How to Insulate a Crawl Space

Choose the right insulation for your home to improve energy usage

Rigid Foam Insulation on Concrete Wall

Bilanol / Getty Images

Project Overview
  • Working Time: 8 hrs
  • Total Time: 8 hrs
  • Skill Level: Intermediate
  • Estimated Cost: $300 to $600

Insulation is important for keeping any home warm while controlling energy costs. One of the most important ways to keep your home warm is by insulating cinder block or concrete crawl space walls and sealing the floor. It's a labor-intensive, time-consuming project if you choose to do this yourself.

Correct crawl space insulation also can prevent your crawl space from becoming a moist environment that grows and harbors mold and mildew. Controlling crawl space temperature and moisture is a dual process that works toward a common goal: a temperate lower area that will keep upper floors warm, while your home's foundation remains in sound condition.

Crawl Space Insulation Basics

Stapling fiberglass batts between joists to the underside of your home's floor presents several problems. While this method does work in the very short term, moisture eventually builds in the crawl space and grows on this highly porous surface. Moisture-laden fiberglass becomes heavier and begins to sag and fall. This outdated method is not just ineffective but regressive, as well.

Instead, the proper way to insulate a crawl space is to take a multi-pronged approach and insulate the walls, but not the ceiling. The best insulation for crawl space walls and a dirt floor is rigid foam on the walls, laying down a plastic vapor barrier over the floor, and then joining the two items with tape. Professionals may use more spray foam insulation than rigid foam, but it can be difficult to know exactly where to apply the spray foam in a crawl space to be effective.

The idea is to condition the crawl space, much in the same way that you might condition other parts of your home. Rather than trying to cut off the crawl space from the conditioned upper floors, you should bring the crawl space nearer to the upper floor's temperature by leaving off the underfloor insulation, by insulating the walls, and sealing the floor against outside moisture and temperatures.

Pros
  • Keeps pests out of home

  • Reduces moisture

  • Conserves energy

  • Adds dry storage space if you can access insulated space

Cons
  • Expensive project

  • Increases home maintenance

  • Reduces air circulation in home

  • Potential loss of storage space if access is prohibited by insulation

Types of Insulation

Two-inch expanded polystyrene foam board is the favored insulation for crawl space inside walls. Rigid foam insulation works for any type of masonry wall: concrete, block, brick, and even stone.

This type of insulation may be faced with silver foil, with the foil facing inward toward the crawl space. Though the rigid foam is the chief insulator, the foil adds a bit more energy savings by reflecting heat into the crawl space. The foil, too, acts as a vapor barrier.

Crawl Space Ventilation

For this type of insulation system, crawl space ventilation to the exterior is sealed off. All avenues to the outside are shut off or sealed with caulk.

An airtight, insulated crawl space will make your house, as a whole, more energy-efficient by keeping your floors warmer in cool months and thus reducing your heating bills. During warm months, this process works in reverse by keeping your floors cooler and reducing the load on your air conditioner.

What You'll Need

Equipment / Tools

  • Caulking gun
  • Utility knife
  • Rake and shovel
  • Flashlight
  • Fine-toothed saw

Materials

  • Expanded polystyrene foam board
  • Construction adhesive
  • Latex adhesive for foam board
  • Silicone caulk
  • 6-mil minimum thickness clear polyethylene plastic
  • Double-sided butyl tape
  • Poly PVC tape
  • Foam insulation

Instructions

Instructions

  1. Prevent Moisture From Entering the Crawl Space

    Assess your crawl space and locate pooled water or any type of moisture that collects in that area. Redirect downspouts that may be forcing water alongside the foundation walls. With a shovel and rake, regrade soil banked up against the outside of the house, so that water moves away from the structure.

  2. Look at Interior Sources of Moisture

    Water can also come from within the house. Inside the crawl space, shine a flashlight around the underside of the house and look for water dripping down. Look at pipes running through or within joists. Pay special attention to areas of prevalent moisture on the floor above: dishwashers, showers, bathtubs, and sinks.

  3. Seal All Crawl Space Doors and Vents to the Outside

    On the outside of the home, seal up any vents, doors, or hatches that open into the crawl space. Load up the caulking gun with exterior-grade caulk and insert caulk into cracks and holes.

  4. Seal Rim Joists and Sills

    Go inside the crawl space during the day. Turn off the flashlight briefly and examine rim joists and sill areas for outside light shining through. With the light back on, seal up these cracks with the silicone caulk. For large cracks, use low-expansion foam insulation.

  5. Insulate the Crawl Space Walls

    Cut rigid foam pieces to size with the saw and attach the pieces to the inside crawl space walls with the latex adhesive. Aim for as few seams as possible by installing large sheets of foam. Seal seams between the boards with PVC tape.

  6. Lay the Vapor Barrier on the Ground

    Lay the vapor barrier over the entire crawl space ground area. Make cut-outs for every protrusion such as piers and posts. Run the vapor barrier up against these protrusions and seal them tightly with the butyl tape. Seal seams between separate vapor barrier sheets. Conclude by running the vapor barrier about 12 inches up the insulated walls and taping the edges down to the insulation.

FAQ
  • Why should you not encapsulate a crawl space?

    Not every crawl space should be insulated or encapsulated, and a crawl space or foundation specialist can help you decide. In addition, insulation (which helps warm the rest of the house by sealing air leaks) differs from encapsulation (which handles ground moisture).

    If you live in a humid area or flood zone, encapsulation especially is not a good idea. For example, if you see large pools of water sitting in your crawl space, closing up the space will trap the moisture, causing more problems such as mold or structural rot. If you don't live in a humid region or flood zone but see standing water, mitigate the root of the problem before encapsulating.

  • How much does it cost to have a crawl space insulated by professionals?

    Depending on how big—and most specifically how tall—your crawl space is, it can cost several thousand dollars for a foundation servicing company to insulate it. Encapsulation is more costly.

  • Should you put plastic over insulation in a crawl space?

    The foam board for the walls is typically considered a vapor barrier, as well. But if you prefer a more aesthetic look, you can put a vapor barrier over the foam board, as well. Don't put a vapor barrier under the foam board, however, because it can become damaged or pierced when installing the foam board over it.