls Along the Roofline

One of the most common questions regarding knee walls is, “How do you insulate knee walls in an attic?” Insulating knee walls is vital for converting your attic into a living space and preventing the loss of conditioned air. The best way to insulate knee walls is to focus on the rafters and the roofline.

While it’s also a good idea to put some fiberglass insulation behind the knee walls, the most important thing is to seal any areas along the roof line where air can escape or get in. During the winter, your heat will escape through cracks or gaps while cold air will seep in, and you’ll have the opposite problem in the summer. You can use spray foam to cover large gaps and caulk to seal thin cracks in your roofline.

Insulating the rafters and adding an air barrier will also significantly improve the comfort level and energy efficiency of your attic. Hiring a professional to air-seal your attic and insulate it is the best way to ensure the highest performance from your attic and roof.

Pros and Cons of Attic Knee Walls

Installing attic knee walls is ideal for those looking to create a “bonus room” in their home. However, there are some issues to consider before committing to this type of renovation. Below we’ll list some of the pros and cons of attic knee walls so you can decide if they are right for your home.


  • By eliminating the acute angles on the sides of the room, routine cleanup becomes much easier.
  • Knee walls make the attic more aesthetically pleasing for you and your guests.
  • Knee walls add support for the rafters, which helps your roof stay strong for longer.
  • They provide an improvement in insulation for your attic.


  • The two-by-fours used to make knee walls don’t hold insulation well.
  • When people build DIY knee walls, they usually don’t seal the seams, edges, and penetrations properly.
  • Poorly insulated knee walls can cause a finished attic to become extremely hot in the summer.

Frequently Asked Questions (FAQS) – Attic Knee Walls

Are Attic Knee Walls Structural?

Knee walls are not a requirement for a structurally sound attic, but they do provide extra support for the rafters. If you have an older home and you’re worried about the roof, installing knee walls may be the best move.

Do All Attics Have Knee Walls?

No. Not every attic needs knee walls, but homeowners who want to use their attic as a living space should consider them.

Can a Knee Wall Be Load-bearing?

Knee walls are not typically load-bearing partitions but can be if the attic layout requires it.


For generations, builders have used knee walls to create a warm, inviting atmosphere in attics across the country. If you feel your home is missing something like a music room or a quiet office space where you can get work done without interruptions, consider installing attic knee walls.

At Attic Projects, our team has years of knowledge and experience with attic renovations and remodels, and we love helping our customers realize their vision for their homes. Contact us today to schedule an appointment.

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