How To Properly Insulate Rim Joists In Your Home’s Basement

Why You Should Insulate Basement Rim Joists In Your Kansas City Home, Plus Our Top Tips

If you have cold floors in your basement or have utility bills that seem too high considering your attic is optimally insulated, it may be time to consider whether it’s worth it to insulate rim joists in your home. A Kansas City insulation company can help you with this insulation project, but to help you determine whether or not this project may be right for you, as well as provide tips for doing this job the right way, we’ve created the following guide. Some things you’ll learn in this article include:

• What a basement rim joist is
• How you can benefit from a project that aims to insulate rim joists
• Why spray foam insulation is the best choice for Kansas City rim joist insulation
• How rim joist insulation should be installed for best results

What Is A Basement Rim Joist?

If you’ve never heard the term ‘rim joist,’ you’re not alone. ‘Floor joist’ is a more commonly known term, but the two are in fact separate framing components. Rim joists (sometimes also called band joists) are the parts of a building’s frame that sit on top of the foundation wall all the way around the perimeter of a building. They support the entire structure of the building and are where the foundation wall transitions to the framing. 

Rim joists are also one of the weakest parts of a building’s thermal envelope and are where the majority of air infiltration occurs. As air in your home escapes out of the top of the envelope (in your attic), it is replaced by air leaking in through the bottom near your rim joists. That’s why sealing and insulating rim joists with spray foam is important if you want to prevent air leakage and increase your home’s energy efficiency.

Is Rim Joist Insulation In Kansas City Worth It?

Why bother to insulate rim joists? Uninsulated rim joists are just one other area where heat loss may occur in colder climates like Kansas City. Rim joist insulation acts as a thermal barrier, preventing air leaks that are common in uninsulated basements. By adding insulation, you’ll not only reduce your energy consumption and subsequently lower your energy costs, but you’ll also make your home feel more comfortable overall during the cold weather that is endemic to Kansas City winters.

Rim joist insulation also makes your Kansas City home more comfortable during summers. Though many homeowners think of insulation as important for keeping their homes warm during winter, its purpose is to resist heat transfer not just in cold weather, but in hot weather as well. Therefore, the better insulated your Kansas City home is, the cooler it will feel during high temperatures without having to turn on your air conditioning. Deciding to insulate rim joists is a significant component of an optimally insulated home. The higher your home’s energy efficiency, the more comfortable it’ll be and the lower your utility costs will be. What more could you ask for?

What Is The Best Material To Use To Insulate A Rim Joist?

Just as the term ‘joists’ brings to mind floor or wall joists, the term ‘insulation’ brings to mind fiberglass batt insulation—rolls of that pink cotton candy-like material that is commonly fitted into cavities between wall joists. You may see these batts in your unfinished basement or in unfinished garages that have not been drywalled yet. This is but one of several types of insulation materials available to Kansas City homeowners and commercial property owners today. In addition to fiberglass insulation, other common materials include:

• Cellulose insulation (blown-in insulation)
• Mineral wool batts
• Rigid foam insulation (board insulation)
• Spray polyurethane foam, also known as spray foam insulation (SPF)

There are more materials in addition to those listed above, but these are by far the most common. Because it’s common to fill wall joists with fiberglass batts, you may not be surprised to learn that they are also often used to insulate rim joists. What you might not know is fiberglass batts do not provide an effective air seal. 

Homeowners with fiberglass batts around rim joists might notice that they are dirty. This is known as the air filter effect, and is a telltale sign that you have a lot of air coming in through your rim joists, causing the fiberglass to become dirty as it acts like an air filter and collects all the dust and debris that air brings in.

To insulate rim joists optimally, you’ll need to use a specific type of spray foam: closed-cell foam.This rim joist insulation’s R-value is around R-7 per inch, making it significantly more resistant to thermal transfer than, say, fiberglass batt insulation, which has an average R-value of about R-3 per inch. In Kansas City, the suggested R-value for ceiling insulation is currently about R-60. You therefore need considerably fewer inches of closed-cell spray foam insulation than fiberglass batt insulation to achieve this guideline.

Though closed-cell SPF has a higher price point per inch, you also need much less of it to achieve your target R-value and make your space energy efficient. This material is also great for filling a tight space that’s hard to reach when a narrow applicator attachment is used.

However, the incomparably high R-value of closed-cell foam isn’t the sole reason why it’s the best material to insulate rim joists. Though this is a major benefit, the true advantage of SPF lies in its ability to act as an air barrier. Commonly used for air sealing, SPF prevents air leakage from occurring, which in turn prevents undesirable air movement. It also acts as a vapor barrier, preventing moisture from accruing and reducing your insulation’s efficacy or creating conditions conducive to wood rot. If pests have been a concern before, SPF can even protect against pest infiltration.

You can use rigid boards to insulate a rim joist, but all seams between the board and the joists should be sealed off with SPF. Once sealed, you can also apply fiberglass batts. However, without spray foam, cavities between your band joists would be susceptible to moisture buildup, impacting the effectiveness of your insulation and even reducing your indoor air quality. To do this job right, you need to use SPF for rim joist insulation.

