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Insulating a Dirt Crawl Space: Best Practices

Wondering about the best way to insulate a crawl space with a dirt floor? Read to learn about problems with vented dirt crawl spaces and best practices for insulating them.

dirt-floor crawl space with fiberglass insulation hanging from the ceiling

Condensation in the crawl space causes fiberglass insulation to sag, making it ineffective at keeping cold air from reaching the floor above.

Most crawl spaces have dirt floors, open vents, and porous, uninsulated walls that allow moisture and outside air to enter under the home. In the summertime, when warm, humid air comes in contact with cold surfaces in the crawl space, condensation forms, which often leads to mold, wood rot, and pest problems. In the wintertime, cold air enters through the vents or the block or stone walls in the crawl space, leading to cold floors above the crawl space, which makes the whole house feel colder.

As a result, building scientists recommend making the crawl space part of the home's conditioned space. The conditioned space in a home is an area that's heated and cooled. This strategy for insulating a crawl space involves fixing groundwater leaks, isolating the home from the earth, insulating the crawl space walls with foam board insulation, and sealing the vents and any outside air leaks.

Keep reading for the four major steps to take when insulating a dirt crawl space. This guide includes the best practices to follow to get a clean, dry, healthy space under your home and a more comfortable living space that's easier to keep warm in the winter and cool in the summer.

  1. Fix groundwater leaks and seepage first

    Problem: If water leaks into the crawl space when it rains and pools in low areas of the dirt floor, eliminate sources of water in the crawl space before you insulate. Water can seep or leak into the crawl space in at least four ways, including a) under the footing (bottom support area of the foundation), b) between the footing and the walls, c) through block walls, or d) through cracks or openings in the crawl space.

    Best Practice: Extend your downspouts, clean and maintain your gutters, and regrade the soil around the home’s foundation so that the soil slopes away from the house and so runoff water from the roof and gutters flow away from the foundation. Some groundwater problems might require installing a sump pump and drainage system - check with your local basement waterproofing professional.

  2. Seal vents and outside air leaks

    Covered crawl space door opening

    An airtight, maintenance-free door helps protect the crawl space.

    Problem: If your crawl space has vents, they were probably added to allow the crawl space walls to allow moisture buildup to escape to the outside. The problem with this design is the natural airflow in a house is from bottom to top - air leaks in the bottom (into the crawl space, basement, or first level) and out the top (through the attic or roof). Instead of venting moisture to the outside, open vents in a crawl space allow in moist air during the summer and cold air during the winter. In the summertime, as warm, humid air passes over cool surfaces in the crawl space, condensation is left behind. This leads to mold growth, wood rot, soggy insulation, and attracts unwanted bugs and critters.

    Best Practice: Install crawl space vent covers on the outside of the vent openings of the encapsulated crawl space. Seal the rim joists with rigid foam board insulation and use foam sealant to seal the opening between the top of the crawl space wall and the sill. Use heavy-duty weatherstripping to create a tight seal around the crawl space door. If the existing door is leaking or deteriorating, replace it with an airtight, maintenance-free door.

  3. Insulate the crawl space walls with a water-resistant material

    Problem: A common practice has been to insulate the crawl space ceiling with fiberglass batts, with the goal of stopping cold air from reaching the first-level floor above. Instead, condensation and gravity can cause the insulation to sag and pull away from the crawl space ceiling, allowing cold air to get above it, against the floor. Having damp, sagging fiberglass batts between the joist bays is about as effective as having no insulation at all.

    Best Practice: Insulate the crawl space walls, not the ceiling. The best insulation for crawl spaces resists damage from water and moisture, including foam board insulation. Avoid using fiberglass or cellulose insulation in the crawl space.

    SilverGlo™, installed by many Basement Systems dealers, is an extended polystyrene foam board infused with graphite that helps increase the material's R-value (measurement of the insulating performance) up to 24% over standard EPS foam board. The SilverGlo™ is also lined with a radiant liner on both sides. This silver lining acts as a vapor barrier preventing the moisture from seeping through the walls from evaporating into the crawl space. Inside the crawl, the silver lining also helps reflect heat back into the space.

    To further reduce heat loss, a crawl space repair professional can add foam insulation over the crawl space floor, underneath the vapor barrier.

  4. Cover the dirt floor with a vapor barrier to isolate the house from the earth

    Crawl space encapsulated with CleanSpace

    Encapsulated crawl spaces with foam board-insulated walls make homes healthier, more comfortable, and easier to heat and cool.

    Problem: The main issue with dirt crawl spaces is the moisture from the dirt beneath the home. Even if you've addressed groundwater leaks, water vapor from the ground evaporates and moves upward into your home. This can lead to rotted floor joists, mold growth, and poor indoor air quality.

    Best Practice: Isolate the crawl space from the earth with a vapor barrier. If you plan to use the crawl space for storage, choose a crawl space encapsulation solution that includes a heavy-duty barrier that’s durable enough to hold up to foot traffic, moving of storage items, etc. The CleanSpace® crawl space liner can be installed over the crawl space floor, crawl space walls, and around any obstacles, creating a sealed and conditioned crawl space under the home.

Insulate your dirt crawl space and make your cold floors warmer with CleanSpace®

Along with causing cold floors, vented dirt crawl spaces make homes uncomfortable, unhealthy, and increase heating and cooling expenses. Conditioning the crawl space eliminates condensation, reduces the risk of mold, energy loss from air ducts, and drafts, and makes floors over the crawl space warmer in the winter. CleanSpace® is an all-in-one approach to insulating a crawl space that includes bringing the crawl space into the building envelope by isolating it from the outside and the ground.

For help insulating your dirt crawl space, schedule your free crawl space insulation estimate today.

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