How To Insulate A Knee Wall And Knee Wall Door?

Are you doing a major renovation, and do you need to install fresh insulation to a knee wall? Or maybe the old knee wall insulation needs to be replaced or improved? We researched how the pros do it, and we have the answer to your question right here.

The steps to insulate the knee wall with fiberglass batt, foam board, and spray foam are as follows:

  1. Measure the sections of the knee wall.
  2. Cut the fiberglass batt.
  3. Apply spray foam on the small holes in the knee wall.
  4. Install the fiberglass batt on each section of the knee wall.
  5. Cut the foam board.
  6. Install the foam board over the fiberglass batt.
  7. Apply spray foam on the edges of the foam board to create an air seal.

While we have outlined the knee wall insulation procedure above, the succeeding sections will provide you with detailed steps and precautions including how to insulate the knee wall door. We also have information that might be useful for your knee wall insulation project. Read on!

An attic is converted into light spacious living room, How To Insulate A Knee Wall And Knee Wall Door?

How to insulate a knee wall and knee wall door

First, we'll start with the steps to insulate the knee wall, and then we will proceed with the steps to insulate the knee wall door.

Safety precautions

Before we begin with the detailed steps to insulate your knee wall and knee wall door, let's take a moment to review the following safety precautions as recommended by the EPA (US Environmental Protection Agency).

  • Anyone not involved in the insulation project should leave the installation area.
  • The area should be isolated for the entire duration of the insulation process. Isolating the attic ensures that there will be no chemical vapors that could leak into adjacent rooms.
  • Everyone that will work on the insulation project should always wear protective equipment while inside the installation area. Protective equipment protects against exposure to vapor, mist, particulates, and dust. The following are the recommended protective equipment:
    • Respirator
    • Chemical resistant gloves
    • Chemical resistant clothing
  • Make sure the work area is well-ventilated.
  • Clean the work area thoroughly after each of these activities: trimming the batt, preparation of the batt, and application of the batt. This prevents other people from getting exposed to dust and particles that may contain isocyanates and SPF chemicals.
  • Handle protective clothing with care when removing them. Caution is needed to avoid exposing yourself or others to toxic chemicals that might be on protective clothing.
  • Clean the area thoroughly before letting anyone, other workers or occupants, into the area.
  • Be cautious when determining a safe re-entry schedule.

If you need chemical-resistant gloves, Gloveworks Orange Nitrile Disposable Gloves are latex and powder-free. Find them on Amazon by clicking here.

Detailed steps to insulate the knee wall

Glass wool insulation

Without further ado, here are the detailed steps to insulate the knee wall:

  1. Measure the length and width of each section of the knee wall. This is the area between the roof supports.
  2. Cut the fiberglass batt based on your measurements. Use a sharp cardboard knife.
  3. Apply spray foam to seal the areas on the knee wall where piping, electrical sockets, electrical switches, etc., pass through. Let dry.
  4. Install the fiberglass batt with the paper or foil side facing the living area or the area where heat will come from. The fiberglass batt should fit snugly between the roof supports.
  5. Cut the foam board based on the size of the knee wall.
  6. Install the foam board over the fiberglass batt, sealing it between the roof supports and the knee wall, and the foam board. Use nails of appropriate length to keep the foam board in place.
  7. Apply spray foam on the edges where the foam board meets the wood surface to create an air seal.

How to insulate a knee wall door

Knee wall doors are often overlooked and can be common exit points of warm air. Follow the simple steps below to insulate your knee wall door.

  1. Install weatherstripping on the knee wall door like you would on any other door.
  2. Installing a latch will allow you to pull it tightly against the frame and weather stripping. This will give you a firm seal.
  3. Measure the exposed area of the knee wall door facing the attic.
  4. Trim the fiberglass batt with a sharp cardboard cutter based on your measurements. Cut as many as needed.
  5. Attach the fiberglass batt to the knee wall door with construction adhesive. The side with paper or foil should face the living area or where warm air comes from.
  6. Trim a piece of foam board. Use the same measurements for the fiberglass batt.
  7. Attach the foam board to the fiberglass batt, sandwiching the batt between the foam board and the door at the side of the attic. Use nails that are long enough to secure the foam board to the knee wall door.

Which insulation material is best for the attic?

Now that we have the steps for insulating the knee wall and the knee wall door down, you might want to know more about your options. You might want to explore other insulation materials that you can use to insulate your attic.

Before we provide a list of insulation materials that you can choose from, we need to take a brief look at how insulation materials are rated based on their ability to insulate.

Insulation materials have an R-value. According to Energy Saver, the U.S. Department of Energy's (DOE) consumer resource on saving energy, the R-value is the thermal resistance of a material.

The R-value goes up as the material's ability to trap the flow of heat gets better. Thus, a higher R-value means the material is better at keeping your house warm in the winter and cool in the summer. The R-value of a material is also influenced by its thickness and density.

We have a list of the different insulation materials below that you can choose from, including the advantages and disadvantages of each.


Man installing fiberglass insulation in the wall

Fiberglass batt and other forms of insulation made from fiberglass are the most popular insulation type. It has an R-value between 2.2 to 4.3.

Its advantages include being cheap, fire-resistant, and easily accessible. Additionally, it is good at resisting moisture, making it a sought-after material in regions with high humidity.

Insects do not like nibbling on fiberglass insulation which is a plus. Finally, it doesn't shrink as it gets older.

Its biggest disadvantage is that it contains chemicals that are harmful to health. We have an interesting article here about the lifespan of fiberglass insulation.


This insulation material is made from recycled paper or wood. Because of this, most manufacturers tout it as a green product—an unsubstantiated claim.

Its main advantage is that it is a cheap material—cheaper than even fiberglass.

Its disadvantages include being highly flammable. It absorbs a lot of moisture—moisture makes insulation virtually useless. Moisture also brings about the growth of mold and mildew.

Cellulose insulation is known to sag and settles over time. Additionally, it generates a lot of dust during installation.

Mineral wool

It is a fire-resistant material that doesn't burn at temperatures below 1,800 degrees. It is very effective at blocking sound from outside your house. On top of that, it is water-resistant.

Its disadvantage is the price— it's more expensive than fiberglass. And inhaling it can cause cancer.

Cotton or denim insulation

A small section of recycled blue jean denim insulation

It has an R-value of 3.7 or better. Moreover, cotton or denim can trap heat. This can help keep you warm during cold winter months.

It doesn't have any safety hazards during handling or installation, unlike mineral wool and fiberglass. Its main disadvantage is that it is quite expensive.


Technician spraying foam insulation

Two main types of foam are used in insulation. Open-cell foam has an R-value of 3.5. Closed-cell foam, on the other hand, has an R-value of 7.

Its advantages include being really good at sealing air leaks. It never loses its effectiveness even after many years. It can also block sound from outside your house.

Foam is impervious to water. Thus, it prevents the growth of mold and mildew. Additionally, it has a polymer that hinders its growth.

The biggest disadvantage of foam is cost. Additionally, it shrinks as it ages. If the foam is not professionally installed, you run the risk of the foam creating openings in your insulation layer.

Moreover, risks from foam installation increase when not done by a professional. Foam is a toxic material until it completes curing, and this can have life-threatening consequences if not handled professionally.

We have an article about blown insulation here that you might find interesting.


It is necessary to insulate the knee wall and the knee wall door. It is a common exit point of heat in your house. Fortunately, the steps to insulate them are quite simple and can be done by an adult without the need for complex tools.

However, it is important to follow safety precautions whenever you do any home improvement projects. And insulating the knee wall and the knee wall door is not an exception.

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