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.

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Episode Transcript

Speaker 1 (00:00):
Hey there, michael Church with Crawl Space Ninja.
We are on week seven of theCrawl Space Repair Challenge and
this week we are going to talkabout rim joist insulation.
Some of the things we'll bediscussing today are what does
the building science folks sayabout rim joist insulation?
What are some of the materialsavailable to insulate your rim

(00:20):
joist?
Should you do open cell versusclosed cell?
I'll give you some informationabout the differences between
those two.
And what about making it dryfirst Is that important?
And also protecting the rimjoists from the outside and some
install methods that we haveused and seen in the past.
For those of you followingalong on our crawl space repair

(00:43):
challenge, you may notice thatwe released an air sealing video
last week, in week six, and Idecided to do rim joist
insulation as a separate videobecause I didn't want the air
sealing video to go longer thanit did.
But this is an important topic.
It's a standalone topic, in myopinion, about basements and
crawl spaces.
Make sure you check that out.
That has nothing to do with rimjoist insulation and, of course

(01:06):
, this is week seven, so there'ssix other videos that you can
watch in this series.
Buildingsciencecom Corporation.
I first learned about theimportance of rim joist
insulation, particularly rimjoist air sealing, from these
folks.
I read the article that I'mgoing to put a link down below.
This is where it is if you wantto type all that out, or you

(01:27):
can just go in the descriptionand click the link down below if
you want to read the fullarticle.
But basically this is ahighlight from the article.
This area they're referring to,the rim joist, is problematic
for several reasons.
First, several framingcomponents, including the sill
plate top plate as it's known insome parts of the country, the
rim joist, which is also calledthe band board in some parts of

(01:49):
the country and the subfloorabove need to be connected with
sealant as an air barriercomponent.
This application is quiteworkmanship sensitive during new
construction.
Now I just want to saysomething real quick.
What they're saying there isthat no one air seals the rim
joists because of all thesecomponents coming together when
they built your house.
But it should have been doneduring the house build.

(02:13):
According to buildingsciencecom, they actually call it a
critical seal.
And then, second, the interiorside of the rim joists is cold,
typically in the wintertime, andhas an associated risk of
condensation.
So if you have a basement or acrawl space that has an exposed
rim joist and it's cold outsideand warm inside that basement or

(02:34):
crawl space, you do stand achance of condensation happening
there, which can be problematicfor a lot of reasons.
Some of the recommendedmaterials we use during a rim
joist insulation are, of course,spray foam kits, like I'm
showing here.
These are available on our DIYstore.
You can get the gun, the sprayfoam applicator, the foam itself

(02:56):
along with the cleaner.
Some companies use atwo-component spray foam.
Typically they can come in amanageable AB tank that you
might be able to pick up at alocal insulation supply
warehouse or you can find a lotof companies will use either a
truck or trailer mount two-foamapplication process and, of

(03:17):
course, the foam board itself.
And one of the things I justrealized I didn't put here is
you can actually use fiberglassor rock rim joist insulation as
well, and I'll get to that alittle later in this video.
But that is an option too.
If you want to use fiberglassor rock wool insulation in the
rim joist, you do not have touse these components that I'm

(03:38):
showing here.
But I'll share with you why Idon't recommend fiberglass or
rim joist a little later.
So open cell versus closed cellI'm going to go over some
differences between the two, butI hope you'll comment down
below if I missed one or two ofthe differences that some of you
spray foam contractors orbuilders might be aware of.
So open cell is softer andlighter where closed cell is

(04:04):
heavier.
Open cell is more flexible.
Closed cell is more dense.
Open cell is better at sounddampening or soundproofing than
closed cell.
A closed cell spray foam is anR7 per inch R value, which makes

(04:34):
sense because it's more dense.
One of the things I do want tosay about closed cell spray foam
we use closed cell spray foamprimarily here at Crawl, space
Ninja and Basement Ninja, andthe reason I like closed cell is
because we're waterproofers andclosed cell tends to stop water
more than open cell.
Not that open cell lets waterroll right waterproofers and
closed cell tends to stop watermore than open cell, not that
open cell lets water roll rightthrough it.
I just want to make thatdistinction.

(04:56):
It's not like you'll see waterrushing through open cell.
It's pretty good at stoppingwater too.
But I did my attic many, manyyears ago with closed cell spray
foam and what always happenswhen you're in the attic you
slip off of a joist and put yourfoot through the drywall
ceiling.
And I just so happened to dothat after I had already spray

(05:16):
foamed the entire attic floorwith closed-cell spray foam and
my foot slipped off and, lo andbehold, I did not fall through
the drywall.
Now I'm not saying that wouldbe the case with every
installation of spray foamthat's closed cell.
But one of the things I learnedfrom that is closed cell spray
foam has some really goodstructural integrity because it

(05:37):
is a heavier, more dense productand I'm not saying open cell
couldn't do that, but I did havethat experience with closed
cell.
So dry it first.
I use this picture here to showyou the importance.
I know this isn't a rim joist.
They're actually insulating anair, sealing the living space