How To Insulate A Rim Joist The Right Way

Now that you know why it’s important to use SPF for rim joists in Kansas City, you’re ready to learn about the installation process itself. We don’t necessarily recommend tackling this project on your own—this guide is for informational purposes only. The more information you know, the better equipped you’ll be to make the right choices for your Kansas City home and the more confident you’ll be about asking your local insulation experts relevant questions.

It’s easiest to insulate rim joists in an unfinished basement, as it’ll be easier to access your building’s framing. Most homeowners invest in a Kansas City rim joist insulation service when they’re in the process of building a new home or are undertaking large-scale renovations and remodeling. There are two main installation methods used to insulate rim joists: SPF only and a combination of rigid boards and SPF. For both of these methods, it’s imperative that you have the right equipment and quality materials to produce quality results.

Required Tools and Supplies

Your Kansas City rim joist insulation installation is only as good as your tools. Kansas City insulation experts have all the necessary tools to get this job done right. These tools include:

• Box cutters
• Tape measure
• Circular saw
• Air purifier and respirator mask
• Safety goggles
• Tyvek suit
• Caulking gun
• Work gloves
• Drop cloth

You’ll also want a shop vac, soap and water to thoroughly clean the area that SPF will be applied to. If any debris remains, it could prevent the SPF from properly adhering to the wood. In addition to the above tools, materials needed include:

• Closed-cell polyurethane SPF (may be supplied within a spray can, but ideally in a canister, hose and nozzle system)
• Acrylic latex caulk
• Rigid board insulation (optional)

Following safety procedures is of the utmost importance when applying SPF. If it comes into contact with your skin, it will be virtually impossible to remove. To avoid damaging your skin, it’s best to leave it there and allow it to eventually fall off, but this could take several days or weeks. The best option of all? Wearing gloves and a suit in the first place.

Likewise, fumes emitted by polyurethane can be incredibly harmful to human health. Chemicals known as isocyanates are the main culprit and may cause lung damage and asthma. In rare cases, exposure has been fatal.

Installation Option 1: Spray Foam

The easiest way to insulate a rim joist cavity is to fill it with SPF. Once protective clothing has been donned, the first step is to do a test application on a scrap piece of cardboard, a piece of wood or even on the inside of a garbage can (lined with a bag of course). This way, if there are any issues with the applicator, they can be corrected without making any mistakes on the joists themselves. It only takes SPF about a minute to dry, so any errors must be corrected immediately.

The foam should first be applied to all of the seams between the rim joist and other lumber and building supplies this joist comes in contact with using a cone-shaped nozzle. This usually involves:

• Seams between the two floor joists
• Seam between the subfloor
• Seam between foundation walls and sill plate

Remember that the goal is to not just insulate these cavities but to make them airtight as well. Once these joints have been filled, the rest of the rim joist’s surface can be covered with foam. A fan-shaped nozzle works best for this job. You’ll need it to be about 2 inches thick when fully cured. It’s a good idea to work on all cavities at a time, going one by one so that by the time you reach the end, the first cavity will be fully cured and ready for a second layer. If any small gaps remain, they may be sealed with acrylic latex caulk.

Installation Option 2: Rigid Foam And Spray Foam

There’s no way around it—SPF is a pricey product. Though it’s expensive, it’s also the most durable and waterproof insulating material available today. However, if you’re working within a smaller budget, you can use a combination of rigid foam and SPF instead. There are three types of rigid boards, all of which can be used to insulate rim joists:

• Expanded polystyrene boards
• Extruded polystyrene boards, and
• Polyisocyanurate boards

All of these materials have a similar R-value and come at a lower price point than closed-cell foam. The drawback of using this method is simply that it’s a lot more fiddly than using spray foam. Cavities must be measured and boards cut to size using a circular saw. This is easy enough to do in large rectangular cavities, but when around pipes or in small, hard-to-reach areas, it’s a little more tricky.

With your boards cut to size, they can be adhered to your rim joists using SPF. Several boards will have to be layered on top of each other to meet R-value requirements. Once these boards have been applied, the gaps between the boards and wood can be filled with spray foam.

Fiberglass may be used to boost your R-value further once SPF and rigid foam have been installed. However, it’s not necessary. If you’d like to add this third material, you’re more than welcome to, but in the end, it really only lengthens the installation process by adding an unnecessary step.

Benefits Of Hiring A Kansas City Rim Joist Insulation Company

Though you can technically insulate rim joists on your own if you purchase all the required supplies and tools, most homeowners choose to use a professional Kansas City rim joist insulation company. There are many advantages to hiring experts to do this job, some of which include:

• The assurance that the job will be done properly and last for years to come
• No need to purchase tools, as insulation experts already have all the supplies they need
• No exposure to toxic chemicals and fumes (professionals have all the safety equipment and training required!)
• Service professionals from a reputable company like Koala Insulation are fully insured, so you’re covered in case something goes wrong
• No need to read up on building codes to ensure that your project is compliant

When done properly, all insulation projects result in an attractive return on investment—even after paying for labor costs. Book your Kansas City rim joist insulation service today with our talented team of experts at Koala Insulation of South Kansas City by giving us a call at 816-929-8255!