How to Insulate a Crawl Space

Insulating a crawl space is an important part of making a home more energy-efficient and comfortable. A properly insulated crawl space can help prevent heat loss in winter, keep the floors above warmer, and prevent moisture and pests from accumulating. Installing insulation in a crawl space takes some planning and effort, but it is a manageable project for many DIYers. Here is a step-by-step guide to effectively insulating a crawl space.

Assessing the Crawl Space

Before beginning any work, thoroughly examine the crawl space to determine what needs to be done. Here are some key things to check:

  • Access – Is there easy access to the entire crawl space or are there any blocked off or hard to reach areas? Access may need to be created.
  • Moisture – Use a moisture meter to check for any damp spots. Look for standing water, mold, or rotting wood. Fix any moisture issues before insulating.
  • Vents – Note the location and size of any vents. More vents may need to be added.
  • Wiring/Plumbing – Locate any wiring, ductwork or plumbing. Assess whether insulation might impact it.
  • Pests – Check for signs of pests like termites or rodents. Deal with any pests before adding insulation.

Once the space has been fully examined, determine how much insulation is needed and the best type to use.

Choosing Insulation Type and Amount

There are several types of insulation that can work for a crawl space. Here are some of the most common options:

Fiberglass Batts

Fiberglass insulation comes in long batts or rolls that can be cut to size. It’s an affordable option that works well for crawl spaces. Aim for an R-Value of R-19 or higher. The batts can be held in place with insulation hangers.

Spray Foam

Spray foam insulation expands to fill cracks and spaces. It air seals well but is more expensive than other options. Use a closed-cell spray foam with an R-Value of at least R-20.

Rigid Foam Board

Rigid foam insulation boards, like XPS or polyiso, have a high R-value per inch. Use at least R-10 boards. Seal seams with tape or caulk. Hold boards in place with adhesive or fasteners.


Loose-fill cellulose made from recycled paper can work for crawl spaces. It achieves an R-Value of R-13 per inch. A moisture barrier is required over the floor.

Preparing the Crawl Space

Before installing insulation, the crawl space needs to be prepped properly:

  • Remove Debris – Clear out any trash, lumber scraps, or other unnecessary items.
  • Pest Control – Have a pest control company treat the space if there are signs of termites, rodents, or other pests.
  • Raise Utilities – If possible, raise any wires, ducts or pipes up off the ground at least 6 inches so insulation can fit underneath them.
  • Install Vapor Barrier – A 6 mil polyethylene plastic sheet should be placed down over the entire floor to serve as a moisture and vapor barrier. Seal seams with tape.
  • Increase Ventilation – More vents may need to be added to prevent moisture buildup. There should be 1 sq ft of vent space per 150 sq ft of crawl space area.
  • Repair Moisture Issues – Correct any standing water issues, leaks, or sources of moisture before insulating.

With prep work complete, it’s time to start installing insulation.

Installing Fiberglass Batt Insulation

Fiberglass batts or rolls are a very common insulation choice for crawl spaces. Here are the steps to properly install them:

Step 1: Plan Your Batt Layout

  • Measure the length and width of the crawl space.
  • Calculate the total square footage.
  • Map out a layout for how the batts will fit to cover the entire area.

Step 2: Cut Batts to Fit

  • Use a sharp utility knife to cut batts to fit your layout.
  • Wear proper safety gear like gloves, long sleeves and eye protection when handling fiberglass.

Step 3: Secure Batts

  • Use insulation hangers, also called stick clips, to secure batts to the floor joists overhead.
  • Place hangers every 2-3 feet along the edges and down the middle of each batt.

Step 4: Fill in Gaps

  • Stuff smaller leftover pieces of insulation into any gaps between batts or hard to reach spots.
  • Spray foam can also seal small gaps if needed.

With the batts installed snugly, the crawl space should now have consistent insulation coverage.

Installing Rigid Foam Board Insulation

Rigid foam boards with high R-values are another great insulation choice. Follow these guidelines to install them:

Step 1: Plan Board Layout

  • Measure space and calculate total area to cover.
  • Map out how boards will fit together like puzzle pieces with no gaps.

Step 2: Cut Boards to Fit

  • Use a utility knife with a sharp blade or a table saw to cut boards to required sizes and shapes.
  • Wear proper safety gear when cutting rigid foam.

