Categories: Insulation

Rim Joist Insulation – Insulating Options

How To Insulate Rim Joists

We’ve written a number of articles on insulating basements including; insulating basement walls, insulating basement floors and basement insulation options. In this article I want to focus on rim joist insulation and the options that are available.

Rim Joist Spray Foam

Spray foam insulation is the best way to insulate a rim joist in my opinion. However, spray foam insulation comes at a serious price point and it is best left to professional installation. Spray foam does a great job not only insulating the rim joist but it also seals all the possible air infiltration points.

Rim joists are typically riddled with nails from siding, trim, decks and porches. The foam also helps encapsulate the nails and screws which helps eliminate any condensation problems from the cold fasteners.

Foam Board Rim Joist Insulation

If you’re looking for a good way to insulate your rim joists that’s easy to do and relatively inexpensive then foam board insulation is your best option. When I insulate rim joists I like to use a minimum of 2 inches of foil faced polyisocyanurate foam board insulation.

Installation of the foil faced polyiso is very easy. All you need to do is cut (you can use a utility knife or hand saw) the foam to fit the space between floor joists, the bottom of sub-floor and the top of sill plate. Typically we will cut the foam about 1 inch smaller so it can be fit into place easily.

In the adjacent photo you can see a piece of the insulation that was cut to fit between I-Joists. The insulation doesn’t have to be cut perfectly because of the next step.

Once the insulation is in place we use a can of spray foam (like Great Stuff) to seal the edges of the foam board to the framing. As you can see above in the photo the spray foam fills the voids and also helps hold the products in place.

Fiberglass Rim Joist Insulation

The last method for insulating rim joists is to use fiberglass insulation. Over the years I’ve come to dislike this method. In fact, our new home was built this way (this photo comes from my own home) for the simple fact that I had too much going on to focus on this detail.

I’ve recently started replacing all the rim joist insulation in my home with the foam board that is shown above. Fiberglass insulation doesn’t work very well in this application. As you can see from the photo it’s very difficult to get the fiberglass insulation installed evenly around all the framing members.

I’ve also seen this application result in mold problems. As I mentioned early nails are very common along the rim joist. Those nails are always very cold and act as moisture condensation magnets! This application is also very hard to seal up and create vapor barriers. The bottom line is it doesn’t work well and I’d recommend you replace your fiberglass rim joist insulation with some type of foam product.

Rim Joists at Bay Windows / Deck Cantilevers

Many readers have asked about special situations where the deck cantilevers out past the foundation (typical at bay windows, fire places, and deck overhangs). If you have a situation like that, then I recommend reading my article: How To Insulate Bay Window Floors.

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

View Comments

  • If you are using I-joists consider the following: precut 1x2 material for the depth of the web of the josit. Install these on both sides of the web vertically with a dab of construction adhesive or a drywall screw. This will create a rectangle space that needs the insulation with out needing to notch the corners of each piece. Finish with the Great Stuff after all the pieces have been installed so you don't waste the straws or clog the can. Saves time cutting the foam board and increases the load carrying abilities of the I-joist end. This idea came from a class offered by Willam Hanlon with Flint Hills technical college in Emporia, KS on energy saving building techniques.

    • Tony - Although your idea would work I don't see the value in this approach. First of all the wood blocking doesn't have nearly the insulating value as the foam. This alone to me is reason enough not to use that approach. Second, the time and effort required to cut blocks, glue and screw them is MUCH more work than cutting the foam. Finally, if your joists are designed properly there is no need for increased shear capacity at the bearing point.

  • Great web site todd!
    I don't have access to the 2 inch foamboard foil faced product . Can I just use the 2 inch foamboard without the foil faced? And, also use the great stuff in a can to fill any gaps like you suggest? If so are there any problems that may occur?

        • John - Every code on this issue is a bit different. In most places it must be protected with some sort of fire barrier, plywood, drywall, etc. But that doesn't mean it has to be directly applied, a properly installed ceiling below is enough to protect it. Recently some of the foam board manufacturers have been getting testing data to support leaving it uncovered, but it's important to follow local codes.

