Knee Wall Insulation

If you are building or renovating a house with an attic, or a top-floor flat where the roof is the ceiling, you are probably facing the installation of knee walls in your space. These walls bring challenges in insulation as the space between them and the roof stays empty and can leak the air from the outside or the rooms below.

What Are Knee Walls and Why Is It Important to Insulate Them?

Knee walls are short vertical walls, typically about three to five feet tall, that adjoin an indoor living space on one side and the unconditioned space of the attic on the other. Even though they aren’t exterior walls, but those in between the interior and empty attic space, they still need to be well-insulated to control the airflow in and out of your living area.

Knee walls exist in houses with vaulted ceilings, lofts, skylights, dormers, bulkheads, or spaces over garages. They are often found in finished attics and are mostly made of trusses and plywood. Many homeowners use them to create extra storage space behind by installing a small door.

Even though knee walls can be useful as storage and create a nice shape for your indoor space, they must be properly insulated otherwise they could cause you lots of problems and money loss on heating or cooling.

If the space behind the knee wall stays poorly insulated, lots of cold air can come in during the winter, and lots of hot and moist air can get trapped there in the summer. All of this can affect your efforts to keep a balanced temperature in your indoor living spaces and bring you high bills and energy loss.

Types of Knee Wall Insulation

There are several options for knee wall insulation. For the walls that are still under construction, you can use rigid insulation materials and methods like:

  • Batt insulation
  • Spray foam
  • Blown-in products
  • Insulation panels and sheets

Insulation is meant to be placed around every knee wall joist after you have finished installing the plumbing, electrical, or ventilation systems. When that’s done, the knee walls can be covered with any finishing material you choose, drywall, panel wood, or bricks.

If, on the other hand, you are dealing with the renovation of already existing knee walls in your house, the ways to go about it are a bit different. The insulation method here will depend much on what material the knee wall is built of or covered with. If there are bricks, you may need to remove only a few, or all of them. If you decide to remove all the bricks, you can use any insulation method, while removing only a few bricks will allow you to use only blown-in insulation or spray foam.

If the knee wall in your space is covered with wood panels or drywall, it could be much easier to remove it all and then insert an insulation material of your choice. However, if you still wouldn’t like to remove all of the drywall, you can cut holes in it, and use blown-in insulation, just like it’s done with the bricks.

What Should You Insulate, the Roof or the Wall?

There are many reasons to include the triangular attic space behind the knee walls in your home’s conditioned space. This will make the insulation challenge and air sealing easier and is actually the only method you can use if that space contains plumbing pipes or ducts.

The air can leak through the knee wall into your living space in a couple of ways. If the knee wall isn’t insulated properly, the air will leak through it but also escape through the floor joist from the rooms below. These two are the most common ways the warm air can leave your indoor space and should be the first targets in your insulation plans.

Many attic spaces are built without or with very little insulation behind the knee walls, and you can fix that by either insulating the wall or the roof and the space behind the knee wall. The second option is more effective, as it will secure your space from air leakage better and longer.

In any case, insulating both the roof and the knee wall would be the best option. In that way, you’d preserve the attic space from moisture, pests, and air leak.

By air sealing, you can minimize air leaks which considers carefully installing insulation around the pipes, electric cables, joists, and ventilation systems, to prevent heat loss and maintain a desired temperature in your home.

Knee Wall Insulation Methods

Maybe the best idea when insulating a knee wall is to use the same material as you did for the rest of the attic. You can choose between spray foam, fiberglass, cellulose, or other insulation materials that may fit your home.

Spray Foam Insulation for Knee Walls

Spray foam is probably the best insulation method to prevent air flow behind the knee walls. When the foam is sprayed on the underside of the roof deck, it is also applied to the cubby behind the knee wall, making that part of the space completely air-sealed.

Another benefit of spray foam is that it can protect your roof from damage, especially from moist and ice dams. If the attic flat is insulated using this method, the knee wall will also be sprayed to insulate the cubby on the other side from the rest of the conditioned indoor space and prevent air leaks inside.

There are different types of spray foam, and their R-value varies from 3.5 to 7. Foam is great at sealing air leaks, and it doesn’t absorb water or moisture. However, it can be very pricey. Spray foam also must be applied by a professional, as the material can be toxic until it's dry and cured, and it can crack if not applied properly. Spray foam is durable, but it’s good to know that it can also shrink with time.

Insulating Knee Walls With Fiberglass

If you’d like to insulate your home with fiberglass, you can use it both for the whole roof or the attic flat. The fiberglass batts should be measured and cut to fit the surfaces and the space behind the knee walls perfectly not to leave any gaps for the air to come through.

If you are insulating the roof deck, you wouldn’t insulate the knee wall itself, but the cubby on the other side of the wall. If you are insulating an attic flat, the fiberglass should be placed on the flat and up the knee wall, and optionally, also on the floor of the cubby.

Fiberglass is a very popular insulation material, and its R-value is between 2.2 and 4.3. It is cheap and easy to find, and very good against moisture, fire, and insects. However, unlike spray foam, materials like fiberglass and cellulose are likely to let some air in as they don’t make a complete barrier.

Cellulose as a Knee Wall Insulation Material

Insulation with cellulose is used when insulating an attic flat, and it’s usually applied as a blown-in method. It is made of recycled wood or paper, so it is considered to be a green insulation material.

It is most commonly applied on the attic floor and to the floor of the cubby behind the knee wall. In some rare cases, the knee wall itself can be insulated with wet cellulose or fiberglass batt.

Cellulose is cheap, but it also has its disadvantages. This material can be very flammable, and it can also absorb moisture which can lead to mold and pests. Additionally, this material is likely to sag and get damaged over time.

Insulation Panels for Knee Wall Insulation

Insulation panels are common in knee wall insulation but are usually not better than the methods mentioned above. Insulation materials are used depending on the knee wall construction and some other factors, so you should find the one that best suits your house or a flat.

When in doubt, it’s not a bad idea to contact a professional insulation contractor and find the best solution for your specific case.

Insulating Your Roof or Attic With Prodex

The all-in-one solution for insulating your attic is Prodex. This insulation material is made of reflective reinforced foil and closed-cell polyethylene foam, which is very stable and durable. Its micro cells don’t collapse under pressure and it doesn’t weaken with time.

Prodex blocks radiant heat transfer, so it’s very good for roof insulation and prevents condensation as an efficient vapor barrier. It is also light and easy to install, and it fits well around the nails, pipes, or cables to protect against any air leakage.

Insulating a Knee-Wall Door

Corners of the attic, especially those behind the knee walls can be turned into a useful storage space. Whether you are building one yourself or renovating an already existing attic flat with a knee-wall storage space, you shouldn’t forget the small doors to the storage, as they can leak more air than you probably can imagine.

When insulating a knee wall door, you can use the same insulation material as for the walls, plus the weatherstrip. Like on any door, the air can leak around its frame and that’s why weatherstripping is useful.

Besides that, just measure the surface of the door, and cut out as much fiberglass batt or apply the required amount of other insulation material on the storage door. After that is done, make sure you secure the insulation on the door and make it stay firmly and last long.


Knee wall insulation and air sealing are crucial, as they will protect your living space from airflow, heat loss, and high heating or air conditioning costs. The good thing is that insulating knee walls and knee wall doors isn’t difficult and can be done with or without professional help.

However you choose to do it, you should follow safety precautions and learn everything there is to know about the specific knee walls in your house or a flat, as well as all the insulation methods and materials that are available to you. After you collect all the necessary tools and information, you will be ready for a successful renovation and insulation project.

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