What is the Best Way to Insulate a Cantilevered Floor Overhang?

Are you noticing cold drafts around your breakfast nook? Is the little overhang in your home more uncomfortable than the rest of your house?

It’s likely because the outside air is moving up through the cantilevered floor. Let me explain what that is exactly as that’s probably not what you call it.

A cantilevered floor is a floor that extends past the foundation. It can be on the first floor or even a second-story bump out, bay window, or a room over a porch, according to the Building America Solution Center.

RetroFoam of Michigan has helped thousands of homeowners with the comfort issues in their homes, and some of those cases included insulating cantilevered floor joists. Using either spray foam or injection foam insulation we have insulated our fair share of cantilevered floors.

We want to give you all the information you need to make your home more comfortable. In our continued efforts to educate homeowners, we will talk about the three top insulation options – cellulose, fiberglass, and foam – as well as each material’s pros and cons.

How to Insulate a Cantilevered Floor

If your cantilevered floor isn’t properly insulated it can lead to a lot of air leakage.

This air leakage doesn’t just make this an uncomfortable area of the home, in some cases, it can cost you money as the air you pay to treat can leak out of the area just as easily as outside air can get inside.

You have options when it comes to insulation. You can use traditional materials like cellulose or fiberglass, or a more modern option like foam which can be sprayed or injected. The method that you insulate the cantilevered floor joists depends on whether you have access to the area.

If there isn’t crawl space access to the overhang, then holes can be drilled into the cavity from below, but we’ll talk more about that in a minute.

Now let’s take a closer look at these options.

Cellulose Cantilevered Floor Insulation

Cellulose insulation can be installed in the cantilevered floor joists two ways – through access from the crawl space or by drilling holes through the bottom of the overhang.

Installing cellulose via the crawl space is fairly simple, as it is just blowing the material into the floor joists. When it comes to installing from below the overhang, your insulation contractor will drill holes into each stud cavity and blow the cellulose in until it is full.

Cellulose insulation is the cheapest option for the cantilever overhang but will need routine maintenance. Here are the pros and cons of cellulose in the cantilever overhang.

Pros:

• Inexpensive option compared to other materials.
• When blown into the floor joists cellulose can get into most nooks and crannies as long as there are no obstructions.

Cons:

• Cellulose can settle up to 20 percent, which is a problem for enclosed cavities where it can’t be raked and maintained.
• Cellulose still allows for air movement, which can blow the cellulose around in the cavity. If there is crawl space or basement access, it will also blow the cellulose into those areas. This air movement will also keep the overhang uncomfortable.

Fiberglass Cantilevered Floor Insulation

Fiberglass insulation can be installed in the cantilever only if there is access from the crawl space or basement.

The best way to install the fiberglass is in batts in the floor joists. The fiberglass must be cut to fit perfectly into the cantilevered floor joists. Here are some other pros and cons.

Pros:

• Suited for standard stud and joist spacing that is relatively free from obstructions.
• Can be done as a DIY insulation project.

Cons:

• Still allows for airflow in the cantilever floor joists.
• Fiberglass can trap allergens, dust, and moisture which leads to mold growth.
• When handled without protective coverings, fiberglass can embed in the skin. It can also be inhaled and can cause respiratory irritation.

Injection or Spray Foam Cantilevered Floor Insulation

Foam insulation can be installed in the cantilevered floor by either spraying from the crawl space access or from the underside of the floor.

The preferred method is to spray the cantilevered floor, but there are instances where injection foam is preferred. Whether spray foam or injection foam is used for your project, both materials create an air seal. They are just installed differently. Now let’s take a look at the pros and cons.

Pros:

• Creates an air seal that will stop air leakage.
• Fills every nook and cranny in the cavity and doesn’t leave any voids.
• Helps to lower monthly energy bills.

Cons:

• Foam insulation is a more expensive option when compared to traditional insulation.
• Not recommended as a DIY project for homeowners without spray foam knowledge.

How to Properly Insulate a Cantilever

Now that you have read about your options, it’s time to make a decision on which insulation will be best for your cantilever overhang.

The U.S. Department of Energy’s Building America Solution Center recommends air sealing the top, bottom, and sides of the cantilevered floor cavity and to ensure there are no voids. The only insulation option that fits that criteria is foam insulation.

If you’d like to learn more about foam insulation for your project, check out the Learning Center on our website.