How to Insulate a Crawl Space

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 your crawl space.

Proper 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 This outdated method is not just ineffective but regressive, as well.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, a better method is the multi-pronged approach of insulating the inside walls of the crawl space with rigid foam, laying down a plastic vapor barrier over the dirt floor, then joining the two items with tape.

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 upper floors' temperatures by leaving off the underfloor insulation and by sealing the crawl space against outside moisture and temperatures.

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


  • 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


  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.