      • Helping a friend encapsulate her crawl space -- 30-year old home.
        We have put the pink foam (we're in GA) in all of the rim joists and are ready to foam around them.
        She is now nervous about using Great Stuff after reading a "precaution" label on the Dow site about cutting all the pilot lights off, etc.
        The crawl space abuts the basement (concrete wall) and has a 3x3 door that we have been climbing in/out each time. She has a gas furnace and water heater approximately 10-15 feet from the door of the crawl space.
        She's been reading about the fumes and flammability of it. THe crawl space has six vents to the outside and we plan on putting a fan in the crawl space doorway to "blow" fumes outside.
        We have all of the protective gear, and would like to get this done this weekend.

  • I recently insulated my basement. I used 2x4 studs and left a space away from my cement pored walls. Used Roxal insulation thats mold free, moisture free, fire free etc. then VB it and accostic seal as well in ther with the headers etc. Question I have is should I have used Rig board insulation first on the walls the stud it?

    But I am doing my wheeping tile outside and I have blue skinned it and used R-value drainage board that acts as a insulation as well pretecting the blue skin etc. I just wonder if this was ok to leave out the rig blue board inside the house? If not what a easy fix now that I have the walls up but there not dry walled yet. Any help would be appreiated. Thanks.


    • Craig - Roxul Insulation is a really good commercial product that we almost never see on a residential project. Having said that, it's certainly a good product to use in a wet environment (basements certainly qualify as wet!) because of it's water and moisture resistance. Honestly I'm not familiar with it's specifications so I can't say for sure if it's going to be ok but I'd guess it will be fine. It's often used in masonry wall construction so it should do well.

  • thanks Todd I do Appreciate the info. Also just a reminder that outside my house I did the wheeping tile and also blueskinned it and used a drainage board that is roxal but for ouside stuff cause I know that some people use Rig board foam and herd it decays later and fills with water so defeats the purpose should look into it I just learning and researching but thanks i have another idea looking at for my floor....using on my floor delcore is like a bubble vB but breeze underneath for basements that don't leak or leak it avoids water from damage the wood that you put on the floor. anyways.....I put that down the rig foam board ontop and then plywood and a foam base with hoodward flooring or limate etc if ya understand what i am explaining to you lol hopefully....anyways check it out let me know if you think its good idea? thanks


    • Craig - Not sure I've heard of Delcore but it sounds like a feasible solution. If you use closed cell foam I think you can do away with the Delcore....either way you're on the right track.

  • Hi Todd lol sorry I gave you the wrong name I from the correct name on this site. Its Delta-floor.

    "•Install Delta-FL plastic subfloor system. The Delta-FL plastic subfloor system can be installed under a plywood subfloor to create an air-gap membrane to keep moisture from contacting the flooring materials. With this system you’d intall the Delta-FL first, tape the seams, then install a layer of 1/2″ plywood and screw it to the concrete with Tapcon screws. I prefer this solution if you plan on using carpet because it’s all screwed down."

    Question is after putting this down can I use a 1 inch foam board on top then put the plywood down and after that put carpet or tile etc ? I thinking this would give you air space underneath to breath and plus insulation foam board would keep the room even warmer and your feet etc. What do you think about this idea?


    • Craig - Honestly I'm not sure. You may run into a problem with the foam sitting on the Delta-FL. You may end up with high bearing pressures between the foam and Delta-FL on it's "ridges". I would actually contact them and get their opinion.

    • Craig,
      Did you find out about the foam on top of the Delta-FL? Looking at finishing our basement. The contractor framed the poured basement walls, and already insulated them (fiberglass), and has installed the vapor barrier. My concern is that the fiberglass is making contact with the concrete poured wall. Looks like I will take that down and put up the rigid foam (two 1 inch layers staggered to provide a better seal). Looking for solution for the floor. Was going to just epoxy/urethane the floor (no mold problems), but I suspect the floor would be just too cold. Again, let me know on the foam on top of the Delta-FL. Thanks.

