How to Insulate Cantilevered Floor Joists - Home Stratosphere

How to Insulate Cantilevered Floor Joists

Learn about how to insulate cantilevered floor joists. Whether you're doing it yourself or enlisting the services of a contractor, it's a good idea to know what's involved, including the cost. Discover the many materials you can choose from and which is best for your project and budget.

Wooden balcony floor mounted on cantilever beams.

Have things seemed a little drafty around your house lately? Is the cold air coming up where you’d least expect it, such as from overhangs and floors? If the chilly blasts are coming from unlikely places, it just might be time for your next home-improvement project…insulating cantilevered floor joists. Probably not what you were expecting. Okay, admittedly, this isn’t an oft talked-about topic, so you may not be familiar with it. But once you’ve acquainted yourself with how to do it, you can roll up your sleeves and get to work, or enlist some mechanically-inclined friends (or a qualified contractor) to help.

We’ll begin with the basics and go from there. Chances are, you’ll be floored (pun intended) when you can check this off your to-do list — especially if it’s a succeeded DIY endeavor and you no longer feel like you’re going through a wind tunnel when walking through your home. If you’re not up to the task yourself, it’s best to hire a contractor. You don’t have to prove anything, and there’s no point in making the project more difficult than it has to be.

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What Is a Cantilevered Floor, Anyway?

When you see descriptions of homes for sale, you’re not likely to see “cantilevered floors” among the top features listed, so for those who haven’t heard of it, here’s a working definition:  A cantilever refers to a floor that extends past the supporting walls or beams of the foundation. While it’s typically found on the first floor, you can find cantilevers at higher points, too. Think second floors that jut out farther than the first, bay windows, or a room situated over a porch.

Why Should You Insulate a Cantilevered Floor Joist?

Wooden joist hangers.

As already mentioned, things can get uncomfortable without insulation. You’ll probably notice that unprotected areas are unusually drafty when cold weather sets in, despite your best efforts to seal doors and windows and keep the thermostat at a reasonable setting. That’s because air is leaking through the uninsulated cantilevered floor joists.

Without insulation, you might be left wondering if you accidentally turned on the AC, but your troubles won’t end there. All your efforts to keep your home adequately heated may be for naught. As cold air finds its way in like an uninvited houseguest, you’ll probably find yourself with energy bills that are through the roof, even though your HVAC system is working properly.

What Type of Insulation Should You Use?

When it comes to insulation, you have options. More traditional materials include fiberglass and cellulose. Homeowners may also consider the more “modern” variety, consisting of foam that’s injected or sprayed. What you select depends mostly on the method you’re using to insulate. Here’s a breakdown of how each material works:


Foam It Green 602 Closed Cell Spray Foam Insulation Kit
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Spraying is usually the go-to method for this installation, but you can also inject it into a floor cavity. Either way, the material creates an air seal. The foam will thoroughly fill open spaces without leaving gaps, so it prevents air leaks and lowers heating costs. However, it’s more expensive than traditional substances and isn’t recommended for do-it-yourselfers (unless you have considerable experience in this area already).

Spray foam is generally more expensive than its more-traditional counterparts. This material comes in two forms — open-cell and closed-cell. The former comes with a price tag of around $1.00 to $1.20 per square foot, while the latter will set you back $1.25 to $1.50 per square foot.

Besides cost, what’s the difference between open-cell insulation and the closed kind? As their name suggests, both consist of tiny cells. With open-celled foam, some or all of the cells aren’t completely closed. This makes the material softer and “springier” than its opposite. Closed-cell insulation is firmer and incapable of holding water.

Closed-cell insulation creates a sturdy barrier for trapping heat, so it’s a popular choice in construction. Open-cell material is more commonly used in furniture since it’s softer and less durable than the closed-cell type.


Man stacks eco-friendly cellulose ecowool material from recycled paper to the floor of the house for preservation and thermal insulation.

There are two ways to install this traditional insulator:  either by drilling holes in each stud cavity in the bottom of an overhang or through a crawl space. At $0.83 per square foot., cellulose is a popular option for many because it’s considerably cheaper than other materials. Plus it’s easy to install because all a contractor has to do is blow the material into the joist.

