Rim joist insulation using spray foam to form an airtight seal.

Insulation is a big topic, with lots of important details that you need to pay attention to.

Missing any critical steps, such as not ensuring a good air seal, can lead to problems down the line.

If you have a cold basement or are noticing drafts along your walls, it’s possible that the cause could be inadequate or incorrectly installed rim joist insulation.

But what is rim joist insulation, and should you go to the trouble of installing it?

Absent or inadequate rim joist insulation can result in the loss of up to 25% of the heat in your home and potentially cause condensation on the interior face of the rim joist. To avoid these problems, you must air seal and insulate the rim joist with spray foam or other suitable materials.

Proper air sealing and the use of insulation materials with high R-values are the keys to ensuring your rim joist is well insulated.

The rest of this article looks at the different approaches to insulating rim joists. It considers their pros and cons to allow you to decide on the best strategy for your circumstances.

So, read on to learn more about this important topic.

What Is a Rim Joist on a House?

Floor framing system with rim joist and hangers used on each joist.
Rim joists provide strength to the floor framing system to prevent it shifting over time.

First things first, what is a rim joist exactly?

A rim joist runs along the ends of the floor joists. It provides lateral support for the joists and sits on top of the sill plate.

The ends of the floor joists are nailed to the rim joist, forming a clean edge along the side of the floor, which helps to prevent any movement of the floor joists and keeps them firmly in position.

The rim joist also helps to give strength to the structure of the house by providing a solid length of timber that the sheathing can be nailed to, creating a strong connection between the floor framing and the wall.

Rim joists are typically made of the same material and have the same dimensions as the rest of the joists in a floor framing system, although they can sometimes differ.

Rim joists are distinct from end joists, which are the first and last joists in a row of joists that make up a floor. Collectively, the rim joists and end joists are known as band joists.

Should You Insulate Your Rim Joists?

A thermal image showing the heat loss from the side elevation of a building.
Heat losses via rim joists can be significant without proper insulation.

The rim joists are a common pathway for heat to be lost in many homes built in the US, so insulating them is one of the best things you can do to improve your home’s energy efficiency and lower your heating bills.

Any construction details that connect building components can lead to air leakage if not finished correctly. This includes the joins between walls and foundations and the interfaces between walls and roofs.

The rim joists, and all the band joists that form part of the floor system, are complicated details to get right when building a house.

They need to be connected using sealant to make the joins airtight, requiring an experienced and skilled tradesman to achieve the necessary seal.

Failure to form a good air seal at these joints can allow cold air through gaps and imperfections, which can cause cold floors and allow drafts along the walls of your home.

This, in turn, can lead to condensation forming on the inner face of the band joists, with the attendant risk of mold and rot.

It will also lead to heat loss, making your basement cold in the winter and costing you more to heat your home.

What R Value Do I Need for Rim Joist Insulation?

If you live in an area that gets significant snow during the winter, one of the telltale signs that you are losing heat through your rim joists (and the other band joists) is fast-melting snow around your home’s foundation.

If the temperature outside is still well below freezing, and yet snow is quickly melting next to your band joists, this is a real giveaway.

The only barriers preventing heat loss from uninsulated band joists are the sheathing and siding on your walls. You’d be lucky to get an R-value of more than R-1 from these materials, which is woefully inadequate.

Rim joists are above grade and should be insulated to the same degree as the above-grade walls in your house.

The R-value recommended for these walls varies for different climate zones. To give you an idea, the recommendation in warmer climate zones (1-4) is at least R-13, but this rises to R-20 in climate zones 5-8.

You can find more information about the climate zones of the US here.

What Is the Best Way to Insulate Rim Joists

There are several options to choose from for insulating a rim joist. Which one you select depends on your specific circumstances and preferences.

For example, you might be on a tight budget, limiting your options to fiberglass alone.

If being able to do it yourself is important to you, you should probably steer clear of spray foam because this requires specialized equipment to be done correctly and safely.

Is having a warranty important to you? If so, then finding a contractor who offers a guarantee on workmanship and materials would be the way to go.

Let’s look at the options.


A closeup of yellow fiberglass insulation.
Fiberglass insulation is relatively cheap and easy to work with.

Fiberglass insulation is an extremely popular type of insulation that is easy to install. Coming in rolls or batts, it is easy to cut to size and put in place between the floor joists.

If you have successfully completed any DIY project, can use a tape measure and a knife, and are reasonably fit, you are more than capable of installing fiberglass insulation.

It is available in Home Depot and other DIY stores and is low cost compared to other alternatives.