  • Hi Todd,

    Great site, I am finding it very useful. Question for you on our home.

    Our house is 6 years old, we bought it foreclosed and someone had broken in and stripped the electrical in the basement. We had that repaired and that is working well.

    Some of the electrical wires/ground wires are running along the rim joists next to the sill. There are also several junction boxes against the sill in certain areas. I want to insulate these cavities before winter. I had considered 2" foam board however I don't have a lot of room to play with that due to the wires, etc. The other option was fiber glass insulation but am not sure this is the best application.

    I am thinking about buying the Froth-Pak spray foam kit and insulating that way however is it OK to foam over the wires? I know the junction boxes need to be accessible so how would I problem mask them off when foaming? How much room would I need to give them around the box?

    Here's an example, you can see junction boxes in the cavity with wires, etc. This is probably the most difficult area.

    • James - Wires get foamed in place all the time with houses that are completely spray foamed. I would just mask the front of the box, foam, then remove the foam in front of it.

  • I'm having a new home built. My spray foam insulation contractor has completed his work and left, and has informed me the rim joist insulation in the basement and between the first and second floor (two story house) were not included in his bid.

    I plan to use the 2-inch foam you mentioned above in the basement. Do you recommend fiberglass between the first and second floor, or some other type of insulation?

    • Chris - It really depends on whether you want sound proofing and if one of those floors in an unconditioned space.

      • Both floors are conditioned. I am assuming if I use fiberglass between the first and second floor (both conditioned) the condensation risk not be there.

        However, it appears you are saying foam would insulated and reduce noise. Is that correct>

  • Todd - one followup question - thanks for being patient with me. I need to install rigid foam in my rim joists in the basement. The walls in the house are insulated to R15 in accordance with where I live (southern Illinois). A contractor has told me I don't need to install 2-inch thick foil face insulation in the rim joits and can go to one 1-inch because the wood rim boards are so thick they give added insulation. That sounds fishy to me. What are your thoughts?

    • Chris - Sorry but I completely disagree. Rim joist is 1-1/2" thick. Wood is only about an R =1 per inch for a 1.5 in your case. Most building envelopes today require at least R19, if you get R=7 per inch of foam you'd end up around 8.5 total with the wood assuming it has no leaks, drafts, etc. Spend the time and money and you'll be far more satisfied.

  • Todd,

    Good information. If one uses foam board w/foil, wont that leave a space between the rim joist and the non-foiled side of the board where condensation problems can occur? Same for non foiled insulation also. Or will proper sealing of the foam board prevent hot air from home reaching the cold rim joist - no condensation issues.

    Any thoughts?


    • Vikram - The whole idea here is to SEAL it well so that warm moist air cannot come in contact with that cold rim joist.

  • When you say - "cut the foam about 1 inch smaller", you mean so that there is 1/2" clearance on all sides?

    The reason I ask is because the great stuff foam spray (red can) is normally only to fill 1/2" gaps.

    Also when spraying the foam, do you place the tube all the way to the back of the joist and then spray? Or do you start about a 1" back (middle of the 2" foam board) so that the spray foam expands all the way out to the front facing end? Just trying to get an idea of the technique since these spray foam cans are not cheap.

    • Jeff - Great stuff will expand much greater than 1/2". If you're worried about how much spray foam you use you can certainly cut them tighter. I've found that joist spacing does vary a bit so trying to cut each one perfectly is nearly impossible. So by making it 1/2" clearance that template will most likely work for all joist bays.

      Yes I put the tube to the back and just slowly out-line the foam and it will expand and seal nicely.

      Good luck.

      • Thanks for the quick reply! Do you also air seal the space between the where the sill plate meets the foundation? I have seen some contractors and energy guys on the web to suggest just using a good caulk like OSI quad for this rather then the spray foam. What is your opinion on this?

        • Jeff - There are several places where caulking is a better choice for air sealing. When we do an Energy Star house all the plates get caulked, so between the sill plate and foundation, between the wall plates and sub-floor, etc. I would do that before insulating the rim joist, then seal the insulation to the sill plate as well.

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