How to Insulate a Crawl Space With a Dirt Floor

Building a house with a dirt floor crawl space was an economy measure, making the construction cost cheaper by not installing a cement floor. Moisture from the dirt can contribute to mold or mildew growing in the area beneath the house. When you insulate a crawl space with a dirt floor, you will reduce heat flow gained or lost through the house floor over the crawl space and reduce the incidence of musty smell in the house. How insulation is installed is determined by whether the crawl space is vented (only the top and sill plate are insulated) or unvented (only the walls are insulated).

Difficulty:

Challenging

Instructions

Things You'll Need

• Box cutter or knife
• Insulation support straps
• Vapor retarder
• Pipe and ventilation wrap

Pre-Insulation

1. Check for and repair any leaks in the crawl space that came from appliances, plumbing or outside drainage. Use wrap-type insulation on pipes and ducts to prevent them from freezing. Clean out any debris, such as that left by construction work, or any stored items.

2. Confirm that the furnace or water heater, if located in the crawl space, are a sealed-combustion design. These units are designed to pull air from and exhaust fumes to the outside.

3. Purchase a vapor retarder made of polyethylene (a heavy plastic) to cover the area of the dirt floor. When moisture evaporates from the dirt floor, it creates water vapor that can lead to mildew and a musty smell. The plastic sheeting helps to prevent the evaporating moisture from entering the crawl space. Tape the seams of the polyethylene as you piece the sheets together. It is important that there be no openings in the sheeting, so take care not to puncture it.

4. Locate and seal outside leaks, including making the door entry air-tight.

Ventilated Crawl Space

1. Purchase rolls of fiberglass insulation wide enough to fit in between the joists of the crawl space.

2. Wear protective clothing and mask when working with fiberglass. Long sleeved shirt, long pants tucked into sock, hat and gloves can provide protection. Cut strips to fit the length of the joist.

3. Push the insulation in place with the vapor side toward the warmer area, which would be house floor (or ceiling of the crawl space). Cut the insulation to fit around cross braces.

4. Secure the insulation with straps specifically designed for this purpose or you can staple chicken wire to the joists. Something is needed to support the insulation even if it fits snugly after installation because it will loosen over time.

5. Install insulation to the sill plate. The sill plate runs around the perimeter of the crawl space and is on top of the cement wall or blocks that support the house.

Unventilated Crawl Space

1. Check local building codes regarding the level of flame retardant needed for wall insulation in a crawl space. This information may also be available through your local hardware store or home and garden center.

2. Purchase blanket insulation with a plan to extend the drop about two feet onto the polyethylene covered floor.

3. Staple the blanket insulation to the sill plate.

Tips & Warnings

• Have your house checked for and protected against terminates before insulating the crawl space.

References

• Dirt Crawl Space Information Center
• Polyethylene Vapor Retarders
• Insulating Floors and Crawl Spaces

Resources

• Read this Article in Spanish
• Crawl Space Insulation