For the most part, it insulates spaces thoroughly enough to block most of the air that may try to escape. It won’t completely stop air leakage though, and it’s also prone to blowing around the cavity where it’s installed. If that area is in the vicinity of a basement or attic, those spaces may become a catch-all for cellulose debris, too.

Another caveat about cellulose: Because of its tendency to settle, creating the possibility of unwanted airflow, it requires regular maintenance. If you’re insulating a hard-to-reach area of your home, this material may not be the best option since it will be hard to get to when you need to rake and maintain it.


Insulating of attic with fiberglass cold barrier and reflective heat barrier used as baffle between the attic joists to increase the ventilation and reduce humidification.

You should only put in fiberglass insulation if you can access the cantilevered floor joist from a basement or crawl space. Fiberglass must be cut precisely to fill in the batts in the floor joists.

This material has a few drawbacks, as it still allows some airflow through the floor and is known to trap allergens and moisture, which can leave you with mold growth. Wearing a mask and gloves is a necessity when handling fiberglass because it can be inhaled or become embedded in the skin.

That said, this material is well-suited for DIY projects and is designed to fit the standard spacing of studs and joists, as long as there’s nothing else in the way. In terms of costs, fiberglass generally runs about $0.91 per square foot.

Insulation and R Values

Floor insulation material in roll.

Insulating materials are given R values to indicate how well they resist conductive heat flow, or the way heat moves through the materials and structures in your home (floors, walls, and ceilings). Throughout the winter, warm air is conducted toward cooler air coming from unheated attics, garages, and basements. Insulation helps you retain warm air where it belongs, keeping it confined as much as possible to climate-controlled areas of your home.

In other words, R values for insulation reflect how effectively they keep warm air contained in your home. The higher the R-value, the more effective the insulation is.

Three factors help to determine an insulator’s R-value:

  • density
  • thickness
  • the type of material used

For the most part, increased thickness will result in a higher R-value. However, with loose-fill insulation, such as cellulose, it can become compressed under its own weight. Thicker loose-fill insulation, then, doesn’t come with a better R-value since it’s prone to settling, anyway. When that happens, it may not live up to its actual R-value.

Where insulation is installed can affect its R-value too. When installing around studs and joists, the potential for trapping heat decreases because air moves more freely around these areas. Therefore, a particular part of a home can have a lower R-value than the rating assigned to the type of insulation in place there.

If you live in a part of the country that feels like the North Pole during the winter, you know how important insulation is. The type and amount you need certainly depends on the climate where you live. The kind of heating system you have and the part of your home that you want to fortify against the elements are other important factors.

More modern materials tend to be the most effective insulators because they re-emit, rather than absorb, heat. Some materials also retain moisture more than others. If you live in a humid area, you’ll want to steer clear of substances that can encourage mold growth.

The US government has divided the country into  8 climate zones. The R-value to shoot for depends on what zone you live in. For warmer parts of the country, a minimum of R30 is recommended, while in colder areas, the lowest suggested R values go up to 49.

Before You Insulate Cantilevered Floor Joists…

Worker measuring the thickness of thermal insulation.

Before we delve into how to insulate cantilevered floor joists, there a few things to keep in mind first.

Consider whether you really have the time and skills to take on this project yourself. If you don’t have prior experience working with insulation, especially the foam variety, this undertaking is best left to a certified contractor.

If you’re enlisting the services of a third-party, it’s wise to get estimates from a few different contractors so you can compare prices. This will also help you to gauge if you’re being overcharged. If possible, seek recommendations from trusted sources, like friends or family members. A contractor has the experience and know-how to decide how much insulation you need.

If you’re naturally the DIY type, it’s still important to be prepared. Measure the space so you know how much material is required, especially if you’re using a material that tends to settle. If you’re handling fiberglass, be sure to have the necessary protective equipment (mask and gloves), so you don’t breathe in particles or risk getting them implanted into your skin.

Clean out the overhang or floor cavity first. You don’t want to end up with dust and allergens trapped inside. Evaluate how accessible space you need to insulate is. If it’s difficult to get to, you will want insulation that’s low-maintenance. Hard to reach spaces may require additional work or creative workarounds to get the job done.