Step 3: Attach Boards

  • Use plastic washers and long screws to screw boards into floor joists overhead.
  • Place screws every 12-16 inches around the edges and down the middle.
  • You can also use construction adhesive to glue boards in place.

Step 4: Seal Seams

  • Use foam sealant or silicone caulk to seal any gaps between boards.
  • Use foil tape to cover all seams for maximum air sealing.

Rigid foam boards take a bit more time and effort to install correctly, but provide excellent insulation value if done properly.

Installing Spray Foam Insulation

Spray foam insulation requires professional equipment, so this is best left to experienced DIYers or professional insulation contractors. Here is an overview of the process:

  • The foam is rapidly sprayed into place using special mixing guns and pumps.
  • It expands quickly to fill every nook and cranny.
  • Layers are sprayed until the desired insulation depth is achieved, usually around 3″.
  • Exposed spray foam should be covered with a fireproof barrier like drywall.
  • Ventilation is critical to prevent moisture issues. More vents may need to be added.
  • Spray foam is more expensive than other insulation options, but provides robust insulation and air sealing when installed correctly.

Finishing the Insulation Job

Once the main insulation is in place, there are a few final steps to complete the job:

  • Install Vapor Barrier – Cover all exposed earth with 6 mil polyethylene sheeting, sealed at seams with tape. This prevents moisture from evaporating up into the insulation.
  • Create Air Barrier – Use spray foam, caulk or expanding foam sealant around any cracks, pipes, posts, or penetrations to seal air gaps. This prevents airflow into the insulation.
  • Cover Insulation – Use rigid foam boards or wood panels to cover exposed faced fiberglass batts or loose fill insulation. This protects against air infiltration and pests.
  • Label Components – Place labels noting insulation type and R-value on joists or posts at regular intervals. This makes it easy for future homeowners to know what’s installed.

With these finishing touches complete, the crawl space should now be fully insulated for improved efficiency and comfort. Monitor periodically for pests, moisture issues or settling.

Frequently Asked Questions About Insulating Crawl Spaces

Insulating a crawl space raises many questions for homeowners. Here are answers to some of the most common queries:

Should I insulate the crawl space walls or floor?

The recommended practice is to insulate the floor (the ceiling of the crawl space). This envelops the conditioned area above in insulation. Insulating walls is not required, though foundation walls can be insulated for added efficiency.

Can I do it myself or should I hire a professional?

In most cases, insulating a crawl space is totally DIY friendly if you are comfortable doing the work. Fiberglass batts are the easiest material for DIY installation. More complex jobs with spray foam or rigid foam boards may warrant calling in a professional insulator.

How much insulation does a crawl space need?

Aim for an R-Value of R-19 to R-30 for crawl spaces. The warmer your climate, the higher the R-Value you may want to achieve. Use the recommended amount for the insulation type – about R-30 for fiberglass batts, R-20+ for rigid foam, 3-4 inches for spray foam.

Should crawl space vents be open or closed when insulating?

Ventilation is important to prevent moisture buildup. Best practice is to add more vents if needed to achieve 1 sq ft of vent space per 150 sq ft of crawl space floor area. Vents should remain open before, during and after insulating.

Can I use faced or unfaced insulation in a crawl space?

Faced insulation (with foil or paper backing) is recommended to serve as a vapor barrier. Make sure to face any batts towards the floor of the crawl space, not upwards towards the subfloor. Cover any exposed faced insulation with rigid foam or wood panels.

How do I know if a crawl space is properly insulated?

Check for consistent coverage across the entire floor with no gaps. Look for proper R-values for your climate. Vapor barriers and air sealing should be in place. No pipes or ducts should be left exposed. Insulation should have some form of covering or protection.


Adding insulation to a crawl space is one of the most impactful energy efficiency projects a homeowner can undertake. The steps outlined in this guide cover the process from assessing your crawl space, choosing the right insulation, proper installation, and finishing the job completely. Thoroughly insulating your crawl space reaps benefits like a warmer home, energy bill savings, reduced moisture and pests, and improved comfort and air quality. With the right preparations and materials, this is a DIY project many homeowners can take on successfully. Your home will be more efficient and comfortable for years to come after insulating your crawl space.