DIY Crawl Space Guide: Air Sealing Subfloor: Week 6

Your crawl space might be the most overlooked yet critically important area of your home. In this sixth installment of the Crawl Space Repair Challenge, Michael Church tackles the crucial topic of air sealing—a process that dramatically improves energy efficiency, indoor air quality, and structural integrity.

The Department of Energy confirms what Michael has seen firsthand: unsealed crawl spaces contribute significantly to moisture problems affecting both your health and home structure. When air freely moves between your crawl space and living areas, it brings along unwanted moisture, allergens, and even radon. Through detailed demonstrations, Michael walks you through the why and how of comprehensive air sealing, identifying 19 different areas throughout your home requiring attention.

From foundation walls to subfloors and ductwork, every potential air leak creates problems. Michael reveals that an astonishing 25% of conditioned air moving through unsealed duct systems simply leaks away—wasted energy you're paying for each month. Using professional-grade materials like mastic spray, foam sealants, and specially formulated concrete crack repair kits (originally developed for NASA), he demonstrates exactly how to create an effective air barrier. The techniques shared work equally well in crawl spaces, basements, and even more visible areas of your home.

Whether you're a dedicated DIY enthusiast or simply want to understand what professionals should be doing in your crawl space, this episode delivers actionable advice to transform your home's forgotten foundation into a properly sealed, energy-efficient space. Ready to stop wasting energy and improve your home's air quality? Watch now, then grab your caulk gun and join the thousands who've already completed the Crawl Space Repair Challenge!

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Episode Transcript

Speaker 0 (00:00):
Hey there, michael Church, crawl Space Ninja.
We are on week six of the CrawlSpace Repair Challenge and this
week I want to talk about airsealing the crawl space.
Some of the topics I'm going tocover today are why you should
air seal the crawl space, someof the materials that we
recommend in air sealing thecrawl space foundation wall air

(00:20):
sealing.
Why is that important?
Air sealing the subfloor aswell as the duct work, if you
have any located in your crawlspace, and, of course, the door.
If you are new to the crawlspace repair challenge, we
recommend you check out the sumppumps and basins video that we
did in week five, and also I'mgoing to put a link to the
others up here on the top right.
So why should you air seal thecrawl space?

(00:41):
Air leakage, according to theDepartment of Energy, can
contribute to moisture problemsthat affect the occupant's
health and the structuralstability of the home.
Sealing cracks and openingsreduces drafts and cold spots,
improving comfort.
The recommended strategy is toreduce air leakage as much as
possible and then provide acontrolled ventilation as needed

(01:03):
.
So, in other words, you want toair seal all of the
non-controlled areas so that youcan control exactly where the
air is coming from.
That way, you're making surethat air is clean whenever it
comes into your home, and a lotof times you can do that through
an ERV or an HRV.
But look here, here are 19different places that the

(01:25):
Department of Energy recommendsthat you air seal inside of your
home, and I'm going to put alink to this down below so you
can check out that article.
Some recommended products weuse are caulk guns and spray
foam guns and brushes, dependingon what we are applying.
We've got sealants like Masticspray, foam, caulk and concrete
crack sealant and then, ofcourse, we've got some things we

(01:46):
use to close up those big gapslike foam board, weather
stripping and backer rough.
Three reasons to seal foundationwalls Inside of a crawl space
on the inside of the basement.
If you've got plumbingpenetration or any kind of air
leaks going in and out of thatfoundation wall, we certainly
want to recommend that you airseal those.
They not only will stop airfrom intruding, but they also

(02:08):
stop water and can add to thestructural integrity of the
house.
Let me know in the comment downbelow if you can think of
another reason why you shouldair seal your foundation walls.
I want to do a quick plug forCrack Fix concrete crack repair
kit that you can get on our DIYstore.
It is a carbon fiber basedrepair kit, great for basements,
crawl spaces, garages, pools,warehouse slabs and warehouse

(02:32):
walls.
Originally developed for NASAand the military, this is a
great DIY kit If you have anykind of cracks in the concrete
or in block around your house.
This is really really goodstuff and we do install it quite
a bit.
Don't forget about the outsidewhen you're air sealing that
foundation wall.
Go outside, see if there's anymajor HVAC penetrations coming

(02:52):
through.
Also, check the outsidefoundation vents.
A lot of times they're not airsealed so you can caulk around
those, especially if you'regoing to seal the vent on the
inside of the crawl space.
Of course, any type of cracksthat you may see on the outside,
that is another great place touse concrete crack fixes on the
outside foundation walls.
Huge place that's overlookedwhen air sealing the crawl space

(03:15):
is the subfloor.
There are a lot of penetrationsbetween that crawl space and
your living space.
By air sealing that subfloorit's going to improve energy
efficiency and help keep thatcrawl space air out of the
living space.
Some of the areas you want tocheck out is the HVAC boots.
Anywhere you see an HVACpenetration going up into the
living space is great.

(03:35):
Electrical penetrations, ventfan exhausts these are real
popular around the kitchen.
Maybe you have an island thathas a vent fan that exhausts
down and out of the crawl space.
Make sure you check for thoseand plumbing penetrations
throughout the entire subfloorarea.
If you don't mind, take just amoment to like this video down
below.
Let us know what you thinkabout this content so far and

(03:57):
share it with your friends andfamily if you feel like it would
be useful to them, and makesure you subscribe to our
YouTube channel and ring thatnotifications bell.
If you have HVAC ductwork goingthrough your crawl space, we
always recommend you seal thoseducts.
As a matter of fact, it isestimated that up to 25% of the
air going through that ductsystem is leaking throughout the

(04:19):
crawl space, the attic, theinterior walls of your home.
So make sure you get as much ofthat ductwork air sealed as
possible.
If you happen to have hard pipe, like they're showing here,
every one of those joints needsto be mastic.
Now, if you happen to haveflexible ductwork, you usually
only have to air seal where itconnects to the boot that goes
into the living space and, ofcourse, the supply side of the

(04:42):
HVAC trunk line.
We see this a lot in crawlspaces where you have sub-par
insulation.
Now, if this was my house, Iwould remove all of that
insulation, air seal as we'reshowing here in this picture,
and then put the insulation back.
That makes some really goodbubble wrap insulation if you
don't want to mess with thefiberglass stuff that is out
there today.
But always make sure you airseal and insulate duct work.

(05:04):
Even in a conditioned crawlspace, it is recommended Make
sure you air seal that door.
This is a good picture of whatan air seal door looks like.
We put weather stripping aroundthe inside of that door and we
do insulate doors when weinstall them to keep that hot,
humid air out of the crawl spaceso that dehumidifier runs less.
Just want to mention my bookCrawl Space Repair Myths Busted,

(05:25):
if you have not had a chance topick that up.
It is available on Amazon andif you'd like to schedule a
consultation with me maybeyou've got a problem that is
unique to your situation thatyou can schedule a 30-minute
consultation with me.
We hope you like and subscribeto our YouTube channel and, of
course, comment down below whatyou thought about this video.
I'm Michael Church.