General Tips on How to Insulate Cantilevered Floor Joists

The US Department of Energy Efficiency and Renewable Energy provides helpful guidelines on how to insulate cantilevered floor joists. Here’s what the experts suggest:

  1. Air seal the top, sides, and bottom of the cantilevered floor cavity. As much as possible, be sure that insulation covers all surfaces thoroughly, without gaps.
  2. Set up a rigid barrier to separate the floor joist from any climate-controlled areas above or below (such as a basement). For extra protection, use foam or caulk to seal any holes or vulnerable spots in the air barrier.
  3. Block off and seal any adjoining open floor joists.
  4. Fill the cantilevered floor cavity with insulation, taking care to line it up as closely as possible with any corners, nooks, and crannies. Do your best to ensure even coverage of all areas to minimize the possibility of air leakage.
  5. Cover the bottom of insulated floor cavities with a solid material that’s rigid and weather-resistant. Plywood and house siding work well for this purpose.

Specific Pointers on How to Insulate Cantilevered Floor Joists

Bay window with white framed glass panels fitted on the red brick wall.

New Construction:  Bay Windows and Overhangs

In colder climes, even your best efforts to insulate can make it difficult to keep cold air from coming up. Ideally, you should shoot for an R30, although a haphazard installation job can bring the rating down to R15 or less. Attempts to insulate cantilevered floor joists often don’t cut the mustard because cantilevered spaces are frequently stuffed with foam or fiberglass once the framing is complete, almost as an afterthought. Taking care of the insulation process while framing is still going on will prove more effective.

Here are some tips that you (or your contractor) will consider when it comes to new construction. The most important thing to remember is to thoroughly seal everything as you go. If you don’t, you’ll be greeted with an invasion of frigid air every time you pass by the offending cantilever, at least during chilly weather.

  1. Add at least a 1″-1 1/2″ layer of XPS (or a layer of Polyiso) foam board insulation to the bottom of the floor joists. It’s not a bad idea to install a thicker layer of XPS. Keep in mind that Polyiso will give you a higher R-value.
  2. To protect it from the elements (and rodents seeking a place to camp out), install exterior grade sheathing below any foam boards that have already been installed.
  3. Install a piece of 1-1/2″ XPS foam on top of the foundation wall inside the basement area. Make sure it’s as closely lined up to the sill plate as possible. You can seal against the sill plate with foam sprayed from a can.
  4. Before sheathing the floor deck, install foam board tight between the joists and against the rim joist. Work from the top side. Use spray foam to seal all the edges. Don’t forget to seal the bottom of the foam piece to the lower piece already installed below the joists.
  5. Install another piece of foam between the floor joists directly above the foundation wall. Once again, be sure to seal around the sides and bottom of this piece.
  6. In the joist bay, install either cellulose or fiberglass insulation.
  7. Now for the finishing touches: Install one last piece of Polyiso or XPS on top of the fiberglass or cellulose. Seal this addition to all the framing and foam.
  8. When you have finished all of the above steps, you can go ahead and put in the deck sheathing.

How to Insulate Cantilevered Floor Joists:  Existing Construction

Keeping your home adequately insulated once it’s been built is important too. In a home that’s already established, you’re likely to encounter spaces that have been packed with insulation, but it may have been done somewhat carelessly. You can finish the job with spray foam or cellulose if you prefer; there’s more than one way to tackle the job.

To complete this task, you will need access to the interior joist bays from the basement. You’ll need to be able to reach the underside of the cantilever from the outside, too. Once you’ve determined that it’s doable, consider these tips on how to insulate cantilevered floor joists on overhangs on existing buildings — retrofit.

  1. From the outside, attach XPS or Polyiso foam board to the bottom side of the cantilever. To accomplish that, it’s recommended that you remove the existing sheathing. If that’s going to be a herculean job, then install foam over the sheathing that’s already there. Keep it tight to the perimeter of the cantilever, and the foundation, too.
  2. To protect the foam, cover it with exterior grade sheathing. You may need to add trim so your handiwork isn’t obviously visible.
  3. From the inside, install XPS or Polyiso tight against the rim joist. Use spray foam to seal the edges.
  4. Pack the joist bay with insulation. You may want to use fiberglass for this, as blowing cellulose tightly enough into space may be difficult.
  5. Install a piece of foam board above the foundation wall, between the joists. Seal the joists, foundation, and deck.
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