DIY Crawl Space Guide: Rim Joist Insulation Week 7

The humble rim joist—that critical transition zone where your home's foundation meets its framing—might be the most overlooked vulnerability in your entire building envelope. Michael Church dives deep into this crucial component in week seven of the Crawl Space Repair Challenge, revealing why building science experts consider proper rim joist insulation a non-negotiable aspect of home performance.

Moisture management stands front and center in this detailed exploration of rim joist insulation options. Michael walks through the key differences between open cell and closed cell spray foam, explaining why the density, R-value, and moisture-blocking properties of closed cell (R7 per inch) often make it the preferred choice for rim joist applications. He shares a compelling personal anecdote about how closed cell foam's structural integrity once saved him from putting his foot through a ceiling—demonstrating the unexpected benefits beyond just thermal performance.

The critical prerequisite of ensuring dry conditions before installation becomes clear as Michael reveals shocking examples of wood rot discovered behind improperly installed insulation. His preferred installation method—combining foam board with spray foam—offers the perfect balance of performance and practicality, particularly when future maintenance access might be needed. Whether you're tackling this project yourself or hiring professionals, this episode equips you with the knowledge to ensure your rim joists contribute to your home's durability and efficiency rather than becoming its hidden weakness. Ready to seal those critical gaps in your home's thermal and moisture barriers? This episode shows you exactly how to do it right.

Chapters

• 0:00 Introduction to Rim Joist Insulation
• 1:21 Building Science Perspective
• 2:41 Materials and Open vs Closed Cell
• 5:43 Importance of Dry Installation
• 7:28 Installation Methods and Conclusion

Transcript

Speaker 1: 0:00

Hey there, michael Church with Crawl Space Ninja. We are on week seven of the Crawl Space Repair Challenge and this week we are going to talk about rim joist insulation. Some of the things we'll be discussing today are what does the building science folks say about rim joist insulation? What are some of the materials available to insulate your rim joist? Should you do open cell versus closed cell? I'll give you some information about the differences between those two. And what about making it dry first Is that important? And also protecting the rim joists from the outside and some install methods that we have used and seen in the past. For those of you following along on our crawl space repair challenge, you may notice that we released an air sealing video last week, in week six, and I decided to do rim joist insulation as a separate video because I didn't want the air sealing video to go longer than it did. But this is an important topic. It's a standalone topic, in my opinion, about basements and crawl spaces. Make sure you check that out. That has nothing to do with rim joist insulation and, of course, this is week seven, so there's six other videos that you can watch in this series.

Speaker 1: 1:11

Buildingsciencecom Corporation. I first learned about the importance of rim joist insulation, particularly rim joist air sealing, from these folks. I read the article that I'm going to put a link down below. This is where it is if you want to type all that out, or you can just go in the description and click the link down below if you want to read the full article. But basically this is a highlight from the article.

Speaker 1: 1:34

This area they're referring to, the rim joist, is problematic for several reasons. First, several framing components, including the sill plate top plate as it's known in some parts of the country, the rim joist, which is also called the band board in some parts of the country and the subfloor above need to be connected with sealant as an air barrier component. This application is quite workmanship sensitive during new construction. Now I just want to say something real quick. What they're saying there is that no one air seals the rim joists because of all these components coming together when they built your house. But it should have been done during the house build. According to buildingsciencecom, they actually call it a critical seal. And then, second, the interior side of the rim joists is cold, typically in the wintertime, and has an associated risk of condensation. So if you have a basement or a crawl space that has an exposed rim joist and it's cold outside and warm inside that basement or crawl space, you do stand a chance of condensation happening there, which can be problematic for a lot of reasons.

Speaker 1: 2:41

Some of the recommended materials we use during a rim joist insulation are, of course, spray foam kits, like I'm showing here. These are available on our DIY store. You can get the gun, the spray foam applicator, the foam itself along with the cleaner. Some companies use a two-component spray foam. Typically they can come in a manageable AB tank that you might be able to pick up at a local insulation supply warehouse or you can find a lot of companies will use either a truck or trailer mount two-foam application process and, of course, the foam board itself. And one of the things I just realized I didn't put here is you can actually use fiberglass or rock rim joist insulation as well, and I'll get to that a little later in this video. But that is an option too. If you want to use fiberglass or rock wool insulation in the rim joist, you do not have to use these components that I'm showing here. But I'll share with you why I don't recommend fiberglass or rim joist a little later.

Speaker 1: 3:44

So open cell versus closed cell I'm going to go over some differences between the two, but I hope you'll comment down below if I missed one or two of the differences that some of you spray foam contractors or builders might be aware of. So open cell is softer and lighter where closed cell is heavier. Open cell is more flexible. Closed cell is more dense. Open cell is better at sound dampening or soundproofing than closed cell. A closed cell spray foam is an R7 per inch R value, which makes sense because it's more dense.

Speaker 1: 4:36

One of the things I do want to say about closed cell spray foam we use closed cell spray foam primarily here at Crawl, space Ninja and Basement Ninja, and the reason I like closed cell is because we're waterproofers and closed cell tends to stop water more than open cell. Not that open cell lets water roll right waterproofers and closed cell tends to stop water more than open cell, not that open cell lets water roll right through it. I just want to make that distinction. It's not like you'll see water rushing through open cell. It's pretty good at stopping water too.