If you use fiberglass, you must install a separate vapor barrier to prevent moisture problems and condensation from forming on the rim joist behind the fiberglass, which could encourage mold growth and rot.

Pros of Using Fiberglass

  • Inexpensive.
  • Easy to install.
  • Readily available.
  • Lightweight and easy to handle.

Cons of Using Fiberglass

  • Poor air seal – unfaced fiberglass batts do not create an air seal (faced varieties are available, though).
  • It is easy to compress during installation, which reduces its R-value.

Mineral Wool

A worker wearing face mask and safety glasses handling a mineral wool insulation batt.
Mineral wool is heavier than fiberglass insulation and has slightly better insulation properties.

Mineral wool is installed in a similar way to fiberglass. It comes in rolls or batts, and you cut it to fit the space between the floor joists.

It is denser and more rigid than fiberglass and is less susceptible to compression, which can cause a decrease in R-value.

It also has slightly better thermal performance than fiberglass, with an R-value of 15 at 3.5 inches compared to R-13 for fiberglass.

Another reason for choosing mineral wool over fiberglass is that it contains a higher proportion of recycled material than fiberglass and is better for the planet in this regard.

Pros of Using Mineral Wool

  • Inexpensive.
  • Easy to install.
  • Easily available.
  • Higher R-value than fiberglass.
  • More eco-friendly than fiberglass, thanks to higher recycled material content.

Cons of Using Mineral Wool

  • Poor air seal – mineral wool is not available with facing.
  • Heavier and more cumbersome to handle than fiberglass.

Foam Board

A closeup of the corner of a piece of rigid foam board insulation.
Rigid foam board insulation can form a good air seal when used in conjunction with spray foam to form a seal around the edges.

Foam board is an excellent choice for insulating your rim joist.

It is slightly more expensive than fiberglass and mineral wool, and you might need to hunt for it on the shelves of your local store, but it can be found.

Although it is easy to install, it’s a bit more difficult than using fiberglass or mineral wool because more care must be taken when cutting to ensure the correct fit between your floor joists.

It is straightforward to form an air seal with a piece of rigid foam board by filling the gaps around the edges of the foam board with spray foam.

Pros of Using Foam Board

  • Fairly easy to install.
  • Fairly available.
  • Can be used to create an air seal if installed with correct caulking around the edges (e.g., spray foam).
  • Better R-value than fiberglass and mineral wool.

Cons of Using Foam Board

  • More expensive than fiberglass and mineral wool.
  • More challenging to work with than fiberglass or mineral wool batts. It cannot be bent around electrical outlets, wires, etc.

Spray Foam

An electrical penetration in a wall that has been sealed using spray foam.
Spray foam is great for sealing around electrical and plumbing penetrations to avoid drafts.

Spray foam is excellent for insulating rim joists because it creates a good air seal as it expands into the small cracks and gaps at the interfaces between different components around the rim joist and sill plate.

It will also deal with gaps around the plumbing and electrical penetrations to prevent the transfer of cold air from the basement or crawl space up the walls and into your house.

Spray foam comes in two types, closed-cell and open-cell. Both types have good R-values (better than fiberglass and mineral wool). Closed-cell is more expensive than open-cell but has a higher R-value per inch and forms a vapor barrier, which open-cell does not.

For more information about spray foam insulation, including the differences between open-cell and closed-cell, why not read our article on the subject here?

Spray foam will stick to the rim joist forming an intimate bond that eliminates the condensing surface on the interior face of the rim joist to prevent condensation problems.

Pros of Using Spray Foam

  • Offers the best performance (higher R-values than fiberglass and mineral wool).
  • Usually comes with a warranty.
  • Closed-cell spray foam forms a good air seal to prevent drafts and condensation.

Cons of Using Spray Foam

  • Most expensive option.
  • Requires a professional contractor to ensure the job is done correctly and safely.

Final Thoughts

Rim joists and other band joists form the edge of the floor framing system in your house.

If left uninsulated, these joists can be responsible for significant heat loss, which is terrible for the environment and increases your energy bills.

There are several options for insulating rim joists, ranging from fiberglass or mineral wool batts, which can be installed by a competent DIYer, through to spray foam, which requires a professional contractor to do correctly and safely.

Insulating your rim joists could save you lots of money in lost heat, so it is well worth the effort.

For more information about how to insulate your home, please read our article on the R-values and costs of different types of insulation, entitled, “Home Insulation R-Value vs. Cost: What’s The Best To Buy?

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