(05:59):
here.
But what you got to be mindfulof is that if you spray closed
cell spray foam on wet wood, itwill create a huge problem, like
I mentioned before.
Those building science folkstalk about, you know,
condensation on the wood whenthere's a major temperature
difference between the outsideand the inside.
So you want to make sure thatthat wood is dry before any type

(06:19):
of rim joist insulation,whether it's using spray foam,
foam board or some type of batinsulation, because if you trap
that moisture in that wood it'sgoing to cause huge problems.
You want to make sure you don'thave leaks from above.
What if this window was leaking, for example?
That could ruin that jobtremendously.
Got to make sure those materialsare wet, even if they're new

(06:40):
construction.
I've seen homes built.
They frame the house in, itrains and then the next day the
insulation contractors come inand throw up the insulation.
Well, did they give thatmaterial time to dry out after
the rain or the high humidityfrom the fog that just rolled in
?
And then, of course, anypenetrations.
Keep in mind that you can haveplumbing penetrations and

(07:02):
electrical wires going through.
Are those sealed real well fromthe outside to make sure
water's not rolling in, forexample?
Or flashing around decks okaycan also cause a lot of rim
joists leakage.
So make sure.
The point of this slide is makesure that wood is dry before
you insulate that rim joist.
This is what I mean fromprotect it from the outside.

(07:22):
This is a great view of what atypical rim joist would look
like.
So this exterior sheathing thatthey're showing right up here,
if that is compromised, ifyou've got some type of siding
that's leaking.
Perhaps your brick is leaking.
Anything that deck that Imentioned where the flashing is
leaking, that has to be fixedbefore I would consider

(07:46):
insulating the rim joists.
As a matter of fact, we just hadone of our sales guys, josh,
call me yesterday where they hada wall that was above ground
that was wet.
It was a foundation, a blockwall that was above ground that
was wet.
It was a foundation, a blockwall that was above ground,
totally wet.
They dry, locked it and it justkept getting wet.
And so, anyway, they thoughthow in the world could it be

(08:07):
from the basement or the crawlspace, because this is the
living space wall?
That was a finished wall thatwas blocked.
Well, lo and behold, there wasa drip edge up near the roof
that was leaking all the waydown that wall that was blocked.
Well, lo and behold, there wasa drip edge up near the roof
that was leaking all the waydown that wall.
That was causing huge problems.
So make sure the outside iswaterproof before you attempt to
put rim joist insulation on.

(08:28):
Just a couple of examples ofwhat your wood could look like
behind rim joist insulation ifit is not dried properly or kept
dry.
This was fiberglass.
The fiberglass does trapmoisture.
We pulled that fiberglass outas part of our crawl space
encapsulation process and foundthis little jewel hidden up in

(08:49):
the joist.
Now I think this was a leakfrom the outside just because of
the fiberglass will preventcondensation, believe it or not.
Fglass and rockwool it doesn'tair seal it, as I mentioned
before.
That's why I don't likefiberglass or rockwool because
it doesn't air seal as well as aspray foam or a foam board
spray foam combination.
But it is a great thermalbarrier.

(09:11):
So if the very least you coulddo is put fiberglass or rockwool
, you're at least gonna get thatthermal break to prevent
condensation.
So I think this was a leak fromthe outside.
You can see that little rustednail on the left side, right
above the sill plate.
So I think there was some kindof flashing issue or leak from
the outside that caused this.
Just another example of whyit's important to stop any water

(09:33):
from coming in before you dothe rim joist.
So the three install methodsthat I've seen done and, of
course, if I missed one, pleaselet me know in the comments
below as the building sciencefolks talk about, use a either
open cell or closed celldirectly on the rim joist, as we
see on the bottom left.
Or you can shove bat insulation, whether it's fiberglass or

(09:56):
rock wool, as you can see.
We've done that in the past onthe top middle.
Or my preferred method I liketo use a foam board spray foam
combination.
The reason why I like thatmethod the best number one I'm
air sealing.
I'm also creating that thermalbreak, unlike the middle picture
where I'm only creating thethermal break.
But secondly, if for somereason we do get a leak from the

(10:18):
outside where we notice watercoming in, that foam board is a
lot easier to take off if I haveto do some kind of maintenance
or repair to that rim joistversus scraping that two
component spray foam off on thefar left.
So that's why I prefer a foamboard spray foam combination.
It is a little bit more work todo it this way.

(10:38):
You know these insulationcontractors.
They can come in and spray foamthe rim joist pretty quick
versus having to cut foam boardup.
As you can see, we had tocustom cut that piece of foam
board and shove it in there andspray foam around it.
So anyway, these are the threeinstall methods that we've used
and done in the past.
Let me know if I missed onedown below in the comments.
We hope you'll like andsubscribe to our YouTube channel

(10:59):
.
If you haven't had a chance tocheck out my book, I do a lot of
education on rim joistinsulation in my book Crawl
Space Repair Misbusted, whichcan also be applied to basements
.
If you get a chance to checkthat out on Amazon, make sure
you subscribe to our YouTubechannel and if you want to set
up a consultation with me, we'vegot that available on
buymeacoffeecom.

(11:19):
I'm Michael Church, crawl SpaceNinja.
We hope you make it a happy andblessed day and we'll see you
later.