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!

Chapters

• 0:00 Introduction to Air Sealing
• 0:38 Why Air Seal Your Crawl Space
• 1:31 Recommended Products and Materials
• 1:51 Foundation Wall Sealing Techniques
• 3:11 Subfloor and Ductwork Sealing
• 5:08 Door Sealing and Final Tips

Transcript

Speaker 0: 0:00

Hey there, michael Church, crawl Space Ninja. We are on week six of the Crawl Space Repair Challenge and this week I want to talk about air sealing the crawl space. Some of the topics I'm going to cover today are why you should air seal the crawl space, some of the materials that we recommend in air sealing the crawl space foundation wall air sealing. Why is that important? Air sealing the subfloor as well as the duct work, if you have any located in your crawl space, and, of course, the door. If you are new to the crawl space repair challenge, we recommend you check out the sump pumps and basins video that we did in week five, and also I'm going to put a link to the others up here on the top right. So why should you air seal the crawl space?

Speaker 0: 0:41

Air leakage, according to the Department of Energy, can contribute to moisture problems that affect the occupant's health and the structural stability of the home. Sealing cracks and openings reduces drafts and cold spots, improving comfort. The recommended strategy is to reduce air leakage as much as possible and then provide a controlled ventilation as needed. So, in other words, you want to air seal all of the non-controlled areas so that you can control exactly where the air is coming from. That way, you're making sure that air is clean whenever it comes into your home, and a lot of times you can do that through an ERV or an HRV. But look here, here are 19 different places that the Department of Energy recommends that you air seal inside of your home, and I'm going to put a link to this down below so you can check out that article. Some recommended products we use are caulk guns and spray foam guns and brushes, depending on what we are applying. We've got sealants like Mastic spray, foam, caulk and concrete crack sealant and then, of course, we've got some things we use to close up those big gaps like foam board, weather stripping and backer rough.

Speaker 0: 1:51

Three reasons to seal foundation walls Inside of a crawl space on the inside of the basement. If you've got plumbing penetration or any kind of air leaks going in and out of that foundation wall, we certainly want to recommend that you air seal those. They not only will stop air from intruding, but they also stop water and can add to the structural integrity of the house. Let me know in the comment down below if you can think of another reason why you should air seal your foundation walls. I want to do a quick plug for Crack Fix concrete crack repair kit that you can get on our DIY store. It is a carbon fiber based repair kit, great for basements, crawl spaces, garages, pools, warehouse slabs and warehouse walls. Originally developed for NASA and the military, this is a great DIY kit If you have any kind of cracks in the concrete or in block around your house. This is really really good stuff and we do install it quite a bit.

Speaker 0: 2:45

Don't forget about the outside when you're air sealing that foundation wall. Go outside, see if there's any major HVAC penetrations coming through. Also, check the outside foundation vents. A lot of times they're not air sealed so you can caulk around those, especially if you're going to seal the vent on the inside of the crawl space. Of course, any type of cracks that you may see on the outside, that is another great place to use concrete crack fixes on the outside foundation walls.

Speaker 0: 3:11

Huge place that's overlooked when air sealing the crawl space is the subfloor. There are a lot of penetrations between that crawl space and your living space. By air sealing that subfloor it's going to improve energy efficiency and help keep that crawl space air out of the living space. Some of the areas you want to check out is the HVAC boots. Anywhere you see an HVAC penetration going up into the living space is great. Electrical penetrations, vent fan exhausts these are real popular around the kitchen. Maybe you have an island that has a vent fan that exhausts down and out of the crawl space. Make sure you check for those and plumbing penetrations throughout the entire subfloor area. If you don't mind, take just a moment to like this video down below. Let us know what you think about this content so far and share it with your friends and family if you feel like it would be useful to them, and make sure you subscribe to our YouTube channel and ring that notifications bell.

Speaker 0: 4:04

If you have HVAC ductwork going through your crawl space, we always recommend you seal those ducts. As a matter of fact, it is estimated that up to 25% of the air going through that duct system is leaking throughout the crawl space, the attic, the interior walls of your home. So make sure you get as much of that ductwork air sealed as possible. If you happen to have hard pipe, like they're showing here, every one of those joints needs to be mastic. Now, if you happen to have flexible ductwork, you usually only have to air seal where it connects to the boot that goes into the living space and, of course, the supply side of the HVAC trunk line. We see this a lot in crawl spaces where you have sub-par insulation. Now, if this was my house, I would remove all of that insulation, air seal as we're showing here in this picture, and then put the insulation back. That makes some really good bubble wrap insulation if you don't want to mess with the fiberglass stuff that is out there today.

Speaker 0: 5:00

But always make sure you air seal and insulate duct work. Even in a conditioned crawl space, it is recommended Make sure you air seal that door. This is a good picture of what an air seal door looks like. We put weather stripping around the inside of that door and we do insulate doors when we install them to keep that hot, humid air out of the crawl space so that dehumidifier runs less. Just want to mention my book Crawl Space Repair Myths Busted, if you have not had a chance to pick that up. It is available on Amazon and if you'd like to schedule a consultation with me maybe you've got a problem that is unique to your situation that you can schedule a 30-minute consultation with me. We hope you like and subscribe to our YouTube channel and, of course, comment down below what you thought about this video. I'm Michael Church.