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What Is the Best Insulation For Your Crawl Space?

fallen foam board insulation

Have your heating bills been painfully high?  Are your floors cold anyway?  Is your home feeling oddly drafty?  These are signs that you need to reevaluate – or replace- the insulation in your crawl space.

Crawl spaces are often victims of “out of sight, out of mind.”  It’s easier to forget this space under your home even exists!  Many homeowners don’t realize it, but your house’s stability and health is heavily impacted by the condition of your crawl space.  This means insulation failure could only be the start of your crawl space problems.

Acculevel is a family-owned and operated company that specializes in foundation repair and waterproofing.  Since our start in 1996, we’ve helped more than 35,000 homeowners preserve and protect their home’s foundation, health, and resale value. 

In this article, we’re going to explore the common types of insulation used in crawl spaces, which ones are most effective, and which ones you should avoid. 


Please Do Not Install Fiberglass Insulation In Your Crawl Space

In other parts of your home, fiberglass insulation is a reasonable and affordable option.  But it should not be in your crawl space.   

Fiberglass insulation will absorb moisture.  This is a major problem for several reasons.

  1. Most builders install insulation batting the floor joists.  When the insulation gets damp, it’s holding moisture against your wooden joists and subfloor.
  2. Moisture causes wood to rot and decay; damaged flooring components cause sagging floors.
  3. Damp wood also attracts insects like termites, who are demolition experts in this field.
  4. Mold and organisms like bacteria and viruses like moisture- and can grow on the wood or in the insulation itself.
  5. When fiberglass insulation collects moisture, it gets heavier.  Eventually, it weighs enough that it pulls free of its fasteners.  Then you don’t have any insulation at all.


falling insulation in crawl space
This photo was taken by an Acculevel project advisor during an in-home assessment for a homeowner. The fiberglass insulation has gotten damp, unraveled, and is falling down.

If you have fiberglass insulation in your crawl space, have it removed and disposed of in a safe manner (you can’t throw it away in your trash).  Then you can replace it with spray foam insulation. 


Spray Foam Insulation is Ideal for Use in Crawl Spaces

Most builders and contractors agree that spray foam is a far better product for use in a potentially damp environment.  Spray foam repels water, immediately attached to whatever surface you spray it on, and is resistant to biological growth.

Spray foam insulation is more expensive than fiberglass, but it only takes a small amount of spray foam to effectively insulate your entire crawl space.  This is because it only needs to be installed in the joist boxes.

close up of spray foam
This photo was taken by an Acculevel team member after installing spray foam insulation in a homeowner’s crawl space.


Why Do You Only Spray Foam In Joist Boxes? 

Think about how your home is constructed.  The foundation was poured or built of concrete block (or brick in older homes).  These are materials that are thicker and less porous than wooden boards.  

The flooring structure built out of wood- that is placed on top of your foundation- is the weakest point.  The diagram below shows the ideal spot for insulation is in the “joist box,” the point where the sill plate and band joist connect.  What’s on the other side of the band joist? Depending on the depth of your crawl space, it’s either solid earth or siding.  

If the joist box is above ground, it’s exposed to the outside air temperatures.  Colder air can sneak its way through an uninsulated board; good insulation prevents this from happening.

Diagram of flooring structure
This illustration is courtesy of NCSU.


Do you have more questions about your wooden flooring structure, or have you noticed sloping or sagging in your floors? We talk about flooring components and repair costs here.


The ground maintains a more constant temperature than air, but it also emphasizes the need for waterproof insulation in the joist box.  If the other side of the band joist is damp soil, it’s more likely the band joist and insulation will be in contact with moisture.


Insulation for Crawl Space Walls Needs to Be Secure

If you have a drafty crawl space, you may also need to insulate the walls.  This is especially true in homes with older foundations, or those made of brick or stone.  (As mortar ages, it can shift enough to create gaps.)  

Sometimes, people will use foam board insulation to cover walls, but this isn’t reliable long-term.  Foam board is usually glued into place, and over time, the glue loses effectiveness.  

fallen foam board insulation
This photo was taken by an Acculevel project advisor during an in-home assessment.  You can see the spray foam insulation is still secure in the joist boxes, but some of the foam board has fallen on the ground.


Blanket Insulation Is An Excellent Choice for Crawl Spaces

Insulating blanket insulation is a better choice.  Blanket insulation is, as the name suggests, a material that is unrolled and attached to the crawl space walls.  

Blanket insulation should be reflective on one side, so that it deflects the colder air back towards the wall.  This material should also be inorganic, so that even if it gets wet, it won’t hold moisture or feed organisms like mold and bacteria. 

Acculevel installs a type of blanket insulation called radiant armor, and it can be in combination with encapsulation, if so desired. 

radiant armor installed in crawl space
This is a photo of radiant armor insulation installed in a crawl space.


Waterproofing Is Often Necessary In Crawl Spaces

This article is supposed to be focused on insulation, I know.  But at Acculevel, we provide whole-home solutions, not just short-term repairs.

Of course, we encourage you to repair the symptom of failing or inadequate insulation.  But we also want you to address the larger issue, which is the moisture in your crawl space.  

Moisture is what’s damaging your flooring structure and causing the insulation to fail.  Replacing damp fiberglass insulation with spray foam provides benefits, but it doesn’t solve the problem of decaying or sagging floor components. 

We detail how to waterproof a crawl space in this blog


Do You Have Other Questions About Your Crawl Space? 

We have a comprehensive homeowner’s guide to crawl space repairs, available free of charge to anyone who has questions or concerns.  This guide is designed to address everything from mold growth to foundation issues, including repair methods and costs. 

Crawl Space Guide link


Would You Like a Professional Evaluation of Your Crawl Space? 

Complete our online contact form or call Acculevel at 866-953-1501.  We have helped more than 35,000 homeowners throughout Indiana and the surrounding areas preserve and protect their homes for the future.

We’ll make an appointment for you with one of our project advisors.  They will sit down and discuss your concerns, your plans for your home, and the symptoms that you have experienced.  Then they will thoroughly evaluate your home, diagnosing the problems and determining the best solution(s) we have to offer.  

Our goal is to provide you with a whole-home solution, a plan that addresses your complaints and will resolve them as efficiently as possible.  We will treat your home as if it were our own, offering 5 star service every step of the way.

If you do not live in our service area, please make sure you are working with a reputable and properly insured contractor.  Verify that they are accredited by the Better Business Bureau, and check their reviews from multiple sources.  

You are also welcome to use our checklist of Questions You Should Ask a Contractor; please use this to protect yourself from potential scam artists, and be certain you are working with a business that is the right fit for you.



Kelly Kater

Over her thirty-year career, Kelly has worked in a wide variety of fields: secondary education, nursing, biology, elder care, the postal service, multicultural development, and academia. She has developed a skill for translating industry-specific jargon into everyday language. Her goal is to share the knowledge and experience of the Acculevel team with homeowners, in a way that is both engaging and informative.

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