Should Floor Joists in Crawl Space Be Insulated?

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Key Takeaways

  • Insulating floor joists or walls depends on if crawl space is vented or unvented.
  • In vented crawl space, insulate floor joists to protect floor above.
  • In unvented crawl space, insulate walls to keep whole space warmer.
  • Insulating floor joists can make crawl space cooler but trap moisture.
  • Fiberglass insulation works best between floor joists as it’s cheap and easy to install.
  • Spray foam insulation is better but expensive and needs professionals.
  • Insulating crawl space walls needs less insulation and can skip insulating ducts and pipes.
  • Always check local building codes before insulating a crawl space.


Crawl spaces are the short spaces under the first floor of some homes. The crawl space has a dirt floor and the building sits on foundation walls and piers. Insulating a crawl space can help save on heating and cooling costs. But should you insulate the floor joists or the walls? The answer depends on if the crawl space is vented or unvented. This article will look at the pros and cons of insulating floor joists versus walls in crawl spaces. It will help homeowners decide the best way to insulate their crawl space.

Insulating a crawl space right is important to protect pipes from freezing. It also keeps floors warmer and saves energy. This article will go over all the options in detail. It will cover the types of insulation to use, how to install them, and the costs. With this information, homeowners can make the best decision for their home and budget. Correctly insulating a crawl space takes some work, but it can really pay off over time.

Should I Insulate the Floor Joists or Walls in My Crawl Space?

What is the Difference Between a Vented and Unvented Crawl Space?

There are two main types of crawl spaces – vented and unvented:

  • Vented crawl spaces have vents on the outside walls near the top. This allows air to flow in and out of the crawl space. Vented crawl spaces help control moisture but make it hard to heat and cool.
  • Unvented crawl spaces are sealed off from outside air. They have plastic sheeting on the ground to control moisture. Unvented spaces stay warmer but need extra care to prevent mold.

If My Crawl Space is Vented, Should I Insulate the Floor Joists?

If your crawl space is vented, it’s best to insulate between the floor joists:

  • Insulating the floor joists keeps the floor above warmer. This helps cut heating costs.
  • It prevents cold air from entering the space from vents. Cold floors won’t be an issue.
  • The crawl space stays vented, so moisture can escape. Trapping moisture can cause mold.

Fiberglass batt insulation fits nicely between floor joists:

  • It’s an affordable option that’s easy to install yourself. Just press it in between the joists.
  • Batt insulation rates around R-13 to R-19. This gives decent insulation for the floor.
  • Use faced batts with vapor barriers against the subfloor. This stops moisture from above.

Spray foam insulation is another option:

  • It air seals and insulates in one step, giving an R-value over R-20.
  • But it’s expensive and usually needs professional installers.

If My Crawl Space is Unvented, Should I Insulate the Walls?

For unvented crawl spaces, insulating the walls may be better:

  • Insulating the walls keeps the whole space warmer, as it’s sealed off.
  • Less insulation is needed compared to the floor joists.
  • Ductwork and pipes in the space stay warmer. No need to insulate them separately.
  • Moisture is controlled by the liner on the ground, not ventilation.

Fiberglass batts can insulate walls in unvented crawl spaces:

  • Use faced batts with vapor barriers facing the crawl space.
  • R-13 to R-19 batts are sufficient for the walls. More is better if the budget allows.
  • Stagger the batts between wall studs and seal up any gaps.

Rigid foam boards also work well:

  • They provide an air sealing barrier and high insulation value.
  • Foam boards rated R-10 or more can insulate walls with just 1-2 inches.
  • Use spray foam around edges for sealing and to hold boards in place.

What Should I Know Before Insulating My Crawl Space?

Insulating a crawl space takes planning and proper installation:

Should I Have Any Vents if Insulating Floor Joists?

  • Some vents are still needed for air flow if insulating floor joists only.
  • But vents should be reduced to the minimum code requirement.
  • Foundation vents should have insulation covers to limit air flow.

How Do I Prepare an Unvented Crawl Space?

Creating an unvented space takes a few extra steps:

  • All vents must be sealed off completely. Use expandable spray foam.
  • The ground needs a vapor barrier like 6-8 mil polyethylene sheeting. Overlap and tape all seams.
  • Concrete walls should be sealed too. Use latex based sealants.
  • A dehumidifier helps control humidity levels in unvented spaces.

What Are Local Building Codes for Insulating Crawl Spaces?

  • Building codes provide insulation minimums for crawl spaces. Learn your local codes.
  • For example, some codes require R-10 continuous insulation on walls.
  • Codes also regulate covering ground surfaces, ventilation, and more.
  • Consulting an inspector is wise to ensure proper installation.

How Do I Know if My Crawl Space Has Moisture Issues?

Signs of excess moisture include:

  • Mold/mildew growth on wood, insulation, or foundations
  • A musty smell in the crawl space
  • Wet spots or standing water on the ground
  • Condensation on pipes, ducts, or other surfaces

These issues should be fixed before insulating crawl spaces.

5 Key Tips for Insulating Your Crawl Space

Follow these tips when installing crawl space insulation:

1. Determine if your crawl space should be vented or unvented

Consider your climate, construction, and goals. Adding a moisture barrier can often allow converting to unvented.

2. For vented spaces, focus on insulating between floor joists

Use faced fiberglass batts or spray foam insulation. Leave some vents for air flow.

3. For unvented spaces, insulate the exterior walls

This allows warm air to remain in the space. Fiberglass or rigid foam boards work well.

4. Carefully seal up unvented spaces and add a moisture barrier

Tape plastic sheeting to the ground and seal all vents. Install a dehumidifier.

5. Review local building codes and have a professional inspect

Get the proper permits and follow insulation requirements.

Insulation Options Summary Table

Here is a comparison of insulation options for crawl space floor joists or walls:

Insulation TypeWhere to InstallProsCons
Fiberglass BattsFloor Joists, Walls– Affordable<br>- Easy DIY install– Lower R-value than other options <br>- Can absorb moisture over time
Spray FoamFloor Joists– Air seals and insulates <br>- Higher R-value– Expensive <br>- Professional installation usually required
Rigid Foam BoardsWalls– High R-value <br>- Also air seals– Can be more expensive than batt insulation <br>- Gaps must be sealed carefully


Insulating a crawl space properly saves energy and makes the space more comfortable. For vented crawl spaces, installing fiberglass batts between floor joists is usually the best option. It’s affordable and not too difficult for DIYers. For unvented crawl spaces, insulating the exterior walls allows the whole space to retain heat. This may require fewer insulation materials overall. Rigid foam boards are one good choice for wall insulation. When in doubt, have an energy auditor or insulation contractor evaluate your crawl space. They can help determine the optimal insulation plan based on your home’s specifics. With the right installation, crawl space insulation can be a great investment toward energy savings

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