Speaker 1: 5:01

But I did my attic many, many years ago with closed cell spray foam and what always happens when you're in the attic you slip off of a joist and put your foot through the drywall ceiling. And I just so happened to do that after I had already spray foamed the entire attic floor with closed-cell spray foam and my foot slipped off and, lo and behold, I did not fall through the drywall. Now I'm not saying that would be the case with every installation of spray foam that's closed cell. But one of the things I learned from that is closed cell spray foam has some really good structural integrity because it is a heavier, more dense product and I'm not saying open cell couldn't do that, but I did have that experience with closed cell. So dry it first.

Speaker 1: 5:44

I use this picture here to show you the importance. I know this isn't a rim joist. They're actually insulating an air, sealing the living space here. But what you got to be mindful of is that if you spray closed cell spray foam on wet wood, it will create a huge problem, like I mentioned before. Those building science folks talk about, you know, condensation on the wood when there's a major temperature difference between the outside and the inside. So you want to make sure that that wood is dry before any type of rim joist insulation, whether it's using spray foam, foam board or some type of bat insulation, because if you trap that moisture in that wood it's going to cause huge problems. You want to make sure you don't have leaks from above. What if this window was leaking, for example? That could ruin that job tremendously.

Speaker 1: 6:37

Got to make sure those materials are wet, even if they're new construction. I've seen homes built. They frame the house in, it rains and then the next day the insulation contractors come in and throw up the insulation. Well, did they give that material time to dry out after the rain or the high humidity from the fog that just rolled in? And then, of course, any penetrations. Keep in mind that you can have plumbing penetrations and electrical wires going through. Are those sealed real well from the outside to make sure water's not rolling in, for example? Or flashing around decks okay can also cause a lot of rim joists leakage. So make sure. The point of this slide is make sure that wood is dry before you insulate that rim joist. This is what I mean from protect it from the outside. This is a great view of what a typical rim joist would look like. So this exterior sheathing that they're showing right up here, if that is compromised, if you've got some type of siding that's leaking. Perhaps your brick is leaking. Anything that deck that I mentioned where the flashing is leaking, that has to be fixed before I would consider insulating the rim joists.

Speaker 1: 7:48

As a matter of fact, we just had one of our sales guys, josh, call me yesterday where they had a wall that was above ground that was wet. It was a foundation, a block wall that was above ground that was wet. It was a foundation, a block wall that was above ground, totally wet. They dry, locked it and it just kept getting wet. And so, anyway, they thought how in the world could it be from the basement or the crawl space, because this is the living space wall? That was a finished wall that was blocked. Well, lo and behold, there was a drip edge up near the roof that was leaking all the way down that wall that was blocked. Well, lo and behold, there was a drip edge up near the roof that was leaking all the way down that wall. That was causing huge problems. So make sure the outside is waterproof before you attempt to put rim joist insulation on.

Speaker 1: 8:28

Just a couple of examples of what your wood could look like behind rim joist insulation if it is not dried properly or kept dry. This was fiberglass. The fiberglass does trap moisture. We pulled that fiberglass out as part of our crawl space encapsulation process and found this little jewel hidden up in the joist. Now I think this was a leak from the outside just because of the fiberglass will prevent condensation, believe it or not. Fglass and rockwool it doesn't air seal it, as I mentioned before. That's why I don't like fiberglass or rockwool because it doesn't air seal as well as a spray foam or a foam board spray foam combination. But it is a great thermal barrier. So if the very least you could do is put fiberglass or rockwool, you're at least gonna get that thermal break to prevent condensation. So I think this was a leak from the outside. You can see that little rusted nail on the left side, right above the sill plate. So I think there was some kind of flashing issue or leak from the outside that caused this. Just another example of why it's important to stop any water from coming in before you do the rim joist.

Speaker 1: 9:37

So the three install methods that I've seen done and, of course, if I missed one, please let me know in the comments below as the building science folks talk about, use a either open cell or closed cell directly on the rim joist, as we see on the bottom left. Or you can shove bat insulation, whether it's fiberglass or rock wool, as you can see. We've done that in the past on the top middle. Or my preferred method I like to use a foam board spray foam combination. The reason why I like that method the best number one I'm air sealing. I'm also creating that thermal break, unlike the middle picture where I'm only creating the thermal break. But secondly, if for some reason we do get a leak from the outside where we notice water coming in, that foam board is a lot easier to take off if I have to do some kind of maintenance or repair to that rim joist versus scraping that two component spray foam off on the far left. So that's why I prefer a foam board spray foam combination.

Speaker 1: 10:35

It is a little bit more work to do it this way. You know these insulation contractors. They can come in and spray foam the rim joist pretty quick versus having to cut foam board up. As you can see, we had to custom cut that piece of foam board and shove it in there and spray foam around it. So anyway, these are the three install methods that we've used and done in the past. Let me know if I missed one down below in the comments. We hope you'll like and subscribe to our YouTube channel. If you haven't had a chance to check out my book, I do a lot of education on rim joist insulation in my book Crawl Space Repair Misbusted, which can also be applied to basements. If you get a chance to check that out on Amazon, make sure you subscribe to our YouTube channel and if you want to set up a consultation with me, we've got that available on buymeacoffeecom. I'm Michael Church, crawl Space Ninja. We hope you make it a happy and blessed day and we'll see you later.