How To Air Seal Your Crawl Space

It’s important to make sure that the crawl space is sealed from the rest of the home. The reason for this is that often the air quality in the crawl space is poor compared to the air inside the home. If the crawl space contains many gaps, spaces or channels that communicate with the home air will move back and forth between the two spaces which will introduce air of lower quality into your home. In this post we give tips on how to seal your crawl space, in particular how to make three important seals in your crawl space to prevent low quality air from the damp dark crawl space from passing up into your home. So without further ado, here are the three most important areas you should seal in your crawl space!

Sub-floor Penetrations

One of the easiest places to seal are the parts of your sub-floor that have penetration holes to allow things like pipes, electrical wires, and air ducts to pass through. Air can pass through these spaces and even though the gaps may be small or undetectable by the naked eye, they are there. What you can do is go on our DIY store and pick up a Spray Foam Starter Kit and spray foam anywhere you have these penetrations. You may need to move wires and ducts around a bit to get at the precise location where the gaps need to be sealed, but with a bit of spray foam the gap will stay sealed for a long time.

Rim Joists

The engineers at recommend that every home with a crawlspace or basement that includes a rim joist insulate and seal their rim joist. If you have a block foundation or a concrete foundation, you most likely have a rim joist. The problem with rim joists is that they are almost never insulated. Due to this, condensation often forms on the rim joist on the inside of the crawl space which can contribute to mold. We recommend insulating the rim joist. But that’s another story and you can find out more about this in this video. We like to seal our rim joists using foam board but you could also use spray foam (just be prepared to lay down a thick layer) to prevent thermal transfer as well as preventing low-quality air from the crawl space from going up into the house. Rim joists are another crucial place to seal to prevent air from passing from the crawl space into the home.


Many of us have central heating and cooling in our modern homes and many times the heating and cooling equipment like HVAC is in the crawl space with ducts. If your ducts are not properly sealed then low-quality air can pass from your crawl space up into the home which can degrade the quality of the air where your family eats and sleeps! Of course, unsealed ducts are leaky ducts, and this means that the conditioned, high-quality air from your home will leak from these gaps in the duct-work. It is estimated that as much as 25% of high-quality air from your home is lost into the crawl space through gaps in the duct work. You can easily seal your duct work using duct sealing tape. You can buy the exact same tape we use in our jobs on our DIY store! It’s an industrial grade high-quality seal tape made for the purpose of sealing duct work and the seal will last for a very long time.

So as you can see there are various areas in your crawl space that need to get sealed in order to ensure that lower quality air from your crawl space does not rise up into the home, and that good quality air does not leak from the home down into the crawl space. These seals are not difficult to make, and once you’ve achieved them the quality of the air in your home may improve dramatically. Of course, you might also enjoy additional benefits such as energy savings as well!

2 thoughts on “How To Air Seal Your Crawl Space”

  1. I’m curious if floor penetrations allow enough communication between spaces to allow radon into the house from an encapsulated crawlspace. I don’t have a radon problem, but I did notice a slight increase when I had a very tightly sealed cralwspace door installed. I believe new pressure differentials is one issue, but wondering if I can slow communication down by sealing the floor penetrations and maybe reduce the radon level in the home.

    1. Michael Church

      Hi Rob, in our experience radon can find a way through even the smallest of gaps so yes seal it the best you can.

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